Car bouncing in reverse!

This phenomenon has happened to a small pick-up and a fast formula firebird…

We throw it in reverse and dump the clutch, and the dang car violently bounces up and down while peeling out, creating the feeling that you just stripped a gear or something serious like that. It’s mainly the back of the car that bounces, and I’m not talking like feet in the air, maybe an inch at the most.

Now, my theory is a detailed one. I won’t drag this out any further than to just say that this phenomenon is due to the wear of the mostly forward travelling car on the tires. The tire wears a certain way depending on which way it is always rolling (this is visible). The tires are “sharp” while travelling backwards and this creates a jump, rather than a smooth peel while going forwards.

actually, this happens to the formula while chirping in 2nd as well… feels like I just blew the transmission.

anyone have any input as to what’s going on?

This is called axle wind-up or wheel tramp, among other things. The sudden torque applied to the axle causes the axle to rotate in the opposite direction of the power applied. Instead of squishing evenly (like from a bump) the spring goes into a bit of an s-curve. Then the spring rebounds, causing the wheel to hop. Then the action starts again; compress, rebound, etc. You get a cycle of chattering. It’s awfully hard on u-joints, and that’s why traction bars were invented.

I had this happen to my 1974 Celica one day, when it hadn’t happened previously during hard reversals. So I started poking around, and found that one of the shocks had sheared off at the lower mounting ring. Replaced the shock - no more jumping in reverse.

The tire wears a certain way depending on which way it is always rolling (this is visible). The tires are “sharp” while travelling backwards and this creates a jump, rather than a smooth peel while going forwards.

I’ve been working on cars for 30 years and I’ve never seen the type of wear you’re describing (I have seen left-to-right wear of that nature, but that’s a whole different animal). Furthermore, when tires are rotated to the other side of the car, they then roll in the other direction, which would even out this wear if it occurred. I think it’s safe to say tire wear has nothing to do with the phenomenon you describe.

GARY: New, soft tires that have not been rotated yet and that have been peeled out a little bit do have this wear. This was on my Formula Firebird, I have seen it with my own eyes. Let’s take a sample tread off the above-described tire. The first part of each little tread that touched the road was “crushed” or smoothed out by the cars’ weight, to make a little round inclination up to the peak of the tread. The end part of the tread was also crushed and smoothed, obviously, but the tread ended in a steep descent to the bottom gully of the tire.

So, basically, the treads on the tire were making claws which were smooth for forward travelling, and claw-like for reverse travelling.

whether this has to do with the subject at hand I do not know. But it’s real. I was pretty astonished to see this myself, thinking there was only left/right wear too.

asknott: Does this only happen in reverse, or anytime? So you are saying that while this jumping is happening, the axle is travelling in the opposite direction? Because when we do it, the car does still travel the correct direction.

When it happens to the formula in 2nd, i think that might be different than what i am talking about. it is much more obvious in reverse.

Wow, this is not the answer I expected, pretty cool actually.

Gary T - I have noticed the “choppy tires” from not getting them rotated too. They feel sharper on the trailing edge.

I had a Chevelle that would wheel hop when I spun the tire off the start. It had an open rear-end (one tire spins). My other two Chevelles with posi rear-ends didnt hop. My 73 amx with posi doesnt hop either.
I think in my case it had to do with the axle twisting and putting pressure on the one side with power and causing the shock and the spring to “bounce”.

Throwing it in reverse and dumping the clutch?

I recently owned a 01 Mustang Cobra that actually had a syncromesh reverse. Most cars do not. No Firebird/Camaro I know of does.

You are stripping gears and putting lots of stress on your drivetrain.

Chirping in 2nd? So this is a manual tranny? Gotta love those big-displacement GM pushrods.

You are definitely beating on this car. But you knew that.:slight_smile:

I used to drive a Chrysler Laser, which was front wheel drive. It would hop when I spun the wheels on dry pavement in first gear (going forwards, not reverse). Any chance it was the same cause? Maybe a matter of the drive wheels hopping when acceleration is away from the rest of the car?

It would seem to me that when acceleration is towards the body of the car (forwards for a rear wheel drive car, or backwards for a front wheel drive car), the force bears down on the drive wheels. When acceleration is away from the body of the car (backwards for a rear wheel drive car, or forwards for a front wheel drive car), the force shifts away from the drive wheels. Could this be enough for the drive wheels to spin for a moment until the weight came back down on them? Repeat enough, and you get a hopping motion.

well, i wouldn’t say DUMPing the clutch, but giving it some good gas.

why do you say i am for sure stripping gears? could you explain? have you experiences this? i would think the clutch would slip before any gear would strip, but who knows…

I don’t understand why anyone would even “put good gas” in reverse.

“Dumping the clutch” means you’re revving your engine and mating it with your asbestos covered clutch that uses friction to spin-up to the engine crankshaft RPM.

If you “dump the clutch” in first gear - in a Firebird - you will wear your clutch (possibly burn it out if your RPM’s were too high) and smoke your tires (wearing them out too).

That is not a good “racing” launch (though impressive).

You are beating on your car.

Corbomite, let’s assume i’m not a moron for a second.

i don’t do this on a regular basis. this happened once while i decided to go in reverse quickly. i didn’t dump the clutch, i guess i shouldn’t have used that word. i just gave it some gas and got a little peel. but instead of just peeling out, the car jumped. so i have done it twice since then just to see again. same thing. jumping and peeling while going backwards, not just going backwards.

i seriously doubt that revving the engine to 2.5k RPM off the line would strip the gears, unless my car was nailed to the road…

it’s a '94 for the record, manual, 4 stage clutch. it’s made to be beaten, within limits.

GaryT has been here (in this thread) and he’ll return.

I was concerned with “dumping the clutch” as the ‘Zen’ of racing is exactly redlining then upshifting. Big V-8’s can handle it.:slight_smile:

Some racers can shift without using the clutch.

How? For any car?

I could have been clearer, I guess. The axle shaft is turning in the direction you had in mind, but the axle housing is doing a wild dance. The housing (that is, the differential, the tube, the brakes, and the middle part of the springs) first reacts opposite to the backward rotation of the shaft. When it has bent the springs as far as it can, the springs whip it back in the other direction (backward, along with the shafts.) When the springs relax a bit, the torque reaction starts the cycle again. While the axle is doing its brutal boogaloo, the wheel hops, so the tire goes through a cycle of bite, slip, bite, slip. You’ll feel it in the seat of your pants. Eventually, you’ll feel it in your wallet.

Here’s a short list of what you may break or may have already broken: u-joints, shocks, spring leaves, ring gear, and the bolts that mount the axle housing to the springs.

thanks asknott. good answer.

asknott: would this same phenomenon occur in a front-wheel-drive car taking off in first? I used to get this if I really romped on it in my VTEC del Sol (especially while turning right, since the weight was shifted more toward the left-front wheel than the right-front, causing the right-front to spin and hop). It really did feel like I was delivering a beating to the drivetrain.

Originally posted by Corbomite
Some racers can shift without using the clutch

At certain ideal RPM’s you can push/pull the shifter to go out of gear, hit the gas to bring the RPM’s up to the ideal RPM of the next higher gear and jam the shifter to bring the car back into gear. There’s little room for error and you have to be almost perfect or damage will ensue.

A safer way to see this is shifting into neutral without the clutch. Say you’re pulling up to a red light in third gear. As you’re slowing down (and the engine RPM is dropping), put some gentle pressure on the shifter. At a certain RPM the shifter will slide and the car will drop into neutral.

:smack: In the upshifting example above, the next higher gear shift should be at a lower RPM so you’d let your foot off the accelerator.

Thanks for the mention. However, it’s not clear to me if there’s a particular item you’d like me to address.