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Fast Ace fork cartridge revalving / tuning

In my previous fork threads one and two I've described spring & oil changes to my 43mm FastAce forks to get the sag roughly correct & a smoother/softer ride; these helped quite a bit, but I knew I'd have to revalve to get them working as well as possible.  This post is ridiculously long, but since it's unlikely you'll be able to replace anything that gets damaged, I figured it was better to be thorough.  The later Zero Showa GK-03 forks have the rebound-on-top design too, so I assume at least some of the internals are similar.

My goals were:
 
1) more comfort & control on sharp impacts
2) damping tunable w/clickers
3) overall damping range tunable by oil viscosity, instead of being locked into using super-thin oil

Due to winter weather I've only had one long and one short ride on the revalved forks, but the harshness appears to be completely gone, replaced by a much softer tire-bounce feeling on impacts (I thought the tire pressure was too low at first, but it was my normal 18 psi). On my 'harshness test section' - a series of deep braking bumps before a corner - all I felt was a rhythmic up/down motion instead of the usual bam-bam-bam, and after several passes the tops of the bumps were nearly gone (usually they just got deeper and deeper). Brake-dive performance (i.e., low-speed compression damping) seems fine even with the compression clickers fully CCW at minimum damping, but rebound damping is definitely weak and pogostick-y with the clickers at min. so I'll be dialing that in over the next few rides.  It's a huge improvement that I'm able to use the clickers at all, indicating normal-range fork operation / adjustment capability (finally!). Once the weather improves I'll run all the clickers thru their full ranges and report back with the results, and if it turns out I need thicker oil, fine; another tuning option I couldn't use before. 

After reading the Race Tech 'MC Suspension Bible' several times, I got a better understanding of how shim-stack fork tuning is accomplished.  Basic fork operation: the clicker circuits control low-speed (LS) damping over a limited range, involving vertical wheel velocities below @ 0.5 meters/second; compression = upward wheel travel, rebound the reverse. Actions affected by LS damping are fork rebound (actually a low-to-medium-speed event), brake dive, acceleration squat (mainly in the rear shock), reducing 'see-saw effects' over rolling terrain, etc. The shim stacks mainly affect high-speed damping (impacts, side deflection, etc.) and LS / HS transition over a much wider range.  During the compression stroke, oil from the lower half of the cartridge tube is forced down by the rebound piston (mounted on the center rod lower end) thru holes in the compression base valve, flexing the shims out of the way & creating compression damping in the process, then out thru holes in the cartridge tube into the slider; at the same time a check valve opens in the top of the rebound piston, allowing replenishment oil to fill the cartridge above the rebound piston to make it available for the rebound stroke to follow. During the extension/rebound stroke, oil from the top of the cartridge flows down thru the damping holes/shims in the rebound piston, while the compression check valve opens to allow oil flow back up thru the cartridge-tube holes & compression base valve into the bottom of the cartridge.  Shim stack design is a huge subject in itself, but basically more and/or thicker shims = more damping unless the piston/valve holes feeding the shims restrict flow & create more HS damping than the shims, like these Fast Ace forks.

These forks are quite decent for OEM. Everything unbolted normally without the grinding or drilling some Japanese forks require, and fit / finish / component quality seems fine (see photo below).  They appear to be early-90's Showa clones, with small-piston cartridges & tiny damping ports.  This fork design - like many early cartridge units - tends toward harshness on medium-to-large impacts which can't be tuned out with the shim stack, though the shims can definitely make things worse.

Here are the complete fork internals:



...and the assembled cartridge, w/locking collar, bottoming spring, main-spring guide, & bumper:



These are certainly not the most complex forks out there, but they're also not tubes-and-a-spring vintage forks either. Be very careful with that center rod - it's hard-coat over aluminum, like a metal corn dog and is very easily damaged.  It's also important to keep in mind that NO spares are available - I'm not even sure Fast Ace is still in business - and I'd recommend wearing your painting clothes and possibly latex gloves, as these forks convert oil into black ink & it's like working on a giant fountain pen.

The disassembly procedure I used was:
1) Remove forks from bike 
2) Remove fork caps & springs (see this thread for details on items 1 & 2)
3) Unscrew fork-slider 'feet' (axle clamps) from sliders
4) Remove cartridges from feet
5) Disassemble bottoming springs/cartridges/pistons/shim stacks

Below is a composite photo of two special tools I used, as referred to below - a Pro-Tek 43mm fork clamp bolted to a short piece of aluminum angle stock, and an OTC 6613 pin spanner wrench with the 3.5mm tips reduced to 3.0mm.



Lower Leg Disassembly: I first marked the foot & slider junction at the front of the foot, for later reference during reassembly.  I removed the setscrew on the side of each fork-slider 'foot' (both of mine were finger-tight!), then assembled the clamp section of my 43mm clamping jig (see photo above) onto the slider, tightened it down quite tight (it might leave a faint mark on the chrome which will come right off, but it won't deform the tube) & clamped it in the vise.  I heated the foot well with a heat gun on low, then used the axle to remove the foot (the LH brake-side foot will need the tube clamp loosened & slider rotated repeatedly).  Once the foot is loose, slide it & the cartridge out of the slider end. With the axle still in the foot, remove compression clicker ass'y fm foot & cartridge with an impact gun if possible.
Reassembly:  Once the cartridge was back in the slider (see below), I lightly coated the low half of the foot threads and upper half of the slider threads with soft-setting Permatex 2B, then torqued the foot back onto the slider (I went slightly past the marks I made above, which told me I was roughly correct torque-wise). I cut a small piece of plastic (anything relatively hard will work, a plastic peg / bottle cap / etc.), coated it with 2B, and pushed it down inside the setscrew holes to protect the slider threads, then put a drop of blue Loc-Tite on both the setscrew and slider threads and tightened it down.  There was something semi-hard in these holes from the factory, but it disintegrated when the setscrews were removed.

Cartridge Disassembly (the base valve assembly threads into the cartridge body, the top cap stays put): The first step is to get the locking collar off the center shaft - getting the top cap off involves turning this collar hard onto the bottom of the tube threads, and getting it off can be hard.  The best way is to double-nut the shaft end with M12x1.0 nuts to hold it while loosening the collar (as mentioned above, the shaft is VERY fragile and should NOT be held with anything other than these nuts or 12mm-diameter-cutout nylon-jaw pliers, which I couldn't find online and would've had to make up).  Then, I arranged two bricks on top of my bench against the bench backsplash with a plastic buffer piece in front, then clamped the cartridge top cap against the benchtop with my clutch-holder Vice-Grip tool (a large Vise Grip with buffers against the cap aluminum should work as well) and engaged the base turning holes with the 3mm pin spanner above. I used repeated short/sharp torquing while pushing the cartridge base end against the plastic buffer and the cap down onto the benchtop (it wasn't super tight, with a small amount of Loc-Tite).  Once the base was off, I removed the thin circlip from the center rod (this is a little tricky, be careful not to scratch the rod coating or bend the circlip), then removed the spring guide / rubber bumper / washer & pulled the rod / rebound valve / top-out spring / plastic centering washer down thru the cartridge cap.
Reassembly:  After assembling the compression base valve/shim stack, I put two drops of blue Loc-Tite on the center flatted-end aluminum tube and tightened it into the base (if the compression piston has been reversed, make sure the check valve shim is free to move against the spring, as with the piston reversed it can get pinched between the piston and tube; the 10x20x.2 check-valve shims were the only ones I couldn't find ANYWHERE, be real careful with these).  I then put two drops of blue Loc-Tite on the large base threads & tightened it medium-hard into the cartridge tube. There's really no way to torque any of these things without some REALLY special (and possibly custom-made) tools.

Damping Valve Removal: Compression: hold base valve ass'y w/pin spanner & turn flatted-circle Al piece w/17mm? or Crescent wrench (threads are Loc-Tited) carefully using short/sharp turns, lubricating exposed threads & reversing direction  as needed.   Rebound: hold damper-rod flatted aluminum end piece in vise (plastic / aluminum jaws or covering) & heat nut (hot, but not scorching), use 14mm socket & impact gun or medium-hard wrench tapping to loosen nut slightly (very tight & Loc-Tited too) / oil newly-exposed threads / turn nut back in / repeat until nut comes off.  Both compression and rebound aluminum center pieces are hollow & fragile !

I was wrong about the shim stacks being set up super stiff; they're actually quite soft (OEM shims are the thinnest I could find, .1mm thick).  Both OEM compression stacks were identical (two 8x20x.1 shims plus 8x12x.3 cap shim), but the rebound stacks were different on each side, surely by mistake:  the LH stack had three 8x20 x.1 / one 8x14 x.1 / three 8x12 x.5 / two 8x11 x.2 shims/spacers, while the RH stack had one 8x20 x.2 / one 8x20 x.1 / one 8x14 x.1 / three 8x12 x.5 / one 8x11 x.2 shims/spacers. This seems like a minimal difference, but that first 8x20 x.2 is EIGHT TIMES stiffer than a .1, so the RH stack had a lot more damping than the LH one.  All pistons had 10x20x.2 check valves with coil (compression) or 'wave' type (rebound) return springs.

As it turned out, I needed to not only replace the shims, but also modify all four pistons slightly, reverse the two base-valve compression pistons, and drill out a number of oil-replenishment holes (which refill various cartridge chambers so oil is available for damping strokes; specs below).  I tried to find hi-flow 8x22mm O.D. replacement pistons - Race Tech Gold Valves would've been ideal - but they don't make them and I couldn't find them anywhere online.  It's possible certain 90's Kawasaki KX dirt bikes used 22mm pistons - perhaps others as well - but I couldn't find out anything for sure, and who knows how well they worked anyway.

The compression harshness problem is apparently quite common on small-piston cartridge forks:  the compression-damping piston holes are too small to pass oil at high velocity/volume, so damping goes thru the roof & the fork hydraulic-locks on sharp impacts even with super-soft shim setups.  There's not enough removable metal to make a real difference enlarging the holes... so I inverted the compression pistons, using the much-larger 'Iron Cross' replenishment  ports as the damping orifices w/shims, & the check valve on the smaller damping holes (which now function as replenishment ports). This doubled the compression-damping port area, reducing HS orifice-related damping by a factor of 4 & resulting in the equivalent flow of an 11.3mm diameter hole (this is VERY non-restrictive, meaning nearly all HS compression damping occurs in the shim stack where it belongs, not the piston holes). The rebound cartridge-replenishment flow is reduced slightly, but since rebound is a medium-velocity event and repl. flow is supplied from four separate sources (rebound stack, compression check valve, and both clickers), there's no increase in rebound damping; in fact it's noticeably less than before, mostly because of the shim-stack changes.  I ended up enlarging the hole entries on all pistons slightly to improve rebound replenishment & damping flows using the Dremel tool, and also drilling out the cartridge-tube holes to .228", and the base valve compression-replenishment holes to .106" to support flow from the new double-size compression ports.  Some other hole specs which might be of interest to a suspension shop:

Compression clicker tube           3.05mm / .120" dia.
Compression piston holes (4)     3.6mm / .143" each
Rebound clicker tube                 2.5mm / .099"
Rebound piston holes (4)           3.5mm / .138" each

Reversing Compression Piston: This seems like a radical procedure, but it's pretty easy to do and works really well, at least on these forks. Normally the shims fit against the side of the piston with holes flush with the shim seating surface, while what I call the 'Iron Cross' (large squarish) openings are recessed; the other piston side has a center circular recess for the mounting shaft and the large openings flush, along with the check valve & spring.  First I filled in the center recesses with 8x10mm shims (one side took two .3mm thick / one 25 and one .1, the other used four .25 and one .1; these may or may not be really necessary, but I didn't want to chance it), then the shim stacks on top.  The other side is easy - just install the check valve & spring as in the photo, but be sure that the check valve shim moves freely on the shaft without getting pinched, as now there's no recess to ensure it doesn't.

The valving I ended up using was (per side):

Compression: one ea. 8x20x.1 & 8x14x.1 shims, two 8x11x.3 spacers to provide adequate deflection
Rebound: one ea. 8x20x.1 / 8x18x.1 / 8x16x.1 shims, one 8x10x.3 cap shim, one 8x10x1.5 spacer

There's little leverage on the small-diameter shims' edges & they must bend sharply for max flow, so I was a bit concerned about permanent deformation of the shim steel at large deflections. The Sandvik 20c 8x20x.1 shims I used (from Suspension Direct and Racing Suspension Products) began to deform slightly at 3.2mm / 35° deflection and my stacks deflect 2.6mm max, so hopefully I'll be OK.  I saw no evidence of deformation in the stock shims, which were all flat as pancakes.

Here are the modified pistons, final shim stack above, check valves, etc.:



... and here's a side view of the complete modified valves, showing the enlarged D-shaped hole entries (compression valve on the left):



The bottoming arrangement is quite simple: a fairly stiff 60mm-long spring between the plastic main spring guide 'torpedo' and the aluminum cap locking collar (see cartridge photo above).  I'd much prefer a hydraulic bottoming cone (as used on both older and newer forks) since it steals less travel, but this spring is definitely effective.  I may try a progressive bottoming spring at some point, which might be good for up to an additional 20mm of usable travel (I've rarely seen over 180mm travel on these 210mm-travel forks up till now, but there may be a bit more since this revalving work).

As far as oil goes, I put 370cc back in; 320cc Red Line LikeWater oil (@ 0.5W) + 20cc Tri-Flow PTFE (Teflon) liquid + 30cc Ultraflon DF-20A PTFE/alcohol solution (sediment layer is @ 60% Teflon).  The Tri-Flow lubricant has some kind of additive to help keep the Teflon in suspension longer.  I also added 3mm more spring preload (12mm total) to improve my sag numbers.  I think next oil change I'll put in significantly less oil (300 - 320cc total) to reduce air pressure near bottoming, but I wanted an identical baseline this time for comparison.

NOTE: If you've read this far and perhaps are thinking about having similar work done by a suspension place with proper tools, you might want to consider having them disassemble your forks & ship them back to you apart, modifying the shim stacks & pistons yourself using common hand tools (I don't think the grinding I did is that critical, but reversing the compression pistons definitely is), then returning them for reassembly / oil / etc.  Even with the extra shipping cost this should save significant money, but most importantly it guarantees they'll be done correctly.  If you do decide to have them do everything, I'd highly recommend talking to the person actually doing the work first; if they haven't the slightest intention of doing what you need done & instead plan on the usual seals/oil/clicker tweak/$$$, you'll likely get the message loud and clear.    Shop around, there are tons of them out there - sometimes local MC dealers & custom shops know of local ones who do good work.

Ray