Augh – Car shopping is so frustrating!

[redpoint5]

I've driven normally on a donut for a couple of months; it was no big deal. Was an AWD Subaru too.

[JSH]

Originally Posted by redpoint5

I've driven normally on a donut for a couple of months; it was no big deal. Was an AWD Subaru too.

Compact spares will wear faster than a regular tire.

Running different diameter tires on a driven axle will cause increased wear on the differential. Subaru requires all the tires to be the same diameter and tread depth on all tires to be within 2/32ths.

[redpoint5]

That's the conventional wisdom. T'was a kid in a truck that did in Subie at 220,000 miles.

[JSH]

It is conventional wisdom for a reason. When I worked for a transaxle manufacturer we did accelerated differential testing by running different size tires.

How much damage you do depends on the difference in tire sizes and the design of the drivetrain. A simple open differential will take abuse much better than a differential with clutch packs like a LSD. The new fancy differentials that can selectively send power to individual wheels using clutches - you really don't want to burn up those clutch packs.

With an AWD car you are stressing two differentials by running an underside tire.

[Piotrsko]

I would think that damage is proportional to lube breakdown. What I know about spiders in either ford or mopar diffs is that the shaft or gear becomes spalled or brinnelled.

[freebeard]

Word of the day.

en.wikipedia.org/wiki/Brinelling

[JSH]

Originally Posted by Piotrsko

I would think that damage is proportional to lube breakdown. What I know about spiders in either ford or mopar diffs is that the shaft or gear becomes spalled or brinnelled.

The greater the mismatch in tire sizes the faster that satellite (or spider) gears have to rotate. While rotating they spin on the cross pin shaft. Again, the larger the mismatch in tire size the faster the gears spin on the cross shaft. I've never seen a satellite gear with a proper bearing, they are just polished metal on metal because they aren't meant to spin at high speed or for long periods of time. Yes, you can easily break down the oil film between the parts. You can also wear the surface of the gear teeth.


We could kill a differential in 20% of normal life with a 10% difference in tire diameter.

[Piotrsko]

Dont forget that except for oil splash, they rarely get fully lubed mostly because they aren't designed to be in constant use which is why the failure is often freebeards new favorite word of the day: brinneled.

Come to think of it, most of the race failures destroyed the spiders, too.

[cRiPpLe_rOoStEr]

Originally Posted by JSH

How much damage you do depends on the difference in tire sizes and the design of the drivetrain. A simple open differential will take abuse much better than a differential with clutch packs like a LSD. The new fancy differentials that can selectively send power to individual wheels using clutches - you really don't want to burn up those clutch packs.

Makes me wonder to which extent the "torque vectoring" on some EVs which use individual engines at each drive wheel could handle it better. BTW did you see the setup fitted to the rear axle 2022 Audi RS3, with clutch packs instead of a proper differential?

[freebeard]

An exemplar would be the Arcimoto FUV; twin motors with differential, limited slip and torque vectoring in software.