View single post by stevegarnjobst
 Posted: 02-07-2016 01:40 pm
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stevegarnjobst



Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 35
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Esprit2 wrote:
Steve,

Nice looking diff, but... have you ever autocrossed a lightweight car with a limited slip differential?

1) Limited slip under power will tend to make the car go straight, meaning you can't start to feed in the throttle until you have the car straightened out. Is Phil Ethier still autocrossing? Ask him about the LSD a PO had put into the Lotus Europa he used to autocross. He put regular gear oil in it, and just prayed for the day the clutches would wear out. When they did, the car got much better at handling the tight courses.

2) With the near-stock 907's modest torque, I don't think you'd have too much difficulty managing the wheel spin with an open diff. With a hot rod 2.2 or 2.5 it would be a different story.

Regards,
Tim Engel


Yup, LOTS of experience with lightweight autocross cars - both with and without limited slip differentials. When set up properly, a car with a limited slip will virtually always be faster than one without a limited slip, assuming the vehicle has sufficient power to spin the tires under acceleration.

The issues you describe are definitely a problem with some limited slip designs, or with limited slip units not properly tuned for autocross use. For autocross, what you want is a differential that behaves like an open diff on corner entry, which minimizes/limits the understeer behavior you describe. However, you want the limited slip to distribute power between the drive wheels when accelerating, such as on corner exit.

With the Gripper unit, diff behavior is adjustable, via ramp angles. The unit permits different diff engagement settings for corner entry/deceleration and corner exit/acceleration. I'll be running the diff with settings appropriate for autocross use.

On a related note - it's fairly common for road racers with older low-powered British cars to run "welded" diffs, which essentially creates a locked differential. Welded diffs display precisely the behavior you describe. It's very difficult to drive a car with a welded diff on an autocross course, as you literally have to break the tires loose and slide the car around every tight corner.

Last edited on 02-07-2016 01:41 pm by stevegarnjobst