what tire size are you running in the rear? also it is quite easy to adjust tire pressures in order to help minimize under steer. or worst comes to worst you can do the poor mans over-steer trick and put less sticky tires out back. I currently run star-specs z1s up front which when autocross-ed have the consistency of a fudge brownie. but in the rear I run PS1s which remain fairly hard. it is only a temporary thing but if all you are trying to do is induce over-steer there are things you can do to accommodate that goal.
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