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Old 10-12-2005, 04:53 AM   #41
Patrick Olsen
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Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
Vehicle:
1997 Legacy 2.5GT
QuickSilver Metallic

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Quote:
Originally Posted by Jonathan
Okay I maybe showing some ignorance here, but what exactly is the difference between a "torque biasing" setup and a true "Limited Slip" setup ?

The objective with each is to apply power to both front wheels, no ? Under extreme cornering situations you wouldnt want your lightly loaded inside wheel spinning helplessly whilest your heavily loaded outside wheel just was just coasting. Dont both differentials accomplish the same task?
A limited slip differential (LSD) does one thing - it tries to keep both axles spinning at the same speed. The most common type of LSD is one that uses friction plates or clutch packs (depending on what you want to call them). If there is any differential speed between the two sides of the diff, the friction will try to "speed up" the slower side (or slow down the faster side, depending on how you look at it). An LSD is always trying to put the same amount of torque to both sides of the diff - it's a "dumb" diff. An LSD can't do anything until the speed differential already exists, and then it will try to "fix" that speed differential.

A torque biasing differential (or TORque SENsing differential, hence the name Torsen) uses a complex interaction of gears, which I can't even begin to describe, to distribute torque to prevent that speed differential from ever occuring by biasing torque to the wheel/axle that has more grip. If you really want to know how these types of diffs work, go to http://www.torsen.com/general/general_faq.htm and then the 2nd question links you to a PDF that describes it in pretty agonizing detail.

AFAIK, Torsen was the first to develop a TBD. There are other companies that make similar types of diffs - I don't know if they pay Torsen to use the technology, or if they've designed variations. Quaife is probably the most famous - it may be they developed it first? I really don't know, I'm just under the impression it was Torsen. Anyway, I know Detroit Locker makes a TBD for the Ford 8.8" rear end (aka Mustang rear end) called the TrueTrak (TruTrac, something like that). The Suretrac diff in the STi is a TBD. Any time you read about a "helical LSD", such as the one in the Integra Type R, that's really a TBD.

For handling purposed, a TBD is better. With this front TBD, for instance, in a corner the outside wheel is going to be more heavily loaded and hence able to handle more torque. So, the diff will bias more torque to that outside wheel, which will tend to pull the car through the corner. Because it only sends as much torque to the lightly loaded inside wheel as that wheel can handle, you're much less likely to get wheel spin on that inside front wheel.

On my Mustang I have a Torsen T-2R, which replaced the factory Traction Lok clutch-type LSD. Assuming the clutches haven't worn out to the point that the T-Lok is worthless (about a yearly occurence in a car that's frequently auto-x'd), when you put the power on coming out of a corner the car is going to want to go straight. The LSD is trying to turn both wheels at the same speed. This effect isn't nearly as bad as a Locker, of course, where the axles are locked together under power, but the effect is still there. With the T-2R, as I put the power on coming off the apex, the outside wheel is more heavily loaded, is therefore able to put more torque to the ground, and therefore will be biased more torque. Again, this helps to rotate the car through the corner, because the outside rear is getting more torque than the inside rear. It's not perfect - I can definitely feel the car start to push/understeer a bit when I get on the gas - but it's much better than the stock T-Lok.

Here's some more good, basic discussion of the different types of diffs - http://www.howstuffworks.com/differential8.htm (continue to pages 9 and 10).

Pat
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