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Air in Pike lowers: a PSA

87K views 141 replies 58 participants last post by  Sneeck 
#1 ·
I realize there is a huge thread on the Pike in this forum, but given that I am subscribed to it and failed to realize this was an issue, I thought I'd make a new post.

Short version: The seals at the top of the lowers on the Pike are very good, and they can trap a substantial amount of air pressure. In some cases, this can completely ruin the performance of your fork. Bleed the air by emptying the air chamber and sliding a thin piece of plastic in between the stanchion and dust wiper until you hear a hiss.

Long version: I loved my Pike when I first got it, but over time, and especially lately, had been experiencing a significant loss of enthusiasm about it. This culminated in a trip to Mammoth where I really just couldn't believe the level of arm fatigue I was getting. I chalked it up to being out of shape and that my Tallboy LTc just wasn't cut out for park riding. Cut to a few weeks later, I remember something about this air trapping issue and decide to check it out. There was significant resistance to compression even with the air cap removed, so I executed the cable tie bleed technique and released quite a bit of pressure.

I had thought it curious that I only needed ~72 psi to get desired sag (~25%) out of the Pike even though I'm 220 lbs geared to ride. After bleeding the lowers, I had to up the pressure to 90 psi in order to have a rideable fork! (Much closer to what Rock Shox recommends for my weight.) On the subsequent ride, my fork was back to it's good old self with the excellent small bump compliance and mid-stroke support I remembered. I'm super happy about this.

So if you own a Pike or took a ride on one and are disappointed, consider that this may be the issue. I was absolutely blown away by the effect it had on the performance of the fork, and I think the air pressure change I had to make speaks for itself. It's a fantastic fork, as long as air trapped in the lowers isn't creating an undesired spring rate that completely alters its behavior.
 
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#45 ·
Unless there is a defective seal in the air cartridge allowing high pressure air to leak into the lowers, there is no reason to purge air out of the lower legs.

Any oil bath fork, coil or air, when assembled, has a sealed lower leg. The oil seal at the top and the crush washers or O-rings at the foot nuts form a sealed chamber. When the stanchion tubes are inserted, at the top, air is trapped in the lower tubes. When the fork compresses and the stanchion tubes travel into the lower legs, the stanchions take up volume, and the air in the lower legs is compressed. Yes, this results in a slight ramp up of pressure in the lower legs. This is normal and inevitable. It does not pose an impediment to performance because at the top of the travel you are starting at 0 PSI and the amount of pressure built up at the approach of bottoming out is just a slight cushion. Depending upon your riding weight and style, the effect will vary, but that is why you adjust the air spring pressure for your particular situation rather than just going by a shock chart.

You can absolutely compensate for this slight air cushion by adjusting your air spring pressure and damping accordingly.

The video that was posted showing the zip tie "procedure" is a horrible example of how to "solve" a problem that does not exist. Worse, it's a good way to create a problem. Notice that the air being "purged" is at the bottom of the stroke. They are venting out the air pressure as the stanchions compress the air. This means when the air spring is charged and the fork extends, the lower legs will be in a vacuum. All this does is provide the temporary effect of having a negative air spring. Sure, it feels plush for a bit, but it needs to be repeated later. Why? Because the vacuum gets filled. In other words, the air gets sucked back into the lowers. Depending on where you are riding, guess what else gets sucked into the lowers? Moisture, dust, or whatever.

I can't see the point in running a zip tie past the seals on regular basis to get some temporary gimmicky effect while risking damaging the seals and contaminating the oil. Simply accept that fact that a slight ramp up occurs inside the lower legs and adjust the air spring and damping accordingly. Then, ride your bike and stop wasting time performing unnecessary "procedures."

Again, if significant air pressure is building in the lowers, that would be different and would indicate a loss of air from a pressurized part of the fork to the lower leg. This calls for a repair of the problem, not a quick zip tie purge.
 
#46 ·
Unless there is a defective seal in the air cartridge allowing high pressure air to leak into the lowers, there is no reason to purge air out of the lower legs.

Any oil bath fork, coil or air, when assembled, has a sealed lower leg. The oil seal at the top and the crush washers or O-rings at the foot nuts form a sealed chamber. When the stanchion tubes are inserted, at the top, air is trapped in the lower tubes. When the fork compresses and the stanchion tubes travel into the lower legs, the stanchions take up volume, and the air in the lower legs is compressed. Yes, this results in a slight ramp up of pressure in the lower legs. This is normal and inevitable. It does not pose an impediment to performance because at the top of the travel you are starting at 0 PSI and the amount of pressure built up at the approach of bottoming out is just a slight cushion. Depending upon your riding weight and style, the effect will vary, but that is why you adjust the air spring pressure for your particular situation rather than just going by a shock chart.

You can absolutely compensate for this slight air cushion by adjusting your air spring pressure and damping accordingly.

The video that was posted showing the zip tie "procedure" is a horrible example of how to "solve" a problem that does not exist. Worse, it's a good way to create a problem. Notice that the air being "purged" is at the bottom of the stroke. They are venting out the air pressure as the stanchions compress the air. This means when the air spring is charged and the fork extends, the lower legs will be in a vacuum. All this does is provide the temporary effect of having a negative air spring. Sure, it feels plush for a bit, but it needs to be repeated later. Why? Because the vacuum gets filled. In other words, the air gets sucked back into the lowers. Depending on where you are riding, guess what else gets sucked into the lowers? Moisture, dust, or whatever.

I can't see the point in running a zip tie past the seals on regular basis to get some temporary gimmicky effect while risking damaging the seals and contaminating the oil. Simply accept that fact that a slight ramp up occurs inside the lower legs and adjust the air spring and damping accordingly. Then, ride your bike and stop wasting time performing unnecessary "procedures."

Again, if significant air pressure is building in the lowers, that would be different and would indicate a loss of air from a pressurized part of the fork to the lower leg. This calls for a repair of the problem, not a quick zip tie purge.
Chris I'm not sure you are understanding the issue. I agree that purging the air at the bottom of the stroke does seem like a bad idea. For whatever reason, air pressure does in fact build up in fork legs, such that even at full extension, there is significant pressure in the fork leg. The anecdotal evidence in this thread suggests it happens on a range of models, and of course, many DH forks have a provision to bleed air out of the fork lowers. You could probably account for why this happens better than I could, but it definitely is an issue than can amount to a dramatic alteration of the spring rate of the fork.
 
#63 ·
I wonder if this is just normal pressure and temperature variations. Do a DH run, entire fork heats up, stick zip-tie in there, sure enough it's going to PSSST a little. When it cools down, it will be slightly negative to the outside ambient pressure, so at some point it will suck in a little more air to equalize, but eventually you'll heat it up a gain and the process will repeat itself. Then think about cold/hot cycles and pressure variations due to altitude and other factors, I have to wonder if this is really varying all that much or just a normal thing that can't be controlled. I guess it would be optimal to equalize it for every condition, at the beginning of ever ride/run, but I'm not sure mine traps the pressure like this, so I wonder also if it's just limited to some forks that seem to seal better than others?
 
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#65 · (Edited)
Re-thinking this, the air space in the lowers goes up at altitude unless it gets released past the upper stanchion seal. Maybe the Fox and Enduro seals are better at releasing that pressure build as you state?

Have FUN!

G MAN
 
#81 ·
I have a defective PIKE. Nothing is wrong with it. I waiting for it to go break, I even proactively zip tie it just to see what happens. Nothing, zilch, nada (knocking on anything made of wood right now). I'm about to do a routine service on it. Crap, given my luck I may just let it go. Surely, as soon as I take it apart problems will begin.
 
#98 · (Edited)
You think??? On what basis? The anodized stanchions are MUCH harder than any nylon zip tie and cannot be scratched as such by a zip tie. The seals are urethane - again, good luck damaging a urethane seal - the most resilient flexible plastic on the planet. You'll do more damage by the hammering out those lower bolts as the friction will cause metal loss/wear at the press fit of the lower shafts into the fork lowers, not-to-mention damaging the plastic crush washers on the lower bolts.

To each their own...

G MAN
 
#104 ·
I just recently drilled and tapped my Pikes lowers to install a pressure release valve used in the Moto industry and the Fox 40s. Went well. I haven't put it all back together yet though. I'm waiting on a custom tuned Avy damper to put it all together. My stock damper cavitated a ton of air regardless of the bladder system.

I'll post pics when I get a chance.
 
#2 ·
I had the same experience but with a Talas 34. I removed the lowers because they were riding harsh, and pressure was released when I knocked loose the shaft. Like you, I had to increase the pressure and they are working much better.
Thanks for the ziptie-under-the-wiper suggestion, I didn't think of that as an alternative to unbolting the shafts.
 
#5 ·
Basically, at the seals where the stanchions enter the lowers, there is a dust wiper and then underneath that, the actual seal. You need to insert something in between to temporarily break the seal and release the air. The thinner the better, and nonmarring so you don't scratch the stanchion. The thin end of a small cable tie works great. Just stick in in between the dust wiper and stanchion and keep inserting till you hear a hiss. Then you are done.
 
#20 ·
If you opened the damper while you still had pressure built up in the legs, it likely wasn't overfilled. That was most likely built up lower leg pressure acting on the bladder. There is no static force on that bladder except for ambient air pressure within the leg.

The only way, that I know of, to tell if it's overfilled is by installing the damper into the upper leg (lowers off), and cycle all the way to see if the bladder hits the ID of the stanchion and restricts any more displacement. It's not like a MX cartridge where you have a big spring instead of a bladder and can feel for top-out force if it's overfilled.


This air burping trick works for any fork as well, not just the Pike. Fox, X-Fusion, Zoke, etc. Film negatives work great too.

With how quickly this happens, and how customer complaints about forks over the last decade or so have almost all related to compliance and arm pump, I find it astonishing that quick bleeders are NOT standard features on all high-end forks now.
 
#22 ·
If you opened the damper while you still had pressure built up in the legs, it likely wasn't overfilled. That was most likely built up lower leg pressure acting on the bladder. There is no static force on that bladder except for ambient air pressure within the leg.

The only way, that I know of, to tell if it's overfilled is by installing the damper into the upper leg (lowers off), and cycle all the way to see if the bladder hits the ID of the stanchion and restricts any more displacement. It's not like a MX cartridge where you have a big spring instead of a bladder and can feel for top-out force if it's overfilled.

This air burping trick works for any fork as well, not just the Pike. Fox, X-Fusion, Zoke, etc. Film negatives work great too.

With how quickly this happens, and how customer complaints about forks over the last decade or so have almost all related to compliance and arm pump, I find it astonishing that quick bleeders are NOT standard features on all high-end forks now.
Definitely. It's something I'll do every ride from here on out.
 
#23 ·
I've been playing with ride characteristics when you equalize when the fork is topped out vs. bottomed out and I noticed:

When equalized with ambient pressure topped out, I still can't bottom out the fork at 25% sag.

When equalized with ambient bottomed out, I can get full travel, but sag sits at 5% when the fork is unweighted. So, I think it creates a vacuum when you equalize bottomed out and then put air in the positive chamber.

Maybe compromise by equalizing at 50%?
 
#29 ·
I've been playing with ride characteristics when you equalize when the fork is topped out vs. bottomed out and I noticed:

When equalized with ambient pressure topped out, I still can't bottom out the fork at 25% sag.

When equalized with ambient bottomed out, I can get full travel, but sag sits at 5% when the fork is unweighted. So, I think it creates a vacuum when you equalize bottomed out and then put air in the positive chamber.

Maybe compromise by equalizing at 50%?
I'd stick to equalizing when topped out, if at all. If you want to use more of your travel @ 25% sag, remove one or two tokens, depending on how much bottom out reserve you want.

A setup method that takes the least amount of work and gives the same result over and over would be best. I just do not see why this equalising should be done with the fork compressed: it's designed to have a certain amount of air in the lowers. Adjusting sag with a guessed amount of vaccuum? No way for me ;). If you equalize the pressure in the lowers with the outside air, all the normal adjustments of the fork are sufficient to get the characteristics you want.
 
#25 ·
BTW:

I know understand why X-Fusion puts "Neutra-Valves" on their HLR forks:

X-Fusion RV1 HLR DH fork review - BikeRadar

We used the pressure releasing 'Neutra Valves' (two small valves on the lower legs to remove air pressure built up inside) while in Spain. When air pressure did build when at altitude, the Neutra Valves proved to work effectively.
 
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