Axle Tramp
Axle Tramp
(OP)
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
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
RE: Axle Tramp
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
RE: Axle Tramp
Kevin
RE: Axle Tramp
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
RE: Axle Tramp
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
RE: Axle Tramp
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
RE: Axle Tramp
RE: Axle Tramp
Sean
RE: Axle Tramp
RE: Axle Tramp
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
RE: Axle Tramp
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.
RE: Axle Tramp
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.
RE: Axle Tramp
RE: Axle Tramp
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.
RE: Axle Tramp
RE: Axle Tramp
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