[Kanye West]

I have a couple of RC4 shocks that I'm going to lighten up the valving on for lower leverage frames. They're both normal OEM spec (as far as I know).

I'll probably have to order some shims, so I'd like to know what's in there first.

Any baseline low leverage stack suggestions are also welcome, though I have a pretty good idea of what I want to do with it.

 

[JohnnyC]

Most of the "low" tunes I have seen are roughly 1/2 of the "medium" tune. So either remove half of the shims or say if the shims are 0.15mm thick then replace every shim with about 2x 0.1mm shims. (1 x 0.15 shim = stiffness of ~3.4 0.10mm shims)

 

[Kanye West]

Gotcha, thanks. That's roughly what I was going to do with it (re-configure with 2x0.1mm shims for each 0.15mm), but the concern with most of these shocks is the total stack height needs to remain the same for the needle position to be right. So if I replaced say 6x0.15mm shims with 12x0.1mm, then I've got 0.3mm extra stack. Not sure if there are extra backers that can be removed to compensate.

Do you know the stock ID and OD's?

 

[JohnnyC]

Last fox shock I worked on had something weird like 1/4" ID shims, OD's don't matter apart from the biggest one covering the ports and the last non-bending/clamp shim has to be the same OD.

Are these bikes low leverage as in 2:1 or more like most DH bikes around 2.5-2.8? If so I would tend towards a little more than "half" the current amount of damping

 

[blindboxx2334]

Most of the "low" tunes I have seen are roughly 1/2 of the "medium" tune. So either remove half of the shims or say if the shims are 0.15mm thick then replace every shim with about 2x 0.1mm shims. (1 x 0.15 shim = stiffness of ~3.4 0.10mm shims)

is there anyway to tell what i have? i bought mine from go-ride with no packaging... ive seen some that have 'm tune' stickers on em, but very few. most of the ones i see dont mention anything of a tune.

 

[Kanye West]

Right, I remember the weird OD and these things being hard as hell to source.

Any idea what the shims are like around the pivot/clamp shim? Usually on these shocks you can cheat a little and just move the extra shims back there to the passive part of the stack. I'd rather get different thicknesses to use though (once I find out the stock stacks).

 

[Kanye West]

Alright, so here are the stock stacks:

All 0.250" ID (huge pain in the ass - I don't even know how to source these)

Compression
.900" x 0.15mm (0.006")
.800"
.700"
.600"
.500"
.400"
.400"
.400"
Backing plate

Rebound
.700" x 0.25mm (0.010")
.700" x 0.25mm (0.010")
.600" x 0.25mm (0.010")
.600" x 0.25mm (0.010")
.600" x 0.25mm (0.010")
.500" x 0.15mm (0.006")
.500" x 0.15mm (0.006")
.400" x 0.25mm (0.010")
.400" x 0.25mm (0.010")

 

[Kanye West]

Doesn't seem like there's a lot to do with the shims that are in there. Can't remove any without changing the whole profile of the stack. Throwing in some 0.1's would be ideal...but where the hell do you get them?

Other suggestions for a low leverage setup using what's in there? I thought about a crossover after the 0.8" compression shim, but that's not doing a lot for high speed. Never had good luck with those on MTB shocks.

 

[troy]

Maybe drill the piston to 8mm ID and use a sleeve, easier than searching for rare shims? You will revalve it anyway, so...

 

[Kanye West]

Maybe drill the piston to 8mm ID and use a sleeve, easier than searching for rare shims? You will revalve it anyway, so...

So, piston goes to 8mm ID, run a taller sleeve through it for the 8mm shims to glide on?

How does the valving AND the piston all get clamped tight then?

 

[Kanye West]

So I pulled apart the 8.5" RC4 I have to put on my Banshee Rune.

WEIRD stack that was in there.

Same rebound stack, but the comp stack was:

.900" x .25mm (0.010")
.900" x .15mm (0.006")
.900" x .15mm (0.006")
.900" x .15mm (0.006")
.700" x .15mm (0.006")
.500" x .15mm (0.006")
.400" x .15mm (0.006")

Super stiff low speed setting. Putting the first two .900's behind the .400.

I THOUGHT both of these shocks were normal OEM spec. Guess not. I wonder which one (if any) is the normal OEM stack?

 

[Kanye West]

On the first shock I removed the .600" compression shim and put it behind the clamp shim. Feels way better.

Also these things both had ZERO grease still in the dust wiper and main seal. Some additional movement there seemed to help a bit on the small stuff.

 

[Kanye West]

Other useful info:

IFP depth for a 9.5x3 shock is 34.8mm (I've found this mentioned a few places).

I also found an IFP depth of 25mm for an 8.5x2.5 shock. This isn't enough depth.

Calculating the IFP displacement for that additional half inch of stroke, it should be 32.1mm (measured 5/8" shaft OD, 1/4" shaft ID, and 1.25" IFP bore diameter). I wish I had a load cell or a sealed active pressure gauge here to verify the ending pressures are identical at those two depths, but on paper they should be.

 

[troy]

So, piston goes to 8mm ID, run a taller sleeve through it for the 8mm shims to glide on?

How does the valving AND the piston all get clamped tight then?

Use 2 shorter sleeves with an oring between them, so it can compress when tightened.

 

[JohnnyC]

So I pulled apart the 8.5" RC4 I have to put on my Banshee Rune.

WEIRD stack that was in there.

Same rebound stack, but the comp stack was:

.900" x .25mm (0.010")
.900" x .15mm (0.006")
.900" x .15mm (0.006")
.900" x .15mm (0.006")
.700" x .15mm (0.006")
.500" x .15mm (0.006")
.400" x .15mm (0.006")

Super stiff low speed setting. Putting the first two .900's behind the .400.

I THOUGHT both of these shocks were normal OEM spec. Guess not. I wonder which one (if any) is the normal OEM stack?

The stock settings have probably changed from year to year, were these standard aftermarket shocks?

I would keep doing what you're doing and moving shims behind the clamp shim, that would be how I do it. If you have time for trial and error then move half of the shims back there for a super light setting, decide which feels better then work your way from there, rather than removing one shim at a time. Ie bracketing

Also, these are standard linear stacks so don't think of it as each shim opening one at a time in progression. Instead think of the whole stack as one big shim that you add or remove stiffness too to change the total compression damping. It is much easier to get your head around what changes to make that way, ie you can add or remove any shim and it will have the same effect whether it came from the top or the bottom. The only way to change the proportion of low and high speed damping is with multi stage or preloaded stacks which add much more complexity

 

[Udi]

The stock stacks change from year to year, and bike to bike - often quite significantly (I've seen everything including dual stage stacks).

One thing I'd suggest also checking is the valving on the neck compression, I've seen variations there also and it can affect compression range significantly.

The other fun thing you can do is lop out the boost valve if the frame has sufficient EOS progression, it generally results in better handling of square edged impacts, particularly high amplitude ones. Just squeeze the circlip shut (mind the reservoir walls) and remove the clip followed by the valve.

Beyond that I think (as already mentioned) all you need to do for lower leverage frames is to move intermediate sized shims behind the clamp shim. I haven't really noticed a need to change the rebound valving for these frames, I think as long as the spring rate is 250 or higher, the stock valving allows rebound rates to be sufficiently low.

I have however noticed that the rebound valving can become quite insufficient in HS on higher leverage frames and needs changing, but not relevant in this case.

 

[Kanye West]

The stock stacks change from year to year, and bike to bike - often quite significantly (I've seen everything including dual stage stacks).

One thing I'd suggest also checking is the valving on the neck compression, I've seen variations there also and it can affect compression range significantly.

The other fun thing you can do is lop out the boost valve if the frame has sufficient EOS progression, it generally results in better handling of square edged impacts, particularly high amplitude ones. Just squeeze the circlip shut (mind the reservoir walls) and remove the clip followed by the valve.

Beyond that I think (as already mentioned) all you need to do for lower leverage frames is to move intermediate sized shims behind the clamp shim. I haven't really noticed a need to change the rebound valving for these frames, I think as long as the spring rate is 250 or higher, the stock valving allows rebound rates to be sufficiently low.

I have however noticed that the rebound valving can become quite insufficient in HS on higher leverage frames and needs changing, but not relevant in this case.

I was thinking about the boost valve thing, particularly for the 6" bike where I'm not launching bigger stuff like the DH bike. Trails with lots of choppy stuff. Does that work well for bikes that have U shaped leverage curves and are slightly regressive at the end? Banshee Legend and Banshee Rune.

The Legend (from a "rough" plot in Linkage) goes from 3.25 down to 2.52 then up to 2.75. 350# spring on that bike.

The Rune goes from 2.84 down to 2.35 then up to 2.5. That's confirmed to be correct. 450# spring on that bike (for now).

I suppose it only costs me oil and a few minutes to get the thing out of there, and it's reversible. Could probably do it without draining the oil even.

EOS progression = ?

I asked Fox, and they refuse to sell 1/4" ID shims, or provide the OEM valving specs, which supremely pissed me off. So, still trying to figure out where the hell to source those. This shock would be amazing if it weren't for that. Customer service at its finest. Basically everyone else in the industry will sell small parts, but Fox thinks they're above it somehow.

I almost wonder if I could make a piston bolt at work that had the same threads but an 8mm shaft, ream out the piston ID, and run normal stacks on there. Looks like there's plenty of material for it, and it wouldn't be so damn goofy. I'm already making a bunch of other extended pneumatic fittings there anyway this week that end up looking real similar. I just don't want to roast the piston if I want to put it back.

Got any specs for the compression exchange valving?

Thanks!

 

[JohnnyC]

The only concern I would have about making a new bolt would be creating a stress raiser at the point where it necks down, especially if the piston bolt gets torqued pretty tight it could potentially break.

I think racetech sell shims in every oddball ID and thickness, check them out

 

[Kanye West]

Right, there'd be some relief there somehow. I've taken that into consideration.

I've tried Fox, SDI, MX-Tech, RSP/Avy (gag), Race Tech...no luck.

 

[Udi]

Those bikes aren't excessively low in leverage, stock tunes work fine on the Legend in my experience and a couple of intermediate compression shims disabled on the Rune should do the trick. I'd say you're over-complicating things here.

The Legend also has a large amount of end-stroke regression (the Sunday is much flatter contrary to popular opinion and mismeasured linkage curves) and thus is a classic candidate for the boost valve to be of benefit. I'd leave it in for that. I can't comment on the Rune but it's easy to play based on travel usage - if you have desired sag and aren't bottoming out, consider removal, conversely if bottoming consider keeping it in and decreasing volume via adjuster or IFP depth within allowable limits.

The Legend also has a decent amount of early stroke progression so does a good job of overcoming the air-preload effect on the large shaft RC4. That shock is a match made in heaven for that frame.

EOS = end of stroke. Now that you've mentioned the bikes though, this (progression) doesn't apply to the Legend and I'm fairly sure it won't apply to the Rune.

I've got a bunch of the stock stack specs kicking around somewhere but I'm lazy, hassle me if you get desperate. I definitely don't think you need to mess around with switching to common shim sizes (and associated piston bolts) here, everything you need to do should be within the range of the parts available in the shock. Unless you have some weird high compression tune, I honestly think some knob turning should net a good setup.

FYI, I'm sure you already know this, but just in case - stock oil on those shocks is Silkolene RSF 10 @ 47cSt, it's way thicker than most shock oils so worth keeping in mind if considering alternatives (most shocks use way thinner oil). Again, excuse me stating the obvious there (you obviously know what you're doing), but I've seen it overlooked before.

 

[Kanye West]

I like the progressiveness of the shock on the Legend a lot actually. The Rune is (and feels) a bit flatter, and somewhat constipated in the midstroke. Average leverage is definitely lower on that bike, so I'll probably focus more on that one. Great bike otherwise. On most runs I don't bottom the thing out either, so I may yank the boost valve just for the hell of it.

Initial plushness is definitely not an issue on the Legend, but repeated mid-stroke chatter can get harsh on it, so just looking for a tiny bit of relief on that. On the lower-leverage tunes I've done on any other shock, that generally provided good results in that area, as long as it had a competent bottoming system. If there's adjustable valving in the HSC adjuster, I may go after that too. I'm also really curious what the arrangement of the HS valve is like in there too. The MX ones are interesting. The spring preloads the shim just below the clamp shim, so it's controlling the speed at which that second shim no longer is the pivot shim, bends back, and allows the actual pivot shim to act. Ends up being more of a mid-speed adjuster.

As for the oil, I'm currently running a lighter one, but just by a little bit (36-40cSt, depending where you look). I had thought they switched a while back during the DHX5 shocks to a lighter Torco oil. Or did they switch the other way? Looks like BelRay HVI 15W is gonna be a good match. The Silkolene stuff is tough to find. I did indeed overlook it, though not for lack of looking. Thanks for that.

I may make the system an 8mm ID for the Rune, because the shims that came in it are so damn weird. Or I can get some shims cut out pretty easily. Probably no need on the Legend because the stack was pretty standard.

Or just get a more normal shock, do one rebuild/shim shuffle, and be done with it..