Fork oil air gap

I am giving my forks a service, I know the book says an air gap of 104mm, and if you have a smaller air gap it will firm the damping.
Has anyone done a smaller air gap, if so how much of a gap did you use.
I am using 10w oil and standard springs.
Sorry if this has been talked about before, done a search but could not find anything..........

Originally Posted by Born again

I am giving my forks a service, I know the book says an air gap of 104mm, and if you have a smaller air gap it will firm the damping.
Has anyone done a smaller air gap, if so how much of a gap did you use.
I am using 10w oil and standard springs.

It will firm it, however, there will come a point where it's so firm that road shocks won't get absorbed - they'll go straight up into the frame and YOU. Also, at some point, there's the possible question of the fork seals giving up. If you're going to experiment, only add enough oil at a time to change the level by say 5mm a go. A bit of experimentation when the forks are out with a graduated syringe will work out how much to add to achieve 5mm at a time.

Are you sure you don't want to try a heavier oil and perhaps a larger air gap to start with? Reducing the air gap will make compression damping start earlier (less air to compress before oil starts moving) but will not affect rebound damping. A heavier oil will affect rebound damping.

[jambo]

A smaller air gap will not alter the damping, It will increase the effective "air spring", the smaller the air gap, the more the air at the top of the fork is compressed during fork travel, This means that at the end of the travel, the effective spring rate will be higher than it would normally be. Don't go wild, a little change would make quite a difference.

Originally Posted by jambo

A smaller air gap will not alter the damping, It will increase the effective "air spring", the smaller the air gap, the more the air at the top of the fork is compressed during fork travel, This means that at the end of the travel, the effective spring rate will be higher than it would normally be. Don't go wild, a little change would make quite a difference.

Sorry, I should have pointed this out too. The air gap doesn't affect the compression damping, but it does alter when it starts to take effect. The air will get compressed more easily to begin with, rather than pushing oil through damper holes.

[jambo]

Originally Posted by chazzyb

Sorry, I should have pointed this out too. The air gap doesn't affect the compression damping, but it does alter when it starts to take effect. The air will get compressed more easily to begin with, rather than pushing oil through damper holes.

If you put oil in the fork without springs and fork caps you can feel the damping by pulling and pushing the fork stanction in and out of the fork lower. The oil is forced through the damper holes by the movement of the fork stanction in the body, not by the air above it.

The air gap affects the spring rate only, effectivly making the final part of travel stiffer like a progressive spring would.

If you were trying to get less dive under braking with adjustable suspension you would probably just up the compression damping rather than messing about with fork oil gap...
</pedant>

Originally Posted by jambo

If you put oil in the fork without springs and fork caps you can feel the damping by pulling and pushing the fork stanction in and out of the fork lower. The oil is forced through the damper holes by the movement of the fork stanction in the body, not by the air above it.

The air gap affects the spring rate only, effectivly making the final part of travel stiffer like a progressive spring would.

If you were trying to get less dive under braking with adjustable suspension you would probably just up the compression damping rather than messing about with fork oil gap...
</pedant>

Hey, no one likes a smart-**** (I'm kidding), even if you are right . Which, thinking about it, you most likely are. My 'knowledge' on the subject is limited, having based it on an old article I found about 'tuning' forks on a racing Laverda Jota.

Originally Posted by jambo

If you were trying to get less dive under braking with adjustable suspension you would probably just up the compression damping rather than messing about with fork oil gap...
</pedant>

You mean oil weight

[21QUEST]

What brand of oil are you intending to use.

At the moment, are you using all the suspension travel?


Ben

[jambo]

Originally Posted by Flamin_Squirrel

You mean oil weight

No I don't. I'd up the compression damping to slow fork dive rather than increasing the effective spring rate with air gap.

Not that the same principle with air gap doesn't apply to cartridge forks too, it's just that I wouldn't start there...

[johnnyrod]

I'd say use 15W but can't say anything on the air gap. Rebound damping is a bit minimal anyway so the extra from the thicker oil will help there too. If you're on the stock fork springs then damping isn't necessarily your problem...