My first adventure re-shiming cartridge forks

Using information available on the net, trading email, picking the local suspension guy's brain (only slightly successfull) and thinking too hard about it, I came up with enough confidence to gut some F3 forks, reshim them, and slap them into my Honda Hawk GT.

The numbers first:

Donor forks: '95 CBR600 F3

compression stack:
    valve
    (1) .15 x 17mm
    Preload shims (.15 x 14mm nested inside a .20 x 17 x 14.1)
    (2) .15 x 17mm
    (1) .20 x 9mm (locking shim)
    base.

Rebound stack:
    valve
    (2) .15 x 17mm
    (2) .10 x 17mm
    (1) .15 x 16mm
    (1) .15 x 15mm
    (1) .15 x 14mm
    (1) .20 x 9mm (locking shim)
    (1) .43 x 11.36 (another spacer before the nut)
    Green = re-used shims, black is new shims

Oil weight: RaceTech US-1 (2.5 / 5 weight)
Oil height: 4.5"
Springs: RaceTech .90 kg/mm (I think)
Spacer: 75mm, to give 20mm of preload when the preload adjuster is in the middle
Rebound adjuster: 12 clicks out from seated. (two turns)

Impressions:

I've only had it out for about 55 miles, a shake out and a couple days of commuting, but so far it just rocks.

My basis of comparison was RaceTech cartridge emulators and 15 weight oil (or maybe it was 10 weight, it's hard to remember, and the cold makes it all thicker anyway. The bike has s Fox shock in the back, which until today was really plenty good.

Emulators are wonderful, they eliminate almost all of the problems with damper rod forks. They make compression way more linear so you don't loose your fillings on pavement cracks, and the front end doesn't wallow and bounce when braking. However, the rebound is still the basic "tiny little hole" oraface damping, so it's pretty non-linear, and you have to use a pretty thick oil to get enough rebound damping, 10, 15, or even 20 weight, depending on the season and the bike. Temperature has a greater effect on heavier oils than on light ones, so the light oil used in cartridge forks don't change nearly as much when the bike warms up, which means the bike's behavior doesn't change.

The bike feels way more stable now, but less harsh. Seriously. Brake dive is vastly reduced, there's no bounce-back afterward, when weaving back and forth or going into a curve I can't feel the bike "settle" into the curve like it used to, it's just there. It has the additional effect of making it turn quicker, even though I actually raised the triples a tiny amount.

It used to be that I'd notice the front end of the bike, the back was just planted, and stable. That Fox shock was doing its job. Now it feels like the Fox is harsh. The front end has improved so much that a quality shock like a Fox feels harsh.

I had a similar experience when I first got the bike. When I got it the suspension was completely stock. The first thing I fixed was the front suspension because it was the thing that felt the worst to me. I was aware of it, which is to say, the back had fewer shortcomings. I got RaceTech springs and emulators and the front was improved a great deal. Unfortunately, that made me notice the harsh, unpleasant rear shock, so I replaced it with a Progressive brand shock I got from some guy on the net. That made things much better. Eventually the shock ate itself (wore out) and the bike felt crappy again, so I replaced the Progressive with the Fox that's on there now, and things were much better again.

Mind you, the Fox is not worn out. It doesn't leak, it's got good damping, it's not bent, it's got a full charge of N₂, etc. It hasn't been rebuilt in a while, but I can tell when a shock is shot.

The front is that good, it makes a good shock seem iffy. Now I have to get a Penske. (another $700 down the tubes!)

Compression Stack

I got the idea for for this compression stack from the net. It makes more sense to me than the other methods, which get complicated with double stacks that hit each other, each one being a long stack of shims of diminishing size. Too fussy, and they don't really address the problem that shim deformation is quite progressive, it doesn't go up linearly.

Preload shims

The preload shims are simply a thicker outer shim and a thinner inner one. The idea is that all the shims above (two in this case) have a preload of whatever the thickness difference is. The intent (and it appears to work) is that the low speed damping can be quite high, since there's that preload on the shims, but once the "break" point is reached the valve opens up quite quickly, the spring rate is actually quite low, what with only the 3 .15 x 17 and the one .15 x 14mm, but the initial force on them is high because of the preload on the two shims on top.

I got the shims from RaceTech directly, you can buy a 10-pack of .20 thick, 14.1mm ID, 17mm OD shims, and a 10 pack of .15 thick, 6mm ID, 14mm OD shims. It'll cost you $32 plus $10 shipping and handling, but you can do it.

The F3 tubes + Hawk sliders are 7.5mm shorter than the Hawk tubes and sliders. This is different because the internal design of the Hawk and F3 tubes is different
Because the overall length is somewhat dependant on the top out spring, it's important to measure from the bottom of the lower triple clamp to the top of the slider. I'll measure what would be using stock tubes and stock position, but I dont' have that now.

The F2/3 fork tubes are 38.9mm (1.53") longer than SV650 fork tubes

If you're using '94 F2 or '95-'96 F3 forks (the ones that the rebound adjuster "clicks") Make sure the adjuster is in one of the "clicks" before you take the fork cap off the damper rod. If it's between clicks, the spring that makes the "click" click will be destroyed and it'll no longer click. Go ahead, ask me how I know this. If it doesn't click, it doesn't matter, it's not functional. The clicks are there to make adjusting easier.