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Discussion Starter · #1 · (Edited)
As the title really. Been off the bike for a looong time and bought some new low end Recon Silver forks only to discover they couldn't be upgraded.

Anyway I susbsequently had some fantastic assistance from Rete and bought a 120mm Reba A5 (Rapid Rebound) and quickly set about upgrading it with the following:
Sid B (.270) RCT3 DNA comp damper
2021 SID C1 Sealhead and negative piston mod.
2 tokens.

I'm around 210 ish fully kitted up and have the fork setup as follows:

2 tokens.
55 PSi
5 click of LSC from fully open
3 clicks of RR reboumd.
Threshold fully open.

Now the forks are understandably pretty sensitive to pressure changes which is due to the smaller air volume from the addition of 2 tokens. 50PSi is bottom out city while 60 PSi reduces available travel (with no LSC set at all - eg fully open) by at least 20-30mm

Been to Glentress two weeks back and Laggan this weekend past and kinda got a good shakedown and the fork provisionally setup; the fork seems to ride great and soak up everything with about 5 -6 click of LSC to stop exessive brake dive or dive on transitions but I'm wondering if I'm using excessive LSC to counter an overly low initial spring rate (SIDB sealhead mod) and I'm sinking too far into mid travel and then have far too much ramp up towards the fully compressed travel. In short it feels great but possibly not getting the best out it by far?

Now from what I've managed to glean - riders with a similar weight seem to be using one token (rather than my two) and running an additional 25-35PSi (80-90 total) which I have sneaking suspicion will lose just a little small bump (if any) but give me far better mid stroke support as the ramp up will be less progressive than when using two tokens? I assume that with a slightly larger positive spring volume that I'll have better luck arriving at a suitable initial air pressure.

I know this is potentially a very subjective question and also very dependant on riding style but I'm pretty sure I'm way off but just would like a quick sanity check before I start changing everything.

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Ok, something is not adding up here for me, you are right in that everyone has their own preferences but 60psi is about half of what I would expect you to have, so something isn't right. What is the negative spring mod? The debonair sealhead on its own has a negligible change to the negative volume so have you changed something else in the air spring? Also a larger neg volume would normally ask for slightly higher pressures, not lower.

I would usually start with minimal tokens and compression damping to get the air pressure in the ballpark, and then dial in the rest. But I do wonder if something is wrong so you need to get to the bottom of that first. If you release all the pressure, does the fork compress to the bottom very easily? Or if you put a zip tie down the LH dust wiper does pressure escape? I wonder if air is leaking in to the lowers, which is a common cause of needing excessively low pressure
 

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Discussion Starter · #3 · (Edited)
Thanks for replying Jonny. I do suspect gremlins hence the post. I can cycle the fork through its' full travel no issues at all when the pressure is dropped out the air spring. All parts were stripped, cleaned and closely inspected for wear/scratches prior to the rebuild so all should technically be good there in terms of sealing. With regard to the air spring... I gutted out pretty much everything under the top piston except the top out bumper and fitted a SiDB 2021 sealhead (with larger o ring) so the negative side of the spring has a fair bit more static volume to begin with. To elaborate the SiD C1 sealhead replaced the existing floating sealhead, wavespring, thrust washer and lower seal that is held captive by the internal circlip.
Pretty sure I only have two tokens in there but now I'm beginning to second guess myself. I wonder if my shock pump is reading correctly? I'll check both tomorrow and report back. I'll also check with a zip tie just to rule out air in the lowers. Also now wondering if I've made some sort of measurment error when filling fluids.
 

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I wouldn't consider the SID C1 seal head a mod, it's more akin to updating the Reba solo air spring to a SID 32mm SL DebonAir spring. It does increase the negative volume quite a bit, especially if you remove the topout rubber bumper cone and replace it with a smaller bumper as well. Also reduces air spring friction, since it uses a bushing instead of a rubber seal.

What doesn't sound right is the very low pressure. The SID SL DebonAir spring allows you to run higher pressure than the equivalent Reba solo air for more mid stroke support and bottom out resistance while maintaining better small bump absorption as well.

60psi at your weight is way off. Similar to the 120mm SID SL Select you should be at 105 to 110 psi.
 

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Discussion Starter · #5 ·
Okies had a check this morning but was waylaid mid investigation. Small tiewrap down stanchion didn't vent any pressure so bonus. Dropped the air out and again managed to cycle the fork thru it's entire travel without much effort. Removed a token with one now remaining and filled it to 90PSi which gives me about 27% sag with me unable to blow through the travel by bouncing whilst riding... Maybe managed to get it to blow thru about 90mm or so with everything wide open. Not had a chance to check fluids or ride it yet but will report back when I get a chance.
 

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If you get to 75% travel while pumping while riding and if the fork feels good otherwise I'd stick with that and perform minor tweaks over various rides.

Also assume you followed the SID SL lubrication guidelines for the upper tube, 3ml of 0w-30 or whatever oil you used for the lowers on top of the air piston. It migrates into the negative chamber and lubes the air piston plus lower seal head shaft interface.
 
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Discussion Starter · #7 · (Edited)
Ah okay missed that re the lower seal bush; at least thats an easy correction. I'm also going to pull the damper and check the oil level just to rule out any overfill errors.

Sanity check on that actually; assuming correct oil fill for the damper is 10mm above the lower Moco body oring seal which is 10mm or so above the lower platform piston that houses the glide ring?
 

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Sanity check on that actually; assuming correct oil fill for the damper is 10mm above the lower Moco body oring seal which is 10mm or so above the lower platform piston that houses the glide ring?
You didn't post a damper part number but assuming it's the 120mm SID B damper and not the shorter SID A version your oil level sounds fine. Do what @JohnnyC7 suggested and cycle the fork with all positive air released. You do have leeway to reduce oil to just above the o-ring level.

Another tuning suggestion is to eventually take the damper apart and remove the preload shim from the stack. At your weight, remove the 0.2mm outer ring but keep the 16mm centre guide shim and move it down the stack. Be careful when you do this while taking it apart, the compression clicker's spring and detent balls have a habit of shooting across whatever room you're doing this work in.
 

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Discussion Starter · #10 · (Edited)
Thanks for the assistance. It's the .270 SIDB damper. The products have come so far since I was last riding regularly (17 years ago) . I'm 47 now so need all the mechanical anti-stack assistance I can get.

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Discussion Starter · #11 · (Edited)
I've spent some time reading up about this and hence redacted the last paragraph of my last post above.

Can I confirm what I seem to have gleaned from my reading?

So LSC is controlled by the needle bleed and when this flow chokes fluid flow goes the next easiest path... Via the compression shims? (Is this all referred to as the 'Base Valve'?) The base valve shim stack partially controls high speed events but there's too much pre-load as stock as RS prioritised a pedal platform over actual fork performance? TBH it's a SID damper so it's tune is going to be aimed at the XC crowd.

So basically I need to remove some preload from the base valve stack to lower HSC? So I remove the ring shim (and perhaps some of the 19mm shims if I subsequently find HSC is still harsh*) and then sort the stack from largest to smallest?

So I've had a quick look at the bottom end of the piston - How do I remove the lower fastener from underneath the piston and is there thread retainer such as red loctite holding it together or does it all come apart some other way? Additionally where are the ball detents and springs hiding?

So apparently my Reba has 'Rapid Rebound Recovery' - Is this just a marketing name for a 'Mid-Valve' and if so I assume the mid-valve also partially accounts for a degree of HSC as it sees far higher fluid velocities than the base valve and that's why the later SID B .270 RCT3 damper (see my query below) has less shims in its' base valve than the earlier .070 SIDB RCT3 damper?

*Think I may have the wrong damper - I bought a 11.4015.544.270 as I have a Rapid Rebound Reba but think I may have been sold an earlier 11.4015.544.070 as 1. The fork is super harsh (like much worse than the port orifice Recon RL it quickly replaced) on anything high speed even with the LSC wound fully open and 2.It has the silver LSC adjuster (and not he black) as per the earlier version. Cosmetic I know on the latter but these two points are enough to warrant me investigating the shim stack this week before we get back out on the bikes at the weekend.
 

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Discussion Starter · #12 ·
So it turns out the difference was only cosmetic. Pulled the RCT3 damper and to be safe I disassembled it in a ziplock given the ball detent projectile stories. So stock shim arrangement and removed the ring shim and moved the inner up the stack. Got a day out tomorrow so I'll see how I get on with it.

Stock arrangement:
Purple Cap Sleeve Textile Rectangle


Resorted (ring shim shown for reference) :

Outerwear Cap Product Purple T-shirt


Also replaced the RP2 with something a little better :
Wheel Tire Automotive tire Motor vehicle Vehicle brake
 

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Rapid rebound recovery is RS marketing term for a rebound configuration with shim, and the cheap one is only an orifice.

The opposite side of the rebound piston needs a check shim so oil can flow through during compression. People can intentionally add shim stiffness and/or spring rate there to create compression damping, known as mid valve.

However, the compression force ratio coming from the base valve and mid valve needs to be within certain range, which is mainly determined by the rebound shaft/piston geometry, to avoid cavitation (an extreme case would cause vacuum under the rebound piston and sucks in lower oil).
 

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Discussion Starter · #14 · (Edited)
Thanks for the explanation. I've been reading up on the ABS+ shim guide on here and seems I might have to do a little playing around with the top stack. If I run into issues I might remove all the top stack shims as a test to see if I can determine if the rebound piston is being flow limited on high speed events as that sees much higher volume flow rate than the RCT stack?

I had the bike out at the weekend and we headed North to avoid the rain and ended up on unfamiliar trails (Heartbreak Ridge, Ballater, Scotland) so couldnt really hit them at speed. However, the McLeod straight off the bat seems to work pretty nice as was essentially unnoticable/invisible compared to the Reba which definitely still needs work.

It would be nice if I could get this Reba playing well with the McLeod.

Edit: Using 5W Maxima Fork oil in the Reba.
 

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Discussion Starter · #15 ·
So after reading up a fair bit and playing about with the fork for a while I decided to pull the RCT3 damper & remove a face shim. When I originally removed the ring shim I moved the centering shim down the stack but I removed it with a face shim today to try and soften the whole thing. I also removed the remaining single token as I think by the time I'm getting good support the fork is becoming overly progressive towards bottom out. So I tried setting spring rate by frequency rather than sag today and arrived at an initial 120 PSI on the front and 150psi (suspect I may need to go higher here in time) on the McLeod at the rear which I'll try and fettle over the next few weeks. The suspension felt supportive without being to springy but not sure how to arrive at the balance between reducing available stroke, grip and rebound as it appears I wont be able to use all the travel. I removed the top out bumper and cut it down so all that remained was the top strip that sits in the piston groove and with the token removed the small bump seems that it may be marginally improved. So I done the standing and sitting hitting a 3" kerb at speed and prior to today this would have sent a fair shock through the bars but now feels tolerable until I get about 4 (I'll need to check this again) clicks away from LSC bleed being fully closed. Will mess around with it over the next few weeks and see if I can further improve how it rides.

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Thanks for the explanation. I've been reading up on the ABS+ shim guide on here and seems I might have to do a little playing around with the top stack. If I run into issues I might remove all the top stack shims as a test to see if I can determine if the rebound piston is being flow limited on high speed events as that sees much higher volume flow rate than the RCT stack?
Removing base valve completely will remove the hydraulic pressure and leaving the air pressure alone to push oil through the mid valve. You will likely experience cavitation during the test.

The alternative is keep the base valve as-is but completely open LSR, which is also a bypass for the mid valve. During testing, focus on the compression feeling and ignore the super fast rebound.

If opening LSR changes the compression feeling a lot, you know the mid-valve has a sizable effect and can address it with less preload/ softer spring. There is a post in the charger 2.1 thread on this.
 

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Discussion Starter · #17 ·
So I wrote quite a lengthy reply only to discover yesterday that it seems to have disappeared! That's a fair point regarding cavitation from pressure drop- I completely missed that as it's almost 30 years since since studying fluid dynamics at Uni so I've essentially forgotten the lot. Interesting that the parallel reverse 'free' bleed through the LSR needle orifice would have a noticeable effect on HS compression; would the characteristics noticeably change on HS events as surely the flow though the LSR needle orifice would flow limit long before the mid valve shim stack or piston orifices?

Regardless I'm going to investigate - If there is a difference noted in response what would the next step be? Remove the compression shim/s from the rebound piston and retest to determine if piston orifices are sufficient then work from there? Piston orifice modification, more float, shim adjustment or reducing spring rate on the clamp spring? I'm assuming piston orifice or float adjustment are the most likely starting points? Whilst ReStackor Pro is not exactly expensive I can't quite, for the time being anyway, justify the outlay.
 

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Regardless I'm going to investigate - If there is a difference noted in response what would the next step be? Remove the compression shim/s from the rebound piston and retest to determine if piston orifices are sufficient then work from there? Piston orifice modification, more float, shim adjustment or reducing spring rate on the clamp spring? I'm assuming piston orifice or float adjustment are the most likely starting points? Whilst ReStackor Pro is not exactly expensive I can't quite, for the time being anyway, justify the outlay.

You can check the pressure balance and “rule of thumb” sections on restackor. I think adding more base valve pressure drop (stiffer base valve shim or closing LSC) is the easier experiment, but an overall linear base valve (shim + LSC) may not be able to generate sufficient pressure for a preloaded mid valve at low speed. Going to a softer spring and/or less preload should be the more robust solution.

You’re right that LSR/LSC will likely both chock with a truly high speed compression event, and the mid-valve has to open eventually. However, mid-valve would look like a solid piston before it cracks open, and pushes additional oil through the base valve. Once opened, the shaft need to displace the same “excess” amount of oil to reach oil volume equilibrium before entering the normal mode of operation. For that brief duration, there is likely no compression damping from ether base valve or mid valves since no oil is flowing through either.
 

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Discussion Starter · #19 · (Edited)
Ened up buying a 2015 140mm DIG valved RCT3 Revelation so will swap out the existing floating sealhead for a 32mm SIDB version and play about with that to see if I can get it to behave any better. It looks like the DIG valve is at least approximating a proper midvalve and also has a small free bleed so at least there's the ability to play around with the midvalve stack.
 

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Thanks for replying Jonny. I do suspect gremlins hence the post. I can cycle the fork through its' full travel no issues at all when the pressure is dropped out the air spring. All parts were stripped, cleaned and closely inspected for wear/scratches prior to the rebuild so all should technically be good there in terms of sealing. With regard to the air spring... I gutted out pretty much everything under the top piston except the top out bumper and fitted a SiDB 2021 sealhead (with larger o ring) so the negative side of the spring has a fair bit more static volume to begin with. To elaborate the SiD C1 sealhead replaced the existing floating sealhead, wavespring, thrust washer and lower seal that is held captive by the internal circlip.
Pretty sure I only have two tokens in there but now I'm beginning to second guess myself. I wonder if my shock pump is reading correctly? I'll check both tomorrow and report back. I'll also check with a zip tie just to rule out air in the lowers. Also now wondering if I've made some sort of measurment error when filling fluids.
What size o ring did you fit on the sealhead if you remember? I have one lying around and tried to fit it inside my 2015 reba with the stock oring but obviously it won't seal the air with the stock one.
 
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