FWD differentials

On a rear wheel drive car with the engine up front and the diff in
the back, the diff gears are lubricated by heavy gear lube…much
thicker than motor oil.

However, in a front wheel drive transaxle, the diff gears are lubed
by the ATF…much thinner than motor oil.

Why the wide difference in lube viscosities?

Some transaxles use ATF, some use gear oil, some use motor oil. One advantage in the thinner lubes is less tendency to get stiff in cold weather. Why the engineers choose one over the other, I don’t know. I’ve never seen even a hint of approval to use a different oil from what the manual recommends, so I assume the differences are significant.

The ring and pinion in the rear differential is usually bigger toothed gears and is less subject to the heat found in FWD transaxles. I suspect that there are specific properties in the different lubricants that the engineers feel work best with the type of gears (FWD type gears–generally smaller toothed) and the amount of heat generated and/or involved.

Some FWD transaxles have both types, but are sealed off from one another to prevent mixing.
Sorry, not an engineer, just fix 'em. :\

Actually, very few FWD differentials, or RWD/rear-engine setups, allow the diff to “share” lube with the gearbox or engine.

The simple fact is that the diff generates very, very few contaminants for its lube- typically there are no clutches or similar surfaces to wear (except in some traction-aiding differentials) there’s no combustion byproducts, very little atmospheric contamination, etc.

Thus, forcing the diff to share lube with the relatively “dirty” gearbox or engine would reduce the overall system lifespan.

Point in fact, while I’m certainly no expert, the only situation I do know of where the diff shares lube with another component, is in the older Lamborghini Countach. Which, as I recall, has a single “oil pan” for the engine, transmission and differential.

I know the Citation/Fiero drivetrain has a seperate diff lube, as does the full-size Toronado/Eldorado…

Enola, what vehicle are you referring to?

Gotta disagree here. While I agree that no modern cars share oil between the engine and the transaxle, the overwhelming majority of transaxles have no separation between the gearbox (transmission) and the differential. The only examples that come to mind that are separated are some Toyota and Subaru models, and then only if the transmission is an automatic.

I was under the impression that, in a RWD vehicle, the engine had
its motor oil, the transmission had its separate lube (Gear lube
for manuals, ATF for automatics), and in the back the Diff had its
own separate gear lube.

In FWD cars, (AFAIK) ,the Engine still had separate motor oil, but the transaxle shared its lube between the gearbox and diff.

I am aware that on some rare examples, like the old Morris/Cooper Minis, the engine and gearbox shared lube.

Knowing that, due to torque multiplication of the gearbox, the
differential gear has the highest load stress of the entire drivetrain, how does the diff hold up to thin ATF, when it should
need the high load characteristics of gear lube?

First:Several auto mfr’s use ATF in both their auto and manual transaxles. My Ford Probe was an example of this. The quality of lubrication one gets from today ATF fluids is distinctly superior to the oil you used to be able to buy, therefore it’s used in both types of trans, and it makes it less likely that the wrong fluid will be placed in a vehicle during assembly

Second: RWD differentiuals usually have spiral bevel gears. Gears of this type produce a shearing motion which requires oil with very specific characteristics. FWD transaxles have straight (or herringbone) gears on the final drive, so the incredible shearing forces encountered in spiral bevel gears just aren’t present. The straight bevel gears which allow the differential to work like a differential are only moving around turns and have a very light load, when they are moving at all. Hence, a good oil with decent lube characteristics, (like ATF) works fine. The only issue then is what type of seals are used.