Fork Suck

[RustyMTB]

Sharing something many of you will know but I reckon a few won't. I've been scratching my head trying to work out why my 150mm Pike is sitting at 120mm & consequently bottoming out on the lightest hits.

I've found my airspring has fork suck. On tear down, I found about half a tub of suspension grease leading to a blockage of the transfer port & build up of pressure in the negative air chamber, sucking the fork down.

The solution is to do a service on the fork but releasing the air spring from the uppers can be very difficult when the pressure is high enough. In these circumstances, you can with great care, take a small cable tie & pinching the wiper seal to create a gap, insert it which will release the pressure in the negative chamber. The release will be audible & then the fork willl extend to its full travel.

I'd say the cable tie hack is a last resort- it's not a genius idea to put anything that could marr the stanchions anywhere near them but if you do need to do this, make sure the cable tie is smooth, the stanchion is wiped clean before you start & use a little suspension oil on the tie itself to lubricate & don't force anything ever.. Anyway, a tip/hack if your fork isn't as it should be.

 

[KennyB]

I had this once and the problem was caused by not extending the fork before installing the foot nut. Extending the fork by adding air to the air spring led to negative pressure in the lower legs and the sucking down. I suggest that your remedy, which lets air into the lower legs rather than the air spring, is an indication that this was a more likely cause.
Good post, by the way.

 

[Mteam]

Sharing something many of you will know but I reckon a few won't. I've been scratching my head trying to work out why my 150mm Pike is sitting at 120mm & consequently bottoming out on the lightest hits.

I've found my airspring has fork suck. On tear down, I found about half a tub of suspension grease leading to a blockage of the transfer port & build up of pressure in the negative air chamber, sucking the fork down.

The solution is to do a service on the fork but releasing the air spring from the uppers can be very difficult when the pressure is high enough. In these circumstances, you can with great care, take a small cable tie & pinching the wiper seal to create a gap, insert it which will release the pressure in the negative chamber. The release will be audible & then the fork willl extend to its full travel.

I'd say the cable tie hack is a last resort- it's not a genius idea to put anything that could marr the stanchions anywhere near them but if you do need to do this, make sure the cable tie is smooth, the stanchion is wiped clean before you start & use a little suspension oil on the tie itself to lubricate & don't force anything ever.. Anyway, a tip/hack if your fork isn't as it should be.

Just a point of order, air trapped in the lowers which you can "release" by pushing a cable tie down the dust wiper seals of the fork stanchion is not the negative air chamber of the air spring, and is not excess air trapped.

The problem you describe is a lack of air in the fork lowers casing a vaccum, the hiss noise is air being sucked into the lowers, not air escaping from them.

You've probably created a vaccum in the lowers somehow - as per KennyB above.

 

[RustyMTB]

Yep, air in , not out, my bad - outcome is the same however, problem fixed.

 

[steve_sordy]

I had this on a fairly new Rockshox Pike fork (160mm travel). A random guy outside a bike shop pointed out that I had 10mm of lost travel. He explained and (with my permission) lubed the tip of a zip tie and carefully inserted it between the wiper seal and the stanchion. There was a slight hiss and the fork raised by 10mm. I thanked him and off he went! I have never seen that problem on any bike I've had since, which has been about 7 years.

 

[KennyB]

I now put a bit of air in the spring to bottom the piston, then attach both foot nuts, screw on a few turns and gently draw the lowers down fully, then torque up the foot nuts. Haven't had a problem since.
PS, this is after a fork service