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Old 12-14-2007, 06:15 PM   #14
Luke's Avatar
Joined: Aug 2004
Location: Idiotville, OR
Oddometer: 4,035
The problem is you're starting from the end. The oil level is measured with the forks fully compressed and the spring removed, however the actual air chamber is created with the spring and spring guide installed and the forks at full extension. In your/creeper's analysis, you're starting from full compression and compressing even more, which isn't possible.

If the oil level is set to 100mm, the initial height of the air chamber is 300mm. That's 100mm + 275mm (suspension travel) - 75mm (spring and guide volume). At full compression, the air height is 25mm. So the air pressure is now 14.7 * (300/25 - 1) or 161 psi.

Repeat for 150mm oil height and you get 75mm compressed, 350mm extended for a compressed pressure of 53 psi.

These numbers aren't quite right because they ignore the volume of the damper rod and the gap between the fork lowers and uppers.

But, to take these numbers further....
The area of the lower fork tube is 2.8 square inches, so the force on the wheel from 161psi is 450# in each leg. In comparison, the force from the spring is 319# (.5kg/mm rate, 15mm preload) With the stock oil level of 150mm, the force is 151#
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