rickc said:

I think there is only one mention of the gear reduction advantage of portal hubs. In the Hummer (real one) it's 1.92:1 so there is a power/torque multiplier through the portal hub to each wheel.

Well, of course the only inherent advantage of the hub reduction gearing is that it takes place further "downstream" than if you set up the same overall ratios in the gearbox, transfer box or differentials. And even then, it's a slightly mixed blessing: on the plus side, there is less torque being put through the diffs and halfshafts, but on the minus side, they all turn that much faster.

That was an interesting article on the Hummer - but they weren't explicit about what kind of LSD (limited slip diff) there is in the "BTM" version. If I recall, the Hummer uses some version of a Torsen diff, which allows "x" times as much torque on one shaft as there is on the other. I vaguely remember that in the Hummer, "x"=6, but I might be wrong.

A normal open differential always maintains the same torque on each side ("x"=1), so if one wheel reaches a patch of ice and only supports (say) 1000N.m of torque, then the other wheel will also only be able to apply 1000N.m of torque, which might not be enough. With "x"=6, the Torsen allows the other wheel to apply 6000N.m of torque, which probably will be enough.

But if you

*lift* a wheel, then the torque at that wheel is zero, and so the torque at the other wheel is still zero (6x0). That's where the Brake/Throttle Modulation comes in: when you apply the brake, the wheel in the air comes under significant torque. Even though that torque is useless on its own, the

*opposite* wheel now gets six times that braking torque. The only penalty (traction-wise) is that all the wheels on firm ground also get braked, which offsets some of the benefit.

On thinking about it, I would have thought that such a torque-sensing LSD system would be an ideal complement for the TorqTrac system, because any computerised braking on the spinning wheel would add six times that torque to the opposite wheel, without any penalty. Still not as effective as a full locker, but it seems to me it would be much more effective than a standard electronic traction control system working through open diffs.