Hello!

I've recently bought my first awd car with limited slip diffs. It's a '92 Mazda 323GTR, which has an open front diff, viscous center and mechanical LSD rear diff.

I have thought about this for a while, and I can't see what would prevent the inside front from spinning like a normal fwd, and then 'using up' all the available torque. Can any of you try to explain this to me?

You are correct that there is nothing stopping you from spinning the inside front, however that doesn't mean there is nothing to limit this wheelspin as soon as it starts. If you spin the inside wheel, and the outside wheel is rotating normally, the front diff will accelerate up to twice the angular velocity of the rear diff (assuming it is not also in the same situation). This will cause the viscous diff to lock up, and torque will be transferred from the front wheels to the rear wheels, thus limiting the amount of inside wheelspin, possibly to the point where it's barely noticeable.

So your torque split shifts rearward in such an AWD system, rather than being transferred to the outside wheel, as would happen in a 2WD vehicle with a LSD.

I would guess that, if you were to fit a LSD at the front, it would initally transfer torque to the outside wheels as per a 2WD design, but once that tyre was also at its limit, the extra torque would then go rearward.

Yeah, I forgot to consider the angular velocity, I was thinking torque only! I guess the viscous diff don't look at torque at all, only rotational speed difference.

So how does the mechanical LDS in the rear work? I've been told it's not a clutch based system, but a purely mechanical LSD like in the RX-7 FD.

If it's not a clutch based system, then it's likely a torque biasing differential.

I will bring back some old discussion here, as atledreier's car is a good example.

His car config may be reproduced on RB4, and it should work as it is, but in lfs is also front/rear power bias slider, which affects torque deliver.
I don't know how, and I don't know what device could change that Mazda, to LFS way of diffs working.
Any advice? If no, I think 4wd systems should be cleared up in lfs.

Achieving torque split (at least in theory) was discussed here http://www.lfsforum.net/showthread.php?t=23929 .