Need help: What shims for a Fox 36 FIT cart?

[300hp]

Ok.

I have a 2009 van 36 rc2. I'm sick of having to run a spring that is really stiff to keep the fork from bottoming. I am going to put better oil in the damper, and I want to modify the compression shimstack by doubling up the face shim and the shim below it.

According to my diagram, stock a 7mm id shim. I don't have these, and I'm having a hard time finding them. I just need two shims.

8mm id shims are really common in moto, and the suspension guy I'm going to work on it with has 8mm id in the width and thickness I want.

Is there any harm in running a shim that is slightly too big id, or do I have a margin of error?

Likewise, is it possible to take, say, a 6mm id shim and open it up a mm to fit?

Ideas please!

thanks in advance,
Hunter

[Udi]

Your suspension guy (or another) may have a shim punch that can be used to properly size a 6mm ID shim to a 7mm ID shim. I would not use a shim without the correct ID.

The stock damping oil (Silkolene Pro RSF 10wt / Fox Red 10wt) is very good and I would suggest sticking with it, it is also a fairly high viscosity for a 10wt fluid (47cSt) so there's every chance that replacing it with another 10wt fluid will result in less damping - even though the difference probably won't be that great compared to re-shimming.

[300hp]

Thanks, Udi!

When you went to a 6 shim stack instead of the stock four shim stack, did you put a spacer on the HS preload spring to take some of the "play" out of it?

I'm getting pretty tired of this fork. Going to break er open, put a new glide ring on the piston, and add a couple shims.

Are there any common problem or failure points that I should look for when I'm in there that could be contributing to my lack of dampening? The cart has a good bleed on it, got a new bladder in june, and does not have any dead travel or audible squishing going on.

I've gathered that the piston shaft can work its way loose, and the piston could be concave, allowing fluid to flow past the shims. Any other common causes I should look for?

Finally, this is the setup I was going to try
16.6 x 0.10
16.6 x 0.10
14.5 x 0.10
14.5 x 0.10
13.5 x 0.10
9.6 x 0.25

Do you still feel that is a good option for more support all around, or have you settled on a better combo?

I'm getting 20% sag, and if I jump off a curb the fork bottoms out. Hydro bottoming is set to full stiff.

Thanks! I never knew a shim punch was a thing. See? That's why I ask the experts.

[kidwoo]

send a pm to johnkranked

he's a guru with reshimming things. He'll usually hook you up for free.

[Udi]

When you went to a 6 shim stack instead of the stock four shim stack, did you put a spacer on the HS preload spring to take some of the "play" out of it?

No, but I generally run the HS preload adjuster closed or very close to it. I think the config you are quoting of mine might have been from quite some time ago on an ~09 40 damper, the '11+ inverted dampers have more compression damping in stock guise - I'm not sure if it is due to a slightly different damper design or stiffer mid-valve, as the actual shim configuration is the same.

How much do you weigh geared up and what spring are you running? If you are jumping off a curb and bottoming out I'd say you need to go up a spring rate, but then you seem to suggest you've already done that.

Your proposed shim config sounds good, I presume you are currently running the stock valving? Try out what you planned and see how it goes, it's definitely a good starting point. The bleed and bladder are important factors on those old dampers, but it sounds like you have all that sorted out.

[300hp]

hahaha it might sound like I have it sorted, but I know it'll be a ****show when I try all this again. I weigh 180 geared up. I've run the blue (medium) and green (firm) spring in the fork. sag is where it should be with the blue spring, and about 10% with the firm spring. Even with the firm spring, it rides all chattery and deflects on roots and such. Which leads me to believe that the softer spring, with some semblance of dampening will be right on. Currently, with the damper out of the fork, rebound half way out, and ls and hs full in, I can easily compress the damper shaft with my hand. There is a *slight* increase in compression from full open, but its really not much. In contrast, when messing with the rc3 damper in my old 55rc3, halfway out, it was quite hard to compress by hand, and nearly locked out when the compression was cranked all the way, which is why I think that either this fork has realllly **** compression, or for some reason the oil is bypassing the shims because of a bad glide ring or warped sealing surface.

Am I covering the right bases, or are there other factors I should be considering?

Thanks man!

Thanks for the advice, Woo, but I could never take someone as biased as you are seriously

[Udi]

Which leads me to believe that the softer spring, with some semblance of dampening will be right on.

Sounds about right. When you find a way to obtain the 7mm ID shims, I'd get a few extras, especially of the 16.6 / 14.5 - maybe even in 0.2mm. A single 0.2mm shim will provide more support than 2x 0.1mm. Also those OD numbers look like my caliper measurements I posted somewhere, I'm sure you can round up to 17 / 15 if they are easier to find.

I'd start with the stack config you suggested, but if you keep some extras handy you can play around. The cycling by hand is probably a reasonable indication if you're going to have a useful amount of damping or not.

I did find most pre-2011 dampers to be quite softly valved in compression, so while it is possible that a glide ring or warped surface is causing bypass, I'd say the damper valving is just too soft. If you have access to appropriate tools, a small preload spacer for the HS spring (while still allowing its seating mechanisms to hold it) could help too.

[kidwoo]

Thanks for the advice, Woo, but I could never take someone as biased as you are seriously

Shims killed my father.

[300hp]

Thanks for the help and good words. I'll report back after doing battle with this thing.

H

[wiscodh]

Shims killed my father.

are you Inigo Montoya?

[kidwoo]

are you Inigo Montoya?

Prepare to be damped.

[300hp]

Shimingo Montoya.

[300hp]

update. Tore her down a couple times. Currently running a 16.7x1.5 face shim. Noticeable increase in compression dampening.

I tore the fork down, installed the new shim on the outside of the stack, refilled and bled the cart, then put everything back together. Even with the new shim and proper bleed, there was virtually no increase in compression dampening. We knew something else had to be wrong, and took the thing apart again.

Upon close inspection, my piston was warped and the shims were not sitting flush on it. I am the second owner of the fork, and when I got it, the mechanical bottom out was at its softest setting. I don't know if this played a role in the damage. Either way, the piston was warped so that the shims would not seal on the piston face, and the fork was in free bleed mode no matter what.

We leveled the piston, and rebuilt with a proper, air free bleed using redline 10wt fluid and the 16x1.5 shim in the stack.

The fork now has a useable range of dampening, and full-in on both adjusters yields a very stiff nearly locked out feel. Currently running the fork with the HS nearly full in, and the LS half way in. at 25% sag, the fork is now much more resistant to bottom out and remains up in its travel during repeated stutter bumps. While the sag is to my liking, a two foot drop to flat ish still fully bottoms the fork. I will probably go up some in spring rate if I have to, but I would prefer to find a way to make the end 10% of stroke more progressive.

if your rc2 fit cart feels like it has no dampening range, and the cart is properly bled with no bladder issues, I strongly suggest looking at the piston. The aluminum is verrrrry soft, and if the stock shims do not sit flat on it, you won't get an effective compression range. Using a magic marker, a flat surface, and varying grades of sandpaper, it is quick and easy to return the piston to flat, enabling the shims to do their job. I'm sure there are really accurate and proper ways to do this, but my home mechanic method has left me with a working fork, and no additional cost.

I am going to ride the setup as is and see how much I bottom out on the trail. Next I will try increasing spring rate if necessary. if that results in ****ty jarring ride quality, I'll add another 13mm shim in the stack to hopefully make the fork more progressive in the last bit of its travel with the softer spring.

Udi, and other smartpension people: Will increasing the amount of bath oil in the lowers from 30ml to 40-45ml safely increase bottom out resistance? I know the fork is not a sealed open bath, like marzocchi, but it seems like this might work to give a little ramp-up at the end of the stroke.

Also, have you guys run two bottom out bumpers, or a thicker bottom out bumper to give a little margin at the end of the stroke? I don't know what caused the initial crunch in my fork, but I would certainly like to avoid it happening again.

Hope this helps some other people with fit carts. Given how little compression dampening teh fork has with the adjusters full open, I would not hesitate to add another .1mm shim farther in the stack. Even in the current configuration, I still have the hs adjuster mostly all the way in to preload the shimstack.

[Udi]

That's pretty interesting, wouldn't have suspected the piston issue. It's been a while since I've pulled apart those dampers, but I can't imagine how it could be damaged in use - sounds more like a machining issue to me. Was it concave or convex?

I think an additional 15ml per lower leg would be safe to try, worst case scenario if you go too far will simply be the seals popping out or leaking, neither of which are immediately dangerous - just messy.

Bottom out bumpers should also be fine to mess with as long as they don't interfere with parts other than the stanchion - probably a bigger concern on the damper side than the spring side.

[Udi]

Something else I was thinking, if the Redline '10wt' you used is 'Medium' (red fluid), it's 30cSt instead of the stock fluid at 47cSt.

Wouldn't make much difference to the rebound/compression dampers as they are both shimmed / variable aperture - however, the hydraulic bottom out mechanism is entirely ported and relies on forcing fluid through progressively decreasing port sizes which are all very small. In this scenario I think it may be worth trying either the stock fluid, or the next weight up from Redline.

This is assuming you've already set the HBO at its hardest setting, of course that's the first step if you haven't.