[Tcoat]

Originally Posted by Edibrac

Okay, this might get long.

A space saver tire on one axle of a torsen differential will rotate at a higher RPM than the larger one on the other side of the differential. We all know this.

A torsen allows the axles of the car to rotate at different speeds through the use of a set of worm gears for each axle. These worm gears are also meshed together with spur gears. So when one axle is spinning faster than the other, like in a corner or different sized tires, the worm gear of the faster axle causes the worm gear of the slow axle to spin and therefore spin the slow axle.

Now the worm gears and the helical gear on the axles have a certain ratio. There is a limit to how much the faster axle can make the slower axle rotate due to this ratio.

In our example there is a speed difference between the axles due to tire size. If the spare is of a slightly smaller size than normal, the torsen acts as if the car is constantly cornering. This is not good for the torsen because the worm gears are rotating constantly, because we have a speed difference in the axles. The worm gears have bearings that are not sized for constant use like the bearings for the axles are. They are small bearings. Rotation in bearings cause friction, which causes heat, regardless of lubrication there is still friction. A couple of corners is not enough to cause enough damage from heat since it is intermittent. However we are talking about constantly having one axle rotate faster than the other for a long time.

So if they can be ran for a short time like this, why do manufacturers advise against it? Simple. People are idiots. The average person might not see issue with running the spare on the drive axle and put off replacing the tire while they continue to drive every day. So instead of saying it might be okay if a bunch of specific conditions are met, they tell the owner to not do it at all.

Simple.

Originally Posted by Tcoat

Exactly! Although you wasted your time writing it out since the response will be more misunderstood, plagiarized first year apprentice text book excerpts saying how you are wrong. There will be not one solid shred of evidence produced to prove you wrong though.

Originally Posted by Gforce

The BRZ does not use the T1. It uses the T2 which fits parallel helical gears and no spur gears. The operating principle is similar. All the other helical torque sensing diffs use the T 2 style solution, more compact but not as robust.

Have you figured out how different the rolling circumference of the tires would have to be to cause a problem in the manner you describe?

Even if the space saver were to be that small (and it would have to be very small) the Torsen or any patent infringement avoiding variation of the Torsen would not wear any more than an open differential would in the same situation. That is to say, the supposed "limited slip" function you suppose would generate heat just wouldn't and reveals a lack of understanding about how Torsen type diffs work. They generate less heat when they are transferring torque than they would just differentiating. They are not relative wheel speed activated. Genius design actually. Bombproof enough for a Hummer.

Differentials are stronger than the axles they drive. You only find differential oil coolers on competition cars.

Your theory is well expressed, and courteous, but isn't correct.

Anybody else care to take a crack at it?

Most of the posts since my challenge are sufficiently infantile as to warrant ignorance, consistent with that displayed.

Called it!