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Fork Preload?

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How do you find the proper preload for forks? I have read over and over about measuring race sag and free sag for determining preload and proper spring rate for the rear shock but never for the forks.

When I installed Race Tech gold valves in my forks they listed a preload number in the data sheet. How they arrived at that and if it is accurate I don't know.

I did read in Eric Gorr's book that he recommended 2" of sag for the forks on the YZ426.

I would appreciate some enlightenment on this subject from some of you suspension gurus.


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What I can tell you is that there is no magical formula or given dimension for fork pre-load. It's a matter of what you prefer and where you want the bike under the conditions that you are riding it. But, in general, most guys set things up at 25 to 30% of total travel.

Now, given that it only takes an additional 25lbs of force to overcome the maximum allowable spring pre-load, (a shift of 12lbs. per fork) it's really not a big factor. In other words - a minor adjustment. In most cases, riders have to swap spring rates to obtain proper sag readings.

However, the other fork spring (made of compressed air) can shift load forces in excess of 320lbs.

At minimum oil level (highest measurement) it takes about 5lbs. to fully compress a fork against itself (no mechanical spring) whereas maximum oil level (lowest reading) it takes a little over 160lbs. (per fork). This is something worth playing with, especially given the forces it can offer for bottoming resistance.

The trick is the ability to maintain height for the fast straight-aways and jumps, but have a front end that will dive so that it load up the front wheel and allow the bike to turn (verses pushing).

This usually means that you have to over-lap spring rates to air pressure rates, allowing for a fork that, overall, would offer a seemingly non-linear progressive rate.

I may have some numbers in the next few weeks given that I'm testing a design that offers just this. Till then, I don't have much to offer except theory and a little math.


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