4-Link CAD program

[FullsizeFun]

Has anybody used this before? It seems like a good program that does a lot of the math for you. It uses Excel. I use MasterCam to design my 4link and figure for pinion angles and driveshaft plunge. but this gives you all the other stuff like anti-squat, roll-axis, roll-center, etc. Use this with another cad program like mastercam or autocad and transfer the numbers.

I dont fully understand what the numbers mean, and how they effect the handling of the truck.
I know Somebody could help me here. I need some basic numbers to go by as far as Anti-squat%, where you want your Instant center and Roll-center, etc.

I am going to 4-link my truck. I is a 92 GMC 2500 extcab long bed.

[FullsizeFun]

I have never driven/ ridden in a linked truck, so I dont know how it feels and what is good and whats not. I want to have as much info as I can to get it right. I am mainly concerned with handling characteristics like traction, sway, and duck-walk or rear-stear.

[Ryan_P]

How did you exactly figure out how to put the number in to design it? I want to build a link but all the math is frustrating, and I want it to come out right, or as close to it. Much help needed and any advice is welcome, thanks!

[Josh_K]

This is probably going to be an unpopular opinion. I would forget about all that hi-teck cad stuff and copy a really good link system that works.

I would try to find some one that knows what is needed in a link system for an off road vehicle and pick there brain to find out what works and why. I am sure that this cad program is the hot stuff for on road cars, but is it really correct for an off road car? I recommend that you get your mind around all the geometry issues that go into a good link system and do it the old fashion way.

I think that this cad program would work great for a F-1 car, but would guess that it will overemphasize issues that arent that important with a off road car and underemphasize issues that are more important.

Josh

BTW, good luck finding some here that really knows how to do it and is willing to give away their trade secrets that pay their bills!

[FullsizeFun]

Thanks Josh,
I understand your point. That's kinda what I am asking here is for some input on what works. I know there is a ton of aspects of the design and handling, and some are more important than others. I know alot of people build 4links without knowing much about it and only focussing on wheel travel. But then what if it handles poorly, like not enough anti-squat, or too much rear-steer. I am just trying to find out a tollerable range.
Another thing, how much pinion angle is tollerable? My design has 3.7 degrees up^, of pinion angle at full-droop.

I actually got this program from a rock-crawling website. They are all over it. I have been reading all that I can find about 4-links, but most of it has been rockcrawling. they have different priorities. The more I learn the more involved it gets. The worst part is I have never driven a linked truck.

[curt]

Look at http://www.race-dezert.com/ubbthreads/favlinker.php?Cat=&Entry=315&F_Board=shop&Thread=3 7729&partnumber=&postmarker= for the lower links 101 thread. It's a very good read. I think a good program can get you close, we drew all our stuff in Autocad and did some analysis but it's like shocks and springs, rarely does it come out of the box perfect the first time even with the programs. I like what we built but it's time to make adjustments and try again...Curt

[Josh_K]

This is one of those times when I wish I could just keep my mouth shut! But I cant, for some stupid reason I am always finding myself trying to help some one out.

Now for the disclaimer, there is more than one way to build a linked rear end. This is just my opinion and I am in no way an expert about this. So, please dont blast me about Mike Smith's latest so and so isnt this or that way. This is just a little of what I have learned to this point. I am not going to tell you every thing. Some stuff needs to be learned for your self but I will lay down some basics to a decent design.

The main difference in link systems depends on the type of car or truck it is going in, 7, or 8 with frame rails or a t.t./truggy without frame rails changes the mounting location significantly. Space frame design is clean sheets of paper with nothing limiting the mounting locations like frames, cross member or cabs. I will use a full frame truck to explain what little I know.

First off any good link system needs to unloads power/torque as the axel moves up ward in its travel form full droop to bottomed out. Another way to say this is that the pinion needs to point further upward as the axel moves upward. I think this is the most important aspect of a good off road linked rear end. If it doesnt do this its junk!

Also, the longer the lower arms are to the upper adds squat to the truck when the power is applied. This does a number of things. Like carrying the front over bumps better, it reduces kicking and flies the trucks nose up in the air. The upper should be around .66% for the lower in length. And the links should almost be parallel to each outer, converging in front of the truck (MAJORE HINT).

This system for a reason I wont explain will limit your travel to around 24+". I will let your figure out why for your self. But I will give you a hit, more travel is not always better because you are losing something to gain the travel.

Now, again this is just my opinion and If you wont more info you are going to have to bring your truck to my shop and pay the going rate.

Good luck,

Josh

[Josh_K]

BTW, toss the cad program into he trash. Its going to give you a bunch of theoretical points to mount the links that you cant get to because the cab is in the way or there is a frame in the way.

Its just a time waster for computer geeks that will most likely never build a working link system.

Josh

[motoxscott]

[ QUOTE ]

Its just a time waster for computer geeks that will most likely never build a working link system.


[/ QUOTE ]

hahaha ... or a way 15 year olds think they can link their truck with their new Lincoln Weld Pak 100.

[FullsizeFun]

Thanks again Josh
Thats all I really needed was some basics to go by. I just needed to know what the most important aspects were for a desert truck. Thanks for the "hints". I know there is a lot I can only learn by doing it, I just wanted to have a better base to start with, so tuning it would be minimized. Who knows, maybe my first setup wont work at all. but now I will know why.
Thanks again

[FABRICATOR]

Josh, you are right on target; especially every part of your first reply. I know one thing you loose with more travel; money!

[ntsqd]

Well, you're right. It is an unpopular opinion, at least with me.

Maybe some background is in order. That program was developed by two guys over on the ColoradoK5.com bbs. One is an Engineer with a very down to earth pratical approach and the other appears to write software code for a living and has a very analytical mind. Both have spent considerable time and energy tweaking the program to be the most useful that they can. Note the rev number is 2.1, they started many, many months ago at 1.0

Tthey are more crawling oriented guys, but the program gives you valid numbers and it has been intended to generate rear live axle linkage designs from the start. It's still up to you to place the pivots such that the truck behaves as intended. The idea with the program is to enable iterative solving w/o cutting a bunch of metal. So you can get close on the computer, and then go out and model it to check that it does what you want it to do.

This Excel file is VERY basic, you do not want to see the program used to design F1 suspension.

BTW, a short upper - long lower arm system by itself does NOT solely determine Anti-Squat.The location of the pivot points is also important. If you will take the time to play with the program a little it will teach you that.

[JESSE_at_TLT]

NTSQD-
Thanks for the background on the development of this spreadsheet. It seems like a useful tool, and this type of software is available for anyone who has the time/interst/ability to use it. Not as a substitute for real fabrication, but as a starting point.

[FABRICATOR]

TS, I don’t think anyone was knocking the Excel program or the value of CAD as a tool. That Excel program is actually very neat and more or less correct. I would question the “percent” of anti-squat reference though. The part that does not compute is that there is always someone with a computer and a CAD program trying to start from scratch with new suspension numbers. And just as often there is someone trying to design an all new suspension from scratch. They immediately get stuck when they try to relate the CAD information to reality. The reason it ends up this way is because of all the variables.

To point out the variables present in many of the questions here; in stock cars to Formula 1, the differences between the cars in nearly any criteria or specification vary by no more than just a very few percent. Off road cars vary by up to several hundred percent in many categories. Part of this is because it covers buggies, trucks, and everything in between. Then we throw in pre-runners and chase vehicles on top of that. Then there are the variables in design within each category of vehicle, the intended usage, power available, driver’s skill level, and budget.

A majority of the geometry questions are really looking for design standards. The fact that you have a choice to do things the way you want or need to, make standards intangible. There is also the omnipresent crossing of technologies such as stock verses race criteria, off-raod at high speed vs. grocery getting, and looks vs performance.

Unless you have built and used that exact make and model of vehicle for the same purpose(s) (another variable) there is no way to give an accurate and reliable answer to a majority of the geometry questions posed here.

[ntsqd]

"toss the cad program into the trash"

Someone was knocking it.

I did say "It's still up to you to place the pivots such that the truck behaves as intended." I think it is a very useful tool to educate yourself on what affects what. For those that want to. Those that don't care won't bother with it, but to turn someone with an interest in it off and tell them to go out and start cutting metal is very, very poor advice.

The percent Anti-Squat terminology has never made sense to me. Percent of what value and where? Define 100% Anti-Squat, and then tell me what that means. Is 101% Anti-Squat actually 1% Anti-Rise? Or is 99% Anti-Squat actually 1% Anti-Rise? So far the whole terminology is not terribly descriptive.

[FullsizeFun]

360 people have downloaded it since I posted it.
I am using this program along with Mastercam to design it. It is much easier to move a pivot point 1 inch on the computer than to do it on the truck. I think I have a pretty good design to start with. I can make adjustments after I drive it. I can post the MC9 file if anyone has Mastercam.

As far as the anti-squat%, I am going to try a few positions for the lower trailing arm, to adjust it between 50-80%. whatever that means.

[curt]

Just to interject a slightly contrarian view, I mulled over the 4-link question for 2 years. I read all the interaction here and elsewhere, bought the chassis engineering book(which is an interesting read-some applicable, some not) and finally sat down to design our system.

Getting around the whole mounting point issue was done by first measuring and drawing the frame, cab, and other items that might be in the way so we started off knowing where we couldn't put it. From there, one of my guys who is very proficient in cad, took the basic parameters that I was trying to achieve drew the parts and checked the angles. Cad provided one huge improvement over the old fashioned way, from the computer to the truck, pinion angle changes and plunge were very close to what we calculated.

We would have never gotten to a finished part without the aid of the computer because there was going to be way too much time investment without first getting a decent feeling that we were heading in the right direction lacking the experience some here have.

All that said, I am glad that Josh got a little riled up cause I think I learned 2 new things in his post and can check our setup to see if we accidently got withing his parameters. We were already fairly happy with what we built except off the line traction, antsquat was designed to be around 105% but with slight fabrication adjustments I'm rechecking that this weekend(but we're building for short course primarily). Some (part-time)geeks do build, and whatever tools that you need to help your understanding of what you're trying to achieve should be tried, as long as you really know where you're going...Curt

[Stephen]

my understanding is that 100% antisquat is where the leverage from the torque on the rear will cause enough upforce on the suspension to negate any drop in the suspension due to weight transfer under acceleration. Basically, when the truck leaves the line the rear would stay at the same ride height despite the front losing weight due to acceration weight transfer.
What this means to me is that the suspension is going to get stiffer whenever it really hooks up which could upset the suspension in rough terrain.
I've linked up the rear of a couple vehicles now but haven't been able to play around with different numbers on one vehicle yet. I have seen this effect in reverse in the front of my K5 though. When I brake hard, the nose doesn't drop much and if I brake hard in the rough, it gets way rougher than it seems like it should. The front has a lot of anti dive which would be anti squat in the rear. The same pricinples will apply in reverse to the rear suspension.

[FABRICATOR]

Re: “Basically, when the truck leaves the line the rear would stay at the same ride height despite the front losing weight due to acceleration weight transfer.’

For all practical purposes, this is not possible in the dirt. There must be squat to allow traction and weight transfer to occur. The increase of all 3 does not stop until a) power levels off, b) you run out of wheel travel, or c) you are happy with the wheelie angle.

Re: “What this means to me is that the suspension is going to get stiffer whenever it really hooks up which could upset the suspension in rough terrain.”

It sure would. Weight transfer and traction go hand in hand to allow acceleration to occur, especially in the dirt. You can’t have one without a significant amount of the other two. Nothing gets traction in the dirt like a car or truck with long travel and soft suspension, be it solid axle or IRS. The comparison to front brake application on a K5 is not very applicable to a vehicle having long travel. The lack of front dive on hard brake application is more from the stiff short travel than anti-squat geometry. It gets rougher on rough terrain because you are using up the travel. It’s not an exaggeration to say that the squat and dive available on a healthy TT is very nearly enough to do a wheelie or stand the truck on its nose. If you are trying to climb hills or accelerate, squat is not only a good thing, it’s a necessity. Anti-squat is anti-traction and anti-acceleration. Except for using up too much travel and/or not being able to see where you are going, the more squat the better.

[ntsqd]

Quoting Herb Adams:
"Because of the springs, the weight transfer can often be seen at the rear of the car as it 'squats''' during hard acceleration......Anti-Squat can counteract the squat force to keep the rear of the car level, and it can actually be made to raise the rear of the car during acceleration.......Because any force that can raise the rear of the car will need to have an equal and opposite force pushing against the pavement, you can use anti-squat to increase the tire loading during acceleration....(ref Chart 9-1) You can see that if the rear tire loading can be increased with anit-squat effects, there will be more rear tire traction available for acceleration coming out of corners."

What I get from this is that weight transfer happens independent of squat. Dragsters have weight transfer and they only have the rear tires for 'suspension' which doesn't make for much squat or anti-squat.
A vehicle with a lot of anti-squat will essentially lock out the rear suspension when under power. For desert racing this isn't a great idea, but that is why I think we see a lot of trucks with little anti-squat.

Anti-dive in the front is the same geometry, except reversed, as anti-squat in the rear. Anti-dive I find easier to visualize. I think of it as a flat cam surface. If the link geometry is anti-dive, the cam surface forms a wedge trying to lift the front. The more anti-dive the shallower the angle. Lots of anti-dive in the front under braking would have the same effect as lots of anti-squat in the rear with power applied in terms of locking up the suspension. There's a reason that motorcycles have those brake caliper rxn rods in the rear. If the rxn torque were applied to the rear swing arm it would lock up the rear suspension under heavy braking.

Stephen, post a pic of your K5. It's not exactly stock in the wheel travel dept.

[FABRICATOR]

That first paragraph is a grotesque twist on anything for the dirt. I am trying to relay what works in the dirt, not comparing apples to oranges and in a totally different environment. Traction is many, many times greater on pavement than in the dirt. That means it is much harder to come by in the dirt. That means squat and dive are way more important and you need way more of it. What’s your anti-squat do when you hit the brakes? What does your anti-dive in front do while you’re hard accelerating? How does it relate to long travel suspension? And how does all the anti-stuff affect how the vehicle takes the bumps? What are Herb's thoughts on that?

That is not accurate about dragsters either. There are two kinds of non-suspended dragsters; short wheel base and long wheel base. The huge tires provide quite a bit of squat for a short car. The long wheel base dragsters have very specialized flexible chassis that raises up to 5 inches or more in the middle! Without that type of chassis the T/F rails would do nothing but smoke the tires. Dragsters also are setup with a weight bias dedicated to acceleration. They don’t have to go over bumps, stop hard with brakes, or turn.

Most all dirt bikes quit using the “floating” rear brakes at least 15 years ago. The calipers are anchored to the swing arm and all works great. I converted a dirt bike to a floating rear brake once. Talk about rear rise and front dive. If you weren’t ready, it would almost throw you over the bars on hard braking. Except for in-board disc brakes, I haven’t seen any floating rear brakes on off-road race cars, have you? IBDB’s work like crap with long travel because the suspension is free to rise all the way up.

FWIW, I don’t think it fair or wise to keep throwing in the nuances of low slung pavement race cars, especially without your own guidance. Differing opinions are fine but doing that just causes more confusion. I can post quotes form several different suspension books in minutes that don’t even agree with each other. Your thoughts would be more helpful than those of Herb Adams.

[ntsqd]

I see it the other way around. Because traction in dirt is so limited, the effects of linkage design have to be extreme for much difference to show up. Subtle changes will only show up on hard packed sections, if at all. I do not see any point in deviating from what's known to work on pavement, only in extrapolating it into the dirt. Some of the fine tune methods & ideas will fall by the wayside as there just isn't the traction available to make them worth pursuing. The basic laws of physics haven't changed, only the available traction has. Just expect that the results of any changes will be less extreme or obvious, which will require a more acute understanding or study of what actually happened.

The dragster I worked on did not have the sliding pillars design that the current frames do. It was a typical rigid trestle design dating to the late 60's. Right as I got out of drag racing the team took delivery of a 'slider' chassis. The purpose of the flexible dragster chassis, as I understand it, is to delay the weight transfer by redirecting some of the lifting force into deliberately bending the chassis. With current tire technology the engine would bog on the line w/o the flex frame design. A slider frame allows the engine to spin the tires slightly at the start. This does two things, it lets the engine come up in rpm's and while at low speed it scrubs the tires clean of any debris it might have picked up from backing up after the burn-out. It might be argued that a change in the clutch tune is all that is required, but that hasn't proven feasible to do. Armstrong came the closest with his staggered 9 finger pressure plate and staged release of the t/o bearing.

About 15 years ago is when progressive linkage first appeared on the rear of m/c's. Look past that. My point was that external forces on linkage, when the design of the linkage is highly biased in some direction, can have the effect of locking out the motion that the linkage supposedly offers.
The Lawwill rear suspension design Schwinn used for a while on their DH frames is a prime example of this. With the brake caliper mounted on the 'toggle link' the rear suspension became rigid under braking. Mert made the bits to to move the brake torque rxn from the toggle link to the frame and the rear became active under braking. If you've seen "On Any Sunday", that driveway is where he worked it out and tested it.

That Stephen's LA linkage could become locked out or head in that direction under braking comes as no surprise to me. I do not think it possible to design LA linkage to have both effective anti-dive and for it to not lock out under braking. You can have offset percentages of each, but not both at the same time. Live/beam front axle linakge anti-dive geometry is a mirror image of rear anti-squat geometry, and the thrust vectors are mirror images as well. So what Mr. Adams has to say on the anti-squat topic also applies, it's just up to us to flip it 180* and extrapolate it into the dirt.

[Stephen]

http://www.offroaddesign.com/wallytub3small.JPG

It's not a TT or even an 8 but with 16" of coilover travel front and rear, it's enough to start getting a handle on how the suspension works. The front suspension was set up around keeping the driveshaft angles usable through the range of wheel travel, not any suspension reaction so I have some drawbacks to live with.

As for the traction on dirt vs. pavement, I think we need to look at it more as a matter of degree rather than 2 totally different extremes. An example of this would be some of the tire limited race cars which really don't have that much traction on asphalt compared to a virtually unlimited dirt stock car on a tacky track compared to an offroad race vehicle on a hardpack maintained road compared to an offroad race vehicle in a silt bed. The point is that all are at different points on a traction line, not totally different, incomparable situations.

The one thing that is a major difference, mostly in cornering is that with dirt, either with a offroad racer or a stock car on a heavy track, we work IN the surface as opposed to working ON the surface with asphalt. When you're digging the tire into the surface some body roll and weight transfer is desirable where you really dont' want that to happen with asphalt because it unloads the inside tires. We want to unload the inside tires to make the outsides dig in and provide more cornering traction. I'm not sure this applies to straightline acceleration though.

No matter what happens, weight transfer will occur on acceleration, that's just the laws at work. Where the suspension matters is what we do with the extra weight on the rear end. If we use it to jack up the suspension which will make the whole vehicle rise up or just let the rear squat and soak up the weight in the springs.

The problem I've seen with having high AS numbers with variable traction surfaces is that you get a lot of bouncing or suspension upset since the upforce on the vehicle changes when traction is lost. This indicates to me that we don't want a bunch of anti-squat for a dirt racer. And this seems to be how most of the successful racing vehicles are set up through either careful design or trial and error.

Just to illustrate that the program isn't perfect (nothing is), I don't think we can assume a single point for the CG on any long travel vehicle. Reason is, as the front suspension unloads, you pick up more and more of the unsprung weight and the pitch action will raise the CG and move it fore and aft. I've started using the program with a range of numbers for CG and fore/aft location to get a feel for what happens in the real world. Hopefully it's one more step in a making it a good model.

That's all I've got for now.

[Josh_K]

Well Thom from looking at your responses to my posts its clear that you dont mind to disagree with people in a blunt way so I will respond in kind.

Thom, I admit that with all your engineering knowledge, your opinions are mostly interesting and accurate, but you are WRONG ON THIS ONE. And I mean 100%, 180 deg. wrong. I think you are way to caught up in the cad programs, laboratory theory, and pavement designs. Off-road racing has nothing to do with pavement of even rock crawling and dosent happen in "theory". It is 100% different and needs to thought of in a "out of the box" method that typically isnt common for engineers.

Lat me clarify one thing before I start. I dont understand the term anti-squat. This is a misnomer. I think what most people are talking about is squat. Squat is when you hit the gas and the rear lowers and front rises and weight is transferred. Anti-squat would raise the rear end and puts weight on the front. I think some one like the Vessels team 20 years ago, when they first developed the linked rear end coined this incorrect term to throw every one off that tried to do a linked rear end. It would ruin a linked rear end to rise under power, diving the front end into bumps and whoops.

Now, I.M.H.O. there is only one thing that is important to an off-road racing linked rear end. SQUAT! I will go through some of the basics of design further on but forget about power transfer, weight transfer and all the other stuff. This is what smart people like to talk about and baffle others with that dont have a clear understand of what is really happing. Just make sure that when you build a liked rear end that the u-joint on the pinion points further upward as the axel moves upward.

Here are some basics. The way to set this up is to mock the axel up in the vehicle at full bump (up) with the lower links on. Make them as long as can fit in the truck. With the driveline in, tilt the rear end back till the u-joints nearly bind. Now place your upper link(s) form the top of the third member to wear ever you get it to fit to the frame. Try to get the top link some where in the ballpark of 60% to 66% (could be more) of the lower link in length. But even more important than upper link length is that you wont the links to be as close to parallel as desired. The reason I say desired is because this and upper link length is how you will control pinion angel change as the suspension cycles though the travel. Now, once you have built the upper as long as possible and as close to parallel as possible. Cycle the axel through its travel till the front of the pinion lowers to the point that it nearly binds and this is the amount of travel that you will get.

If this is a space frame design the links could be nearly the same length but then you would wont a diverging (separating) angel on the links as they go forward.

With your typical 1350 u-joint you should come out with around 24" to 28" of travel. If you have less travel you could have too much pinion angel change. More travel could mean to little pinion angel change. Of course you could have some custom u-joints made that give more angel for more travel too. This is also figured with a drive shaft that is about 54" long.

Now spend some time talking to other that have done this. If what they are telling you differs greatly form what I have said here, walk away and look at some else truck to get ideas about mounting locations.

This is just the tip of the iceberg. So have fun wasting metal and remember, all you have to be is just a little smarter that the steel.

Thats my opinion. Hope it helps.

Josh

[mexracer10]

By far the most accurate and simple way of explaning it, Its sound smarter than all of the other threads ever posted concerning 4 linked rear ends. In every other web site desert rides, desert rangers and here terminolgy has been criss crossed every which way and thousands of theories have been expressed correct or not. but this is the easiest to understand. I just hate it when people try to sound smart by using fancy 5 doller words in explaning something they know very little about. Just makes for more confusion. Answer 1 question and have 5 more.

[curt]

[ QUOTE ]
Lat me clarify one thing before I start. I dont understand the term anti-squat. This is a misnomer. I think what most people are talking about is squat. Squat is when you hit the gas and the rear lowers and front rises and weight is transferred. Anti-squat would raise the rear end and puts weight on the front.

[/ QUOTE ] Exactly, what I AM talking about is to raise the rear end under power to provide more rear tire traction, or at least apply more pressure (less squat) under acceleration. ANTISQUAT the term, from the places I've read about it is indicating a condition of negative squat. So whatever term gets applied, it is a potential source of EXPERIMENTATION... Harris dirt chassis, one of the largest producers of IMCA roundy round dirt and asphalt cars have been doing it for years to create a very desirabe traction condition on a smooth surface. Now I'm not saying this is smart or even desirable on a desert truck due to all the other factors, I just don't know.

A couple of opinions and facts from a small truck perspective....
When you are lighter it's harder to get traction since there's less weight transfer in the traditional ways.

4-links "seem" to have more trouble "dropping out" unless the hangtime is reasonable due to the lighter components vs the preferred valving to do all the other stuff it needs to be good at (Heard it from both King and Sway-A-Way). If the truck, under power, would help push the suspension out it might be a desirable thing, then again it might just force the nose into the next whoop. Just don't know without some experimentation.

Mechanical traction is achievable using different methods of suspension design and/or set up. No matter who's information, knowlege, data or whatever some people do better than others at optimizing the equipment they have. The same 20% of teams win most of the time in any motor sports, they must know something or they must be willing to try about any hairbrained scheme to see if it works.

On to our problem......
What I'm trying to cure is an unacceptable amount of traction loss off the line in a shortcourse truck. Since I switched to a 4 link from a little bitty stock length Toyota leaf design, working the angles on the computer, it appears that the short more uphill lower pivot creates more mechanical traction at the cost of wheel travel. The back end is waaaay softer than it was, with far more squat but less traction going from a standstill to 30 or 40mph. For 2 years we had the holeshot for almost every moto(15-20X) at the shortcourse, since the 4-link(4 motos), we haven't gotten any. The truck handles way better for everything else but the loss of the holeshot in an 8 lap (1-1.5 miles per lap) race means you're probably not going to win. I've got 10 years experience with the A-Arms but since I don't have the years of link experience of others and am trying to think outside of the box to help our competetive efforts, I'm willing to learn from thos that have done that as well as experiment with concepts that might be outside the normal. I remember last year when RG won Sears Point, he made a comment that he and his own shop had helpep the Childress team with the suspension design so maybe the two disciplines can learn something from each other...Curt

[Stephen]

Ummm, Josh, don't look now but I think you're agreeing with us about the desirable torque reaction.

I think you misunderstand the physics of acceleration though. Weight transfer takes place no matter what you do. It's whether or not we let the springs compress with this weight transfer that's decided with the suspension geometry.

As for engineering having no place on a desert race vehicle, if you really think that and you compete, you're not going to remain competetive. Every motorsport in the world has had guys with that opinion and they all get passed by teams with better engineering. Do you think NASCAR is dominated by guys that gave up trying to understand suspensions? It's a different medium but you still need to think. Good engineers put people on the moon in the 1960, why the hell can't they figure out how to make a truck go fast in the dirt? Do you really think that desert vehicle are too hard to figure out?
I'm done on the engineer bashing/defending. If you don't want to think about why a suspension works, don't do it. But, don't bash those that do want to understand more.

Curt, how about putting multiple mounting points in for the rear suspension? That way you could test some different setups to see if you can get your holeshot back.

[FABRICATOR]

It's O.K. if Thom stirrs the pot...and slow 4WD crawling is a very different world.
Re: "No matter what happens, weight transfer will occur on acceleration,"
Where do you think the acceleration comes from?

I just wonder why there is endless extrapolation from pavement cars by so many, rather than reverse engineering of an existing and truly remarkable technology. Design or research should come from successful off-road racing vehicles not pavement race cars or books about them. Off-road suspensions do not need to be engineered from scratch like many here attempt to do. When a single vehicle can do a wheelie from acceleration, stop on a friggen dime, make fairly decent turns, plus take huge multiple bumps in stride, it means there is not much left to extrapolate. For its intended purpose, the level of performance that has been reached is not far behind that of Formula 1. Except for perhaps push rod or linkage suspensions, off-road suspension technology has exceeded what pavement racing has left to offer. The best suspensions out there are not perfect and could use refinement, but the basic requirements are a done deal. As far as desert racing, to argue whether squat is even useful or desirable or not, or how to apply more pavement rules to the dirt, or what provides traction, is stone age discussion, not pertinent engineering.

[Stephen]

I think you just hit the nail on the head. We (I say that loosely knowing what I'm after and guessing the rest) need some standards to build to. That suspension you're talking about is the one we all want. What makes it tick? What happens when we apply it to a truck with a V6? What happens when we put it in a truck with a frame? What happens when we use a smaller tire? What happens if we can't have 30+" of wheel travel?
With some standards to build to and some idea of the principles that make the best suspensions work, we can apply the principles to our own vehicles. If all we know is one TT had upper links 66% of the length of the lowers and it works, we have a hard time making that work on a different vehicle.

I think a lot of this is arguing but really agreeing. It seems pretty well known that we want the rear to squat under acceleration but if Curt's suspension does that, why does he lose the dragrace starts he used to win?

[partybarge_pilot]

Curt

Part of your lack of traction off the line might be due to the fact that your CG has moved forward with the switch from springs. Do you have corner wieghts from before the switch?

With sprigs roughly 50% of you susp wieght is behind the axle. 4link= 100% in front....

[Josh_K]

I think some of you are not getting it! I think you are trying to make this a way bigger deal than it is. All you need is squat to happen when you hit the gas. Its that simple. If you have a stock V6 you might wont 3 to 4(w.a.g.(wild as guess)) deg of pinion angel in one foot if you have more power you might wont less pinion angel change per foot. Set your links up with a diverging angel and the truck will squat when you hit the gas. Its that simple. It might squat too much thou, but a cad program is not going to tell you how much is good or not enough either. That comes from experience.

Every one can talk about all the pavement this and that or rock crawling they want but its got nothing to do with off road racing.

And by the way NASCAR does have engineers and good ones. But there not that great because the geometry that is in those cars goes back to the 60s and there scared big to make any changes to the chassis to improve safety. They dont know what it will do.

Josh

[hoeker]

curt, did you ever compare your trucks pivots to the CORR pro-lite standardized chassis? i'm not saying it's the best possible short course rear suspension but i can' t remember when a non-standardized truck won a pro-lite race.

great reading guys, this thread is getting good!

[Kritter]

"And by the way NASCAR does have engineers and good ones. But there not that great because the geometry that is in those cars goes back to the 60s and there scared big to make any changes to the chassis to improve safety. They dont know what it will do. "

The reason they dont want to make them more foldable in a crash is you lose rigidity of the car and then EVERYTHING IS A VARIABLE and then the car becomes unsafe...


Somebody with a good grasp of the engineering principals and great knowledge of 4 link suspensions can design and build one cut and dry...

Tuning an infinite variable race car that is now "safer" that goes 200 mph is not cut and dry no matter how much knowledge the engineers or drivers have..when dealing with a large number of variables...it isnt cut and dry and hence the reluctance from nascar engineers to change up the specs on the cars. They want something cut and dry as the cars are now...just like a 4 link.

I dont have a need to build a 4 link, dont want to build a 4 link, but if I did I would look at ones that work(whatever that means), compare them to theory and go from there...like any good designer would do. I wouldnt go outside and get the torch and stickwelder and start wasting metal until I got it right...when I could bang something out very close to what I would want with the ability to adjust it to perfect with a little bit of time on a computer.

"Never enough time to do it right the first time but there is always enough time to go back and fix it the second time"

A 4 link is not a big deal when you go copy somebody elses....it is a big deal when you start from scratch and want to know the whys and hows that anybody who likes to design and build things usually likes to know...

althought my ol lady as well as plenty of other people im sure think "I dont give a **** how it works...as long as it works" but a lot of people dont think that way...and I would hope people building race cars/trucks dont think that way.

[Stephen]

[ QUOTE ]
I think some of you are not getting it! I think you are trying to make this a way bigger deal than it is. All you need is squat to happen when you hit the gas. Its that simple. If you have a stock V6 you might wont 3 to 4(w.a.g.(wild as guess)) deg of pinion angel in one foot if you have more power you might wont less pinion angel change per foot. Set your links up with a diverging angel and the truck will squat when you hit the gas. Its that simple. It might squat too much thou, but a cad program is not going to tell you how much is good or not enough either. That comes from experience.

Josh


[/ QUOTE ]
So what happens if we have parallel or diverging links but no little or no pinion angle change? I'm not asking to be an [I have been warned], just curious about how that works since you're relating the suspensions performance under power to pinion angle change.

Just to fuel the software fire, theres a company in new mexico (Auto-ware.com) that has a lot of performance analyzing software for suspensions and engines. It's mostly geared toward stock cars but as long as the limits aren't exceeded by our wheel travel numbers, they could be useful to model our stuff too. The front suspension software could be the cat's butt for building a-arms.
The reason so much asphalt and dirt stock car modeling comes up is they're the only ones with the number of cars to drive the R&D required to make up the software. And as long as we use the models to get what we want, who cares what the intended application was.

[Josh_K]

I really dont think that I am that smart, but I guess that I must be a genius. Because this is so simple to me.
Making this simple: If your upper link is shorter than your lower and both links are parallel, when the axel move up and down the pinion angel will change. Period! Thats it.

Now the reason I talk about squat and being on the gas has everything to do with torque. When you hit the gas the bottom of the tire move away form the front of the truck. The opposite effect of the torque applied to the axel trys to twist the axel house top rearward. This twisting pulls on the upper links that are attached to the frame and if the link is positioned were it is point up (as they all do) you will pull the frame down and now the truck is squatting under power the nose is up. Bring on the whoops.

Now the next question is; Are you using up your up travel like this? Yes this is true. You better adjust you shock so the axel doesnt pack and drops out for the next bump.

Do springs have some thing to do with all of this? Yes, a lot.

This squatting under power also aids in the truck flying nose high in the air. Have you ever wondered why a rear engine buggy typically flies nose low and front engine truck can fly nose high?

This is my opinion; I attribute this mostly (and I say mostly because a lot of thing are happing at the same time) to the fact that the rear end is under power as the front tires leaves the lip of the jump. If the power stays on as the front tires are in the air the torque will keep the rear end in the squatted position not allowing it to push the rear end down as the rear end is rolling up the face of the jump, remember that the front end is in the air so if the rear end unload it would start the motion that move the truck into a ever increasing nose low position as the truck flies through the air. Buggies swing arms cant do this so that is what I attribute buggies fly nose low to.

I am done now. Have a nice day.

Josh

[DPpatrol]

Time to define what anti-pitch geometry(whether its anti-squat or anti-dive) really is.

Percent anti-pitch is the percent of the weight transfer which goes through the suspension arm as opposed to the spring. Say we have a car with 0% anti-squat. What this means is during accleration foward, all the weight transfered from the front wheel to the rear wheel will go through the spring, the springs will therefore compress and there will be squat of the rear end. Now if we have a car with 100% anti-dive all the weight transfer will go through the suspension arms and none will go through the springs. Therefore under foward acceleration the rear springs will see no additional load, therefore they will not compress and there will be no squat. 50% anti-squat means half the load in the arm and half in the springs and so on.

What we see here is that squat and anti-squat affect only the pitch of the car and have no direct affect on weight transfer, thus wheel loads, and thus traction. Weight transfer under foward or rearward acceleration (longitudinal acceleration), is only affected by 4 variables: wheelbase, cg height, mass of the car, and acceleration.
Weight transfer = mass * acceleration * (cg height / wheelbase)

The reason anti-pitch geometry affects weight transfer and traction is because a when a car pitches (squats or dives) the cg height drops. As seen by the weight transfer equation, the larger the cg height, the more weight transfer. So anti-squat prevents lowering of the cg under foward acceleration which has the effect of increasing weight transfer to the rear of the car in comparison to a squating car.

Curt, this may be why you are no longer getting the holeshots. You yourelf said that your truck squats alot more with the link suspension than with the leaf springs. As shown above squat = less weight transfer equals less traction at the rear wheels under forward acceleration.

However, as I believe Josh and Chuck are trying to say, when designing a linked rear suspension there are far more important variables to consider than just anti-squat such as travel and pinion angles. Also anti-squat values go all over the place during suspension travel, especially when you consider a vehicle with over 20" of travel.

Also like I said earlier anti-squat cause wheel loads to pass through the suspension arms instead of the springs. As one can imagine this is bad news in an off-road car (can you say harsh ride).

I asked a similar question about anti-pitch geometry on this board a while ago and in my opinion Chuck's statement about them in an off-road vehicle sums it up:

"2. Forget the anti-dive. This would translate into poor stopping and anti-smooth ride.
3. Forget the anti-squat. This would translate into poor traction and anti-smooth ride. "

This has been the longest post ever so I'll shut up now.

jason

[DPpatrol]

"It is 100% different and needs to thought of in a "out of the box" method that typically isnt common for engineers."

Josh,

first of all there's no need assualt engineers as a whole, there are alot of dooshbag ones and a lot of good ones just as there are alot of dooshbag fabricators and good ones.

Also since when is "copy(ing) a really good link system that works" considered thinking "out of the box". I would consider trying to understand why a really good link system works, and then improving upon it a better example of thinking outside the box.

jason
stuck in a box

[Josh_K]

Ok, ok, not all engineers are bad. But you are doing a classic engineer foopaw. You are talking at such a high level that a average person cant take the information that you have given and put it to use. I have a deg. in aerospace maintaince and that gives me some training to understand what you guys talking about but I am willing the bet that what you have said here has gone over the heads of the vast majority of the people that will read it.

So I ask you. What good is all that info when the users cant make any since out of it?

Josh

[DPpatrol]

"What good is all that info when the users cant make any since out of it?"

I agree completely. I tried my best to explain it in simple terms, however if I failed and ended up just complicating the matter with engnineering "foopaw" I appologize. If I came off as trying to speak arrogantly at a higher level, that was unintentional. My aim was to try to explain a difficult concept as simply and correctly as possible. The problem is that vehicle suspension and kinematics ARE a difficult concept and they only begin to start to make sense when you work with them hands on.

jason

[rommel]

I'll agree with Chuck that most of us weekend-in-the-garage fabricators are wasting a lot of time trying to re-invent the 4-link. Our time would be much better spent working out refinements on a known system that works well!
However, I don't know Mike Smith, or Craig Stewart, or Nestor Berardi, or any other big-time fabricator, and I don't have access to all the desert teams to pump them for information. Even if I did, I respect the fact that they have $$$$ and time into getting to where thay are on the learning curve, and I don't expect them to give anyone else a head start.
I'm sure that everyone that's reading this would love to have all the specs of the setup that Chuck spoke of earlier, so that we can start playing with the adjustments to make it work on our trucks, rather than playing with a blank CAD screen!

[ntsqd]

First off, I've been working a long string of 12's & 14's with two hours of daily commute tacked on top, so if I was uncivil I apologize.

The reason I keep hammering on the pavement design stuff is that the physics of linkage geometry is laid out for you. All you have to do is read it, chew on it, and try to grasp it. Now hopefully you have a a foundation to stand on. The reason I've been saying we need to extroplate that to our application is that the design goals of a pavement car and a desert racer are NOT the same, but with an understanding of what effects what AND what we want the design to do maybe we can build something that works for the use. I guess I haven't been very effective in conveying both parts of my message.

It is, btw, a foolish designer that expects a brand new to him/her design to work perfectly the first time out. Sometimes we get lucky, but don't count on it.

[ QUOTE ]
It's O.K. if Thom stirrs the pot...

[/ QUOTE ]

Something I appear to be good at!

[ QUOTE ]
Making this simple: If your upper link is shorter than your lower and both links are parallel, when the axel move up and down the pinion angel will change. Period! Thats it.

[/ QUOTE ]

Why do you want pinion angle change? I am NOT mocking you, I'm curious how you've found that it matters. This is something that the local dirt roundy racers are also concerned with.
BTW, terms like "packing" causes me as much consternation as "Anti-Dive" causes you. I'm not saying either is more or less correct, just that once we have a commonality of terms we can communicate far easier.

[ QUOTE ]
snippage.....
I asked a similar question about anti-pitch geometry on this board a while ago and in my opinion Chuck's statement about them in an off-road vehicle sums it up:

"2. Forget the anti-dive. This would translate into poor stopping and anti-smooth ride.
3. Forget the anti-squat. This would translate into poor traction and anti-smooth ride. "

[/ QUOTE ]

This is what I was saying, or trying to anyway, a while back. That is that excessive anti-squat or anti-rise will lock out the suspension motion. What we do not need when trying to avoid that MayTag is for the suspension to go rigid under braking or power.

Personally I think a little Anti-dive and Anti-squat can be employed, but no where near what is used on pavement. Just a little Anti-Squat might be enough for Curt to gain back his holeshots. And just little Anti-Dive might allow one short course truck to drive deeper into a corner than his/her competitors.

[curt]

[ QUOTE ]
Curt, how about putting multiple mounting points in for the rear suspension? That way you could test some different setups to see if you can get your holeshot back.


[/ QUOTE ] We are going to add another couple of front upper link mounts to play with more and less squat. Originally I tried to design for a slight anitisquat condition but made a bad assumption on my cg height so ended up with squat. Since reading all of this, I'm now considering instead of anti squat maybe just less and more squat. We have a data logger so I'm going to the track and try a bunch of starts to see if I can get an aproximate test. With dirt being so variable it's questionable as to whether I can apply any kind of scientific pinciple to make a conclusion unless the differences are fairly large.
[ QUOTE ]
Part of your lack of traction off the line might be due to the fact that your CG has moved forward with the switch from springs. Do you have corner wieghts from before the switch?
With sprigs roughly 50% of you susp wieght is behind the axle. 4link= 100% in front...

[/ QUOTE ]
Yes, I have them. The truck was about 51/49 front heavy. Haven't checked since we redid the rear though.
[ QUOTE ]
curt, did you ever compare your trucks pivots to the CORR pro-lite standardized chassis? i'm not saying it's the best possible short course rear suspension but i can' t remember when a non-standardized truck won a pro-lite race.

[/ QUOTE ]
I agree that the best starting point is to look at a P/L truck to try and figure out what they've done that we haven't...Patty will be at Crandon and will try to get some pics of Kincaid and Huseman's setups for me. There are definately things to learn from them but they have some huge advantages over a non std chassis truck that effect the race outcome as well like motor set back and the fact the HP figures have gone up by about 20% in the last 3-4 years (250 up to 300+) with EFI and multivalve technology that wasn't available to them prior to that.
[ QUOTE ]
What we see here is that squat and anti-squat affect only the pitch of the car and have no direct affect on weight transfer, thus wheel loads, and thus traction. Weight transfer under foward or rearward acceleration (longitudinal acceleration), is only affected by 4 variables: wheelbase, cg height, mass of the car, and acceleration.
Weight transfer = mass * acceleration * (cg height / wheelbase)
The reason anti-pitch geometry affects weight transfer and traction is because a when a car pitches (squats or dives) the cg height drops. As seen by the weight transfer equation, the larger the cg height, the more weight transfer. So anti-squat prevents lowering of the cg under foward acceleration which has the effect of increasing weight transfer to the rear of the car in comparison to a squating car.

Curt, this may be why you are no longer getting the holeshots. You yourelf said that your truck squats alot more with the link suspension than with the leaf springs. As shown above squat = less weight transfer equals less traction at the rear wheels under forward acceleration.

[/ QUOTE ]
The first and second paragraphs seem to contradict each other (Only pitch, no direct effect on weight trsfr/prevents lowering increasing wt trsfr) I'm wondering since cg height seems to be one of the largest effect variables of antisquat if I first checked the starting line results to see if A/S is a benefit and if I can get to a 100% figure at ride height and less than that at extension. This would possibly minimize the argument of A/S making the truck pack the front end in the whoops since the suspension would potentially be extended beyond ride height and the CG height would have increased to a point that the truck would be in a squat condition again. If I'm correct, squat/antisquat are not definite variables but a curve as the suspension travels through its arc. I have it modeled in cad so checking it should be simple after I determine an accurate CG height figure instead of the incorrect guesstimate I've been working with.

In no way am I tring to be argumentative, I'm just struggling with some of the concepts so keep it coming, I'm getting new ideas with these posts. I have the up pinion angle, the converging angles, and the 60ish% sized upper link so now am in the refining process for how it all works together. I guess it's time to quit thinking and start sweating, need more data and testing to draw any conclusions..Curt

[DPpatrol]

Curt,

In my opinion the 2 paragraphs dont contradict eachother. Squat and anti-squat dont directly affect weight transfer. They directly affect cg height and cg height directly affects weight transfer. So I would say anti-squat and squat indirectly affect weight transfer I guess, but then again it really is just semantics and probably not really that important as long as the concept is understood.

If you have some scales (which I assume you do since you gave a weight distribution) and the means to lift one end of your truck a few feet in the air, it would probably be worth your while to measure the CG height. It might also be interesting to take out the rear springs and measure the CG height with the truck in full squat. I would be very curious how much the CG height would change.

You are correct in that anti-squat is not a finite variable but more like a curve during suspension articulation.

It would be pretty cool if you were able to put some of this theory into practice in your race truck. I hope after you try everything out you share the results with us. I know I would be very interested, and I am sure the people reading this post would be too. Best of luck with the experimentation.

jason

[curt]

Yes I have scales, doing the cg height is going to take a bit of time, I have to fabricate some solid bars to go in place of the shocks to keep the suspension from moving on it's own for an accurate measurement. When I have all that done, I think I will get measurements at: ride height, full compression both ends, and full compression/full extension each end if I can handle all that in one bite...Curt

[Stephen]

Curt, how about just sticking a blcok of wood in the bumpstops and then running a ratchet strap around the frame and axle? That worked for me in one previous CG height session. I did have to raise one end of the truck up a bunch to get accurate numbers. This was a hill climb truck (pikes peak style) so it was probably close to your configuration.

I'm not sure we want to apply short course suspension to open desert suspension directly. It looks to me like the tracks are smoother (not smooth, just smoother) and the shocks are set up way different. But it's a really close data point to look at for sure. Quite a bit better than any other motor sport.

Another problem here is that I think the CG we need to be working with the the CG for the sprung weight which is hard to weigh since we rarely have the truck fully ready to go without front and rear suspension. You can figure the overall CG and subtract the weight and location of the unsprung weight though.

[curt]

For me it's a little more complicated. At our last race, we bent an upper arm badly and when you back up it raises the front end all the way so I'm fixing that first so I can get to an accurate ride height again. The other problem is when you jack up the front, it doesn't settle back down until the truck rolls cause of tire scrub, so once I get the accurate ride height, I'm going to cut some 1-1/4" tube and put it in the shock tabs so it will hold the height and tire spread while I jack it up to put it on the scales.

I made a rookie screwup, until it was mentioned, I didn't even remember I had not scaled the truck since before I did the rear so I really have no idea what the weight distribution is. My issues could easily be a weight distribution issue even before considering any of the other factors we've been talking about. Two years of learning with what I had, and then I forget to go back and do the basics http://www.race-dezert.com/vb3/attachments/old/images/graemlins/frown.gif It's a pain to get all this ripped apart so if you think of anything else I should be checking while in scale mode, post it...I've almost completed the new upper arm, so by tomorrow I should be back to a baseline setup again...Curt

[ntsqd]

A couple people touched on something here with regard to finding CG height that I think needs more exposure.
If you are looking for the at-rest height of the CG then you need to come up with some method of locking out the suspension while at ride height. Otherwise when you tilt the vehicle the springs will compress and you'll not get the number you were looking for. You'll get a number, but it will not be a good or valid one.

Some of the pavement guys (there he goes again....) are so anal about spring rates that they have solid bars that replace the C/O's while the car is at the shop or even on/in the trailer. The car only sits on springs while at the track. Something similar would be ideal for finding the height of the CG.

[ntsqd]

[ QUOTE ]
I'm now considering instead of anti squat maybe just less and more squat.

[/ QUOTE ]

Just to be clear; Lack of Anti-Squat is Squat. Lack of Squat is Anti-Squat. You can talk about it either way, but for some reason whom ever laid down the original terminology (WAY b4 Vessels) chose "Anti-Squat". I'm guessing the reason is that anti-squat is a linkage induced action whereas Squat is based on weight transfer.

[ QUOTE ]
What we see here is that squat and anti-squat affect only the pitch of the car and have no direct affect on weight transfer, thus wheel loads, and thus traction.

[/ QUOTE ]

I'm not sure this is right. Weight transfer happens proportionate to acceleration, nothing we design will stop that. This is where a circular argument can come in though so I'll attempt to steer clear of it b/c we can design linkage to counteract the weight transfer. So now we design a rear linkage with XX% Anti-Squat (or 100%-XX% Squat if you prefer).
The linkage itself can generate no force, it needs input from somewhere to generate a force that either acts with or against the rear suspension compression that happens due to weight transfer. Or it may even be possible for it to be neutral and neither act with or against the force from weight transfer. That force comes from the driveshaft, specifically from the torque created in the axle housing caused by the pinion gear trying to climb up the ring gear.

If the design has some degree of anti-squat then the linkage is generating a force that acts to oppose the compression of the rear suspension. That force needs something to push against, which happens to be the rear tires. So if the weight transfer to the rear has no compressive effect on the rear suspension then the amount of anti-squat is exactly equal to the force trying to compress the rear suspension. Which means that the downward force on the tires is exactly the same as if there were no compression resisting force.

Curt, you are right, the IC moves around in an arc. I don't know what the CG moves in, but I suspect that it is more of sphere or ovoid sphere than simply an arc.

[Stephen]

To sum up TS's post here, basically if you don't have any traction you don't get any weight transfer or any squat/lift out of the rear suspension. If it's really loose the weight that's on the back of the truck is all you have to provide traction.

[91yotaprerunner]

wow i just read this whole thread and i really appreciate everyones input. my mind is like a sponge and i love to soak up as much information as i can and u guys just taught me so much. thanx again

[curt]

[ QUOTE ]
[ QUOTE ]
I'm now considering instead of anti squat maybe just less and more squat.

[/ QUOTE ]

Just to be clear; Lack of Anti-Squat is Squat. Lack of Squat is Anti-Squat. You can talk about it either way, but for some reason whom ever laid down the original terminology (WAY b4 Vessels) chose "Anti-Squat". I'm guessing the reason is that anti-squat is a linkage induced action whereas Squat is based on weight transfer.

[ QUOTE ]
Actually I understood the concept, but what I was trying to explain was that if squat was necessary maybe for my application I don't need/want as much as others need. We're doing the weight thing tonight to get a gigure of CG height at rest and with the suspension locked down in a compressed front then rear condition to plot a CG height curve for lack of a better term if it all works out. I don't know if any of this will be useful but if I have the data I might be able to make use of it...Curt

[rokker]

[ QUOTE ]
wow i just read this whole thread and i really appreciate everyones input. my mind is like a sponge and i love to soak up as much information as i can and u guys just taught me so much. thanx again

[/ QUOTE ]

Ditto!

[billymanfroy]

Curt,

I'm thinking you're overlooking the most obvious yet hard-to-quantify variable there is in racing. You can measure and slide rule and calculate and weigh and chat and guess and experiment and worry and discuss and adjust all you want on your truck, but there is an infinite number of variables you cannot control: The other guys' (gals') trucks. Everyone is stepping up and sometimes that means they take bigger steps than you. That's racing. That's life.

Broc came and swept the floor with all of us. Lesson learned. You could put twin screw dually diffs or tank treads on your truck, but unless you include an IRL or NCT motor, he'll be handing your a** to you every single time he shows with DOUBLE the horsepower he had before. You can move your links .5" one way or the other, but I'd personally be looking at mounting them to a rocket engine.

Oscar has a freakin' 3.4 vs. his old 4 cyl that couldn't go a lap without the pickup choking off the fuel. Nobody really noticed him much when he showed up to EVERY race and limped around. Those days are gone. I talked to him recently and he's only been able to use 2 of the 4 gears in his auto tranny. When he regears for all 4, you'd better get ready for another heaping spoon full of familiar a**.

Don't forget Josh Nie (akaDesertLab).

Then there's us. I never got a holeshot in my entire life. Never. Not once. Those days are gone, too.

I am not trying to discourage you from really dialing in your suspension, and I agree that it's a lot easier to make things worse on accident than it is to make things better on purpose. (As always, we're only a phone call away, and the shop is only 5 minutes from your house.) I'm just saying that you have to allow for the competition to learn from all of the beats you've handed them. You'll get bitter like Fred if you expect to win and have everyone else sit on their duffs.

BIlly

[ntsqd]

[ QUOTE ]
Actually I understood the concept, but what I was trying to explain was that if squat was necessary maybe for my application I don't need/want as much as others need. We're doing the weight thing tonight to get a gigure of CG height at rest and with the suspension locked down in a compressed front then rear condition to plot a CG height curve for lack of a better term if it all works out. I don't know if any of this will be useful but if I have the data I might be able to make use of it...Curt

[/ QUOTE ]

If you can lock out the suspension at ride height, weigh the truck level. Then tilt it as large of an angle as you manage and weigh the low axle again. Some trig from there to figure the CG height.

[curt]

Actually Billy I never forget anything....We all just work with what we have and when we need more, then we find a way to pay for it or learn to do it....Other than a fairly rude post was there any useful advice you'd like to offer?

Tom, I used a calculator at http://www.longacreracing.com/articles/art.asp?ARTID=22 I was only able to get the 10" of lift and the figure placed my CG height at about 26" which is within an inch of what I had guessed before, but I don't buy it the squat figures vs what the truck does don't add up.

This weekend, I'm going to get an engine hoist and try to lift the truck as much higher as possible to increase the accuracy, then I'll work on the front to rear weight when one end is squashed and the other extended just to see what kind of data that might produce. I also want to see what my corner weights do when I change the tire pressures or when I increase the preload a fixed amount just for future understanding...Curt

[ntsqd]

Use this if you think it might help:

What I advised a local guy who is linking his Nissan to do is buy some bar stock that is same size as his linkage mount tab holes. Use the bar stock to extend the actual pivot points out to the side of the truck. Stretch string from the center of the axle end past the center of the frame end and tie it to something up front. Where they cross is your real IC. Now use masking tape on the side of the truck and mark where your CG is.

Stand back and look at all of this. Maybe cycle the suspension incrementally and readjust the strings. Could get some poster board and plot the IC curve.
I'll bet you'll find something.

I didn't learn that in a book, and no computer taught it to me.

One builder I talked to likes to put his IC's in front of the truck, by 18" + I'm still working on how that leverage influences how the truck behaves.

Good tech find btw, I've marked it.

[91yotaprerunner]

whats IC, i kinda got lost

[WorkInProgress]

I think IC is instant center.

[Josh_K]

"I didn't learn that in a book, and no computer taught it to me".

W.T.F. I am totally confused now. I thought that all these roundy round cad programs and Longacer calculators and things HELD ALL THE ANSWERS!!!

Now you imply that some of this knowledge that you have didn't come out of a book, and a computer wasn't holding your hand when you figured it out, but instead it was good old seat of the pants intuition.

And for the others that don't speak engineer, I don't know what i.c. stands for but in plain English it is an imaginary line that is projected forward form the top links and the lower links. Some say that the imaginary line should intersect at the rear of the engine crank some say 18" forward of the bumper. Some times in some situations it could be parallel and never meet or even a expanding angel and going away for either. On my 8 truck it is about 30' in front of the bumper and this is due to the relationship of the upper and lower links length to each other.

Josh

[billymanfroy]

[ QUOTE ]
Other than a fairly rude post was there any useful advice you'd like to offer?

[/ QUOTE ]

Sheesh! I'm starting to feel like the penguin!

Other than giving you COMPLETE access to John's design to our 4-link nearly two years ago and offering to help design yours so that you could get the best out of your truck while allowing for the compromises inherent in long travel/short arms and offering a full shop two miles from your house and several people who would be willing to help ANY time and offering to help dial the thing in once it was done, yes, I do have one more thing to offer: You can use our fork lift if you'd like.

I forgot in my 1st post to mention Stevie Millward. We did his 3-link from design to completion - including the 2 days it took to get the laser cut metal back - in about 1/10th the time this thread has been active. He'll be racing soon, too.

Oh, and one more piece of advice... Maybe you should rethink the gearing... You don't need a theoretical top speed of 145 mph. on a 1-mile shourt course. What gear do you feel comfortable with again? 5.48 or something like that? Ours was 7.00:1 for the last 2 years and we STILL didn't use 3rd much.

Good luck.

Billy

[ntsqd]

[ QUOTE ]
"I didn't learn that in a book, and no computer taught it to me".

W.T.F. I am totally confused now. I thought that all these roundy round cad programs and Longacer calculators and things HELD ALL THE ANSWERS!!!

[/ QUOTE ]

See, you've fallen into the very trap you've been warning about!

Seriously, all of those things are tools. Tools don't make decisions, they help you make them. You still have to understand what any number result might mean, and be able to implement it. Engineers put numbers on everything. That isn't the only way to do things, but it's how they do it.

[ QUOTE ]
but instead it was good old seat of the pants intuition.

[/ QUOTE ]

Fingered it out on my own. Once in a while my noggin is good for more than a place to hang my hat (curved bill thank you very much).

[ QUOTE ]
And for the others that don't speak engineer, I don't know what i.c. stands for but in plain English it is an imaginary line that is projected forward form the top links and the lower links.

[/ QUOTE ]

Let me see if I can take some of the engineer out of it. "IC" stands for "Instant Center". If you start at the center of a link's rear pivot point and extend a line or a string thru the center of the forward pivot and then take it WAY out in front you have one reference line. Do the same for the other link on the same side of the vehicle. Where they cross as viewed from the side, assuming they ever do, is the Instant Center.

[ QUOTE ]
Some say that the imaginary line should intersect at the rear of the engine crank some say 18" forward of the bumper. Some times in some situations it could be parallel and never meet or even a expanding angel and going away for either. On my 8 truck it is about 30' in front of the bumper and this is due to the relationship of the upper and lower links length to each other.

[/ QUOTE ]

Might be some confusion here. A VERY rough Rule of Thumb for CG location is at the crank centerline and the face of the flywheel. Usually that will get you in the ballpark for CG location, but only just that.
Where the IC should be at static ride height, and where it should go in compression or extension is the $64 question. I think most only worry about static ride height and have other priorities that overrule where the IC moves to during suspension cycling.

[bee-line]

I think some of you are not getting it! I think you are trying to make this a way bigger deal than it is. All you need is squat to happen when you hit the gas. Its that simple. If you have a stock V6 you might wont 3 to 4(w.a.g.(wild as guess)) deg of pinion angel in one foot if you have more power you might wont less pinion angel change per foot. Set your links up with a diverging angel and the truck will squat when you hit the gas. Its that simple. It might squat too much thou, but a cad program is not going to tell you how much is good or not enough either. That comes from experience.

Every one can talk about all the pavement this and that or rock crawling they want but its got nothing to do with off road racing.

And by the way NASCAR does have engineers and good ones. But there not that great because the geometry that is in those cars goes back to the 60s and there scared big to make any changes to the chassis to improve safety. They dont know what it will do.

Josh


thread revival #2 today!

I am halfway done building my lower links & I'm reading up as much as possible on this stuff & this theory has always stuck with me...

so, I am ***-u-me-ing that since the pinion angle is moving upward (it's natural way for it to move because of the wheels turning the opposite direction) at bump, that the rotation of the rear axle pulls the chassis down. Is this assumtion correct, or is there something that I am missing, and the pinion change is a side effect of what I am missing?

I now think I understand why anti-squat is a traction reducer in the rough, because it effectively stiffens up the suspension under power, thus not letting it follow the terrain & put down power...on a straight-line drag ster, I can see why it would be beneficial, though.

so now, I am messing with everything on CAD & checking pinion angles & such...I'm going to try for about 5º or so of pinion angle change from ride height to full bump (about 12") engine is a stock 4cyl ranger, 6.20:1 gears, spooled.

...I'll probably have some more questions, but for now, if anyone can confirm my assumptions, or set me straight on what I'm missing, I'd really appreciate it.