[RockyMtnRay]

Originally Posted by molex

In that case, isn't it a benefit that the two differentials do not communicate?
If the differential's job is to send the proper tourque to the outside wheel in a turn so that it can rotate around its axis the proper number of times, and the differentials don't communicate with each other, why doesn't each differential send the proper tourque to its own outside wheel, thereby negating the negative effect of turning with 4wd on? I mean, if each diff is it's own unit and does not rely on data from the other diff to know what's going on, each wheel should be free to revolve the number of times necessary to make a successful corner, provided that the diff is sending the proper amount of tourqe..... I would think.... or is the system just not complicated enough to provide that type of parity?

The latter...just not complicated enough to handle it...the missing link is a center differential.

Reason: in a turn, it's rare that any 2 of the 4 wheels makes exactly the same number of revolutions...the outside front makes the most revolutions and the inside rear makes the least. Depending on the radius of the turn compared to the vehicle's tread and wheelbase, the inside front will sometimes make more turns than the outside rear and sometimes less...but always less than the outside front. The outside rear always makes more than the inside rear. However, with just a simple transfer case in the middle, both the front and rear driveshafts are making identical revolutions...yet since it's seldom that the inside front/outside rear pairs of wheels are turning at the same rate, stress will build up. It's kind of like having a locked axle except the locking is diagonal front/rear instead of side to side.

However, if you make the system a bit more complicated by putting a differential in the middle, then the rear and front driveshafts can turn a different number of revolutions through a turn and there's no stress buildup. The front and rear axles are effectively decoupled from each other. This center differential is the key piece in "full-time" 4WD systems and all AWD systems.

Clearer now?