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Propedal Clunk.

3K views 19 replies 7 participants last post by  Dougal 
#1 ·
Has anyone found a good solution to the perceptible clunk from some propedal shocks? It appears to be freeplay in the valve which let it slam shut at the start of the stroke rather than close by itself before the stroke begins.
 
#2 ·
Dougal, this may sound silly, but have you ensured there is enough gas pressure? Should be 300 PSI.

The other thing too is that some of the dampers are valved to have very firm propedal settings while others are not.

When I bought the DHX5.0 air for our Ventana tandem, I purchased an older damper model that used the round knob clicker vs the new lever style. I wanted to be able to tune the compression damping, not lock it out.

PK
 
#3 ·
Don't know if you are referring to a RP-23 or DHX but...
I've re-valved my RP-23 a few times. It came with a medium compression tune and I lighted it up for my fly weight, but I've never noticed any "clunk" before or after modifications. The spherical propedal valve is a spring loaded valve that is always "closed" until the compression stroke opens it up and lets the oil flow. I can't think of anything that would cause a "clunk" unless there was something wrong with the shock. I haven't ridden that many other RP-23's but never noticed a clunk. Fox valving has been all over the map from 05' to present though. I loved my 06' DHX and hated the 08 DHX (The one with the lever instead of the dial). Any clunking I've had in the past was related to loss of pressure in the IFP chamber. Everything goes to hell real quick when that happens.
 
#4 ·
I've come across it twice now. Both on just R dampers. One a coil, the other a float.
On the coil it was the propedal valve, it could be felt even compressing it by hand. I ended up swapping a non-propedal bolt into it to solve the problem.

I haven't taken the float apart yet, compressing it by hand (air can removed) suggests it's got most of it's gas pressure if not all. I'll give it an oil change and recharge later on. Besides force on the shaft I can't think of an easy way to measure the gas charge that's still there.
 
#5 ·
Probably way off, but maybe something with the DU-bushing for the shock? Had a similar issue with a friends fox float and the clunk was triggered by the initial force to open the pro-pedal damper, it was causing the bushing to move and clunk first before beginning the stroke.
 
#6 ·
electrik said:
Probably way off, but maybe something with the DU-bushing for the shock? Had a similar issue with a friends fox float and the clunk was triggered by the initial force to open the pro-pedal damper, it was causing the bushing to move and clunk first before beginning the stroke.
Always worth checking, but I can guarantee no play in this current one.
A vanilla R is going back on for the new years day riding.:cool:
 
#7 ·
I remember some old threads from back 2005 (maybe?) regarding this clunk.

There was no solution but send it back to Fox and I think Push plain removed this valve. Maybe someone found any solution and want to share it now that is water under the bridge?
 
#8 ·
could be the treaded hysteresis!

I don't know where I came across this , but I saw a video of Darren at Push describe this effect that all shocks have.
When the piston changes direction and the compression circuits and the rebound circuits "switch" on and off, you can get a momentary "lull" or "delay" in the in the shock dampning.
I have this on a DHX 5.0 with Propedal, that came on a used frame I bought. It can feel like a loose bushing or loose headset. It drives me crazy!
It might become more noticable as the shock nears rebuilding time.;)
 
#10 · (Edited)
Dougal, what is the bike? As I mentioned, some of the ones I have fixed had really deliberate initial compression settings making the PP very effective. These were on Rush's.

Basically the setting is too digressive enough, or not linear enough in how it was shimmed.

Feels great with no pedal bob, but not to nice over small hack.

If the rod pressure feels good, meaning the IFP seems to have decent pressure, it is very possible the valving or basically the fluid has entrapped air. That too will make those shims knock, just the same as no N2.

Stroke the damper with the bleeder port at 12:00, let it set for 5 minutes, then crack the bleeder to dump any trapped air. Be quick or get wet. Cracked, and within a 1/2 second retighten it.

Very common for a Fox damper to have air forced past the main seal into the fluid. One reason you need to run IFP pressure super high.

PK
 
#11 ·
The stock FLOAT and Van-R shocks use a spring loaded plastic poppet valve built into piston bolt. This valve acts as a one-way valve so that the Propedal effect can be created.

The knock or slight gap in damping whether audible, or felt while pushing on the seat, is from the tolerance between the bolt and the plastic poppet. Shocks with higher levels of Propedal from the factory tend to exaggerate this noise or feeling.

In your case, you've either come across a couple that are a bit out of spec, or have highly preloaded compression valving.

It generally has no effect on the ride quality, but it some cases we do exchange the bolt for a different one when it shows up exaggerated on our final dyno inspection.

Darren
 
#12 ·
Aha, thanks guys.

Paul, this is a second hand shock, originally off a GT I-drive (identified from the shock hardware it came with). I suspect it was sold because the frame turned up it's toes. Why else would you sell a shock with all the shock hardware (right down to bolts)?

It's been tested on an Intense Uzzi SLX, but you don't even have to ride it to feel the knock, fast push/pull on the seat will do it. I have already tried a basic bleed as you've suggested above, it helped but didn't cure.
Time to stop procrastinating on the net and pull it apart then.

Darren, would a shock for an I-drive count as high level of propedal?
 
#13 ·
Dougal,

Depends on model year and model really. There should be designations on the shock.

Anyway, I'm suspecting it's the bolt. If it were other things such as bleed and/or low gas pressure you'd have symptoms that would be more obvious.

I don't have a fix for you as we haven't really looked into one...rather just switch it out on our end to not tie up too much time on the repair/tune. Not too mention that it's not very common. It maybe just a quick scotchbrite debur of the plastic poppet, or possibly running the correct drill bit size into the bore of the poppet seat. Not sure, but you could try starting there.

I have this on a DHX 5.0 with Propedal, that came on a used frame I bought. It can feel like a loose bushing or loose headset. It drives me crazy!
Fox switched to a grooved replenish seat in the reservoir back a few years ago. Quite common in dampers to reduce the area that a shims seals on, therefore lessening the chance of it to not seal properly causing gaps or clunks. If yours is this older model then that is most likely the cause. You can generally check this by increasing and decreasing the propedal using the external dial. If it get s better the softer you set the dial than this is most likely the culprit. With our services on these older shocks it's procedure for the techs to hand the reservoirs over to our machine shop to have the groove installed.

Darren
 
#14 ·
PUSHIND said:
........Fox switched to a grooved replenish seat in the reservoir back a few years ago. Quite common in dampers to reduce the area that a shims seals on, therefore lessening the chance of it to not seal properly causing gaps or clunks. If yours is this older model then that is most likely the cause. You can generally check this by increasing and decreasing the propedal using the external dial. If it get s better the softer you set the dial than this is most likely the culprit. With our services on these older shocks it's procedure for the techs to hand the reservoirs over to our machine shop to have the groove installed.

Darren
Thanks for this info Darren. Guess I should send the shock in!:thumbsup:
 
#15 ·
PUSHIND said:
The stock FLOAT and Van-R shocks use a spring loaded plastic poppet valve built into piston bolt. This valve acts as a one-way valve so that the Propedal effect can be created.

The knock or slight gap in damping whether audible, or felt while pushing on the seat, is from the tolerance between the bolt and the plastic poppet. Shocks with higher levels of Propedal from the factory tend to exaggerate this noise or feeling.

In your case, you've either come across a couple that are a bit out of spec, or have highly preloaded compression valving.

It generally has no effect on the ride quality, but it some cases we do exchange the bolt for a different one when it shows up exaggerated on our final dyno inspection.

Darren
He may also take a look at how many of the small PP spring preload shims were installed. Often decreasing the preload will work well also. I can't say I have ever needed to replace the piston retaining bolt to solve this but a bolt shy on drilling depth may need replacement to optimize the OEM PP settings.

The preloaded on the compression stack tunes to some extent how digressive the valving works. Very possible they left out one shim, which is a very noticeable change when testing on the bike.

It's very possible that the blends in the damping curve are too abrupt.

If I remember correctly, the one I drive I pulled apart had broken compression shims from what I thought was a too much PP spring preload. Replaced the shims, and softened the PP setting and the guy thought he had a different bike.

PK
 
#16 ·
He may also take a look at how many of the small PP spring preload shims were installed. Often decreasing the preload will work well also. I can't say I have ever needed to replace the piston retaining bolt to solve this but a bolt shy on drilling depth may need replacement to optimize the OEM PP settings.
The shocks that he's referring to don't utilize PP spring preload shims so that couldn't be the issue. As I mentioned, the problem he's experiencing is pretty rare.

Darren
 
#17 ·
Bit of an update, I found the problem and it was instantly obvious.

I assumed from the old "unscrew the aircan and push the shaft" method than the gas charge was still good. But it wasn't.

This shock had somewhere along the line lost enough of it's gas charge that it sucked itself full of air. Enough that venting the IFP valve sent the piston right down the bottom of the shock and still left pressure in the body of it. The oil came out as bubbly as a freshly opened coke can, similar colour too. It's back together as it should be and with no perceptible clunk so the propedal valve is indeed fine. To keep it that way I may need to invest in some high pressure nitrogen gear, 300psi probably isn't enough to keep the seal head pressurised in hard use but might be okay on the wife's intense.

Sitting in the top of it was a small but fragile wire clip, I have no idea why it was there and I've never seen one in the parts diagrams before. If anyone is interested I'll throw up a photo.
 
#18 ·
Dougal
For years this has been an Achilles Heel of this style damper.

On paper they look great, but truth be told they are a maintenance pig to keep them working properly.

The concern is on account of the air spring driving air past the sealhead, thus emulsifying the fluid. Additionally, as the seal wears, fluid will weep past the seal into the air spring on account of the pressure differential.

These are a coil over design with an air spring.

A true Airshock, which in Foxes case goes back to the Dual Airshox of the 1970's, was adopted in the design of the ALPs series. The ALPs went a step further by installing a seperator or IFP when compared to a Dual AIrshox.

In the Alps, the gas pressure applied to the fluid is also the spring pressure. While not perfect at preventing this contamination of the fluid with air, I believe it was not a caused by blowby of the IFP, but rather the main seals inability, along with the shafts finish that air was drawn in under the damper compressing.

Granted the later styles of dampers such as the RP series have many features, this is nice, but they are a lightwieght performance product that require a fair amount of routine maintenance, not normally accomplished by the home mechanic to work properly.

For a rider that puts 50 miles a week on their bike should have the shock rebuilt every 6 months. Less riding should be at least once per year.

Getting back to the N2 pressure, 300 PSI helps to make the damper work well, but also adds harshness to the initial bump compliance and increase the end stroke ramp up.

Most of the riders I have done rebuilds for are offered a means to tune the IFP pressure for best ride quality, but are also informed about proper service intervals. Big guys require more pressure, lighter riders less.

If Fox had plans to design a lesser performing damper, with greater longevity, the sealhead needs a redesign. The single "O" ring, while not a typical "O"ring, does work well, but could use a second seal to help.

PK
 
#19 ·
Dougal said:
Bit of an update, I found the problem and it was instantly obvious.

I assumed from the old "unscrew the aircan and push the shaft" method than the gas charge was still good. But it wasn't.

This shock had somewhere along the line lost enough of it's gas charge that it sucked itself full of air. Enough that venting the IFP valve sent the piston right down the bottom of the shock and still left pressure in the body of it. The oil came out as bubbly as a freshly opened coke can, similar colour too. It's back together as it should be and with no perceptible clunk so the propedal valve is indeed fine. To keep it that way I may need to invest in some high pressure nitrogen gear, 300psi probably isn't enough to keep the seal head pressurised in hard use but might be okay on the wife's intense.

Sitting in the top of it was a small but fragile wire clip, I have no idea why it was there and I've never seen one in the parts diagrams before. If anyone is interested I'll throw up a photo.
Wow, that's weird. Glad you got it sorted out. I've been running my IFP at ~225 psi of air (not N2) for several months and it's been fine so far at my 140 lb weight. The RP23 has been my first air shock in the last 3 years (previously coils), and I'm actually quite impressed how good it feels when tuned right.
 
#20 ·
Here is a picture of the offending wire clip. I have no idea where it's from or what it was for, it's slightly smaller than the diameter of the bore and was found between the piston and seal-head.

The other two pictures are from another shock (vanilla R) which had "rebound issues". It originally had the same symptoms as the Float R, low charge pressure and sucked itself full of air, but there was more to be seen inside.

Anyone got a source for fox shims yet or do I need to cut my own?

Paul, I agree completely with the air intrusion problems, I recall somewhere a rumour of fox increasing charge pressure to 500psi to stop this. Back in the good old days RS sent me a warranty shock with no charge pressure at all, I had to ride it until the next one arrived. That one sucked itself completely full of air in just one ride.
 

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