Axle Tramp

[riancalder]

I'm hoping someone may be able to assist with a few ideas on how to cure a tramping problem in a rallycar. The car is an older V8 Holden Commodore (aus) with approx 400bhp. (approx 1300kg, live axle, coil sprung, upper and lower diff links, rally tyres)

Presently, when accelerating the car suffers from severe tramping. We have tried increasing the shocker rebound, installed stiffer springs (not sure on actual stiffness) and installed less giving (nolathane) bushes in the diff links. Although slightly improving the problem it has not cured it.

Our next idea is to increase the compression resistance of the shocks (requires rebuild). Due to regulations we cannot install tramp bars or move the position of the factory links.

My question is can tramping be caused by excessive movement of the driveline? ie. if we installed more rigid engine and g/box mounting could this help the problem?

Any other ideas for a cure?

Thanks for any help.

Rian Calder

 

[GregLocock]

The ususal mechanism is a torsional resonance in the driveline, from the wheel right back to the engine, which sends a torque pulse back to the wheel, in a positive feedback loop.

Are you allowed to add a shock absorber to suppress pinion nose nodding?

I worked on the Lotus Carlton - we cured axle tramp by using a 'rubber up tube' prop shaft. I think some older Falcon's may have used one.

I doubt that new engine mounts will help unfortunately, I think the mode terminates at the crank.


Springs and shocks may help a bit, but not as a complete cure.








Cheers

Greg Locock

 

[c2s]

Axle tromp starts as a slight side to side vertical oscillation. This then progresses into a walking motion, where one side is going up and rearward, while the other is going down and forward. Some Fox platform vehicles had this phenomena. To solve it two methods were devised. One is a rubber snub that's put between the axle and the leaf spring, limiting wind up. The second solution was to put shocks angling for and aft (in addition to the two regular shocks). The latter was chosen for marketing reasons.
Kevin

 

[riancalder]

Thanks to both replies.

Greg, can you please expand on what a "rubber up tube" propshaft is?

The two replies seem to describe two different mechanisms for the tramp/tromp. When I think about it, I can see the second occuring for a leaf sprung vehicle as the diff would move up and back when the suspension is compressed, but I don't think it would occur in a four link coil sprung set up like in my particular case? A shock absorber on the pinion nose would help in the first but not in the second mechanism. Do many production cars actually utilise this set up?

Unfortunately we cannot consider this an option due to the class rules.

Do you have any thoughts on increasing the downward angle of the pinion nose to a max of 4 degrees? Would it provide some rotational resistance to the differential? I'm not sure on the current angle but it is likely to be quite shallow due to raised rear suspension.

Thanks again.

Rian Calder

 

[GregLocock]

"can you please expand on what a "rubber up tube" propshaft is? "

The single piece propshaft is made of two concentric tubes of steel, one to the front, and one to the back, with an intermediate concentric rubber tube in the overlap to provide isolation between the two.

" The two replies seem to describe two different mechanisms for the tramp/tromp. "

Don't bet on it, the truth is that axle tramp is very poorly understood and is fixed in different ways on different vehicles, nobody has a really good handle on it in my experience.

"When I think about it, I can see the second occuring for a leaf sprung vehicle as the diff would move up and back when the suspension is compressed, but I don't think it would occur in a four link coil sprung
set up like in my particular case? "

It still happens but the frequency is much higher because it should be reacted by the suspension bushes in compression, but often people make the arms non-parallel which induces a tendency to rotate in the axle.


"A shock absorber on the pinion nose would help in the first but
not in the second mechanism. Do many production cars actually utilise this set up?"

I don't think so. In mechanical terms it is very similar to Kevin's mechanism - basically you are damping out rotation of the axle, and teh 4 shock absorber setup is the only time I can remember seeing shock absorbers used for a driveline problem.

" Unfortunately we cannot consider this an option due to the class rules."

"Do you have any thoughts on increasing the downward angle of the pinion nose to a max of 4
degrees? Would it provide some rotational resistance to the differential? I'm not sure on the
current angle but it is likely to be quite shallow due to raised rear suspension."

My gut feeling is that there is probably an optimum setting for the pinionnose angle, but we never explored it as that angle is set for a production vehicle by the 2nd order excitation from the Hookes joints.

I doubt it will be a complete cure.

Cheers

Greg Locock

 

[GregLocock]

"teh 4 shock absorber setup is the only time I can remember
seeing shock absorbers used for a driveline problem."


Sorry, I should have said " a rear wheel drive driveline problem", they are used in front wheel drives to cure similar problems. I fitted one to the FWD Elan to cure wheel tramp.

One other cure for axle tramp I have seen : thicker halfshafts.

One way you might be able to damp axle rotation within the rules is by moving the lower point of the shock absorber so that it partly reacts axle rotation. This will affect your handling unfortunately.




Cheers

Greg Locock

 

[riancalder]

Thanks for the ideas Greg. Let the experimentation begin...

 

[SWB]

Back in the 60's it was found by the Pensky team that alterations in a damper's low-speed damping characteristics (less damping) can eliminate these problems and that sometimes it has nothing to do with the axle "winding up." Carroll smith also makes reference to this fact in one of his books, possibly Tune to Win. I would consult with your damper manufacture about this, assuming they are not the OEM of the car..

Sean

 

[emanuelhung]

not being familiar with ausy cars, does your car have leaf springs or link bars? if it has link bars and you have raised the hieght of the car, as U.K. rally cars do, have you altered the angle of the link bars and you are now creating to much grip, and shearing the contact patch of the tyre. IE grip/spin/grip/spin ,leading to axle tramp. let me know if i can be of any help.

 

[riancalder]

emanuel,

The rear end is 4 link with coil springs. Unfortunately, work commitments have meant forgetting about the cars problems for a while, but when time becomes available we will start experimenting again. Thankfully, we have a bit of time up our sleeves before next season.

Our planis to remove the link diff brackets and reweld them on with the pinion angled slightly downward. Thanks for the offer of help and I'll let you know how we go.

Rian

 

[JonnyRochester]

I think you have all missed the mark slightly. What is happening is that the car is lowered slightly, and the lower links are no longer harizontal but at an angle. The angle of the lower links causes the car to squat under acceleration. A bit burst of power on takeoff causes squat so the wheels are lifted up off the ground so you break traction, and just as traction is regained the axle is sent back up into the body of the car only to loose traction again. Thats the way I understand it, nothing to do with pinion angle or shocks. Shocks can only hide the problem, not fix it. Its all about geometry.

The common fix (to reduce squat at least), is to relocate the bottom link position on the diff a bit lower. This brings the bottom link back to horizontal or even a bit past that. No welding is needed because there are bolt on kits avaliable. Try Whiteline first, or ask them if they don't make it. I'm sure its legal for rally too.

 

[BEATH]

My understanding is that axle tramp (live axle)is caused by driveshaft torque unloading the right side tyre so that it looses traction. When traction is lost driveshaft torque reduces and the tyre is forced back into contact with the road, driveshaft torque increases the tyre is lifted and the cycle repeats.
There is a solution involving focusing the suspension links on a point on a line drawn through the diff centre to the right (in the horizontal plane) at an angle to the car centre line, the tangent of the angle being the diff ratio.This is difficult to explain without a diagram.
The problem with this solution, aside from not being allowed by regs, is that axle tramp reappears under brakes. The fifth link used on some speedway cars comes close to this solution, although I have never seen reference to using the diff ratio to find the correct location for attaching this link to the car frame.

 

[JonnyRochester]

I'm sorry, but I can't agree about the diff ratio thing. Tram is cause by torque and some movement in the suspension. Torque is not a result only of the diff ratio, but of the gear in the gearbox used and the engine rpm and the engine size and design. Therefore there can not be one solution for a diff ratio, because changes with so many other things. Also, tramp does not happen on just one wheel from the body of the car being rotated sideways. Stand next to a car trying to spin them up and tramping. Both wheels will be doing it. It is only with drag racing cars that the engine can twist the body sideways on its suspension to any significant degree. The issue is of the diff rotateing about the axle, or moving up and down. With a 4 link setup, the diff can not rotate, but moves up and down.

 

[BEATH]

Johnny I will try to explain what it means.Looking at the car from the right hand side, imagine a lever arm solidly attached to the rear axle housing and attached to the car frame a distance y forward of the axle.Torque about the wheel axis will be the driveshaft torque times the diff ratio,call this D*R where D=driveshaft torque and R= diff ratio. The force at the end of the lever arm resisting this torque will be D*R/y.
This force can be used to resist the tendency of the driveshaft torque D to lift the right tyre.How?.
Facing the front of the car,imagine the attachment point of the lever arm offset to the right of the car (your left)by an amount x. The force at the attachment point D*R/y multiplyed by x generates a torque which opposes the tendency of the right wheel to lift.For this torque to be equal to D we can say (D*R/y)*x=D
or D*R/y=D/x
which reduces to R=y/x
that is to say if the diff ratio was 3 to 1 make y three times x and the system is in balance.
A four link system with the links focussed on this point would behave in an equivalent way.

 

[JonnyRochester]

You may be correct in part with this theory, but you need to get out of the office and see what realy happens. The torque aplyed to the vehicle in this way is not realy that great comparted to other things. If it was, you would see vehicles rock to one side every time they accelerate. In the real world, vehicles only rock to one side like this in a noticeable way with drag cars, and thats because they deliberately use soft suspension without anti-roll bars to get maximum grip. If this type of axle tramp did actualy occur, then it would be easily controlled by fitting a stiffer anti-roll bar. But if you try this, you will see it does not fix the problem, because the axle tramp is in fact caused buy the axle housing moving up and down (with 4 link), or twisting (leaf springs).

 

[GregLocock]

Once you go over about 300 hp in a rear wheel drive car, even if it has IRS, you will begin to see the sort of axle tramp that is due to the torsional compliance of the driveline. This has the ability to wreck engine mounts and other things, if you keep doing it.

BEATH's hypothesis is the other sort, wheels lifting due to poor anti-squat geometry on live rear axle cars, doesn't seem to be much of a durability problem on the cars I've worked on - lifting the inside wheel and losing traction that way is pretty harmless. I don't think a bigger sta bar will cure it since the problem is lack of vertical compliance, not too much. Milliken says increasing the front sta bar may help, since that will hold the car flatter on the road without hurting the rear wheel rates.

I haven't sat down and puzzled through Beath's explanation of a geometrical solution, but we use a similar idea to mount the diff in an IRS or 4wd, so that the diff pitches but does not roll in response to torque. That is why the diff bushes are either offset or of different rates.











Cheers

Greg Locock