Wheel hop / Tramp in IRS

[Ron364]

I am trying to cure a wheel hop /tramp issue in an IRS sedan. The cause seems to be a resonance issue from the wheel spin / then traction twisting of the rear axles. The new 2010 Camaro solved it by using different axle sizes left to right, as did the Cadillac CTS-V. The frequency of the tramp seems to be around the 6Hz range. Problem is complicated by using Koni FSD dampers that use a tiny oil pump that after a few oscillations, results in a rebound hole opening and rebound force decreasing for ride comfort ...hence frequency damping is reduced on rebound for a selection of frequencies approaching 10Hz (hitting holes at a fast rate). I feel that this reduced rebound force is making the tramp worse (let's agree to call it tramp, under acceleration). Any thoughts before I try different rebound settings? Fabricating new axles is the main aim, but this rebound issue is stopping progress.

 

[Ron364]

Thanks Greg and Mike. Interesting to also learn that if the diff does NOT have a LSD, that the problem is reduced with just one wheel spinning away without the tramp, or at least less tramp. Mike, it seems that some cars react better if the driveline is stiffened, perhaps with stiffer mounts aft of the engine. In your analogy, this would mean more energy is being stored in the "coiled spring/mouse trap". The wet road behaviour is much more violent than dry road, consistent with your idea that dry road tramp doesn't get a long enough time to release all of its energy.

However, if the mechanism stores the energy and never reaches resonance, then no tramp occurs and all is well. So the trick seems to be either dampen the vibrations (rubber tailshaft) or make the whole driveline so stiff that it keeps absorbing the forces and doesn't dare vibrate. Solid mounted engine/gearbox racing cars may well have less tramp too. I could have added a big anti-sway bar to the front, or smaller rear bar to keep the rear tyres more level under power to reduce the chances of wheelspin. Gee, getting complicated now and I was hoping a big axle one side would fix the blighter! Suggestions on things to try would be nice.

 

[FeX32]

It's interesting how it always comes down to fundamental laws, in this case fundamental solid elasticity.

"All complex things come from simple relations put together in different ways"


Fe

 

[Ron364]

The Lotus Carlton propshaft setup that Greg helped design is shown in good detail in this link

http://www.lotuscarlton.co.uk/prop.htm

Owners report that it certainly works well, thus indicating that the source of the energy windup "problem" is the engine and gearbox. In addition to the rubber mount midway along the two piece prop, there is rubber between the two "shells" in the rear half that seems to effectively stop any vibrations getting to the diff end. It is still not clear how the rubber "sandwich" can transmit the torque ... might have a spline as well perhaps.

Solutions for tramp that seem to work for the infamous IRS Cobra Mustangs of the 1994-2002 era suggest that replacing the rubber cradle mounts with a delrin or aluminium bush together with very stiff bushes in the upper and lower arms works well. I don't understand why this "cure" works well for the Mustang IRS yet does not work on other cars like the BMW and Commodore/Pontiac.

 

[TheDarkSideofWill]

MikeHalloran

The tuning fork analogy doesn't _quite_ do it for me.
Tuning forks exchange energy between elements while resonating, but not much energy is required to start them.

This system works more like a relaxation osciallator, where energy is alternately pumped in and released, and there's a lot of energy going in.

Think of the strain energy that's stored in >whatever< is deflecting. When wheelspin just starts, there's some high level of energy stored, possibly similar whether the road is wet or dry.

The energy release stops when the wheel regains traction.
... which in the case of a dry road, is when there's still a lot of energy stored in the system. I.e., there's still a lot of windup in the axles, or deflection in the motor mounts, or whatever.
... and in the case of a wet road, is when there's very little energy stored in the system. I.e., most of the stored energy has been dissipated in wheelspin, and nothing is still wound up or deflected.

... and the difference between the two stored energy states is much greater in the wet road case.

Ron364

Thanks Greg and Mike. Interesting to also learn that if the diff does NOT have a LSD, that the problem is reduced with just one wheel spinning away without the tramp, or at least less tramp. Mike, it seems that some cars react better if the driveline is stiffened, perhaps with stiffer mounts aft of the engine. In your analogy, this would mean more energy is being stored in the "coiled spring/mouse trap". The wet road behaviour is much more violent than dry road, consistent with your idea that dry road tramp doesn't get a long enough time to release all of its energy.

However, if the mechanism stores the energy and never reaches resonance, then no tramp occurs and all is well. So the trick seems to be either dampen the vibrations (rubber tailshaft) or make the whole driveline so stiff that it keeps absorbing the forces and doesn't dare vibrate. Solid mounted engine/gearbox racing cars may well have less tramp too. I could have added a big anti-sway bar to the front, or smaller rear bar to keep the rear tyres more level under power to reduce the chances of wheelspin. Gee, getting complicated now and I was hoping a big axle one side would fix the blighter! Suggestions on things to try would be nice.

Continuation of this analogy would be that everything in the powertrain that has drive torque or tractive force acting on it stores energy and contributes to the oscillation.
As the driver advances the throttle, increased engine torque stores elastic energy through deformation of the engine and transmission mounts, the subframe mounts, the control arm mounts (as tractive forces deflect them), the differential mounts and even a bit in the bodyshell (althogh its stiffness is MUCH higher than that of the rubber).
When the tries break loose, torque on the entire driveline suddenly drops, everything "unwinds" and all the elastic energy dumps out through the spinning tires. Once the elastic energy is released, the torque on the driveline drops to the point at which the tires can get traction and the cycle repeats.
IE, in addition to stiffness of the mounts for just about everything, axle tramp also depends on the difference or ratio (probably ratio) of the coefficient of static friction to coeff of dynamic friction of the tires--which is why it's worse on wet roads, as stated above.
But it's ALSO related to the mass and energy of the driveline. IE, the ridiculously heavy flywheel contributes as well. The first gen CTS-V's had something like a 45# flywheel... and people would wonder why they blew up differentials all the time.

I don't think that simpy stiffening the powertrain mounts would completely eliminate the problem for any level of stiffness short of bolted firmly to the bodyshell. Extra stiffness simply increases the frequency. The energy must be dissipated by a damper of some sort.

I'm working that issue on my project car: 1987 Fiero with Cadillac Northstar engine. I'm using four GMPP urethane transmission mounts to mount the powertrain. The mounts are very stiff, but they do not have much damping, which makes them prone to oscillate. So far this only manifests as rapid powertrain lash oscillation at parking lot speeds in first gear, but I haven't accumulated enough miles on it yet to be ready to drop the hammer and see what she'll do. IE, I don't know if I have an axle tramp issue yet or not.

The Fiero brings an interesting extra element to the equation... it has pro-squat rear suspension geometry (yes, *pro* squat), which relates tractive forces to vertical suspension motion. Thus a stiffer suspension damper *DOES* help reduce axle tramp on that car.

 

[NormPeterson]

Once the elastic energy is released, the torque on the driveline drops to the point at which the tires can get traction and the cycle repeats.

Might not be quite that simple. Operation on the "back side" of the tires' mu-slip curve might be a bit different than it is on the low side of the slip where peak mu occurs.


Norm

 

[TheDarkSideofWill]

Yes, there may be hysteresis in the slip curve...
I don't think there's much conceptually wrong with what I said.

Now I'm interested in a driveline torsional damper for my Fiero... any suggestions about how to implement one or what product to use?

 

[MikeHalloran]

In your case, Will, i.e. Fiero + Northstar, I'd start by adding some cantilever 'wings' to the bellhousing to amplify any torsional oscillation, and shock absorbers at the wing tips to damp the oscillations.

Next question: Have you even got room to do that?

If no, next thing to try is shock absorbers tied to a couple of head bolts, but they need to be stiff, and I don't think messing with head bolts after engine assembly is a real good idea.



Mike Halloran
Pembroke Pines, FL, USA

 

[TheDarkSideofWill]

I remembered that the stock V6 mounts use this:

http://www.fierostore.com/Product/Detail.aspx?s=52898&d=319&p=1

I can use the body mount for the original torque strut (*NOT* damper) and fab a bracket to use the power steering pump mount for the engine side of the damper.

That damper is very stiff compared to a suspension damper, and will be much further from the axle centerline than it was stock... but will also be dealing with about twice the torque.

Should end up mounted just to the right of the coil pack.

 

[GregLocock]

I spent 4 years sorting axle tramp on FWD cars, so my experience should be directly relevant here. The trick is to fit a fairly light damper - we often used the clutch damper, to damp the torquewise motion of the engine&trans assy.

These were about 0.6 inch dia. Fit it where the engine moves the most.

This will damp out the judder during acceleration.

Unfortunately the exact rate of the damper is a bit of a gamble, I had a box of 6 different ones to try.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Ron364]

Interesting that the use of shockers is mentioned. A good solution to tramp in an IRS car similar to the one I am working on can be found at

http://www.hopnot.net

Although it treats the effect and not the cause, horizontal dampers do seem to work and are relatively easy to fit. The tramp videos on that site are somewhat frightening, but enlightening. I hope to trial this approach soon.

Greg ... despite its complexity, the Falcon XR6T also tramps much the same as a VE/G8 and BMWs etc. I suspect another design tool is to use a wider wishbone angle to give more longitudinal control if space allows.

 

[ebola]

Hi.This is very good reading for us, drag racers from east europe.We race compact european cars on concrete airfields, and most of us still utilise IRS suspensions. All our research on IRS under load is empirical,trial and error way.What works for us is stiff torsion bars, nothing else.We dont use any rubbers or PU bushings at all - all suspensions are rose joints or spherical bearings, cars are light, and tracks are dusty.We run mid 8 quartermiles today here.
Currently Im looking for good source of information - scientific research, books or anything about chassis/suspension vibrations which would help me to create model/equation for interaction of suspension/chasis and engine/trans vibrations.For what I see today, this is clearly a mystery even between very advanced race chassis builders.Ive talked about this issue with quite a few known race car builders, and realized, that this issue never been researched properly.I have heard rumours from 6 sec dragster crews about how loosening motorplate bolts helped alot with tireshake, so on and so on, but never met confidence in knowledge on this issue.I do not know, maybe it has been done in states, or somewhere in high budget motorsport shops, by space level engineers.If you guys know good reading anywhere, please let me know.I have basic knowledge on non-linear dynamics and mechanics, and ready to study further mostly in self education purpose.Cheers.Andrius

 

[FeX32]

If I read you correctly there are many studies on dynamic interaction of suspension and chassis or engine and chassis.
Some can be found here.

http://www.sciencedirect.com/scienc...57e357c4346f714eef1c6114b2935958&searchtype=a

Other scientific journals also.

Most researchers nowadays use dynamic simulation software like ADAMS or others to speed things up.


Fe

 

[FeX32]

one eg.

http://papers.sae.org/2003-01-0859

Fe

 

[ebola]

cheers for links.

 

[CobraVertOps]

Wow, just found this forum looking for solutions to wheel hop on my 03 Cobra convertible. Some great input here.

I haven't found a solution to my wheel-hop yet but plan keep looking until solved, and will post my results. Meanwhile I'd like to add some simple observations from the Cobra IRS, may help lead others in right direction or generate some healthy what-the-hell-are-you-talking about type discussion:

- my wheel hop is same frequency as my clutch chatter. I'm guessing 90% chance they're related
- my wheel hop can be induced from a 2nd gear dead start with a 5k RPM clutch dump (burnout). That tells me it isnt due to tires losing traction and then gaining traction since tires would have to be spinning constantly. 5 grand in 2nd is like 50mph so no way tires are getting traction. This agrees with comments above that it gets worse when its wet btw.
- wheel hop only occurs when loading between rear tires is about equal, like doesnt happen going around a corner. I suspect this is what engineers were onto with the Camaro mentioned above and different halfshaft diameters.

Looking at it from a simple physics point of view (spring, damper, mass) there are a lot of "springs" in the system: motor mounts flex, clutch springs compress, drive shaft twists, differential and control arm bushings load, body flexes, tire sidewalls flex, etc. Most of these arent really damped either.

Most of the stuff I've seen online says I need to replace all the IRS bushings with Delrin. This is really expensive/hard to do, and I also hear doesn't always fix it - especially on a convertible.

So, that tells me that bushings are just part of the problem, but maybe not the main "springs" in the system. Some spring or probably combination of springs is loading up, and then something is releasing that force by binding, or basically going non-linear to introduce oscillation.

To eliminate an oscillation you need to remove the spring, acheive critical damping of the system, or change the resonant frequency from the input. My gut says driveshaft has the most potential to be a spring, but I havent heard anything is better than Ford Racing Alum shaft, which is what I have.

Anyway, here's what I intend to experiment with next:
- pinion angles. Best if zero degrees or at least offset front to rear while under load. Will use shims to make zero both ends if possible. not sure how to measure differential movement under load though, so may just be trial and error
- adjustable shocks. Will install with different settings side to side
- weld extra supports on the rear tie rod mounts, and get beefier tie rods. These look weak to me.
- build a pinion snubber. This will be easier than replacing all the bushings.

Any input appreciated.
thansk,