Tuning Fox Float RP2

Let me get this out of the way- I service forks and shocks at work and for fun, but am just learning about custom tuning dampers.


This, be warned, is what happens when my girlfriend goes out of town for a few days:

My Float RP2 feels the same as what every one on mtbr.com says: wallowy through the mid-stroke, blows through travel.  I've ran it almost entirely with the Propedal engaged. But then it's just not that plush or responsive.  Especially compared to my new Minute Pro 120 fork.  Now that the shock finally took a shit on my last descent down Crooked Trail, it's time to figure out how to rebuild this thing and maybe tune it a little better.  The damper hasn't been maintained in 5 years of hard use, and finally the seals let air into the damper.  The damping became very inconsistent very quickly with lots of slurping noises.  It's not worth the money to send it to Fox or PUSH for a rebuild because it's almost the same price (for me) as much nicer replacement shock.

Goals:
1. Firmer Propedal for road climbs (Propedal on RP2 was more of a "trail" setting)
2. "Trail" tuned damper in full open- more low and especially mid speed damping, digressive curve at the high end to counteract low-volume progressive air can.
3. Replace critical damper seals: IFP seal, shaft seal, damper body seal.

I'm not going to go step-by-step through the rebuild.  Floats are very similar in design to RS Monarch/Ario and RS provides great instructions with photos.  Practicing on something that's actually meant for end-user service would be a good start.  I also had the privilege of tearing apart a few old, broken Floats at the shop. Filling the IFP chamber is the trickiest part, I cut 5/16x24 threads onto a truck tire tubeless valve and used the rubber gasket from inside a high quality schraeder cap to seal it. I'll have to remove the valve to get the air can on and off.

For the sake of simplicity, unless I can scrap something obvious from one of the old shocks, I'm going stick with the basic (but not very simple) piston and shaft setup of the RP2.





 
Here is the obligatory shim-stack photo: (note, I'm not that grungy- those rags are washed and clean, just stained)

FAQLoad has a good explanation of the RP2(3) damper.

The belleville washers push on the outside of the thick aluminum shim between the glide ring/piston and my thumb (left).  That thick washer is separate from the washer axial to it.  The large washer slides on the inner washer; I have them offset slightly so the the difference is visible.  The large washer completely seals the ports through the main piston, so it also acts as a check valve under rebound to create a separate rebound circuit. Compared to a normal shim stack, once the belleville washers are compressed to vertical, the thick aluminum shim can't flex any further.  However, since the whole shim moves rather than just the upper edge, it's hard to say how it affects the damping curve.  My guess is that it allows a lot of fluid to move at high speed. 

There's also a secondary compression piston (right) after the main/belleville piston.  Fluid enters the shaft if enough force exists to open the Propedal needle valve (or if the adjuster is set to Open), and can exit the shaft either at the secondary compression piston or at the rebound orifice.  The secondary piston is closed completely by shims and so only opens at a certain force.  It would be hard to guess whether the main piston with it's highly preloaded belleville washers or the secondary piston's shim stack opens first, though given that the propedal system is designed to control low speed damping, and given the high preload on the belleville washers, my guess is that the LSC shim stack on the secondary piston opens first.

So in Propedal mode the low speed damping is controlled by the very peripheral parts of the main piston's ports.  With the Propedal open, the LSC is controlled by the same part of the main piston as well as the upper-shaft rebound orifice.  Mid and high speed damping are controlled by the belleville shims on the main piston and the shim stack on the secondary piston (depending on the propedal needle preload force).  I'm have no idea why there are two pistons working in tandem like this.  It seems to me that the belleville shim set-up increases friction because of the extra moving parts and might make the damper less reactive because of it's on-off nature.  I also might be full of shit.

Without changing the whole layout I think I have three options:
1.  Increase preload on the Propedal needle valve.  This is two birds with one stone- firmer propedal setting, also more LSC in full open.  I need to make sure the propedal adjuster doesn't travel too far- adding too much preload could bottom out the propedal coil spring when the platform is engaged. The propedal lever was getting a little loose anyways so I'll at least add a small amount of preload (1 or 2 of the tiny shims if i can find some)
2.  Add a shim to the secondary piston.  The belleville washers on the main system seem to me like kind of an on/off sort of setup because as soon as that thick washer lifts off the piston the oil flow is massively increased.  I want it digressive as I mentioned earlier to counteract the progressivity of my small volume air shock, but I think it might be digressive too soon, need more velocity-dependency through the mid-speed range.  If I add shims to the secondary piston it might increase mid-speed damping, or it might just put more pressure on the main piston, and cause it to open earlier, so I don't think this is a good option.
3.  Add more preload to the Belleville washers so the mid-speed impacts are damped through the more traditional shim stack on the secondary piston.  I'm assuming that since they bothered to design the secondary piston and shim stack in the first place, the shaft orifice at the propedal valve is big enough that it can pass more fluid than what can escape the rebound orifice. As the FAQLoad page mentions, 3 belleville shims are used on the High tune RP2's.  I have two, for the medium tune, so maybe some extra preload on the two will be a middle ground.  The problem here is that adding a washer without subtracting one of the two preload shims that already sit behind the secondary piston will restrict how far the main piston's valve can open, which could lead to high speed spiking.  I really need a half-diameter belleville shim to replace one of the preload shims.  Some shocks are going to be torn apart tomorrow!

Am I totally wrong on this?  Please enlighten me if that's the case. 

Science of Flow (non-Newtonian)

I love to complain about trail flow.

In fact, it's one of my favorite things to talk about, especially while riding.  If you ever went riding with me, by the third time I yelled" Ughhh decreasing radius blind corner what the fuck!", you'd have already turned around to ride it uphill instead, figuring that would be much more pleasant.

As an amateur trail builder and enthusiast, I feel like I have a small right to complain.  I've built a small bit of good trail, a fair amount of mediocre trail, and some trail so bad that we had to fix it before anyone else saw what we had done.  I guess I've never been able to learn any other way, but it sure works quickly.  So in other words, I'm an expert in what bad trail looks like.

All mountain bikers can feel the difference between good and bad trail.  But is a slightly awkward corner really worth complaining about?  You're still out in the fresh air, on your bike, and no one's telling you what to do.  You have to appreciate the time someone took to build it.  The weeks that someone took to build it.  It's because we stand on the shoulders of inventive, motivated giants, that we can see the finer points of trail building so well now- both in the negative examples, and the plethora of beautiful, so subtly-executed trails.  The real problem with bad trails is not a rider's enjoyment but the durability of the tread.  Anytime you have hard braking, cornering, and descending at the same time, the tread will blow out quickly.  That's my motto: It sucks to brake hard, corner, and descend at the same time.  Now Keith Bontrager will be quoted on the Internet for the 7-billionth time: "Pick two." (Since less than half of the world has Internet access this is probably at least the second time I've done so)

But the magic of a great trail is worth striving for.  And for the record, I'm not talking about the pump-track type of trail in which no corner goes un-burmed.  There's a place for that kind of trail, and I'm always impressed by the ambition and engineering cred of the builders and the skill of the riders.  But for most mountain bikers, where shared use is a reality and maintaining a wilderness feel and a safer appearance keeps the trails open, this isn't realistic.  A simpler trail doesn't have to come at the expense of flow and challenge.  It's those very trails that use nothing more than a ribbon of singletrack, with just enough grade reversal before the corners, crossing over roots and rocks that are just barely rideable, corners just tight enough that it's very hard to ride fast but doesn't suck to ride slow that are truly great trails.  These are the trails that I'm in love with.

Like everything else, I learned the hard way about tread width- at first it feels right to dig that nice deep backslope, make absolutely sure there's no false toe, and to inslope all the corners.  But wide trails are fast trails and managing speed is the hardest thing about trail building. They're also ugly trails-not only is there a lot of dirt showing, but twice as much (or more) left over dirt to be dispersed.  A high-use trail that crosses or turns on a very steep hillside needs some width.  But for every other situation, let's have narrow. Turns out it takes half the time to build them, too.

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Lately I find myself much less critical of trails.  In fact it's been at least 4 rides since the last time I cursed a corner halfway through.  I would like to believe that it's an indication that I'm much more at peace with the world (I mean, this world, Earth), or my existence in it, or even that the anti-anxiety herbs are working and sleep makes me that much more pleasant of a person.  But I've looked over the evidence again and again and there's only one answer:

I put real tires and a well-damped 120mm fork on my bike.

When I started mountain biking I realized immediately that a five inch fork and some knobby-ass 2.3's turned awkward or sketchy into just fun.  I used to tease my dad (who to my knowledge never has used an index shifter) that not only did my bike have index shifting but also indexed-rolling tires.  I proudly rocked (as opposed to ran) a 200 dollar fork on a bike I paid $380 for, new. Later I developed this road racing problem and all my mtb tires got lighter and faster, and my forks got shorter, even to the point of riding a '99 SID (which that they never should have put disc tabs on, that was just scary). I went tubeless and kept dropping my pressure to try to squeeze a little actual traction out of the fast rolling tires (Michelin Dry2 was the rockbottom equivalent of the SID).

Anyways, a few weeks ago I put 2.4 Conti Mountain King black chili Protections on front and rear.  They fit great on my 26" Anthem.  They actually hook up better at 30-35 psi because the knobs bite so much they need support.  I ride in loose country--Missoula, MT--and an open tread pattern with real knobs is the only thing that hooks up.  Sure, better riders can milk a lot of speed out of some startlingly bald tires, and I'm probably not night-and-day faster than I used to be, but there is a massive difference in the level of control at high speed and the ability to use your brakes.  Magnum rubber = more fun, more control, less trail damage.  Plus, it's a great excuse to be the last up the hill! 

Soon: can you guess what this means?