Rear Wheel Hop Problem

[Mike Cook]

When breaking hard without clutching, our car wheel hops terribly in the rear. Basically the rear is just resonating. When the clutch is pulled in all the problems go away. We are using the Taylor Diff. There seems to be alot of slop in the CV joints, however the diff only has about 10-15 hours on it and I'm not exactly sure how much slop can be expected. We have tried all damping settings, chain tight, chain loose, different roll stiffnesses and just can't seem to figure it out. We also aren't using any anti-squat in our geometry. Has anyone encountered a similar problem. Clearly it seems to be drivetrain related, but I'm not exactly sure what is happening.

Mike

[Mike Cook]

When breaking hard without clutching, our car wheel hops terribly in the rear. Basically the rear is just resonating. When the clutch is pulled in all the problems go away. We are using the Taylor Diff. There seems to be alot of slop in the CV joints, however the diff only has about 10-15 hours on it and I'm not exactly sure how much slop can be expected. We have tried all damping settings, chain tight, chain loose, different roll stiffnesses and just can't seem to figure it out. We also aren't using any anti-squat in our geometry. Has anyone encountered a similar problem. Clearly it seems to be drivetrain related, but I'm not exactly sure what is happening.

Mike

[Daves]

You did not mention whether you are downshifting or not.

Wheel hop under braking may be caused by your damping or too much anti-dive in your suspension geometry.

Or you may simply be downshifting without using the clutch. You must always use the clutch on downshifts.

[Patrick W. Crane]

Not necessarily... As has been poited out in another thread, the gear ratios are so close together in fsae cars that the extra braking from the engine when hard on the brakes is so minimal that you can pretty much skip the clutch. If i have nice straight line braking, i pretty much never use the clutch.

Not saying that isn't the problem though...

[Colin]

this might be a stupid question, but I thought I'd ask just in case, your not just getting compression lock on a down shift are you?, because that can cause serious tramping in the rear end somtimes

[jonno]

What's the preload like in your diff? We ran the Quaife for the last few years, but in the new one for our '04 car we didn't take the preload out. This caused the unladen rear wheel to resonate when cornering (the diff was too hardcore for the application). Taking most of the preload out resolved the issue.

[Buckingham]

Are the rear tires locking? Are they locking at the same time? Duct taping a university video camera to your main hoop might help you sort some of this out.

[Mike Cook]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> this might be a stupid question, but I thought I'd ask just in case, your not just getting compression lock on a down shift are you?, because that can cause serious tramping in the rear end somtimes

</div></BLOCKQUOTE>

I'm sorry Colin But I don't exactly understand what you mean by compression lock.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jonno:
What's the preload like in your diff? We ran the Quaife for the last few years, but in the new one for our '04 car we didn't take the preload out. This caused the unladen rear wheel to resonate when cornering (the diff was too hardcore for the application). Taking most of the preload out resolved the issue. </div></BLOCKQUOTE>

Hmm, I have never considered adjusting the preload in the diff. During skip pad exercises, we have had a real problem with spinning the inside real wheel, do you think taking the preload out would help this?

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
Are the rear tires locking? Are they locking at the same time? Duct taping a university video camera to your main hoop might help you sort some of this out.
</div></BLOCKQUOTE> We run an inboard brake. Do you think that the diff is doing some weird biasing thing under braking and the wheels aren't brakeing together?

Thanks for the responses,
Mike

[Denny Trimble]

Sounds like too much rear brake bias, or downshifting without rev-matching. But I'd have to drive the car to really tell... http://fsae.com/groupee_common/emoticons/icon_smile.gif

[scotty young Taylor Race]

Gentlemen,

It appears that there are four items to address in this thread:

1. Wheel hop
2. Wheel spin
3. Diff Preload
4. Brake Bias

First, Wheel Hop: At the competition in Pontiac last year, Scotty and I observed many cars in all the events. The ones that were having problem with wheel hop were universally those that were trying to downshift without the clutch. Since there is no neutral through which a blip (rev matching) to occur, with the transmission going instantly to the next gear, the ONLY way to eliminate the huge jerk is to disengage the clutch during the shift and match revs before reengaging the clutch. We saw 3"� of air under the tires of some of these cars! Check out the system used by UT Arlington. Their shift lever is a two-stage lever, disengaging the clutch with initial motion, shifting on the follow-up motion. Their shifts were smooth as glass.

Shifting too early in the braking process increases the violence of the hopping. Delay the shift until approaching the turn-in point. The differential, whether it be a Salisbury (plate-type), Cam and Pawl, or Torque Biasing (Torsen and Quaife) has no effect on this straight-line phenomenon. However, the halfshafts will behave as undamped torsional springs, making the problem more apparent. The solution for all of this type of wheel hop is a well-trained driver, using the clutch for all shifts.

Second, Wheel Spin: A torque-biasing differential can only bias a maximum percentage of available torque to the wheel with better traction. In the case of Quaife-type diffs, the percentage varies in a linear fashion from a 50-50 split (like an open diff) to a maximum transfer of 80% of the applied torque. This means that if the inside wheel is dynamically unweighted so that less than 20% of the weight is on the inside, it will spin. If the inside wheel becomes airborne, all drive will be lost. The best way to reduce wheel spin when working with an existing chassis is to reduce the rear anti-roll bar stiffness, and increase the rear spring rates, and reduce rebound damping. Softer front springs coupled with increased anti-roll bar can be brought in, also. In the initial design, wider track, lower CG helps.

Third, Differential Preload: This is primarily used for Salisbury (clutch type) differentials, and in formula cars is used mostly for effecting handling. EG more preload = more push. Preload in Torque Sensing differentials is primarily to eliminate slop, which makes the differential erratic in power-on, power-off situations. Using preload to eliminate wheel spin in a low-powered formula car with Salisbury differential is a band-aid, resulting from poor design.

Fourth, Brake Bias: If too much rear brake bias is used, any small disturbance in the track can cause lockup, and erratic braking. Set brake bias initially with a torque wrench. Roughly the front wheels will do roughly 60% of the braking. With a single rear brake, the rear brake will do 40%, 30% will be done by each of the front wheels.

Final update: Just spoke with Brian, the driver of the UMD car. The problem is only with the clutch engaged. When the clutch is disengaged during braking, the deceleration is smooth. This leads me to think that the ECU is not entirely shutting off the fuel with throttle off, leading to "Loping"� of the engine, which causing the entire driveline to go into a resonant tensional vibration. Brian indicates that they will test this. They will also explore brake bias as a possible culprit

Hope this helps!

Craig Taylor
Taylor Race Engineering Inc

[mtg]

There's no need for using the clutch on a downshift. It takes a little practice, but once you get it down, its faster and has less of a chance of upsetting the car under braking.

With dog engagement transmissions, when the gears are unloaded, the dogs will slide between gears with only a few pounds of force at the shifter. You just have to use the pedal on the right to unload the gears.

Diff type does have an effect on straight line behavior, and preload in "torque sensing" (ie. Torsen and Quaiffe) diffs has the same effect as in clutch pack and Salisbury diffs.

[DJHache]

We have noticed some of the same problems Mike has. Under heavy braking the rear wheels seem to hop.

I thought that this could be due to the rear wheels trying to turn the engine during braking. At a certain point in the braking the speed (and therefore the velocity and the moment applied by the rear wheels) has dropped to a point where they can no longer turn the engine and the engine starts fighting back, leading to wheel hop.

The rear unsprung mass oscillates on the suspension and the tire and has a natural frequency at 10-15Hz. If you adjust your tire pressures and compression and springs you might be able to delay the onset of wheel hop (if I'm right).

Also, if you drive on a really bumpy parking lot then one of the rear wheels can become totally unloaded and lock up intermittently giving you a similar problem. As dhaidinger said you could use a University or freshman owned camera to see if your wheels are locking up. Or wheel speed sensors if you have them.