Axle tramp - lsd?
Discussion
Hello,
I've taken advantage of the slippery weather to investigate my car's grip from a standing start and practice balancing that against the slightly torque-shy power delivery. It seems to suffer a bit of axle tramp on anything more than mild wheelspin and I'm a bit surprised at this. I've only seen axle tramp occur before on a racing Mustang, and I assumed it was because they have basic and stiff suspension. My car on the other hand ("Cossie Merc"
has 5-link suspension that's comparatively soft. So it was rather a surprise to discover it does something so unruly, and it feels like it's going to break something!
Is it because it has LSD? It's only something like 15% LSD, but additionally it has "ASD", which means when it detects wheelspin it hydraulically locks the diff up, up to as much as 100% (although I'm not convinced the diff might be a bit worn out - any way of telling?). Could this ASD be the cause? Is it likely to break something when it happens?
I've taken advantage of the slippery weather to investigate my car's grip from a standing start and practice balancing that against the slightly torque-shy power delivery. It seems to suffer a bit of axle tramp on anything more than mild wheelspin and I'm a bit surprised at this. I've only seen axle tramp occur before on a racing Mustang, and I assumed it was because they have basic and stiff suspension. My car on the other hand ("Cossie Merc"
has 5-link suspension that's comparatively soft. So it was rather a surprise to discover it does something so unruly, and it feels like it's going to break something!Is it because it has LSD? It's only something like 15% LSD, but additionally it has "ASD", which means when it detects wheelspin it hydraulically locks the diff up, up to as much as 100% (although I'm not convinced the diff might be a bit worn out - any way of telling?). Could this ASD be the cause? Is it likely to break something when it happens?
Can you get someone to watch the car from the out side or failing that try braking hard in the wet..
axle tramp is the axle moving about, which can happen under braking as well. on my old jago i thought the front wheels were falling off under hard brakeing but it was the rear axle tramping. a couple of extra anti tramp bars on the axle solved that... what you could have is the lsd snatching which could feel the same, but would only happen on wheelspin rather than braking as well. I had something that felt like axle tramping on the chim but turned out to be a bearing on the diff.
If someone is watching the car can they see what the axle and the wheels are doing?
axle tramp is the axle moving about, which can happen under braking as well. on my old jago i thought the front wheels were falling off under hard brakeing but it was the rear axle tramping. a couple of extra anti tramp bars on the axle solved that... what you could have is the lsd snatching which could feel the same, but would only happen on wheelspin rather than braking as well. I had something that felt like axle tramping on the chim but turned out to be a bearing on the diff.
If someone is watching the car can they see what the axle and the wheels are doing?
Quite often the problem is caused/exaggeratted by the fact that your axle is held in place by soft rubber bushes, which will load and unload as the tyres grip and then unload. This process causes the axle to move in it's mountings and then the problems just recycles itself.
Solutions? New bushes, more tyre pressure, more power, poly bushes, less tyre pressure, rose jointed suspension arms..... take your pick!
Solutions? New bushes, more tyre pressure, more power, poly bushes, less tyre pressure, rose jointed suspension arms..... take your pick!
Fitting an LSD is not really going to fix the fundamental tramp problem. You can get tramp under braking - no LSD is going to fix that.
On a vehicle I worked on we found that during the acceleration and braking manoevres (verified with the use of very high speed filming) the wheel moved to it's full extent of longitudinal travel and started to compress the damper top bushes. Their rate was lower than the dampers' rate at the frequency of wheel movement. As dampers have non-linear speed to damping curves this meant the issue was seen only at certain speeds. The solution was to change the top damper bushing rates which fixed the problem for that application.
On this vehicle I would make sure that there are no obvious faults before swapping parts, but I think it is most likely an uncontrolled compliance problem.
On a vehicle I worked on we found that during the acceleration and braking manoevres (verified with the use of very high speed filming) the wheel moved to it's full extent of longitudinal travel and started to compress the damper top bushes. Their rate was lower than the dampers' rate at the frequency of wheel movement. As dampers have non-linear speed to damping curves this meant the issue was seen only at certain speeds. The solution was to change the top damper bushing rates which fixed the problem for that application.
On this vehicle I would make sure that there are no obvious faults before swapping parts, but I think it is most likely an uncontrolled compliance problem.
High speed filming of suspension under load sounds interesting! I've no axle tramp under braking luckily.
You've just made me recall that the back end can sometimes feel a little bit wayward under hard acceleration in low gears, as if the suspension is wandering a little. It's not always noticeable but it's the first powerful rear driver I've owned so maybe it's normal. I've had it looked over and no obviously worn bushes or anything were noticed. The rear suspension is self-levelling with struts instead of dampers and the damping done by spheres, so maybe a problem lies here? It doesn't ride stiffly which I thought was the sign of worn/failed SLS?
I track dayed it and it seemed perfectly fine (until a crappy Pirelli out).
You've just made me recall that the back end can sometimes feel a little bit wayward under hard acceleration in low gears, as if the suspension is wandering a little. It's not always noticeable but it's the first powerful rear driver I've owned so maybe it's normal. I've had it looked over and no obviously worn bushes or anything were noticed. The rear suspension is self-levelling with struts instead of dampers and the damping done by spheres, so maybe a problem lies here? It doesn't ride stiffly which I thought was the sign of worn/failed SLS?
I track dayed it and it seemed perfectly fine (until a crappy Pirelli out).
Tramp is the setting up of a resonant frequency in the axle/suspension system. What basically happens is that some input encourages the wheel to jump up, traction breaks, the wheel kicks back down, traction is restored, and the input repeats itself - all of this happening at the resonant frequency where the suspension (particularly the dampers) are unable to quell the input.
it's normally associated with beam axles, but can happen on independent systems. There are several modes depending on where the resonance occurs, and they can be very severe. It normally occurs on either acceleration or braking (not so common on both), but can even occur at stable speed on a wavy surface. It can be affected by tyre/road grip, payload, surface smoothness, and power/braking delivery.
So the trick is to identify the resonant frequency and move it. High-speed photography might be fun, but even for experts it would be difficult to interpret. In your case I would suggest:
- First question is "how common and how severe?" Is it really a problem that you need to worry about?
- It could be a design fault, though unlikely on a Merc. Have you browsed the forums?
- Is your car standard or modifed? changing the weight of the car, different unsprung weight (e.g. differnt rims), could have moved the frequency back into a zone from which the Germans had carefully engineered it out.
- The lsd setting could be (and probably is) part of the problem. The sudden change of power delivery/non-delivery would encourage a resonant frequency. However, again I would be surprised that it got through design & development as a common, significant issue.
- I would be more tempted to look for a part worn beyond limits. Bushes are certainly worth considering - some mode of hub movement (such as rotating around the axle line) could lack the control that it had ex-factory, and that's triggering the problem. You should also consider whether your dampers are tired.
- Next, and less likely, might be that the suspension arms are cracked & flexing.
- Only then consider upgrading components - stiffer bushes, higher spec dampers, changing the lsd setting.
- Finally, consider additional anti-tramp linkages.
So I'd recmmend the first steps are:
- Sussing out whether others suffer it, or is it just you?
- Getting the car up on the ramps and checking bushes, dampers and arms.
it's normally associated with beam axles, but can happen on independent systems. There are several modes depending on where the resonance occurs, and they can be very severe. It normally occurs on either acceleration or braking (not so common on both), but can even occur at stable speed on a wavy surface. It can be affected by tyre/road grip, payload, surface smoothness, and power/braking delivery.
So the trick is to identify the resonant frequency and move it. High-speed photography might be fun, but even for experts it would be difficult to interpret. In your case I would suggest:
- First question is "how common and how severe?" Is it really a problem that you need to worry about?
- It could be a design fault, though unlikely on a Merc. Have you browsed the forums?
- Is your car standard or modifed? changing the weight of the car, different unsprung weight (e.g. differnt rims), could have moved the frequency back into a zone from which the Germans had carefully engineered it out.
- The lsd setting could be (and probably is) part of the problem. The sudden change of power delivery/non-delivery would encourage a resonant frequency. However, again I would be surprised that it got through design & development as a common, significant issue.
- I would be more tempted to look for a part worn beyond limits. Bushes are certainly worth considering - some mode of hub movement (such as rotating around the axle line) could lack the control that it had ex-factory, and that's triggering the problem. You should also consider whether your dampers are tired.
- Next, and less likely, might be that the suspension arms are cracked & flexing.
- Only then consider upgrading components - stiffer bushes, higher spec dampers, changing the lsd setting.
- Finally, consider additional anti-tramp linkages.
So I'd recmmend the first steps are:
- Sussing out whether others suffer it, or is it just you?
- Getting the car up on the ramps and checking bushes, dampers and arms.
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