Wheel turns when not in gear

smessing · 06-12-2009, 11:43 AM

I have the rear end of my 80sc up on jack stands to check out a noise. I have the car out of gear and start it up, when I see the passenger side rear wheel slowly start spinning. I can stop it by hand, and when I let go, it begins to spin again.

I have never noticed this happening before. Is this normal?

Thanks,
Scott

nhromyak · nhromyak's Garage

Mine does this too.

I think the input shaft on the trans is spinning and somehow (guessing with trans fluid) spins the gears.

kodioneill · kodioneill's Garage

100% normal. Drag on the clutch disc.

smessing · 06-12-2009, 12:33 PM

Excellent news. One less thing for me to be worried about.

Thanks for the responses.

911pcars · 06-12-2009, 01:00 PM

Most 911 diffs are the open-type. That is, no limited slip unless it's a factory option. Open diffs don't have friction disks, and they normally act that way. Limited sip diffs can used friction disks or a gear assy (Torsen) to provide limited slip characteristics.

Major drawback with open diffs: If one of the drive wheels doesn't have any traction, the drive goes to that tire.

A limited slip diff. applies driving force to the tire with traction while still allowing differential wheel speeds when the vehicle makes a turn. With both drive wheels in the air, if your diff. has limited slip, rotating one wheel will rotate the other in unison and in the same direction of rotation.

To complete the trifecta, a locking diff is as the name implies. Both drive wheels are locked together which makes for even distribution of driving force but also results in interesting tire scrubbing around turns.

What you're observing is normal stiction between the unloaded gears in the diff.

Sherwood

yelcab1 · 06-12-2009, 01:00 PM

Even out of gear, in neutral, the input shaft and the output shaft are loosely connected via the friction on the bearings of the gears. The wheels will spin a little bit ...

longhornchris04 · longhornchris04's Garage

As others said, its the friction on the bearings on all the gears. Now why you would only get one side I don't know (or perhaps you just didn't check the other side).

I get the same behavior in my ford focus. I always start the engine after an oil chainge while still on jack stands to make sure I haven't screwed something up. Boths wheels start to spin up but a light tap on the brakes stops them cold.

Daviboy · Daviboy's Garage

Would not worry about it after recently changing my clutch cable and while having back end up on jack stands while having rear wheels refurbed, I started the car and my right rear wheel was turning out of gear. Put her in 1st and 2nd could stop both hubs with my hands popped her into 3rd just idling and again could stop hubs by hand. After putting rear wheels back on took her for a drive all normal so would not worry about it....

T77911S · 06-16-2009, 05:06 AM

normal. and dont think that if you stop one wheel from turning, it is in neutral, it can still be in gear. so dont let the car down unless you want to chase it across the yard with the floor jack under it trying to jack it back up before it hits a tree or the shed.
no, I did not do this, but i know someone that did. he even told me, can you believe it. that one would go to my grave.

longhornchris04 · longhornchris04's Garage

If you have an open differential, then even in gear you can stop one side... the open diff will just cause the other to spin twice as fast. This is exactly what happens when one wheel is on ice (or airborne??? sorry, that belongs in the offroad forum). In an open diff, the max torque applied to both wheels is whatever the max torque that can be applied to one (the one slipping).

I believe the same thing will occurs with certain types of limited slip differentials, specifically worm-gear / "torque-bias" diffs (torsen & quaife for example) where the torque to the wheel with grip is a multiple of the torque to the wheel that is slipping. Zero (o.k. - CV and wheel bearing resitance) times X is still zero for all values of X - so the free wheel will spin faster.

If you have a clutch-type or viscous limited slip then you shouldn't be able to stop the wheel in gear. These differentials are setup to ensure a certain percentage of torque always goes to each wheel, with the remainder being divided per road conditions. In actuality they are designed to limit the speed differential between the two wheels - if it gets too large the slower wheel gets "dragged" up to speed.

Still, as a rule of thumb I wouldn't ever take a car off jackstands with the engine running, much less running in gear.