Measure the distance from the wheel spindle to the tail piece when the rear is off the ground.
Divide by three, and then use that figure as the reading for when you're sitting on it.
It's far easier to have a friend help doing this!
Then ride it and see what you think. Softening the rear will slow the steering a bit (unless you're hard on the brakes and the rear is unloaded of course) but will probably make you faster as the rear tyre will get an easier life and give you more grip...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
