[WojtekHHH]

Hello, my first post here.
I've bought a used fox dhx shock. Tried it and there was a significant knock when compressing shock. It was noticeable when I compressed shock outside the frame, I mean by hand, so there is no way that it is caused by bushings or mounting hardware or spring. I changed every oring, seal and the blue plastic ring on shaft. Disassembled shim stacks - found out they were wrong so made it right. Everything was tightly assembled and the shock was carefully bled. There is no other sound than the knock or pop. It annoys me really badly. I' searched most of the internet and reached out to local bikeshops. Nothing. Maybe you guys have any solution to this problem? Maybe one of you know a possible cause? Please, it is driving me crazy for 2 months now haha.
Cheers from Poland!

 

[Kanye West]

When is the knock occurring exactly? When motion starts? When motion stops? Same two questions, but in rebound or compression? Or both?

Unless you have some slop in the bushing or piston band, the culprit on those shocks is usually a distorted face shim, or a sub-optimal sealing face for the face shim on the piston. There are 4 spots for this in most shocks (main piston C and R, bridge piston C and usually a check valve in R).

Depending on the generation of shock, that might have the boost valve in the reservoir which is another port control mechanism that's worth looking at too.

 

[StiHacka]

Knock, knock!

Who's there?

Fox DHX RC4.

Spoiler

SHOCKING!

 

[WojtekHHH]

Well, the knock starts when a motion is applied in both situations (rapid compression- knock, further compression, stop, return-knock, further return). Actually there is some play in the bushing and shaft. To be honest if that's the cause, I don't know what to do about it because it is not replacable. Thank you for your reply.

 

[Kanye West]

Well, the knock starts when a motion is applied in both situations (rapid compression- knock, further compression, stop, return-knock, further return). Actually there is some play in the bushing and shaft. To be honest if that's the cause, I don't know what to do about it because it is not replacable. Thank you for your reply.

Depends on the shock. Some of them with 1/2" shafts use a normal eyelet DU bushing. RC4's had a couple different diameters.

If it happens on both directions, the other thing it can be is a worn glide ring. They "float" a little bit in the groove of the piston OD, so if the soft polymer wears, the piston band can shift back and forth on load reversals. Defects or inconsistent wear in the ring can cause one side to "catch" more and create a heel/toe situation in that groove which could cause some noise. Just one more possibility.

Next thing which may (or may not) be telling is to see if there's any variability by moving the adjusters to full closed or full open. Example: If the adjuster is fully opened and the noise goes away, it's likely the shimstack that's associated with that adjuster isn't opening and slapping shut because there's enough flow through the adjuster/needle. Just another way to isolate. If the adjuster is fully closed and the noise goes away, well....then maybe the needle is shifting around somehow and has a worn o-ring, and closing it off shoulders the needle and constrains it. Haven't seen that one yet though.

 

[WojtekHHH]

Depends on the shock. Some of them with 1/2" shafts use a normal eyelet DU bushing. RC4's had a couple different diameters.

If it happens on both directions, the other thing it can be is a worn glide ring. They "float" a little bit in the groove of the piston OD, so if the soft polymer wears, the piston band can shift back and forth on load reversals. Defects or inconsistent wear in the ring can cause one side to "catch" more and create a heel/toe situation in that groove which could cause some noise. Just one more possibility.

Next thing which may (or may not) be telling is to see if there's any variability by moving the adjusters to full closed or full open. Example: If the adjuster is fully opened and the noise goes away, it's likely the shimstack that's associated with that adjuster isn't opening and slapping shut because there's enough flow through the adjuster/needle. Just another way to isolate. If the adjuster is fully closed and the noise goes away, well....then maybe the needle is shifting around somehow and has a worn o-ring, and closing it off shoulders the needle and constrains it. Haven't seen that one yet though.

Wow, that's provably the best explanation I have ever received. I will check all these elements you mentioned. Diameter of the shaft is 15.9mm so that's not a standard shaft. I remember the blue ring was a bit loose tho. Thank you very much, I appreciate your knowledge.

 

[WojtekHHH]

Update: the noise is present ONLY when damper is returning. Clearly I wasn't paying enough attention before.

 

[WojtekHHH]

You mentioned face shim

the culprit on those shocks is usually a distorted face shim, or a sub-optimal sealing face for the face shim

and sealing face for the face shim. I have really basic knowledge about valving etc. but couldn't find these names on fox site either. Could you explain or point which shims are you talking about?
Tomorrow I'm doing the last ever service, so wanted to also check that out.
Thanks

 

[troy]

Sealing shim, as a shim right next to the piston.

Those 2 are sealing/face shims:

 

[WojtekHHH]

Thank you, Sir. Gotta check them. I will post if I manage to solve the problem.

 

[Kanye West]

Update: the noise is present ONLY when damper is returning. Clearly I wasn't paying enough attention before.

First place I would look would be the face shim and piston surface of the compression adjuster. Also sounds like you have a boost valve shock based on the shaft diameter, so one more mechanism to inspect.

 

[buckoW]

Do we know if it’s bled properly?

 

[WojtekHHH]

First place I would look would be the face shim and piston surface of the compression adjuster. Also sounds like you have a boost valve shock based on the shaft diameter, so one more mechanism to inspect.

That's the main suspect tbh. I don' have tools to unscrew piggyback to take a look at the boost valve. If that's the reason for the knock, I can't do much about it right now.

Do we know if it’s bled properly?

Yes I'm sure it is. Spent around 30minutes doing the bleeding process. No "sobbing" sound, no dead stroke. I think that can't be the case. Thank you doe

 

[WojtekHHH]

Okay, writing that after full check-up. The result is a completely silent shock. What I noticed there was some gunk marks on shims, so sanded them down using 2000 sandpaper. Then i noticed this fat oring in sealhead was moving up and down creating some kind of noise. There wasn't a lot of grease around it - filled the grove with suspension grease. There was also slight play on blue glyde ring That @Kanye West mentioned. It was changed to brand new 2 weeks ago but I still thought this might produce some sound. Imo the biggest difference was made by properly cleaning and fitting tightly shims on the piston. Can't tell you how it performs on a long run - still waiting on mounting spacers for my frame.
Thanks to all of you for pointing out plausible causes. I hope this thread will help some inexperienced riders like me.