According to the petitrc chart the rear purple springs have a spring rate of 3.30.
The main reason that nobody uses these very springs might be their spring rate, they are pretty stiff. On the 2WD car I never had to use stiffer springs than the blue ones in the rear. I even found the purples to be very stiff for an SCT with Pin Points on carpet.
People use the springs they see on other people's set-up sheets, which is so much easier than starting from zero. Yokomo's springs are neither better nor worse than other manufacturer's springs and come at a decent price. Blacks and purples are really stiff, but greens, oranges, red fronts and yellows should be sufficient for almost any track. I have yet to try my Avid RC spring sets, Yokomo blues, red fronts and yellow rears were all I really needed so far with the B-Max2 from loose dirt to high bite carpet.
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