[Ithnu]

Has anyone ever blown the rod on their Boxxer motion control?

I believe I was running too heavy of an oil and it caused the blue rod to bend and it seized the fork, causing a pretty bad crash.

I cut the outside of the motion control cylinder off since I need a new one anyone and its easier to see what happened.

I put in 12.5 wt because I got it rebuilt at a race and 6wt was put in and it was toping out. I figured it was too light of an oil, but now I think there just wasn't enough in there and the 12.5 wt was too thick.

 

[foxracing]

Holy smokes!
Never did such thing before.
Hope you didn't get hurt in the crash, man.

 

[Ithnu]

Any help out there?

 

[davep]

Any help out there?

What kind of help do you want? You need a new MoCo assembly.
I am not sure that a thicker oil wouId cause this sort of damage. That is the low speed portion of the damping that is bent, and it should blow off, unless somehow it got stuck shut, or you had it completely closed (it should still blow off). I would check to make sure that the rebound rod did not make contact with the end of the MoCo. I am not sure of the spacing in there at full bottom (I am sure it is close), but it could possible happen if your stanchions are not even in your crowns

FYI 'weights' of fork oils mean nothing. There are '5 weight' oils that are more viscous than '10 weight' oils..but I have to wonder why you would want more damping on that fork? At 190lbs, I have rebound at full fast, the high speed compression full fast, and the platform set at maybe 1/3. Any more low speed and the fork is way way to harsh. I know of several people who have removed the secondary high speed (black box) stack to lessen high speed damping with good results.

The orig damping oil in that fork is Torko 7.5 wt RFF..I would suggest you find some at a moto shop and replace whatever is in there.

edit: top out would not be caused by low oil unless there was litteraly no oil in that fork. The rebound piston is at the bottom of the damper cartrage (connected to the rod that is bolted to the fork lowers), thus it is always in oil. Too little oil would cause a loss of compression damping in the first part of the stroke (there is also a top out spring in both legs to prevent harsh top out). Either you just needed to adjust the rebound, have your legs un-level so that one is bottoming/toppong before the other, or there is something wrong with the rebound damper head (might have come unscrewed from the shaft)..I would certainly check the rebound assembly for problems/dammage.

 

[Ithnu]

I already ordered a new assembly and I use motocross oil in it. As for rebound, I'm trying different settings to see what I like.

It sounds like from your description, and my recollection of the crash, that it had to have been the bottom rod hitting the bottom of the MC....

I just ran to my garage, and sure enough, there is a faint mark on the metal piece at the bottom of the MC that is the same diameter as the top of the damping rod.

So you're right, it couldn't have been the oil, thanks.

You mentioned the topping out might have been a rebound rod problem, that is the same rod below the MC correct? This is starting to look like an answer.

The only thing I can't figure out is how after I changed the oil it wasn't topping out anymore, but the problem must have still been there since it broke anyway.

And how do I make sure its put back together correctly this time, since the other shop guy obviously screwed up.

 

[davep]

Without seeing the fork, it is hard to know where to start..go to sram.com and find all the tech documents on the boxxer, it shoud help you with how things go together. My best guess is that the rebound piston head has come partially unscrewed (I could be totaly wrong) from the shaft. To get to it, pull the stanchion out of the lower, then there is a circlip/snap ring up inside the stanchion bottom that needs to be removed (make sure you put it back in the same way (sharp edge down i think). Then the entire rebound assembly will slide out of the bottom of stanchion. Check the tech documentsorque, but the piston should be tight on the shaft (most likely a little loc-tite).

As i mentioned before, make sure the stanchions are at the same height in the crowns. If the damper side is lower, it will be forced shorter at bottom than the spring side (the damper side will bottom, the spring side will not).

I understand that you are using MX fork oil, but there is NO standard for oil 'weights'. Manufacturers do what ever they want, and call it whatever they want, so one company's 10 wt can be thicker that another mfg's 15 wt. Look for cSt, centistokes as a true measurement of viscosity....check here: http://www.peterverdonedesigns.com/lowspeed.htm for some great oil info. His mountainbike suspension page has some great info about what is in (and relabeled and sold for more $$) current mtb forks as far as oils.

 

[Ithnu]

That's for all the help, I'm going to take the rebound apart this evening and I'll see what's going on in there. Based on what you have been saying and I've been looking at, I think you're on the right track.