[Doc]

Thanks for the correction. I had a brain fart...I was indeed talking about the Auburn cone-type LSD, not the Eaton.

The makers of posis love to throw around words like "torque sensing". Ever looked at the inside of a posi? Where is the torque sensor? There isn't one. This kind of description is just a fancy way of describing (marketing) some basic physics that is occuring inside the posi.

Let's talk about open diffs for a moment. We all know the example where you've got one tire without traction (in the mud/snow/puddle/whatever) and one tire with traction (on good pavement)....the slipping tire contiunes to spin while very little torque is being applied to the "good" tire. Note that there is actually SOME torque that is applied to the "good" tire. (If there wasn't any power going there at all, then every car would get stuck in the slightest little puddle or slick spot on the road). In and open diff, what little torque is available comes from the friction and drag that is latent in the axle assembly and the diff. Needless to say this is pretty low, but it is good enough for most cars on average street conditions.

Enter the posi. Posi is of course a trade name for a limited slip diff. A posi has design features that increase this torque available at the good tire. However, it is still possible for a posi to slip. It just means that for slip to occur, this increased torque must first be delivered.

Let me better explain what I was talking about. Functionally, a cone-type posi and a disc posi work the same way. The internal springs in the posi are preloading a set of clutches. In the Eaton, it is a disc type clutch. In the Auburn, it is a cone clutch. These clutches are what resist "differentiation" and slip. And in either case, a posi is basically an open diff with either kind of clutch added to it.

The clutches in a posi provide a certain minimum amount of torque to overcome. Given enough of a difference in torque between the two wheels it will eventually slip. This happens every time you drive around a curve or corner. However, whereas the latent friction in an open diff provides very little torque at the good tire in a slip situation, a posi will provide more.

Now, in the above paragraph I lied a little bit. In addition to the "static preload" of the clutches, there is also another factor. That is basically a percentage of the input torque (bias). This percentage is a design factor of the Posi. But the rule still holds: In order for it to "slip", the torque difference must exceed the holding power of the clutches. It is therefore the stength of the clutches (preload and bias) that determines just how effective a given posi is at resisting wheelspin.

Let's do an example. (I'm just going to be making up numbers here)

Suppose we've got a car, and one rear wheel is on an oily slick and has zero traction. The other is on good pavement and has excellent traction.

An open diff has a little resistance to slip. Maybe 25 ft-lb could be delivered to the good wheel, regardless of the rest of the car/engine. Not good.

A stock posi would be designed to have a high preload. Maybe 200 ft-lb, and a bias factor of 0.1. In this example, it requires 300 ft-lb PLUS 0.1 x torque delivered by the engine/trans/gears in order to overcome the clutch. If we assume there's 1000 ft-lb available at the axles for a given throttle position, in this case 300 ft-lb would be available at the good wheel...clearly a lot better. But, note that even though the driveline is cranking out 1000 ft-lb, there's still only 300 MAX avaiable at the good tire...the clutch will just slip away the rest. (This is why a serious drag car will run a locker or an outright spool, becasue those will transfer 100% of torque, period. If we had a spool or a locker, there'd be all 1000 ft-lb at the good tire!)

What happens if we have a posi with a higher preload? (stronger springs, such as the Moroso Brute). This will increase the preload torque. For example, we might have 400 ft-lb plus 0.1*DT. That gives us a higher torque (500 ft-lb) at the good wheel.

What happens if we have a posi with a higher bias factor? (this would be a disc posi with more discs in it, or a cone posi with a steeper angle on the cones). This gives us a higher driveline torque factor. For example, 300 + 0.3xDT. Again, this gives us more power available at the good wheel. The higher the bias factor is, the higher the torque delivery limit is. In this case, 600 ft lb. (And the more you step on the gas, the better the performance difference between this and a standard posi)

...Basically if you raise either factor sufficently high you would have an equivalent to a spool. The higher these factors might be, the more torque the posi is capable of delivering to the good wheel in the case of a slipping condition.

Practically, it makes more sense to raise the driveline torque factor (bias). This is becasue of what happens when you turn. During normal street driving, you turn corners under low-throttle conditions. This is when the driveline torque factor falls low and the static preload becomes the governing force to overcome to ensure proper differentitation. If you had too high of static preload, then you'd be chirping tires every time you turned a corner (just like if you had a spool)--and I have heard this happen on a car running the Moroso Brute. On the other hand, if you have a posi with a relatively LOW static preload but a fairly high bias factor, turning corners is no problem, but when you romp on the pedal you get much better performance. Too high of a static preload will wear out the posi quickly.

SUPPOSEDLY, this is why a steep cone posi (Auburn) is better than a disc posi, becasue it has a higher bias. That means whenever you hit the gas hard, it becomes more "spool like" and gives you better traction than could be provided by an OEM type posi. The disadvantage, of course, is that they are unrepairable once they wear out.

I was hoping that someone had acutally used one of these, and could provide a real world comparison. This stuff all sounds great on paper (and especially in maker's literature) but nothing beats actual experience.