reverse anti-squat?

[sszuch]

I haven't been able to find any information regarding suspension set ups on rear engined cars. I have a 64 beetle with IRS (independant rear suspension) I use for drag racing. The problem I am encountering is that the car does great wheelies (good show for the crowd), but I feel the 60 ft. times suffer because of the lifting instead of forward motion. What I would like to do is get the suspension working in a forward fashion, similiar to what traditional anti-squat suspensions do. My initial thought is to move the forward pivot point down, effectively pointing the rear trailing arms forward and down. The way I see it, the reaction in the gearbox is trying to rotate the transaxle around the axles, lifting the nose of the gearbox, and lifting the car.

The first step would be to try to make a simple model of the car. I would eventually like to design a tube chassis and optimize this 'forward thinking', but I am not sure where to begin.

Could this be approached as simple vector analysis? While thinking out loud, the first answer is 'yes', but it is obvioulsy more involved than this. I am sure I would have to model the front suspension and the chassis stiffness to get a reasonably accurate answer. So, before I too involved, am I on the right train of thought or is all of this jibberish? I would like to get some thoughts and opinions from those more experienced (almost everyone!) on this unique set up and the feasability of what I'm trying to do.

Thanks,
Steve

 

[GregLocock]

First of all bear in mind that anti or pro squat does not give you something for nothing. If you momentarily increase the vertical wheel load at some point in time, some time later the wheel load will drop back to less than the static value.

Havaing said that if you are traction limited early in the run, and, anyway because late wheelslip is less damaging to ET than early wheelslip, you may still come out ahead.

What you are describing is conventional antisquat. So you want to lift the line of action through the wheelcentre and the ic of the svsa higher relative to the cg.

Vector analysis and free body diagrams are fine, if you have no significant compliances in the arms.











Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[sszuch]

Thanks Greg,

I think I understand most of what you are saying and how traditional anti-squat set ups work. I'm not readily familiar with the svsa term and by compliances in the trailing arms, I am assuming you mean bending.

Bear in mind that this is a rear engined/transaxle car. I am thinking that a traditional anti-squat set up will make things worse by 'helping' the lifting reaction of the nose of the tansaxle and rotate the car about the rear wheels, which is what I'm trying to avoid. My first thought is to counter act the reaction at the transaxle, so the resultant vector would be directed more forward than up.

Am I correct in thinking that the wheelie is time wasted off the line when there could be more forward motion or am I putting too much effort in minimal gain?

 

[GregLocock]

If you want to do this yourself then you'll need ot find out what svsa means.

compliances means rubber bushes, typically.

"Bear in mind that this is a rear engined/transaxle car. I am thinking that a traditional anti-squat set up will make things worse by 'helping' the lifting reaction of the nose of the tansaxle and rotate the car about the rear wheels, which is what I'm trying to avoid. My first thought is to counter act the reaction at the transaxle, so the resultant vector would be directed more forward than up. "

That is wrong. Antisquat doesn't care about trivial details.

"Am I correct in thinking that the wheelie is time wasted off the line when there could be more forward motion or am I putting too much effort in minimal gain?"

I suspect so. If you are competing against similar cars and they are faster and don't wheelie then I guess you have a case.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

It uses power to lift the front.

The trade off depends on how much power and how much traction you have. you need just enough weight transfer to not lose traction, BUT if you lift the wheels and still lose traction, further weight transfer just wastes power to lift the wheels higher, which slows the car and reduces stability, which may further slow the car. It also increases aero which can slow the car if the wheels are to high for to long.

The rear engine placement makes VWs prone to wheel stands.

The front gear box mount is a weak link and should be reinforced with a steel strap over the front of the box and bolted to the chassis rails or horns.

A little anti squat to keep things fairly level should help if for no other reasons than it will reduce camber changes.

To reduce wheel stands, you need to move weight forward, preferably the engine/transaxle unit. Easier said than done in an air cooled VW chassis.

Another easy option is to move the wheels back. This is often done on off road buggies using VW type IRS rear suspensions.

I would expect minor compliance in the rubber that surrounds the outboard end of the torsion bar. I would also expect some in the so called spring plate or trailing arm that runs from the hub to the torsion bar. These are about 5" deep, but 3/16" thick from memory. They need some form of reinforcement to prevent flex in the lateral plane, but still be allowed to twist a little due to camber changes at the hub, but not at the torsion bar end.

It's 20 years since I played with a Beetle, so my memory might be a little off, or there may be more recent developments.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sszuch]

Thanks again Greg, I now know what svsa is and I am one step closer.

"That is wrong. Antisquat doesn't care about trivial details."

Does this mean that the +Z load at the transaxle will just be reacted out by the chassis and not affect the line of action from the trailing arm?
I have been thinking about this for some time and and cannot convince myself that this load is trivial since the transaxle is solid mounted to the chassis, as suggested by patprimmer, which is why I'm here asking for guidance.

"I suspect so. If you are competing against similar cars and they are faster and don't wheelie then I guess you have a case."

It is not that they do or don't wheelie, I am just trying to optimize the set up. Most of what I see is "monkey see, monkey do" in the way the current pro stocks are set up and I just feel there is a better way. These cars do not employ anti-squat suspensions. If the basic theory still holds true with a rear engined car, I'm suprised no one has tried it yet on a VW, especially when there is a decent amount of money on the line.

patprimmer,

Traction so far is not an issue, but I am not putting out considerable HP at this time. What I've noticed is that swing axle cars that do wheelies will typically unload the rear suspension in the middle of the wheelie, drastically changing the camber as well as traction loss. I have personally seen a car that unloaded so bad that all four wheels were off of the ground shifting into second. These problems are typically "fixed" by the use of limiting straps and stiffer torsion bars.

My car now is pan based bracket racer with a raised type 2 transaxle and the supports you have suggested, but plans are to design a custom chassis and enter the pro stock ranks. Compliance issues should not be a problem, I'll probably either use a clevis or heim style joint. As for now, if it does become an issue, I think someone makes some busings from metal (I can't recall the material right now).

Currently, the trailing arms are pointing up at a relativly small angle (I don't know for sure, I'll have to measure) but is pointing well forward of the CG, which I suspect is just forward of the rear wheels due to the lack of weight up front. Dynamically, this angle changes as the car squats further lowering the line of action from the CG. This would mean for anti-squat, as I understand it, the trailing arm would have to point up at a relatively steep angle (60*?). The car lifts the front wheels about 2.5 feet and only stops because of the wheelie bars.


Sorry for the long post, my intent is to get some feedback instead of wasting time using trial and error.

 

[GregLocock]

I did think of another reason why wheelies are bad, which you haven't mentioned - aero drag.

If you have halfshafts then the antisquat equation doesn't care about how your engine/gearbox/diff is mounted - the only reaction of importance is at the wheel centre, forward.

This is then reacted into the body via the instantaneous swing arm, whose inclination will control your squat.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

You're probably giving away tiny bits of time due to things like increased aero drag and the possibility of the driver lifting slightly as this happens. If nothing else, the last thing you need is more upward migration of the CG under forward acceleration once you've got just a shred of daylight under the front wheels.

"What I've noticed is that swing axle cars that do wheelies will typically unload the rear suspension in the middle of the wheelie, drastically changing the camber as well as traction loss. I have personally seen a car that unloaded so bad that all four wheels were off of the ground shifting into second."

Sounds like they're hitting the wheelie bars with enough rotational momentum about the Z-axis to get the whole business to start rotating about the wheelie bar casters instead of about the drive axle, which then lets the car's own rear suspension go into droop. Once you've reached the point of wheelie bar caster contact, in some (many?) respects you're now driving a FWD drag car, so you may be able to adapt some of the tricks learned there.

Perhaps longer wheelie bars, or a design with some compliance/springing/damping (perhaps even position-sensitive?) might help. The thought here is that the more gently and progressively you can make use of the bars, the less load comes off the drive wheels. There's probably a limit (or at least a region of insufficient return) based on any amount of added weight, and it would certainly add its own demands on tuning to suit conditions.

I do wonder what the anti-squat is doing as the front wheels are lifting - is it progressively increasing or is it dropping off - since the "at-rest" kinematic construction is obviously out the window.

This is a case of a too-rearward CG for the available grip and total torque, so are there any "little things" which, taken together, might be enough? Even if it's only a few inches of driver seat and controls relocation. Out of curiosity, is this car using a relatively light or heavy flywheel (this can be considered a component of "total torque")?


Norm

 

[sszuch]

As I mentioned earlier, the trans reaction was giving me trouble because I couldn't convince myself either way and I wanted a second opinion from people more experienced than me. Thanks Greg for clearing that up. I didn't consider the increased aero drag as a major influence because the wheelie is over at the end of first gear, probably less than 40 mph. While this does increase aero drag some, I figured this was only a bonus when the wheelie is abated and 60 foot times improve due to a more efficient launch.

In the chassis design, I was thinking of trying to incorporate some way to tie the forward trans mount to the cage at an angle to react it out in a more forward fashion. This way, there will be some forward +X component along with a +Z similiar to anti-squat, does this make any sense?

Hello Norm,

I should have clarified, but the car I cited in the previous post did not have wheelie bars. What you have mentioned has been another something I have considered. I am not sure how the rear suspension is reacting when hitting the bars hard on launch. So far, my car does fine concerning traction. The car launches, hits the bars, and the suspension unloads at the top of the wheelie. This is ok for now because I am running out of steam and gear, the car starts nosing over and it is time to shift. Add more power and torque, and things will change.

As far as what the anti-squat is doing, in a perfect world nothing should happen after the front wheels are unloaded, correct? I would imagine that it would drop off as the suspension compresses and lowers the IC below the CG. But, if I design the chassis correctly, I would think this is where fine tuning comes in.

The flywheel is 12.5 pounds, I believe stock is about 16-18 pounds and the car is about 1500 pounds (give or take). I understand a flywheel as an energy storing device because of its rotational inertia. It takes enery to get it moving as well as trying to stop it. I didn't think it was additive as "total torque", only the torque from the engine and gear multiplication from the gear box to the rear wheels.

 

[NormPeterson]

Aero drag isn't likely to be a huge influence, though I imagine that it could amount to ~0.03g averaged over its duration. Enough difference for a close race.

The point about looking at the flywheel in terms of its contribution to a total torque is that the output from rotationally decelerating it on clutch engagement is a torque delivered over that length of time. Inertia doesn't cause acceleration through a powertrain; its (temporary) torque being the measure of its conversion to useful work does. The bottom line is that a heavier flywheel (or a more abrupt full clutch engagement, for that matter) would be more likely to yank the nose skyward as in the 1-2 shift mentioned.


Norm

 

[GregLocock]

When the front wheels start to lift off, and you have no wheelie bars, what prevents the car from doing a backflip? Do you back off on the throttle?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[sszuch]

Hello Greg,

Personally, my car has wheelie bars. The car launches very straight and stays on them pretty hard all the way through first gear.

Other cars that don't have wheelie bars usually have enough torque and horsepower to initiate and sustain a wheelie, but not overcome it. Although, I have seen pictures where aircooled beetles without bars have actually rolled completely over.

I have seen and talked to other drivers that forgot their bars on occasion, and yes, they do lift when the stars shine too bright.

 

[GregLocock]

The chances are that you'd get better launch if you didn't need the wheelie bars as all the weight transfer would be onto the driven wheels.

So modifyng the antisquat to keep the front wheels on or near the ground seems like a good idea to me.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BillyShope]

sszuch, the wheelie isn't affecting your 60 foot times as much as the load reduction on the driving wheels due to the wheelie bars. The goal is to eliminate any loads on the wheelie bars and have the front tires just "kissing" the track surface during launch. This means moving weight forward. I would assume you've moved the battery forward. I had a Ghia and was able to mount the battery up in the "nose." In your case, you might have to fab an external tray and move the battery forward of the bumper area. You probably don't want to increase overall weight, so the next steps are going to be more difficult. If you have enough success moving weight forward, the elimination of the weight of those wheelie bars might be the final increment.

I would suggest that you take a look at the "Getting Started" section on the first page of my site. It's written for the non-engineer, but you can skip over the basics and get to the explanations involving matters such as lines of constant percent anti-squat. With IRS, of course, these lines converge at a point essentially on the rear wheel centerline.

http://home.earthlink.net/~whshope

 

[sszuch]

Thank you BillyShope,

I have previously read many pages on your website, which, in a way, prompted the initial question here. I understand that anti-squat suspension optimizes traction, which at this time is not as much of a concern. Rather, I am more interested in trying to optimize the forward motion of the launch.

Unfortunately, I don't have the benefit of the huge moment arm that exists with front engined cars. When it is time for any VW to go on a diet, traditionally the front end is where most of it comes from. In my car, the battery and an aftermarket fuel cell have been relocated forward of the front beam, but are still within the confines of the fiberglass front end. I'm not sure if it would be legal to have the battery exposed in front of the car.

Another option I have thought of was to remove as much weight out of the back and add it back to the front in the form of lead bars (or shot) at specific locations. The weight of the car will not change, only the CG location.

The difficult part of this project is that all of the normal convienent places to remove weight also works against the 'CG forward' train of thought. For example, many people, including the previous owner of my car, removed the front brakes to save weight. Cars seem to slow and stop fine with only the rear brakes. Would there be any forseeable handling problems with this philosophy reversed? How would a car respond with only the front brakes? I wouldn't imagine this would be a problem since the front brakes do most of the work anyway, correct?

 

[GregLocock]

Front brakes only is fine, it is generally safer than rear brakes only, although that depends on where your cg is a bit, and of course on your tires.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.