Front Fork Bottoming

1 reply to this topic
  • roylo

Posted September 14, 2009 - 07:45 AM


I want to add a small amount of oil to my front forks to help with bottoming out. Ive been informed that adding oil with increase the stiffness of the fork towards the end of the compression stroke. Is this correct and will adding oil make my rebound stiffer aswell because I dont want it to be.

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  • grayracer513

Posted September 14, 2009 - 08:03 AM


It is generally true that adding oil will make the fork stiffer overall because it reduces the volume of captive air within the fork. This air becomes a de facto part of the spring rate, so the less of it there is, the higher the effective spring rate.

It won't change the rebound rates, or any of the damping, for that matter, but it can have the effect of making the fork seem harsh. This is especially true of the '06 and up fork. As little as 10cc per side change in oil level is reported to be enough to make the fork significantly harsher or mushier.

If you have an '05 or later model, you have twin chamber forks, and you have a little trick available to you that the earlier forks don't. You can use a heavier oil in the outer chambers and run it at a slightly lower level. All of the valving in these forks is within the inner oil chamber, or cartridge. The outer chamber oil serves only to do 3 things: lube the fork, adjust the effect of the aforementioned air spring, AND it does the "braking" when the fork nears the bottom in the last 30mm or so of travel. The bottoming brakes in a fork are simple cones and/or a series of holes, and heavier oils make them work better.

If you run a 5wt fork oil, you can try a 7 or 10wt, and run it at 330cc per side instead of the standard 350cc.

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