×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Wheel hop / Tramp in IRS

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

(OP)
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?

RE: Wheel hop / Tramp in IRS

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.

RE: Wheel hop / Tramp in IRS

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

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
 

RE: Wheel hop / Tramp in IRS

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.  winky smile


 

Mike Halloran
Pembroke Pines, FL, USA

RE: Wheel hop / Tramp in IRS

(OP)
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?

RE: Wheel hop / Tramp in IRS

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

RE: Wheel hop / Tramp in IRS

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).

RE: Wheel hop / Tramp in IRS

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.

 

RE: Wheel hop / Tramp in IRS

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

RE: Wheel hop / Tramp in IRS

(OP)
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?


 

RE: Wheel hop / Tramp in IRS

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.

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

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.

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

What vehicle are we talking about here? You've never told us ...

RE: Wheel hop / Tramp in IRS

(OP)
Pontiac GXP / G8

RE: Wheel hop / Tramp in IRS

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

RE: Wheel hop / Tramp in IRS

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
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
 

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

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
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
 

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Wheel hop / Tramp in IRS

(OP)
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?

RE: Wheel hop / Tramp in IRS

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
Fe

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

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?

http://www.vacmotorsports.com/catalog/bmw---guibo--driveshaft-flex-disc-for-various-bmws.htm

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

"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

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

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.  

peace
Fe

RE: Wheel hop / Tramp in IRS

(OP)
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).

 

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Wheel hop / Tramp in IRS

Good post Ron.  

peace
Fe

RE: Wheel hop / Tramp in IRS

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

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

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"

peace
Fe

RE: Wheel hop / Tramp in IRS

(OP)
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.

RE: Wheel hop / Tramp in IRS

Quote:

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.


Quote:

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.

RE: Wheel hop / Tramp in IRS

Quote:

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

RE: Wheel hop / Tramp in IRS

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?

RE: Wheel hop / Tramp in IRS

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

RE: Wheel hop / Tramp in IRS

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.

[img]http://will.saturnet.net/pictures/101_0033-1.jpg[/img]

RE: Wheel hop / Tramp in IRS

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: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Wheel hop / Tramp in IRS

(OP)
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 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.

RE: Wheel hop / Tramp in IRS

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

RE: Wheel hop / Tramp in IRS

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/science?_ob=ArticleListURL&_method=list&amp;_ArticleListID=1794823152&amp;_sort=r&_st=13&view=c&_acct=C000228598&_version=1&amp;_urlVersion=0&amp;_userid=10&md5=57e357c4346f714eef1c6114b2935958&searchtype=a

Other scientific journals also.

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

peace
Fe

RE: Wheel hop / Tramp in IRS

cheers for links.

RE: Wheel hop / Tramp in IRS

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,

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close