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Old 12-14-2007, 10:07 PM   #16
NSFW
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I'm glad someone started this thread. I will do mine soon since my left fork has been leaking.
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Old 12-16-2007, 07:23 PM   #17
meat popsicle
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Quote:
Originally Posted by Luke
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#
So the difference is not 2x - it's 3x more pressure with the highest recommended oil level (100mm) vs. the lowest (150mm), although my manual says the highest recommended level is a bit lower at 110mm.

Thanks for the additional information Luke. Where did you get the numbers?
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Old 12-16-2007, 08:49 PM   #18
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Quote:
Originally Posted by meat popsicle
So the difference is not 2x - it's 3x more pressure with the highest recommended oil level (100mm) vs. the lowest (150mm), although my manual says the highest recommended level is a bit lower at 110mm.

Thanks for the additional information Luke. Where did you get the numbers?

Don't take my results as exact. As I mentioned, there were a couple of things that I ignored. I mainly did it to show the method used to figure out the pressure.

I figured the spring volume from measurements I made the last time I opened the forks. There had been a couple of discussions on spring-cutting here and I was wondering about coil binding, so I measured the wire diameter and the coil gaps. I used those numbers (and spring length, ID and OD) to calculate the volume. The volume of the spring guide was a guess from what I remembered of its size.

Next time I have my forks open I'll do enough measurements to get more accurate numbers. If anyone else knows the ID of the fork upper, OD of the spring guide or depth of the fork cap, post up.

Really, the best way to do this is to slap one on a suspension dyno, but they're a bit beyond an amateur's reach.
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Old 12-16-2007, 09:04 PM   #19
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Originally Posted by Dotbond
Rocket scientist eh???

Electrical Engineer

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Old 12-17-2007, 09:24 PM   #20
meat popsicle
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I was interested in the relationship between pressure and oil height through the fork's travel so "exact" does not help me much. Although I am not trying to become a suspension tuner I thought it best to at least understand this relationship so that I don't consider it linear and over adjust the air spring during my own hack-tuning sessions.

A little (change) goes a long way eh? It may also help me understand the effect of fork oil height changes on different parts of the stroke as well as fork handling in specific situations. But I am day dreaming now...
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