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Torsen differential

EXP Jawa

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Rick
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1999 Cobra Convertible, Electric Green
That was posted here a few weeks back. There a couple issues with that video: 1) it shows a crossed-axis Type-1 gear set, which Torsen used extensively in the 80's, but is only used on military applications today, so it isn't applicable to the Torsen differential models than are and have been used in Mustangs. 2) the narrator is still hanging onto the "worm wheel" theory to describe how it prevents slip, but in practice, this isn't correct.

At the heart of it, Torsen differentials work just like any other limited slip device - because they generate friction internally, and that friction is what resists wheel spin. Resistance to wheel spin is what allows a differential to apply more torque to the tire with better traction. That's what we refer to as "torque biasing"; the ratio of which it can distribute torque is called the "torque bias ratio" (TBR). Boiled down, the more friction that can be generated, the higher the TBR.

What sets helical gear differentials (Torsen included) apart from typical plate-clutch differentials is the way in which generates friction, and - just as importantly - the behavior in how it increases that friction with the amount of torque applied in a progressive manner. That means that it behaves openly when no torque is on the system, but generates a high amount of lock-up when you get on the throttle. The actual friction is generated by the gearing, being pushed under load against the internal walls of the housing. The gear geometry (helix angle, pressure angle) cause reaction loads on the gears, creating high thrust loads. The higher the torque you apply, the greater those loads, and higher the resulting friction that is generated.

I suppose that the old worm & crown wheel theory would work if the helix angle was very high - like 85 degrees or so, like a screw jack. In fact, that's how a screw jack can lift up without being pushed down by the weight on it. But in this case, the gearing helix angle is a lot lower (say, 45-50 degrees), so it isn't nearly high enough to prevent the back driving of one side to the other. If all of that where true, if you lifted the car off the ground and spun one wheel forward, it would lock across and push the other wheel forward. But what really happens is the gearing drives through and spins the other wheel backward.
 
 




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