Wheel hop / Tramp in IRS
Wheel hop / Tramp in IRS
(OP)
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.
RE: Wheel hop / Tramp in IRS
Cheers
Greg Locock
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RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
You are almost right about FSD except they don't start stiff and go soft at high frequency - they start soft and go firm if the input is long enough. Net result however is that you are right - you are not getting small amplitude rebound damping. If performance is a driver - as indicated by the fact tramp is an issue - then normal dampers would be a better (and cheaper) solution. Next I'd go to any bushings in the suspension ( including the shock ) to stiffen them up.
RE: Wheel hop / Tramp in IRS
So stiffening the engine mounts may produce more benefit than stiffening the suspension, and if the engine is in decent balance, will produce fewer undesirable side effects.
Mike Halloran
Pembroke Pines, FL, USA
RE: Wheel hop / Tramp in IRS
1) They are both very experienced engineers and Greg in particular with specialist knowledge of drive lines and vibrations.
2) The origin of drive line vibrations can be very obscure and removed from where the symptom is obvious.
3) In a front wheel drive transverse mounted engine I very recently had 2 supposedly different problems. The first to occur had a symptom identical to rubber insert in an engine mount being pounded out or the mount loose, ie lots of backlash evident on clutch being engaged or disengaged. The other which developed later was symptoms of a tyre de-laminating, ie severe vibration at all speeds but increasing greatly at certain speeds to the point that drivers vision was disturbed. Both emanated from the decay of a tripod design type inboard CV joint. Lucky I replaced the CV before I did the engine mount.
Regards
Pat
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RE: Wheel hop / Tramp in IRS
Set up the connections so the chains have a little slack.
Then go try and induce some wheel hop.
It's a tough job, but someone's got to do it.
Mike Halloran
Pembroke Pines, FL, USA
RE: Wheel hop / Tramp in IRS
More "tramp" experiences out there?
RE: Wheel hop / Tramp in IRS
Maybe changing the amount of anti squat could have some effect on the wheel hop?
Tim
RE: Wheel hop / Tramp in IRS
Front-drivers can have the same issue in the front end. My front-drive car (VW Mk5 Golf chassis) has horrible front-end suspension hop if the front wheels start spinning on wet pavement. I haven't done anything about it because for what I use the car for, it doesn't matter, but I've seen this matter discussed elsewhere on the interweb, and the problems and solutions might be related.
I have a funny feeling that it has to do with fore/aft compliance in the linkage combined with damper settings combined with torsional compliance in the powertrain (engine/trans mounts in my case, that plus differential mounts in your case).
RE: Wheel hop / Tramp in IRS
From what I understand of the Camaro's design, there was a lot of compliance built into the rear suspension and subframe to allow for a nice ride to go with the Conestoga wheels.
RE: Wheel hop / Tramp in IRS
Norm
RE: Wheel hop / Tramp in IRS
Re motor/gearbox torque, it seems that stiffer poly mounts for the diff does help, but introduces considerable harshness.
I'll report back after testing with better rebound control using one large and one std size axle. Do Porsches tramp?
RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
The engine mounts are one of the biggest springs in the problem, as Mike says.
Mustang live axle has or had two shocks per side, partly to solve this issue.
It is primarily a torsional wind up of the driveline, so things that act in the vertical plane won't necessarily work.
The other thing you can do is damp the driveline by using a rubber up tube propshaft.
I suggested the engine mounts as they are the easiest thing to change in a car that might affect this problem, they certainly aren't the best approach in production.
Cheers
Greg Locock
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RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
Mike Halloran
Pembroke Pines, FL, USA
RE: Wheel hop / Tramp in IRS
It was never required but my next point would have been the trailing arm bushes and arm stiffness in the longitudinal plane.
I know it was not a Porsche, but the layout of the later model Beetles was similar.
Regards
Pat
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RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
I also fabricated a substantive sub frame to stiffen the floor in the area and I would think reduced floor pan flex by a factor of at least 5 and possibly 10.
That is not to say I am against your axle idea. Just throwing in my personal experience on a somewhat different example. I would think Porsche used a more sophisticated and integrated version of my approach.
Regards
Pat
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RE: Wheel hop / Tramp in IRS
Cheers
Greg Locock
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RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
The Americans were obsessed in curing tramp at the axle end of things, but that is inefficient. I doubt they really understood it properly.
I worked on the Lotus Carlton, which was based on the Opel Omega, which the Holden guys used as the basis for the VE. You presumably have the later suspension but that makes no odds, since the suspension isn't really part of the problem.
Cheers
Greg Locock
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RE: Wheel hop / Tramp in IRS
The 2007+ VE has a pseudo double wishbone rear with pivots that do not lie in the same plane ..hence rubber bushed. The new CTSv and Camaro have fixed the tramp with axle offset sizes. I wonder if they trialled the rubber prop before going with the new axles?
RE: Wheel hop / Tramp in IRS
Although, my older golf tramped significantly.
Biggest differences between the 2 are; engine (presumably mounts), much stiffer suspension, lower ride height, tires and wheels (much less compliance in the GTI), electronic XDS diff, and a few other things that don't cross my mind right now.
Fe
RE: Wheel hop / Tramp in IRS
Suspension layout is irrelevant to this problem.
Cheers
Greg Locock
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RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
Are you referring to this type of coupler?
ht
The last 2 generations of 'Vettes have 2 of these in the torque tube and they don't help with the tramp issue. Shocks, tires, diff mounts, engine mounts and ride height all do have an effect to varying degrees.
Greg - Would I be correct in suspecting that shaft was tuned specifically for that car?
RE: Wheel hop / Tramp in IRS
What do you suppose is happening to the water/what is it doing/where is it going, as tramp is happening?
On dry pavement you'd have to be transferring some rubber.
Norm
RE: Wheel hop / Tramp in IRS
It's got something to do with the stick slip phenomenon at the contact patch, but I don't know what exactly.
No the RuT was whatever the manufacturer gave us, probably off a Jag or Merc.
Cheers
Greg Locock
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RE: Wheel hop / Tramp in IRS
This would make a good study project for a keen masters degree student. Keep those thoughts coming, that is why we have forums.
RE: Wheel hop / Tramp in IRS
This combination can cause imbalanced forces when the wheels loose traction.
Fe
RE: Wheel hop / Tramp in IRS
This explains why some have raised the resonance frequency using stiffer engine mounts, or stiffer props, or rubber insulated props, others have used stiffer poly diff mounts and others have used axle diameter offset to diminish the reactions and possibly cancel each side's vibrations out. Overall solution is probably a mix of all of those ideas.
Competing solutions ...stiffer mounts = more noise and harshness. Can someone expand on how fluid engine mounts work ...tempted to fill them with polyurethane.
Re transaxle setups ...much stiffer with no prop, hence resonance is higher than can be generated by wheels spinning. FWD cars can be solved using stiffer mounts.
Greg ..re the prop joint. Have you seen the brilliant "Thompson coupling", the first true constant velocity CV, google is your friend, also on youtube.
Comments please ...(as he ducks for cover).
RE: Wheel hop / Tramp in IRS
Cheers
Greg Locock
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RE: Wheel hop / Tramp in IRS
Fe
RE: Wheel hop / Tramp in IRS
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.
Mike Halloran
Pembroke Pines, FL, USA
RE: Wheel hop / Tramp in IRS
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.
RE: Wheel hop / Tramp in IRS
"All complex things come from simple relations put together in different ways"
Fe
RE: Wheel hop / Tramp in IRS
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.
RE: Wheel hop / Tramp in IRS
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.
RE: Wheel hop / Tramp in IRS
Norm
RE: Wheel hop / Tramp in IRS
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?
RE: Wheel hop / Tramp in IRS
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
RE: Wheel hop / Tramp in IRS
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.
[img]http://will.saturnet.net/pictures/101_0033-1.jpg[/img]
RE: Wheel hop / Tramp in IRS
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
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RE: Wheel hop / Tramp in IRS
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.
RE: Wheel hop / Tramp in IRS
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
RE: Wheel hop / Tramp in IRS
Some can be found here.
htt
Other scientific journals also.
Most researchers nowadays use dynamic simulation software like ADAMS or others to speed things up.
Fe
RE: Wheel hop / Tramp in IRS
http://papers.sae.org/2003-01-0859
Fe
RE: Wheel hop / Tramp in IRS
RE: Wheel hop / Tramp in IRS
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,