Differential Questions

[The Dude]

Rencently there have been several people posting on badly toasted differentials due to mismatching tires. I know that the center differential in a Subaru can be damaged by mismatched tires, but these posters reported severe damage in the front and/or rear differentials as well.

 

A center differential is unique to AWD cars, but the front and rear differentials are similar to the units found in FWD and RWD cars. FWD cars aren't sensitive to mismatched tires, so why do the front and rear differentials go out in the Subaru? I can understand a locking rear differential going out, but these sounded like open differentials.

[porcupine73]

Maybe part of the issue is heat cooking/breaking down the diff gear oils? I think part of the issue may be that one front wheel and one rear wheel on the opposite side are kind of coupled b/c of the center diff and that might be what toasts the diff between the hypoid and ring gear or whatever? That's my impressions but maybe I could research first.

 

FWD cars with transverse mounted engines/transaxles might have a straight or bevel or maybe even herringbone gear type incorporated diff which is not the same as the hypoid type diff's in Subaru's?

[firstwagon]

I haven't heard of many rear diff failures other then normal wear in older high mileage cars. I doubt it's any higher then any other 2 wheel drive car. Mismatched tires won't damage a open rear diff in any car.

Some have limited slips in the rear which can wear out over time. That doesn't stop the car though, it just loses the limited slip function.

 

The front may be different as it's incorporated in the transmission and four wheel system. Still I don't think it's that common a failure. According to the PO, the entire drivetrain is original on my Legacy and it's 15 years old now. Seems reliable enough to me.

[cannonball]

With an open diff tire size shouldn't matter, but with an LSD it does.

[Manarius]

The AWD system is a unique creature. The speed difference front to back is what kills the AWD system as a whole. If you have one different tire, while that may not burn out a specific differential (although, front ones like burn out thanks to mismatching tires), it will make either front or back spin faster than the other and thus it fries the system. This is why we advocate that your car have 4 MATCHING tires if you have AWD.