Mid-Engine

[racer951]

Searching hasn't yielded too many promising results.

The basic reasons for a mid-engine setup should be better weight distribution. What are the exact pros and cons of it and how do they weigh out? How does it affect different wheelbase cars (would it work better on a 100 in wb car or 120 inch wb car)? I know it depends on the capabilities between the front and rear suspension, but assuming the front is nearly equal to the rear, how does it play out?

[Hollowpoint]

This is exactly what I've been wondering, as well. I'm sure similar weight distributions (front wheel/rear wheel) can be had given the seating/fuel/wheelbase arrangement, so I'm guessing it depends whether you want the weight more toward the center or toward the outside. Dondel and Marking just changed their setups to rear engine from mid engine(anybody know if they changed any other weight/wheelbase factors?).. and then there are successful mid engine cars like Ebbert/Castro. I'm curious to what the pros are cons are, as well.

[la2baja]

I don't work on mid engine set ups but the few pro's and con's I know are...
Weight in the middle of the truck is the biggest reason for this set up. Getting all that weight behind the driver is a huge factor on a good working suspension. Front suspension doesn't have to work as hard. The rear on a 3 or 4 link handles the pounding with the extra weight much better than an A-arm or beam suspension in the front. (more leverage). A big downer about the mid engine is the extra $$ to arange the motor, transfer case and drive shaft. Most of the trucks are designed from the ground up in order to keep the clearances of the drive system and rear suspension.
Also working on a mid engine set up is tight. real tight. I've taken a close look at a few trucks out there and they really max out the space available in there. Probably not fun to work on.

[Hollowpoint]

Now I realize the question was about trucks..was thrown off by the word "car" in the first post. My curiosity still falls under engine placement, so please give input as it refers to mid-rear comparisons, as well. I can't see this being that far "off topic."

[la2baja]

As far as a car (class 1). The cars I work on are both rear engine. They are running the more common stub axles to hubs on trailing arms set up. (not a truggy w/ live rear axle) Our fuel source is right behind the cockpit (literally inches from the drivers back) our weight is distrubuted to the rear of the car where our suspension is capable of handling quite a bit more weight. The mid engine cars are definetly spreading the weight more evenly between all 4 corners. I can't recall where the last mid engine car I saw had the fuel cell. If the cell is rear of the motor I would think the car is going to dispalce weight pretty close to ours. Not sure about that though. I'd have to see.

There should be some car builders lurking somewhere that can actually explain the way this works and it's pro's and cons far more specifically then I can. I'm just a Monkey with a wrench!

[racer951]

Actually I was referring to buggies, but general discussion on the topic is good too.

A 5 car guy with a mid-engine says it helps through the whoops but it pushes in the corners. He said you have to finish your braking before the braking bumps and set up the turn, then get back on it. I think there's a different driving style involved for sure.

Usually the cell is smashed between the seat/firewall and the motor and all the weight is pretty much stacked throughout that area between briver and tranny. That's how I'd do it.

[FlyHiFlyLo7]

What wins?

[FABRICATOR]

What wins?

Front, mid, and rear. It's been that way in the dezert all along and it's still that way today.

[5racer]

iv driven the same car with the motor in rear and then switched to a mid engine a class 5 .like everyone says it has pros and cons it did help with the notorious 5 car kick but also mad it corner completely different .for my driving style i could never get ahold of the cornering like most drivers i like to steer with the rear and the gas .now I'm not saying that one could not get it to steer that way with enough tinkering but i was in someone Else's car .and the owner had no prob driving it like it was .my new class 5 is rear engine IL deal with the kicking .

[Dezertpilot]

I know that Criswell now has a mid-engine setup from the prevoiusly rear engine setup and I think that is the person who told you that as I have heard the same about that car. I have also heard that a rear engine car like ours (524) has better traction with the weight over the rear wheels. With the mid-engine setup I believe you do not get as much traction going up rocky whooped out hills or just hills in general for that matter. I also imagine in a buggy you would have to possibly whip the car into corners like v-drive trucks?

[Philip]

i have been told that it pulls more torque/hp strait to the wheels? would this be true? Logicly i thought about it and it seems that there would be less (whats known in electrical terms, sorry i dont know the physical term for it) "resistance". I. E. Engine - Tranny - drivetrain - gears/axle - tires, As apposed to Engine - tranny - CVs - tires (in a much smaller space).

Sorry if my post is a little choppy and hard to read but i hope i got my question across correctly...

[mosebilt]

the main benefit of a "mid-engine" configuration is you get e low polar monent.cars with a low polar monent corner and steer better(usually),where a car /truck with a high polar moment(i.e. trophy trucks/rear engine cars) will handle better and be more stable in the straight aways.low polar moment=weight in center of car,high polar moment=weight spread out in car,i.e.-two spares in extreme rear of car,fuel cell in rear,engine in front.high polar moment cars will loose you in the straights,but low polar will catch you in the corners.

[Josh_K]

Just becouse a truck has a front engine lay out doesnt automatically mean that it is nose heavy, My 8 truck carry about 55% of the weight in the rear full on fuel and 55% in the front when empty on fuel. It is pretty close to balanced.

I think the real issue is driver preferance. If you like the feel of a buggy do a mid to rear engine car If you like a truck build a front engine truck/truggy.

I personally am use to a front engine truck that is linked and I know how it handels and how to drive it. It can hammer thro some things that kick rear engine cars becouse of there rear weight enertia. Then there are times were those rear engine cars can flat put it down on a truck/truggy.

[5racer]

the main benefit of a "mid-engine" configuration is you get e low polar monent.cars with a low polar monent corner and steer better(usually),where a car /truck with a high polar moment(i.e. trophy trucks/rear engine cars) will handle better and be more stable in the straight aways.low polar moment=weight in center of car,high polar moment=weight spread out in car,i.e.-two spares in extreme rear of car,fuel cell in rear,engine in front.high polar moment cars will loose you in the straights,but low polar will catch you in the corners.


that is some nice alum work on that car bye the way .who ever did it great job

[Outcast]

Like others have said, it comes down to personal preferences. Looking at Class 1, for an example, you can see competitive machines with front, mid, and rear-engine layouts, capable of winning on any given day.

However, what many racers have found is that while a front, mid, and rear-engine vehicle can have the same front to rear weight bias, as measured at the tires, they will still have very different handling characteristics due to where the mass is concentrated within the vehicle. This is the “Polar Moment of Inertia” that some refer to.

The best analogy that was explained to me was this. Picture a tight rope artist on the rope with the long balance poll they typically carry. Why do you suppose they use that poll? What would happen if they used a poll that weighed the same but was only 4 feet long instead of 15 or so feet? You see, when you spread mass over a broader area, it becomes easier to balance. Also, getting mass to ‘overhang’ where forces are acting on the body, lessens the effects, said forces have on the body overall.

Now apply that to a desert racer blasting along a straight and think about how you want the sprung mass to be affected as little as possible by what the suspension is doing. Having a greater polar moment of inertia helps to keep the sprung mass more stable and forces the suspension to do more work. Thus the desire for an increased polar moment of inertia.

However, everything’s a compromise and that comes into play especially during braking and turning. Vehicles with high polar moments of inertia tend to exhibit more weight transfer under breaking and accelerating. This is not necessarily a bad thing for accelerating with a rear wheel drive vehicle but can reduce braking ability/stability if too much weight is transferred to the front.

Also, vehicles with a high polar moment are often more difficult to turn and more difficult to control in turns because of the increased momentum involved in rotating the vehicle around the turn.

Thus, there can be too much of a good thing with increased polar moment and you have to decide is the application worth the pros and the cons involved. Case in point: I can see why it would be more desirable for a desert racer but less desirable for a short course racer ;).

Feel free to flame correct, bash, what have you but a lot of times I think people confuse “high polar moment” vehicles with those that simply have ALOT of rear weight bias. There is a difference, be aware of that.

Also think in terms of CG height as well. Which allows for lower overall CG, front, mid, or rear-engine? That, of course will depend on your design and application. However, knowing that teams have gotten front, mid, and rear-engine vehicles to be successful, while both would be given great consideration, I would personally prioritize CG height over polar moment of inertia. Suspension tuning can cover up alot of design compromises. I think we see more of that in off-road racing than anything.

[Josh_K]

No flaming here, I am trying to understand. Take my truck for instance, I have the 2 spare tire hanging way off the back of the truck to level the weight bias out. Does this make a high or low polar moment? I know it aids the truck in the rough and it also aids the rear to slide out in a slide (tread off) but I am not sure of the techincal language that should be used for this situation. It also dosent kick in the rear much but this from about 4 years of shock tuning.

Josh

[la2baja]

I've never heard of a polar moment. I am soaking it in. I understand most of it. I may never understand all of it. I have given myself a headache. I am still thankful for the knoledge!

[Josh_K]

I've never heard of a polar moment. I am soaking it in. I understand most of it. I may never understand all of it. I have given myself a headache. I am still thankful for the knoledge!


This is a Josh original.

"If you ever think you have it all figured out, your about to make a huge mistake"

[Hollowpoint]

That would be a higher polar moment of inertia.... say compared to mid engine TTs. But then again, all class trucks are going to be, because of the stock engine location rule.. and even more so from the loading of the tail end of the truck to achieve a better overall weight distribution. Baldwin, and the ex-Taylor/Scaroni TT go even further by placing the shocks behind the axle.

My question for the last 2 trucks: if they proportion that much weight to the extreme rear, what do they do about the frontend weight and engine placement?

Sidenote: front engine/rwd benefits from a 50/50 proportion, rear engine/rwd tends to be somewhere around 60/40... in general.

[Josh_K]

Here is one more about the rear shock set up that doesnt meet the eye (but I dont know if its true). There is one thought pattern that says that since the shock mounts point to the upper and rear (generally) when you hit somthing and it shoves the piston up, the force also then pushes the the shock body up, and in trun the truck bed cage induces the front to pop, helping to carry the front over bumps.

Now, I dont know if there is any truth to this and if there is, how much more front popping force is there when compaired to a standard link and shock set up?

The only way I know to calculate this (with out having a Skunk Works computer) is to build a test truck with both shock mounting location and simply drive it. Do nothing to the truck except move the shocks and valve them acordingly.

Josh

[Outcast]

"If you ever think you have it all figured out, your about to make a huge mistake"

That's a good one, I really like that. If, I might be so bold as to add to it. Don't buy anything from someone who claims to have it all figured out ;).

Okay, if I may, using Josh's 8 truck as an example. As he stated earlier, with full fuel, its about 45%/55% front to back weight bias. (That's pretty awesome considering a stock engine location and that a stock short-bed full size truck is like, what, 65%/35% front to back?) Now picture how the components are laid out and Josh correct if any of this is wrong. The engine, which I think we'll all agree has a high concentration of mass is up front, is pretty far forward by "race vehicle" standards because of the rules. Then you have the cab, which houses the driver, controls, a large part of the cage, and other various things, perhaps tools and what not. Also the tranny is located under the cab so in a fore/aft since it contributes to a good concentration of mass there too. Thus the cab area has another good concentration of mass. Then behind, there's probably the radiator and fuel cell? Then you have the spares located behind the axle. Looking at that and how the components are laid out, I would say that the sprung mass is pretty well spread out so I would say that the truck has a relatively high polar moment of inertia. Placing those tires out back is probably a key factor there. You've got a significant concentration of mass hanging out back there and that helps manipulate both the CG location and the polar moment of inertia. It also helps to force your rear suspension to do more work and also creates additional momentum that allows you to easily step the rear out in turns. Conversely, having a significant weight concentration (engine) close to the front suspension forces it to do more work also.

Question: Do you notice any difference in the ability to step the rear out as the fuel goes down and the weight bias moves forward? Does it get harder because less weight "in back" doesn't create the momentum to step out? Or does it, perhaps, get easier because less weight "in back" reduces traction and makes it easier to slide the rear. I am curious to see if the movement in the CG, though it is not alot of movement in your case, changes any effect of the high mass concentration you have in back due to the spares.

Now a couple of hypothetical situations, think of how the truck would act differently if you decided you wanted to convert it to a TT and move the engine behind the cab, Gordon Style, with the radiator(s) basically in the same location fore/aft but in the back window instead of the bed. Now since you've moved the engine, you don't have to hang your spares so far back so you move them up right on top of the engine, or maybe you get a little "out of the box" and put them under the hood right in front of the firewall, to achieve a "perfect" 50/50 front to rear bias. Now with this scenario, your front to rear bias isn't really all that different than what you had before. But think about how different the truck would act now that the majority of the sprung mass is concentrated in the middle. You would have a much lower polar moment of inertia. How would the truck handle rough stuff? How would it turn? What changes would you think you would need to make in your suspension? If it got off the ground, what would it act like in the air?

Now think what would be the case if you converted the truck to rear-engine buggy style with the engine hanging off the back. And since you might want to "maximize" traction you kept your spares in back too, on top of the engine. You also moved your radiator and fuel cell a little back and cut the back of the cab to move your seat back some, all trying to get more weight on those drive wheels for traction and maybe perhaps allowing you build some lighter arms up front to reduce some un-sprung weight. Now, in this case, you still have weight concentrated together but no longer in the middle of the vehicle. So the sprung mass doesn't have as high of a polar moment as it did when it was a class 8 but now has a much greater rear weight bias. And it might even have a slightly higher CG because the engine has to "set up" a little bit so it doesn't hit anything in back. Now think about how the truck would handle differently than in the previous two scenarios. Also think about how some handling characteristics might be very similar (less rear bucking ;)?)

Now about the idea where on a "Nezzie" style rear suspension with the shocks pointed up and back actually pushing on the chassis enough and causing it to rotate. I can see the logic there but, in my mind, I don't think enough force would be transmitted that way to have a noticeable effect on the chassis like that, especially if the suspension is tuned well. I believe that the "nose up" attitude has more to do with CG placement and mass concentration(s), overall suspension geometry, and spring/shock settings. That's my opinion. I would agree the best way to know would be to try the different setups on the same vehicle and perform under the same conditions.

[Josh_K]

Yes, I do notice the change as fuel burns off. It slides out a little less and at handels the big g-out a little better. The big hits are becouse I am spring just slighlty light in the rear and I dont mind the sliding out just as long as there is a tractor to pull it back over. All in all I like the lay out of my truck and the only clear improvemt that I can clearly see is to lower the ride height. But this would call for nothching the frame an thats a class 8 no no.

I agree with all of your exampeles and I clearly think engine location comes down to driver/owner preferance. I dont think that the "next level" is gonna come form chassis design. I think the next level in off road racing is is gonna be reached becouse other areas are improved. But I am cheating becouse I have seen the future "next level" in off road racing.

Josh

[Gadzooks2]

I belive if your weight is distributed 50/50 front to rear on both axles, then your polar moment of inertia is dead nuts in the center regardless of how your motor/trans/spares/radiator etc. is configured. So If you build a mid or rear engine car, the "polar moment" isn't any better or worse if you balance the car out front to rear. :cool:

[Outcast]

The polar moment of inertia of the sprung mass is also effected by how much the mass is spread throughout. You can have the same CG location with everything lumped towards the middle and with stuff spread out, the difference will be the body with mass spread out will have a greater polar moment of inertia. Just because two vehicles have 50/50 weight distribution that does not mean their sprung masses will have the same polar moment of inertia.

[Josh_K]

weight x arm (inches) = moment. This is the fomula for figuring ac cg out and it applies to polar moment too.

Basically the futher the weight is out the less of it you need to balance the car. Polar moment is not about CG it is a the ratio of weight and where it is at to balance a car and how does the arm that the weight is on effect the cars handling.

I frankly would not have coined the term "Polor Moment" for this ratio I would have called it somthing like "The ratio of weight muliplied by arm vs handling properties".

Josh

[younggunracer]

I love my mid engine 10 car but you take one thing and you lose the other. It turns in the wash and other places better then anything I can pass anyone there. But when it gets rough my *** starts walking and its hard to get traction at times. Give and take boys. Trucks are a little diffrent all i know is robby's mid-engine is real real fast