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AWD DSM torque split/distribution

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jtmcinder

DSM Wiseman
5,402
90
Nov 4, 2003
Iowa City, Iowa
Before we get to the details of how much of the engine torque is sent to the front and rear wheels, we need to make a clear distinction between two things. First, there is the torque split between the front and rear, which is entirely determined by the gearing of the center differential. Second, there is the actual, moment-to-moment distribution of torque to the front and rear. Note the two different words used: split and distribution. That's key to really understanding all this.

The torque split for all stock DSMs is 50/50. The center differential is a straight-forward, spider-type diff with equal gearing to the front and rear outputs. This never changes as you drive, regardless of when or whether any limited-slip devices kick in. The torque split is set by the gearing, never changes (unless you buy a new diff), and is always 50/50. Period. End of story. Good-bye. (To quote Shrek.)

In contrast, the torque distribution can vary infinitely, going as high as 100/0 or as low as 0/100 (assuming extreme conditions and a center viscous coupling [VC] that's in good shape). [Note: I follow the convention of listing the front torque first, so 100/0 is all-front and 0/100 is all-rear.] Torque distribution is how much power is being used by the fronts and rears. If your fronts are on clean, dry pavement and your rears are on oiled glass, then all of the torque will be used by the fronts, so the momentary distribution will be 100/0 - assuming that the limited-slip device on the center diff can handle the transfer. Conversely, if your rears are on pavement and your fronts have no grip, then - again assuming that your VC manages to lock - all of the torque is being used in the rear for a 0/100 distribution.

The distinction between torque split (set by gearing) and torque distribution (which also depends on grip and the action of limited-slip devices) is crucial to understanding what is going on. Again, the torque split never changes, since it is set by the gearing of the center differential. In contrast, the torque distribution can vary from moment-to-moment from just about all-front to all-rear, depending on which set of wheels has more grip and is, therefore, using more of the available torque.

Thus, the only hard-and-fast statement that can be made about our cars is that the torque split is 50/50. No general statement can be made about the torque distribution, because that changes from moment-to-moment.

- Jtoby

The purpose of this post - here in the Newbies Forum - is to answer what appears to be one of the most-often-asked and most-often-answered-incorrectly questions about our cars. For more information, see these tech articles:

http://www.dsmtuners.com/forums/art...17-diffs-lsds-part-1-types-differentials.html

http://www.dsmtuners.com/forums/articles-drivetrain/177206-diffs-lsds-part-2-types-limited-slip.html
 
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