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Wheel hop / Tramp in IRS

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Ron364

Ron364

Automotive
Nov 18, 2010
34
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.
 
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The vehicle is rear drive. How will stiffer front engine mounts change the rear tramp Greg? I am looking for hints for rebound settings. Anyone had experience with Koni FSD?
 
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Ron,
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.
 
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I think Greg is hinting that strain energy (and especially gravitational potential energy) is being stored in lifting one side of the engine with axle pinion torque, and that that energy store is significant relative to whatever could be stored in the axle and suspension bushings.

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
 
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I agree with Mike and Greg for several reasons.

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
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
 
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Easy way to check; chain the engine down to the frame at all four corners, so it can't lift.
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
 
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Thank you to Mike, Pat, GT and Greg for the replies. Good web site to share info. Back to the tramp issue on IRS with respect to rebound settings. I see that BMW 135i, AMG C63 also have the same tramp issue, masked by electronic "traction control" ...now that's not engineering! Seems that GM's Camaro and CTS-V are the only ones to tackle the source of the problem rather than mask the effects. I would appreciate feedback on tests to fix the tramp with rebound rates. I have swapped the FSDs for "normal" rears now ...waiting for a wet road to test on. The mini oil pump inside the FSDs is ingenious ... increases rate over "corrugations" hence builds fluid pressure with time. GT6racer ..re-reading Koni web site agrees with you ...gee they make a description hard to follow though.

More "tramp" experiences out there?
 
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General question which might be related. Does tramp generally have anything to do with the side view IC locations?

Maybe changing the amount of anti squat could have some effect on the wheel hop?

Tim
 
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IRS designs generally absorb the torque reaction of the differential within the chassis (because the diff is mounted to the chassis and its torque reaction doesn't go through the suspension linkage). There still can be some effect, but it doesn't work the way it does with a beam axle. The hubs on an IRS see purely thrust (from the wheels acting on the ground), no torque from the halfshafts.

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).
 
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Tramp in a non-live axle suspension is generally caused by bushing compliance.

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.

 
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And I think that one of the approaches taken by an aftermarket company working with the new Camaro involves filling the holes in those nice voided bushings with some stuff that's rather stiffer than air.


Norm
 
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Norm, the "void fillers" have been tried using poly and have no impact at all on the tramp. The cause is a resonance setup in the axles and is solved by using one larger axle to offset and hence dampen a torsion wave. If any Americans read this, please chime in with experiences driving a new Camaro or CTS-v. However, torque control in the ECU is still used to minimise the problem in those cars via traction control. No wheelspin = no tramp. I was hoping to use rebound to get close to a solution then change one axle rather than two. Camaros use 45mm and 30mm axles or thereabouts, with 45mm one hollow. Treating the cause rather than muffling the resonance seems a better way to go. Amazing to me that BMW & Mercedes haven't figured out that the cause is in the axles, hence so too is the solution.

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?


 
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The Camaro issue was addressed by System Engineering principles with a good Adams model used to prove the solution and guide the requirements specifications. You won't solve it in any manner with damping, that's the mechanic's solution, not the engineer's solution. Its not a compliance problem either, B-I-O-N. The driving cause is in the mu-slip curve (the ENTIRE curve) shape and also the elasticity of the drive train. Writers have mentioned one of these elements but not the other. You'll need some component testing to see this for yourself. The handwavers and right brained contributors can't help you with this one. And, the solution works for solid axled, high torque drivelines, too. That includes the Amps, the Watts and the Volts.
 
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Once you exceed about 350 Nm in a RWD IRS tramp is quite common in production cars, especially,but not only, when accelerating out of corners in damp conditions. I've only worked on it on two cars, in both cases the shocks needed MASSIVE recalibration to solve it and we did it other ways.

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


New here? Try reading these, they might help FAQ731-376
 
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Thanks Greg. I thought of a rubber "doughnut" over the axle rather than the driveshaft to dampen the vibrations. The forces must be enormous though as there is a lot of thumping going on. The prop (driveshaft/tailshaft) has a rubber connection (much like Alfetta) and it tears if tramp continues too long, indicating that the vibrations go through the axles to the diff and prop. Hence the rubber on the axle(s) seemed a reasonable thing to do for a trial. Not sure how to attach it though to stop rotation. Surely BMW must have experienced the same issues? Using ECU ... even if T/control is turned off, to limit torque perhaps to the 350 value, is not the best engineering solution, a bit like waving the white flag. Camaro limit torque as well as big & small axles.
 
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The torque limiter on the Camaro is to save the transmission. The tire properties were respecified to minimize power hop. Reverse power hop (in reverse gear) could break some very expensive parts. If you get on the backside of the mu-slip tire response its self generating. So, that's compound, load rating, wheel width, pressure, etc. You have to give up some stuff, too.
 
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Just tested the IRS car with higher rebound ... using std shockers instead of FSD ... result was no difference in amount of tramp. Hence using polyurethane bushes makes no difference, higher rebound makes no difference ... indicating that the only idea worth following is the offsetting of torsion resistance in the axles. The big trick is to figure out the torsion ratio left to right. Diameter to the fourth power, means a bit of trial and error testing is needed. Tramp freq is around 8Hz, and wet roads do make some difference ...tramp freq is higher. Any Americans online care to measure up a CTSv ...I think around 50mm/30mm using hollow axles. Always keen to hear other experiences. Still keen to know why Porsches don't tramp.
 
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