Rear Shock
First extend the shock completely and measure from the axle to a vertical reference point on the chassis. This is L1.
Take the bike off the stand, put the rider on board in riding position. Get an assistant to balance the bike or have the rider hold onto something, compress the suspension about 25 mm (1") and let it extend very slowly (slowly is the critical term). Where it stops, measure the distance between the axle and the vertical reference point on the chassis again. Do not bounce. This is L2.
Next lift up on the suspension and let it drop very slowly. Where it stops measure again. Do not bounce. This is L3. The reason L2 and L3 are different is due to stiction or drag in the seals and bushings. (If there were no friction in the seals or the linkage L2 and L3 would be identical.)
Half way between L1 and L2 is where it would come to rest with no friction. Therefore L2 and L3 must be averaged and subtracted from L1 to calculate true Static Sag.
Static Sag = L1 - (L3 + L2)/2
To adjust Static Sag use the preload adjusters.Spring collars are common, and some benefit from the use of special tools. In a pinch you can use a blunt chisel to unlock the collars and turn the main adjusting collar. If you have too much sag you need more preload; if you have too little sag you need less preload.
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http://www.howellracinginc.com