A Arm questions for the pros... ahem FABRICATOR :)

[geoff]

Myself and 3 other engineering students are designing centermount A-Arms for a prerunner toyota. The lowers are 24" long, the upper length is yet to be determined, and is not restricted by the frame rail as it is front halfed. That being said, we are using a program called Front Suspension Geometry Pro, its VERY cheap and VERY useful. I recommend it to anyone designing a suspension, whether its your first time or if you are a fabricator... That being said...

the lowers are parallel to the ground. We have yet to determine the kingpin (spindle height) and the location of the upper arm's mounts. Obviously this is what matters. After reading all of FABRICATORS posts and NTSQDs posts on the matter, i went and watched desert people 1 and 2 for the past cpl of hours, playing alot of the sickest trucks in slow-mo. What i noticed was exactly what FABRICATOR describes: negative camber at full bump, zero camber at static, and negative camber at full droop.

The only way i can get this to work out is by making the upper arm virtually parallel to the lower arm. If the upper arm is angled down towards the lower (to give it an IC) there *must* be some positive camber at some point in the camber curve, and FABRICATOR says no positive camber is good camber.

My main questions are as follows:

1) Do we want parallel arms to avoid any positive camber?

2) With the setup i have on my computer screen now, i get -8.4 degres camber at full bump, and -6.171 degrees at full droop. How much are we aiming for? The overall travel is 20 inches, 8 inches bump and 12 inches droop

3) What kind of spindle height should be considered acceptable? I see no advantage to going to a taller spindle... if there is one id love to hear it

here is a pic of whats goin on at bump

http://www.full-race.com/490/fabrication/spindles/bump.JPG

and droop

http://www.full-race.com/490/fabrication/spindles/droop.JPG


thanks for the input

[CRAIGHALL]

Geoff I am by no means a pro ,but before tou can determine your pivots you should know the spindle your using so you can get the kingpin inclination angle,then you can move the outer upper pivot along this line to get your spindle height pivot to pivot,then play around with the inner pivot-Look at the truggy upper pivot is defenietly an inch or two lower.

[91yotaprerunner]

I know this doesnt help u any, but i was curious to where u are going to school at. I plan on majoring in mechanical engineering next year when i leave off to college but am still working on my decision where to go. Also the main reason I want to major in mechanical engineering is because i want to learn all the fundamentals for suspension design and everything else with trucks. Is the mechanical engineer major teaching alot of things to help u in this area?

[geoff]

i go to school at ASU, and to be honest i have learned nothing about suspension or anything except for material strenghts and stress analysis. If you want to learn about building/designing you need to have a drive to do it on your own, you definately wont learn it here... i believe many other schools that have SAE classes might be better suited. good luck!

[geoff]

this was made assuming i am using a t100 spindle with the lower balljoint flipped to the other side of the spindle, to make the lower arm parallel to the ground. I already know the spindle, but i cna move the upper arms ball joint (spindle height) up from her if i like.

[91yotaprerunner]

ya i was actually lookin at Cal Poly Pomona, they have the off road club and stuff like that, but i have already been playing around with suspensions and stuff, so far i have built the suspension on the front of my yota, but thats about it.

[geoff]

if you are going to do any front susp analysis, go to
http://www.auto-ware.com/

tell them youre a student and you can get that program for 60$, i learned a LOT from spending hours with it.... dont take any of its advice however. I believe cal-poly would be a great school for this...

[91yotaprerunner]

ok thanx, i dont want to hi jack ur thread though

[FABRICATOR]

1) Have you tried tilting the arms together on the inner ends? This should give you a wider choice of camber change.

2) It looks like you are getting plenty of negative camber. Before you finalize the numbers it would be prudent to throw in some spindle designs and check bump-steer. More camber change can mean more pain figuring out the steering.

3) The usual reason for a tall spindle is to make room for a big bodacious UCA and/or buy some room for shocks. It is also sometimes used to raise the inner UCA pivot point.

[geoff]

1) i have tried tilting the arms together on the inner ends, but i keep getting some positive camber right as the wheel starts to go into droop, i am trying to minimize any positive camber. I beleive the upper needs to be parallel to the ground at static to have zero positive camber, is that correct?


2) i agree i am getting a lot of negative, what is considered better, somewhere around 4 degrees?? 2 degrees???


3) i could go to a taller spindle, i just felt it might be weaker that way, although there is no harm at all in doing this... the shocks wont be an issue becuase we are doijng a J-arm to make space for both shocks, and i have no problem raising the UCA pivot, i just didnt see the benefit to doing that. Can you enlighten me? thanks =)

[FABRICATOR]

1) Yes and no. If the LCA is exactly parallel with the ground and you are starting from exactly zero camber, it will go slightly positive. If it is set up with slightly negative at ride height, it may lessen negative but doesn't have to go into positive. A starting point of having the LCA perfectly level at ride height along with zero camber is nice for reference but not an important goal. You still need to factor in axle location on the spindle, wheel location on the axle, and tire size.

Are your outer pivot points straigh up and down in relation to each other? It looks like they are in the diagram; they shouldn't be. If they are it's adding to your positive camber problem. The lower must be out from the upper; back to spindle design 101.

2) Somewhere around 4 is nice but again it's not critical and watch out for the steering.

3) The taller spindles are not weaker but they are heavier. They are also a bit more tolerant of slop in the upper bushings and put less load on the UCA.

[geoff]

1) so you recommend giving the truck a bit of negative camber at ride height? The axle location on the spindle is oriented such that it is in front of the lower ball joint

2) yeah i have to build the lowers, attach them to the spindles this weekend, then i can start playing around with the uppers and start worrying abotu steering...


3) you are definately correctly in saying that our diagram had the outers vertical, i fixed that it was a data error on my part. thanks for catching that mistake, it has already cleared up a couple things.

4) im not worried about slop in the uppers as there will be heims, but how does it load the UCA less?

thanks again...

[FABRICATOR]

Tiny amounts of negative or positive camber are not really important. Again, optimize geometry and arrange component positions accordingly. I would lean (no pun intended) toward negative if any. A taller spindle simply has less leverage from all wheel load directions on the UCA. More load is carried by the LCA.

A big advantage to having an “open” environment for the upper arm length and location is that you can often start by designing and even installing the spindle, lower arm, shock(s) and spring(s). Then “fit” the upper arm to match. Then you need to find the “sweet spot” for your tie rod’s inner pivot point that gives you minimum bump steer. Good luck with it.

[MNotary]

Tons of iterations with that program........

Did you edit the snapshots on your first post? The outer vertical thing.....

If Bob Sheaves would take 5 minutes and give us some numbers for a-arms....

HEY Bob......!!

[V8Ranger]

Geoff,

I don't know if it would be of any help to you but I drew up links for the UCA, LCA, upright and frame mounting points and assembled them in SolidWorks which allowed me to cycle everything and measure the camber through out the travel. I found that being able to cycle the mocked up suspension in the assembly allowed me to better visualize the geometry through out the travel. If you'd like I can email the parts and the assembly to you. Solidworks was also nice because I could sketch out the front end and check out all of the other suspension geometry (instant centers, roll center, etc) I have never used the suspension program you have but maybe it has a similar feature.

Matt

[geoff]

i would love to see your files, if you could, please email them to me at geoff@full-race.com thanks i really would like to see it in solidworks, as it is somewhat difficult to see all changes at once in ths program, it breaks everything down into 2D, nothing in 3D.

Thanks!!

[geoff]

OK... here are the changes i made today --

1) i made the spindle taller
2) i moved the mounts for the upper a little bit higher.
3) i played with the tie rods to get a nice bump steer curve, i think i should increase toe-in however as it is still *very* small amounts at full bump and full droop, i think it would be helpful to have a bit more toe-in than we have now, simply to help the truck straighten out a bit


Now i am really unsure about one more thing, caster... what kind of caster do i want the system to do?? the input has helped a lot so far.

For now, here are the results:

at ride height, the lower arm is parallel (i have learned this isnt that important), the upper arm is angled down slightly, and the spindle is appropriately tilted inward

http://www.full-race.com/490/fabrication/spindles/simulation/rideheight.jpg

at full bump, there is a camber change of -5 degrees
http://www.full-race.com/490/fabrication/spindles/simulation/bump.jpg

at full droop, there is a camber change of -3.5 degrees
http://www.full-race.com/490/fabrication/spindles/simulation/droop.jpg

of course the truck will be strapped to have less then the 22 inches of travel shown so there will be slightly less camber change in each direction


In regards to the bumpsteer....

at ride height, we of course have zero toe change...

http://www.full-race.com/490/fabrication/spindles/simulation/rideheightsteer.jpg


and full bump, there is only .043 degrees of toe in, this seems like too little to me... however we can certainly change this:
http://www.full-race.com/490/fabrication/spindles/simulation/bumpsteer.jpg

and at full droop we have .195 degrees toe in, of course this can also be changed.

http://www.full-race.com/490/fabrication/spindles/simulation/droopsteer.jpg


so what do you think of this fabricator? Some values for toe-in at bump and droop would help =) ... and as noted, no need to be so quiet bob sheaves http://www.race-dezert.com/vb3/attachments/old/images/graemlins/wink.gif

[geoff]

i didnt edit the snapshots, the most recent pictures are right below this post tho, much more accurate this time.

[AaronDixon]

Your inner steering tie-rod pivot should be on the line between the UCA and LCA inner pivots. Verticaly, it should be placed at the same height ratio as the spindle. try that...

[FABRICATOR]

I don’t see toe-in at ride height?? Full up and down numbers look good. As long as you have any toe-in you’re OK, especially if the LCA’s are mounted with rod ends and not rubber bushings.

Castor has everything to do with spindle design. This is why I mentioned knowing where the axle is on the spindle and where the wheel is in relation to the axle. The statement <font color="blue">"and the spindle is appropriately tilted inward” </font color> ” is ambiguous. This tilt is a part of the spindle design. If you are going to build a spindle from scratch and dump the OEM numbers, you will need to come up with a new combination that will provide desired steering and “stay straight” characteristics.

[CRAIGHALL]

Placing the inner pivot on the line of pivots is only a good starting point. Geoff you need to move your outer pivot (tie rod) further out not on center of the spindle as you have no ackerman in your steering,then you can place your inner pivot.The first thing to be completely designed should be the spindle.

[geoff]

there is no toe-in at right height. the graph only shows toe change as the suspension cycles.

The numbers we have are rough numbers measured off of the 2wd t100 spindle, which is what we will be using for the time being. When i ambiguously said it is appropriately tilted inward, that just meant i had the correct measurements of this OEM spindle.

For now, the purpose of this post was just to get a ballpark idea of what kind of curves we should be looking for. Once the lower arm is built and attached to the spindle, i will start to plug many more numbers into the program to more accurately reflect our design. Dont think i will be using any of these number in the final design, i am just trying to figure out what we are going for in terms of overall behavior as the system cycles.

can you give me an idea of castor range to shoot for too? thanks, i hope io am not asking too much of you

[geoff]

thanks craig. The spindle we are using is already designed, just a t100 spindle, with the ball joints all moved up 3 inches.

the outer tie rod pivot in the pictures is not accurate becuase i had the spindles at the shop, not at my house so i had to guess, ill have more accurate numbers later tonite. any more input/recommendations is welcome =)

[FABRICATOR]

There are too many variables and lack of information to get that specific with toe-in. Too little is better than too much, avoid toe-out on the street. If some of this seems vague it’s because usage and construction are vague. I can throw numbers out there, but no one can tell you the optimum settings unless they have 100 percent of the information or the exact same setup and have thoroughly experimented with it.

If you are going off the T100 spindle then the numbers are basically already set. That spindle is designed to work with a specific king pin inclination, camber, and castor. There have also been previous discussions on camber, castor and toe-in on this board. Modifying the spindle will change the numbers.

There seems to be a constant tendency to add to the front end numbers. Except for wheel travel, adding to anything is more often than not, a mistake. Going to a taller tire already changes things from original. My advice is to start at OEM specs and work from there.

Depending on year and model, T-100 spindles (stock) run at:

1.55 to 2.66 degrees castor,
.41 degrees camber,
.09 to .25 inches or .19 to .56 degrees toe-in.

[MNotary]

1.55 to 2.66 degrees castor,
.41 degrees camber,
.09 to .25 inches or .19 to .56 degrees toe-in

Why does the castor and toe-in have a range?

Toe-in is set when the tie rods are adjusted....????

Toe-in changes through the travel...called bumpsteer...and no bumpsteer is best.???

Ackerman is important especially on a front engined vehicle...... It needs to turn....

Scrub??

Itereation upon iteration...come on Bob...cough up some numbers.....

Toe-in gives stability in a straight line with both tires on the ground...how about those one wheeled landings (bump steer) and the car hooks and rolls....

[CRAIGHALL]

Caster could increase as you bottom out to help keep it straight,but it seems you would want the minimum at droop so as not to wonder when hauling butt on the big rollers and it droops out?

[FABRICATOR]

"Why does the castor and toe-in have a range?"
Ask Toyota, it's their spindle and their specs.

Re: "Toe-in is set when the tie rods are adjusted....????"
Yes at ride height, where it goes from there is up to the rest of the design.

Re: "Toe-in changes through the travel...called bumpsteer...and no bumpsteer is best.???"
No, toe-in is an alignment between the two front wheels, bump steer is between the wheels and the steering system. No bump steer is best.

Re: "Ackerman is important especially on a front engined vehicle...... It needs to turn...."
Yes, especially on pavement. Ackerman is applied by changing toe.

Re: "Scrub??"
Scrub?

Re: "Itereation upon iteration...come on Bob...cough up some numbers....."
?

Re: "Toe-in gives stability in a straight line with both tires on the ground...how about those one wheeled landings (bump steer) and the car hooks and rolls.... "
Yes, but more does not make it go straighter...is that a one wheeled landing in a straight line? lol

[FABRICATOR]

Oops, my bad. A couple of these questions were more practical than first realized. (was working with concrete at the time)

Re: "Why does the castor and toe-in have a range?"
The ranges given cover different spindles as used on the T-100. However, even for a given spindle the manufacturer will have different settings depending on vehicle model. Things such as wheelbase, GVW, even engine type (weight), can require different settings. Geoff may need to find out which one he has and start from there. This is also why actual numbers cannot be specified.

Re: "Toe-in changes through the travel...called bumpsteer...and no bumpsteer is best.???"
With a solid axle there is no connection between bump steer and toe-in. Bump streer affects steering direction. For all practical purposes, with independent suspension there is no connection between bump steer and steering direction. You could say there is only bump toe-change. This is because one side is always a mirror image of the other. But technically speaking they have traditionally been two different animals.