Effect of air pressure in forks

[KarlP]

I'm curious what effect air pressure has in the forks of a '99 CR250?
It is powered by a mighty KDX220 motor, so I feel O.K. asking here.

I have speed bleeders on the bike and find that I like just a bit of pressure in them. If I bleed them during a ride they don't feel right until after about 10-15 minutes of hard going.

The forks have been revalved and re-sprung. I've spent a lot of time working with clickers and really like the way everything works.

If I knew what part of the comp/rebound circuits were effected by air pressure in the forks, I'd be inclined to move the clickers in that direction and use the speed bleeders more often for a more consistent ride.

(I also notice the left leg has consistently more pressure in it than the right. Just an aside....)

Any help would be greatly appreciated!

[island220]

Sounds like forks are not same height regarding pressure in left leg, pull front wheel,remove axle, using just axle make sure there is no binding when placed in clamps, rotate and slide from side to side in clamps, by applying pressure up or down to fork you will feel it bind or slide freely, I will go through this process after a good crash also,

[canyncarvr]

Air pressure increase will relate to oil level increase (lower number). Sub-tanks equate to a real low oil level...without changing the oil level.

If you like them with more pressure, raise the oil level (if the valving suits you).

What part effected? Considering what's going on makes sense to me. The forks are compressed, the rod displaces fluid in the cartridge, said fluid is forced past the valving, that fluid (then out of the cartridge) starts to pressurize the 'air shock'. It's going to be 'softer' on the beginning of the stroke, increase as the stroke increases.

I think..........

Different air gasps? How about one fork seal letting a bit of air pass? I suppose there would be some oil with it. My forks are hardly ever 'the same' in that regard..no trace of oil nowhere. Maybe the oil level isn't the same in both forks?

Could be a bleeder leaking a little, too.

[canuckhybrid125]

You do know that the bike is supposed to be elevated on a stand before you bleed the air right?

[canyncarvr]

You do know that the bike is supposed to be elevated on a stand before you bleed the air right?

With race sag on a set of forks running maybe 30mm, it's not hard to unload the front-end enough to top the forks out. Don't need a stand. I'm sayin the bike does not need be elevated on a stand, the forks just need to be unloaded.

..which isn't always true, either. I change pressure in my forks from time to time..sometimes bleed 'em with 'some' sag to 'em. Depends on how I want it to 'feel'.

[KarlP]

That is what I was looking for...

I'm going to try adding a bit of oil to each leg through the bleeder hole. Maybe 5 cc at a time?

I'm not too concerned with different pressure build up in the legs. I'd rather take the approach of running zero pressure and have it consistent

[canyncarvr]

I'd approach it from a linear standpoint, not volume. The original setup is height, not volume. Adding an unspecific volume won't tell you how to change the oil height. Yeah..you could figure it all backwards from what you put in I guess. For some reason (or no reason at all), I'd prefer to do the figuring first.

'Fer instance, I don't know the diameter of your forks. Figuring for a 42mm diameter tube, every 5mm will take 6.59cc of fluid.

Round it off to make it easier..say 6.5cc per 5mm in tube length (oil height).

So..if your last oil setting was 120mm, you put in 13cc and like it, you know that NEXT time you set the oil level to 110mm.

Better check the math on that.

My radii may not be squared!

[Indawoods]

Don't like your math CC... it may be fine for forks with nothing taking space up in them... but rods, valves etc... take up volume!

[canyncarvr]

Yes...but the oil level is SET with the biggest part of that space-taker-upper list not IN there...the spring. As far as the valves and all that..they're already submerged. Don't matter.

Add a teaspoon to a pail of water, you add a teaspoon to a pail of water. Put a big rock in the bottom of the pail? ..the teaspoon's still a teaspoon.

So take off some for the rod.

Say 5cc per 5mm.


Oh. Yeah. That's what he said already........

I bet HE did his OWN math.


Or, do it emperically next time a fork service is done. Measure the height change with a 5cc fluid add. ...if anyone is really that worried about any of it.

That wouldn't be me.

[KarlP]

I checked with the guy who did the forks in the first place. He set the level to the bottom of the range, his recommendation for woods use.
Which means diddly to me at this point, 'cause I'm not going to check the current level. I'm just going to keep adding oil in 5 cc increments until I go past the point that works well. I have a fork service interval due very shortly anyway at which time I WILL check what level I ended up at.

I know, butt backwards.

[canyncarvr]

Sounds perfectly reasonable to me.



..which might alone be reason enough for you to re-think it.