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Question for Boxxer Guru's - Udi, Sprocket, etc.

KevinB

Chimp
Jan 2, 2008
6
0
Well I tried the nyloc nut mod that Udi and others described in the "blackbox speedstack tuning" thread last year - seems to work great and it was pretty easy (I have an '07 Boxxer Race). Thanks guys!

My question is about a "clicking" noise that I now have in my forks. If I have the LSC cranked up anywhere past about halfway, and push hard on the forks, I get a sharp click at the very beginning of the rebound stroke. It wasn't there before and it is kinda annoying. I "think" I have tracked it down to the little silver rotating piece at the bottom of the MC cartridge that covers up the blowoff ports - if the LSC port is mostly closed, at the start of the rebound stroke oil pressure pushes on this piece, and the little spring that holds this piece closed is compressed very quickly, and the little piece clicks/snaps against itself. I think that when the regular LSC orifice is open enough to let oil freely flow back and forth through the LSC port, oil pressure isn't high enough to compress the little spring, and the little silver piece stays put. Sorry, this is a really crummy explanation, but I was wondering if this is normal - but it didn't happen before I took everything apart. Kinda wondering if maybe something small fell out and I didn't notice. :shocked: The other problem is that I changed all of the oils etc., at the same time (first time I had it open since I bought it) so I don't know if it is somehow due to using a different oil from RS, but I used a good synthetic fork oil and stuck stick with the spec'd. oil levels and weights.

Anyway here is a picture of the piece I'm talking about:
First is the bottom of the MCU with the little silver plate closed; second is the same thing with it open - when I snap it open with my fingers it clicks loudly. I think this is what I can hear (and feel) when the fork is together. Anyone have any ideas on how to get rid of this clicking, or if I have done something wrong? Thanks.
 

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toodles

Turbo Monkey
Aug 24, 2004
2,697
881
Australia
Check that the spring on the underside isn't doubled over on itself - that might be causing the noise.
 

davep

Turbo Monkey
Jan 7, 2005
3,279
0
seattle
The only other thing that I can think of is some sort of vacuum being created when the port opens....if both surfaces are extremly smooth they might 'stick' together (think 2 glass plates with a bit of water in between). This was reported as the cause of some of the early DHX's making noise.
 

Udi

RM Chief Ornithologist: “I Brake for Birds”
Mar 14, 2005
4,738
843
Basically, the oil that gets pushed into the motion control assembly on the compression stroke has to exit past the one way valve (spring loaded plate) on the rebound stroke, and as soon as the oil has drained the valve will snap shut again. You probably knew that.

Now when you wind on more compression damping, the size of the open port (forget about the one way valve for now) is smaller, which means on the rebound stroke there's also less of an orifice for the oil to return through. That means that the one way valve will probably open more when the compression damping is wound up a bit and therefore close harder too.

That'd explain the noise being more noticeable when the comp damping is wound up. But yeah, at least on my fork it's barely noticeable... I can hear it if I really listen for it, that's it.

I've got a custom spring setup for the return valve though, ditching the stock coil and using a dust-wiper spring (which might have made it quieter or something). That only came about because the stock coil on mine seemed to keep tangling itself up and stopping the one-way valve from closing properly though. I'll post some pics of the setup sometime, gotta find em.. or will snap new ones next time the fork is open.
 

KevinB

Chimp
Jan 2, 2008
6
0
Thanks for the replies. I'll check the little spring again, but I think it appears to be functioning fine. I think I agree with you Udi, the sound is from the one way valve opening very quickly as the rebound stroke starts. I pretty much suspected this but since I'm kinda a suspension moron I'm glad to get a second opinion. At least I can be pretty sure nothing is broken, or put back together wrong. I guess mine is just really noisy. I'll try increasing the rebound damping a bit and see if that helps, and I was thinking about trying 2.5 wt. oil in the MC instead of the stock 5 wt. The sound definitely became noticeable after I changed the oil, so maybe playing with the oil weights might help.

cheers
 

Udi

RM Chief Ornithologist: “I Brake for Birds”
Mar 14, 2005
4,738
843
I'd probably advise against both of those ideas. Increasing rebound is likely to be detrimental to performance as well as unnecessary if you're running ample compression damping (the boxxer really works best with minimal rebound damping IMO); and in my experience, lighter oil tends to equal more damper noise, which is no fun if you're one of those people bugged by noise.

If anything heavier oil will do a better job at dampening any noises that the return valve makes... but more importantly I've found it helpful in getting the right amount of compression damping. I remember saying this lately in a recent thread, but basically there's only one "right" position for the compression adjuster on the MC boxxer and that's 2 complete clicks back from closed (if there are partial clicks there, the adjuster needs to be adjusted/corrected which is easy enough, just ask).

3 clicks back leaves an amount of damping that's only slightly better than negligible (not enough really), and 1 click back or less results in lockout. The trick is to stay at 2 clicks back and use different oil weights to get the amount of damping spot on for your weight and taste. I'm running and recommend Silkolene Pro RSF 5wt (~26cst), but preference might sway that down to 2.5wt or up to 7.5wt.

In short though, forget the noise and adjust for the right performance is my advice.
 

KevinB

Chimp
Jan 2, 2008
6
0
Thanks Udi, I'll look up that other thread (I think iremember seeing it). I have a partial click so I'll take it apart and adjust that.

I do like the boxxers a lot. I guess I'll just have to get used to the clicking!