Fork oil height and removal of oil? Two ?'s.


4 replies to this topic
  • slowxr

Posted March 15, 2006 - 11:27 AM

#1

I just went from 125mm of fork oil to 110mm on my Xr650. My bike use to be really plush on sharp and fast bumps, now it's hard and I feel every little bump. It feels ok everywhere else, just terrible on the fast bumps.
Is this caused by oil height or did I screw something else up?

Also, is there an easy way of lowering the fork oil without taking the forks apart? Maybe a small diameter tube through the air hole?
Thanks

  • weskc35k

Posted March 15, 2006 - 01:37 PM

#2

Have you backed off the compression clickers.
A syringe with a small diameter hose will get the excess out and you will also know how much you took out.
Was your front end soft before?

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  • Jon-D

Posted March 15, 2006 - 05:25 PM

#3

when the oil height was raised the "air spring" inside the forks was decreased which resulted in a harsher ride on the choppy stuff, but also increased bottoming resistance (G-outs for example). The oil is considered an incompressible fluid while the air is considered compressible to an extent. By having a larger air/oil ratio, the fork will be more forgiving on the small choppy stuff, but have less bottoming resistance.

The simplest way of changing your fork oil height (provided you do not know how much oil changes X amount of height within the fork) is to leave your forks on the bike and with the bike on a stand remove the caps to allow the forks to fully compress. Remove the fork spring and measure oil height and adjust as necessary. If you know the volume of oil it takes to change oil height then you could simply remove that amount to lower the oil height as indicated by weskc35k (there may be a bleed bolt at the bottom of your forks to remove the fluid). A word of caution, make sure the bike is stabilized when compressing the forks or the bike will fall over.

BTW, what improvement were you looking for by increasing the oil height?

  • slowxr

Posted March 16, 2006 - 09:39 AM

#4

Well before I made the change my bike was fairly plush and "cushy". On the smooth trails, jeep trails and sand, the bike felt really stable. I never jump but had the bikes valving changed to reduce the "fast" hits the were coming through the bars when stock.
The tuner told me he set it up w/5 weight at 110mm. When I removed the spring(carefully to reduce the amount of lost oil), I measured the oil level. It was around 127mm! I know I lost some oil on the spring but it was supposed to be close to 110mm.
Anyway, I put the oil level back at 110mm and now I'm having this problem. Really hard on sharp hits. It's very dramatic compared to the way it was.

I'm thinking I either screwed something up on the reassemble or I should put the oil level back at 125mm? Seem reasonable?

Also, I don't think I ever bottom my forks at the 127mm oil level. This was just an oil service after 120 hours on the oil.

  • qadsan

Posted March 16, 2006 - 10:34 AM

#5

Did you use the 'exact' same brand and weight of oil that was previously used?

5 weight is simply a range and not all 5 weight oils have the 'exact' same viscosity. Some 5 weights are significantly more viscous than other 5 weights and some are significantly less viscous than others. If you find that the adjusting the volume doesn't resolve your issue, then look for the cSt (centistoke) viscosity measurement listed on the fork oil or get this number from the manufacturer of the the oil you're currently using and try a different 5 weight that has a lower or higher cSt number. My guess would be you'll want a thinner fluid to get back your plushness, so in this case you'll want an oil with a lower cSt number.

Here's just one example of how 5 weight fork oils can differ. Bel Ray 5 weight fork oil has a viscosity of 20.5 cSt @ 40°C and 6.2 cSt at 100°C. Honda's 5 weight fork oil has a viscosity of 15.06 cSt at 40°C and 3.45 cSt at 100°C. They're both classified as 5 weight fluids because they both fall into the 5 weight range, but as you can see, their actual viscosity is significantly different, especially at lower temperatures.





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