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MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Shim stack taper, what's the point?
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[QUOTE="SoCal Steve, post: 252024, member: 23612"] The reason to use a taper in the shim stack is to give a more progressive damping curve. In other words, for the fork to be able to react differently for differing impacts; high speed vs low speed etc. A completely straight stack I would predict would be overly harsh on small stuff, because it will take a certain amount of force to get any deflection, and then once it is open it won't give much increase in resistance. More shims of the same diameter at the valve face serve like an additional preload.... it takes more force to get them to move at all. When the stack is tapered, each shim starts to pivot off of the next one; until that next one is deflected and then they pivot of of the following one, etc. So for small, rather low speed, impacts the deflection may not move all the way thru the stack. The clamping shim, or the last shim, creates the final pivot point for the stack. Normally there would then be a hardened washer to follow so the clamping shim never deflects. So if the pivot is a 12mm od., and the stack begins with 24mm ods., then the outer 6mm of each side of the stack will be off of the valve face with the highest speed hit. But if there were 16mm ods in the middle, the 24s would pivot off of the 16s for lesser impacts so only the outer 4mms would be deflecting. So bottom line, the higher speed the hit, the further thru the stack deflects and the more fluid flows. This is what we want to make the damping progressive. For low speed impacts we want low deflections for a controlled feel and not blowing thru the stroke or bottoming. For high speed hits, we want the valve to flow much more so the fork can compress and absorb the impact and the feeling is not rigid or harsh. Make sense? Steve [/QUOTE]
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MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Shim stack taper, what's the point?
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