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Rebound Adjustment on the Shock Reservoir

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I wanted to take a quick moment to talk about the three-way adjuster which has the (H/L) compression and rebound adjustment placed on the shock reservoir. I have been getting a lot of questions about how it works, and how it’s possible. There have also been questions about how it functions without causing cavitation, or shock body side refill issues. The topic is also relevant for the understanding of non-adjustable, but shim controlled return circuits, rather than typical check systems.

So let me start by saying, yes, you can regulate the return of the rod charge and effectively adjust a segment of the rebound damping in this way. That is what we are doing, and this is what others have successfully done for a long time, sometimes without even knowing it.

I first heard about experiments with return circuits in the 90's after a phone conversation with Ross Maeda. The story went (forgive me if I overlook a detail Ross this was a long time ago). Ross made a mistake on a build in a fork, and he noted the fork worked much better from the rider’s feedback perspective. He did not expect this as it was a simple change. When he inspect he discovered his check valve had been limited. He told me that he thought this was crazy, and I agreed. But, as history dictates, he tried it again. Later, it became a mod he did. After that it appeared in KYB production parts. No one mentioned this as being a radical idea, and many tuners scratched their heads (myself included). I took notice when Showa started copying this in TC forks.

The watershed moment came to me when KYB introduced the PSF 1 fork. I was always left with the question, where does the rebound damping come from on this fork? The midvalves have massive holes in the piston! Try drilling 2 x 3mm holes in each of your traditional fork pistons and see what you get!

And then it occurred to me, the low speed rebound actually comes from the return circuit on the compression adjuster.

So presently the design is regulating a percentage of the positive force of the reservoir against the shock rod (rod charge). This is a small force, and it’s not the same as the potential force adjustment possible in the clevis. In a practical sense it adds control in the very lowest speeds and loads during the transitions to compression and rebound. In R&D applications we have perhaps even gone into negative territory. Without pushing limits one cannot know the boundaries. While this is not something we sell as a solution I’ve been curious from an investigative standpoint. We can do this because we have the main piston ultimately limiting the speed of return and unlike a compression force; the rate of return is essentially fixed. Our risks are far lower and it’s easy to know your upper limits of velocity as it relates to rebound.

Thanks for reading.

Jeremy Wilkey 


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