Anti / Jacking forces and wheel hop 2

[TMAPV]

Can someone provide some input as to why an an anti force induces wheel hop? This seems to be prevalent in drag cars, and from what I've noticed is near limit behaviour. What I don't understand is why would anti-squat, so rear, produce this wheel hop during acceleration, and this seems to be also a thing for anti dive BUT in braking, so upward forces on body and downward force on tire/hub. Also struggling to understand why it only appears at the tire grip limit. Interesting as it might also explain why antis can reduce grip if it increases the tcp load variation due to wheel hop

 

[GregLocock]

Trouble is with drag car tires you are into very weird and non linear behavior. So all the normal stuff goes out the window.

When we started putting turbos in FWDs, and when RWDs hit 400 hp (roughly) we started getting a lot of problems with axle tramp (so called), particularly in slightly greasy to wet conditions. This sounds similar. You can often get some insight into the possible mechanism by studying the video, and estimating the frequency of the 'vibration'.

Antisquat in particular has given me a problem with harsh gearchanges on a RWD, they really thumped the car around. The small changes I could make to the geometry unfortunately didn't get rid of this and we had to soften the calibration off.


Cheers

Greg Locock


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[BrianPetersen]

There are aspects of this that I cannot figure out.

Anti-squat with a live rear axle is a different ball game from the various wheel-hop issues associated with independent rear suspension. Most IRS designs don't have significant anti-squat - the forces involved when the drive shaft is coming from a diff fixed to the bodyshell are a different situation from the "anti-lift" from the brakes. (It is possible to build anti-squat into the design of an IRS, but only with side effects that are obviously bad.)

There have been previous threads on here about wheel-hop on rear-drive IRS-equipped vehicles, with no apparent conclusion or fix.

I've heard of "brake hop" because the rear axle side-view instant-center was too short (too much braking "anti-lift" in the rear) and yet ... Practically every front-wheel-drive rear suspension design has an insanely short rear-wheel side-view instant-center (pivot point of, for example, the twist-beam axle, or of the main trailing arm), and I've never encountered wheel hop under braking on any of them.

I have certainly encountered front-axle wheel-hop on my various front-drive vehicles (mostly VW, some Honda, some Fiat, all with MacPherson front suspension), more frequently on wet pavement (wheelspin seems almost certain to be followed by nasty wheel-hop). I've never attempted to do anything about it, instead simply backing out of the accelerator or shoving in the clutch pedal when it happens, in order to stop it before breaking something. Certain VW manual transaxles are notorious for breaking the cross-pin in the diff from this.

 

[GregLocock]

"no apparent conclusion or fix"

Fixed it twice on RWD. Once with a rubber up tube propshaft that decoupled the roll mode of the engine and gearbox from the rear wheels, basically the stick slip from the tires was bouncing up the driveline and back down again to cause another cycle of stick slip. The other time I can't remember the solution, probably the same. FWD cars usually needed more damping in the engine mounts or a separate little shock absorber. I used to have a bag of them and spend a happy day on the track running through the different tunes.

I have had problems with wheelhop under braking caused by some bad modal alignment. Solution is to move the recession mode away from the wheelhop frequency and add some damping and or snubbing (less likely to be successful) to the bushings. This is a mandatory fix as the energy involved is sustained and enormous, which may be why you don't see it in production cars.

Cheers

Greg Locock


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[gruntguru]

Can someone provide some input as to why an an anti force induces wheel hop? This seems to be prevalent in drag cars, and from what I've noticed is near limit behaviour. What I don't understand is why would anti-squat, so rear, produce this wheel hop during acceleration, and this seems to be also a thing for anti dive BUT in braking, so upward forces on body and downward force on tire/hub. Also struggling to understand why it only appears at the tire grip limit. Interesting as it might also explain why antis can reduce grip if it increases the tcp load variation due to wheel hop

If I push my fingertip along the desktop, I can make it judder if I get the angle just right. If I drag the finger along with the contact point trailing, it will never judder.

- Pushing the fingertip ahead of the mass of my arm is an "anti" setup.
- The judder is the result of stick-slip behaviour which occurs near-limit with friction oscillating between static (higher coefficient) and dynamic (lower coefficient).
- The role of "anti" is to create an unstable relationship between normal force and longitudinal force. The cycle goes something like this:
1. tyre sliding (dynamic friction)
2. "anti" produces a jacking force (which obviously increases normal force and therefore longitudinal force) but "anti" also produceds a displacement of the wheel relative to the vehicle in the direction of the force. This creates an opportunity for
3. temporary transition to static friction ("stick") which rapidly increases both normal and logitudinal forces until
4. downward movement of the wheel reverses pushed up by the highly compressed tyre and reduced compression force in the suspension spring
5. back to "1. tyre sliding"

je suis charlie

 

[TMAPV]

If I push my fingertip along the desktop, I can make it judder if I get the angle just right. If I drag the finger along with the contact point trailing, it will never judder.

- Pushing the fingertip ahead of the mass of my arm is an "anti" setup.
- The judder is the result of stick-slip behaviour which occurs near-limit with friction oscillating between static (higher coefficient) and dynamic (lower coefficient).
- The role of "anti" is to create an unstable relationship between normal force and longitudinal force. The cycle goes something like this:
1. tyre sliding (dynamic friction)
2. "anti" produces a jacking force (which obviously increases normal force and therefore longitudinal force) but "anti" also produceds a displacement of the wheel relative to the vehicle in the direction of the force. This creates an opportunity for
3. temporary transition to static friction ("stick") which rapidly increases both normal and logitudinal forces until
4. downward movement of the wheel reverses pushed up by the highly compressed tyre and reduced compression force in the suspension spring
5. back to "1. tyre sliding"

je suis charlie

gruntguru, that's the kind of reply I was looking for, that analogy is perfect too!

Why does this not happen due to springs forces, even if antis are zero, we can still have a vertical force from spring and the drive force?

For point 4. wouldn't the wheel/tire hop up or move back up only once we go over the tire force peak causing the jacking force to fall?

 

[moon161]

Greg, not enough power involved for hop or stick slip, but the 82 Benz 240 D I used to have and the 300D I do have a bucking mode if you add throttle too quick in first or without enough slip in the clutch. Tall heavy motor will get everything bucking at about 2-3 Hz. Some versions have small shocks stabilizing the engine but mine doesn't.

 

[TMAPV]

Antisquat in particular has given me a problem with harsh gearchanges on a RWD, they really thumped the car around. The small changes I could make to the geometry unfortunately didn't get rid of this and we had to soften the calibration off.

Greg, how did the harsh shifting get amplified with antis in your scenario?

 

[GregLocock]

Just the torque pulse as the high revving engine got braked by the inertia of the car during the gear change. Sent a bit of a bang through the IRS. The theory was that the bang was caused by the antisquat locking the suspension up (in effect, I'm not a big fan of antis) and transmitting the traction force pulse directly into the body instead of keeping it in the suspension where it belongs.

We couldn't change the antisquat enough to make a big subjective improvement. this was a double wishbone with some seriously weirdly angled hinge lines at the body end of the arms, due to theories.


Cheers

Greg Locock


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[gruntguru]

Why does this not happen due to springs forces, even if antis are zero, we can still have a vertical force from spring and the drive force?

Because spring force is only proportinal to vertical displacement. The vertical force generated by anti is proportional to longitudinal force so any increase in LF generates an increase in VF which generates an increase in LF and so on . . . Meets the definition of instability. Of course this escalation can only continue until the cycle is broken - probably by the wheel moving upward and reducing VF at the contact patch.

For point 4. wouldn't the wheel/tire hop up or move back up only once we go over the tire force peak causing the jacking force to fall?

The tyre (longitudinal) force peak is a moving target because VF is increasing courtesy of the anti geometry. Of course LF/VF for a tyre is not entirely linear so there may be a point where LF exceeds the static friction limit in spite of further VF increase.

je suis charlie

 

[TMAPV]

Quote (TMAPV)
Why does this not happen due to springs forces, even if antis are zero, we can still have a vertical force from spring and the drive force?
Because spring force is only proportinal to vertical displacement. The vertical force generated by anti is proportional to longitudinal force so any increase in LF generates an increase in VF which generates an increase in LF and so on . . . Meets the definition of instability. Of course this escalation can only continue until the cycle is broken - probably by the wheel moving upward and reducing VF at the contact patch.

Quote (TMAPV)
For point 4. wouldn't the wheel/tire hop up or move back up only once we go over the tire force peak causing the jacking force to fall?
The tyre (longitudinal) force peak is a moving target because VF is increasing courtesy of the anti geometry. Of course LF/VF for a tyre is not entirely linear so there may be a point where LF exceeds the static friction limit in spite of further VF increase.

The instability comment is interesting way to think about it, as it's essentially positive feedback.

And I guess pro-squat (so jacking force in other direction) would be in more of a cycle instead, ie Increase in LF would reduce load on tire, therefore reduce LF, which increase load, which increases LF and back to start

 

[gruntguru]

Well - its a loop but feedback is negative so it is stable and will settle at a value.

je suis charlie

 

[GregLocock]

I think the general term is self excited oscillation. Typically the amplitude increases until some boundary condition changes at which point it assumes a steady state.

Cheers

Greg Locock


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