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Subject: HML: Re: diffs
I find it easier to think in terms of torque instead of power when talking about diffs. What we care about is the torque going to each side. The short explaination...
An open diff always distributes torque equally left to right. If one wheel has very little traction, little torque gets applied to that side. Hence, you get little torque to the traction side.
A limited slip wants rotational speed to be equal. However, with little load on them, they function only on preload. They can allow a little more torque to go to one side but usually only about 50-100% more. With a no traction wheel, 100% of little is still little. Of course you can have a "built" LS that changes the ratio...
The Torsen allows a torque split of around 4:1. Hence, the traction side can receive 4x the torque going to the slipping side.
BTM works by adding resistance to the system. By adding brake drag to the slipping side, you increase the torque going to that side. Hence, torque to the other side (traction) also increases. Since an open diff splits torque equally, BTM is of marginal benefit. It does work in a very limited sense but only because the open diff is not ideal. Don't count on it.
With a LS (like the factory Trac-Lock on the Jeep), BTM works better because the device is adding resistance to the low traction side already.
BTM works well with the Torsen because it allows a much higher torque biasing. A little brake drag on the low traction side translates into 4x torque to the traction wheel. With geared hubs, this is 8x torque...more than enough for most situations.
As Hal points out, the theory behind BTM has been around before Torsens and before Hummers. It does work on open diffs and LS units..you just have to use light brake pressure. BTM just works really well with the Torsen + geared hub design.
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