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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Wheel hop / Tramp in IRS
- Thread starter Ron364
- Start date
MikeHalloran
Mechanical
Porsche engines are rigidly coupled to the transmission and differential housing. Their motor mounts are absent from the tramp dynamics equation.
Mike Halloran
Pembroke Pines, FL, USA
Mike Halloran
Pembroke Pines, FL, USA
patprimmer
New member
When I had tramp issues with an air cooled VW Superbug and Type 3, I cured it by restricting up and down movement of the front of the gear box and by increasing the stiffness of all the chassis hard points where engine or transmission are mounted.
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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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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- Thread starter
- #23
Thanks Pat. Harder bushes in the Pontiac makes no difference to hop, despite what the suppliers say. The poly bushes in the cradle certainly sharpen up the response, but the hop remains the same. Arm flex is also a non issue with this car, leaving only the axle sizes as the cure. Has anyone driven a 2010 CTSv? The new look Camaro has a similar but slightly different rear end to the Pontiac GXP and cured the hop with axle sizes. Just found out the CTSv has 55mm and 35 mm hollow axles ..torsion ratio about 6 to 1.
patprimmer
New member
My results where from extra mounts of much lower compliance, like maybe only 5 to 10% of the original compliance. Basically solid steel bolted through a piece of 1.2" thick heavily reinforced conveyor belt rubber, not just poly bushes. In my experience, poly bushes can actually make it worse as they rebound so well.
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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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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GregLocock
Automotive
Well you are in luck, I worked on the chassis/driveline that forms the basis of the G8, many years ago, as it was the donor vehicle for the Lotus Carlton. Rubber up tube propshaft was the cure.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
- Thread starter
- #26
Greg, the Australian Commodore VE is the G8 ...did you work on the VE? Rear end is 98% same as new Camaro, also designed by the Australians. Camaro has the big/small axles, but VE missed out. Camaro doesn't tramp, VE/G8/GXP does. Can you elaborate on any cure found ...what does "rubber up tube propshaft mean" given that G8/GXP has a rubber doughnut at the diff end and it definitely tramps anytime the wheels spin, especially on wet roads.
GregLocock
Automotive
It's a propshaft that comprises two concentric steel tubes, with a rubber element compressed between the two that transmits the torque. Fairly common in luxury RWDs in the 80s.
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
New here? Try reading these, they might help FAQ731-376
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
New here? Try reading these, they might help FAQ731-376
- Thread starter
- #28
Thanks Greg. The Lotus Carlton rear end was used on the 2003-2006 Commodore ..semi trailing with a toe link. However, no rubber sandwich prop. The 2003-06 tramps worse than the later VE. A variant was sold in the USA as Pontiac GTO 2 door, with same horrible tramp issues too. Did the rubber prop fix the Carlton tramp issues?
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?
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?
My mk6 GTI does not tramp much at all. In fact it only did so very very mildey once on dry pavement coming out of a hard corner in 2nd gear. Every other time the wheels broke loose they just spun...
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.
![[peace]](/data/assets/smilies/peace.gif "[peace] [peace]")
Fe
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
GregLocock
Automotive
The RUT prop was used on all production Lotus Carltons to cure powerhop. Holden would not have picked it up, they probably didn't know about it, and it would have cost too much.
Suspension layout is irrelevant to this problem.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Suspension layout is irrelevant to this problem.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
- Thread starter
- #31
Greg or others, can you explain why the tramp is considerably worse on wet roads? Once the tramp begins, it seems to gain momentum (resonate)but is worse on wet roads than dry, despite the dry roads generating more twist on the axles before wheelspin occurs? Also, would the use of rubber doughnuts at the gearbox and the diff ends of a prop with a central bearing (hence split prop) behave similar to the Carlton with its overlapping splined props with rubber in the overlap? The subject vehicle has the rubber doughnuts hence attempt to stop vibrations getting to the diff and they do not fix the tramp.
LionelHutz
Electrical
Ron - I doubt the axle twist causes it, so the fact the axles twist less on a wet road is irrelevant.
Are you referring to this type of coupler?
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?
Are you referring to this type of coupler?
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?
NormPeterson
Structural
"Once the tramp begins, it seems to gain momentum (resonate )but is worse on wet roads than dry"
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
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
GregLocock
Automotive
Both production cars were worse in damp to wet conditions.
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
New here? Try reading these, they might help FAQ731-376
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
New here? Try reading these, they might help FAQ731-376
- Thread starter
- #35
Lionel, yes those are the rubber mounts at the gearbox end and also at the diff end of the prop. If resonance in the prop was the cause, I would have thought those rubber joints would reduce it somewhat. Norm, the tramp produces a broken rubber strip ..wheelspin then the wheel is in the air etc continues until you lift off the throttle. Hence a massive force is compressing the spring, that is why I was trying to change rebound to control it better (made no difference). I'd like to understand the cause mechanism. It is strange that the reduced friction forces in the wet cause a bigger hop problem! Only clue here is that the wheels are spinning faster when they let go and the outcome is a strange up/down at the wheel and it feels like a sideways force acts on the rear subframe, as the car does shake sideways. Mechanical sympathy makes you lift off quick though! It feels as though there is a massive out of balance force, like a huge wheel imbalance.
This would make a good study project for a keen masters degree student. Keep those thoughts coming, that is why we have forums.
This would make a good study project for a keen masters degree student. Keep those thoughts coming, that is why we have forums.
Think about it a bit more. Is the force under accelerating that the wheel torque creates strictly horizontal? I think not. And is the suspension at the fornt end exerting the same force to the ground under acceleration as it usually does. I also think not.
This combination can cause imbalanced forces when the wheels loose traction.
Fe
This combination can cause imbalanced forces when the wheels loose traction.
Fe
- Thread starter
- #37
The mechanism: well, try this as it seems to fit info from Greg, Pat and from others outside of this forum who have stumbled on solutions. The drivetrain is a big tuning fork. The engine has rubber mounts (actually fluid filled on this example), the gearbox has only one rear rubber mount, and the split tailshaft has a flexible middle joint, the diff is rubber mounted, and so to the axles that havew links rubber mounbted. Treat it as a huge tuning fork with the axles as the two prongs. Like any tuning fork, when excited (by the wheels/axles breaking traction) it will resonate. It continues to resonate until the cause is removed or dampened by other means (putting your finger on the end of a tuning fork ceases it)
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).
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).
GregLocock
Automotive
Thompson coupling - not a bad idea but the claimed increase in efficiency is tiny. It's basically a slight refinement of a double cardan. I've got no argument with the kinematics but the whole thing is completely oversold.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
MikeHalloran
Mechanical
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.
Mike Halloran
Pembroke Pines, FL, USA
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
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