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Old 10-03-2007, 01:41 PM   #9
Patrick Olsen
NASIOC Supporter
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
1997 Legacy 2.5GT
1996 Impreza coupe


Originally Posted by Element Tuning View Post
We recently upgraded to a Cusco 1.5 way rear diff (Thank you for getting that setup for Element Tuning) after blowing the OEM STi unit. Without any track time to break in the rear differential I showed up Sunday to race to a big surprise. The lockup was so aggressive that I had people coming up to me asking if we had converted our car to rear wheel drive as I was drifting huge all over the place. Man this was unexpected but definitely a step in the right direction to get rid of corner exit understeer. This was definitely exacerbated by my inability to build any heat into our rear Hoosiers as temps never broke much above 160F given the short race format. The aggressive lock up also had the front pushing on entry and massive oversteer on exit...
I'm trying to wrap my brain around this one. It seems to me that a very tight rear diff would cause power-on understeer, not oversteer. In the worst case scenario, RWD drag cars use a spool (which keeps both rear wheels locked together). Road racers generally don't use spools because the car will understeer like a pig under power as both rear wheels have to spin the same speed. Similarly, a very tight rear LSD is trying to make both rear wheels spin at the same speed, so the rear of the car is trying to push the car straight.

I think in your case, Phil, you've got enough power to spin the rear tires, so when you tromp on the gas the inside rear tire tries to spin as fast as the outside rear tire (due to the tight diff) and goes up in smoke, causing the power-on oversteer. In my car, which has probably 1/4 the power of yours , when I get on the gas I can feel a decided shift to understeer because of the tight Cusco rear LSD I've got.

I'm heading to Shenandoah this coming weekend. I don't think I'll be running 1:34s.

Pat Olsen
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