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The 2.5 chassis is good on higher bite carpet, but i have found the flex of the 2.25 helps in low grip or on asphalt. Last year at reedy i used a 2mm and it was great! I have actually used black rubber o-rings under my upper deck to get the car to flex more at my local track. It was a little more "loppy" but very consistent and provided great traction. Keep in mind this was only for low grip situations, such as the begining of a night racing or new carpet. I had even noticed in one of the pics from the last event Viktor had removed the steering post brace to get a little more flex out of the car.