In message , at 15:32:20 on
Tue, 28 Oct 2014, Recliner remarked:
If the diff is locked both driving wheels rotate at the same speed. If
one is spinning, very little power is "lost" - the only place it can be
dissipated is warming up the tyre/road surface and if slippery/icy
that'll be very little. The rest of the power inevitably goes to the
wheel with grip.

Agreed, but the opportunity is also lost to put some limited power
through the wheel with low adhesion.

In practice the power gets applied through as many of the wheels as do
have adhesion.

But the key thing is you can't have a locked diff on a normal road
car, as it would ruin the tyres and make the car undrivable on curves.
Off-road vehicles can usually lock the centre diff and possibly the
rear diff, but that's a conscious decision taken when driving
off-road, not a default setting. So if a car unexpectedly hits a
slippy patch, even cars with lockable diffs will be unprepared. But
ASC is ready and waiting to spring into action at all times.

The times I found locked diffs the most useful was driving on snow/ice
in the heart of winter where you could pretty much expect never to come
across a patch of dry road. Those are the sort of conditions where
modern hatchbacks with front wheel drive just sit there literally
spinning their wheels.
--
Roland Perry