Fork Shim Stack

Discussion in '3rd & 4th Generation 1990-1997' started by Lovie, Apr 10, 2022.

  1. Lovie

    Lovie New Member

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    I'm currently working on a Gen 3 (1990) fork overhaul and looking to re-shim the standard valves, I know I could and probably should get some RT gold valves but I want to see what can be achieved with the standard setup.

    I'm convinced that after 32 years and 60,000 miles the current shims are passed their best, the tarnishing on them seems to confirm they are less than perfect. As a base setup I'm aiming for slightly (≈25%) stiffer than standard so will replace both stacks like-for-like and then swap/add some 0.1mm and 0.15mm shims to create the desired stiffness. But before I do I'd like to confirm if my current setup is indeed standard, can anyone confirm standard shim stack for a 1990 or any other Gen 3 year?

    Current Rebound Stack: 9 x Ø17 x Ø6 x 0.1mm
    (clamp) 1 x Ø8.5 x Ø6 x 0.2mm

    Current Compression Stack: 5 x Ø17 x Ø6 x 0.1mm
    (clamp) 1 x Ø8.5 x Ø6 x 0.2mm

    Shim 1.jpg
    Shim 2.jpg
     
  2. squirrelman

    squirrelman Member

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    see my pm
     
  3. Terry Smith

    Terry Smith Member

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    I don't know the specifics of the 3rd gen shims stacks. I can tell you that mathematically a 0.15mm thick shim is 3.4 times as stiff as a 0.1mm shim.

    With Gold Valves on my 5G I use 6 x 0.15mm x 17 main shims for rebound, and 2 x 0.15mm x 17 main shims for compression. The latter is one less that RT recommend. Those valve bodies in your 3G actually look pretty decent for flow, and much less restrictive than the HMAS valve bodies in the 5G.

    Personally I would not worry about tarnishing on the shims as long as they have clean flat surfaces.
     
  4. Lovie

    Lovie New Member

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    Thanks for the info guys

    Agreed, the 3-port valve looks much less restrictive than the 4-port HMAS, there’s an article by Peter Verdone discussing this point. This is why I don’t see the point in installing gold valves. When I got the bike the fork internals were standard and when I removed the rebound valves the damper rods were neatly mushroomed over the retaining nut, as if from factory, so I guess I’m safe to assume that this is the standard valving and Mr Honda got it a little bit wrong.

    I’ll keep you posted on my progress, at the moment I’m planning a slight increase on overall stiffness and the addition of tapered stacks to get a bit more of a digressive damping curve. If I’m feeling adventurous I might go as far as a pre-load shim on the compression side and play around with chamfering the ports to reduce turbulence, but that might come with Mk2.

    Probably preaching to the choir but I've picked up a lot of useful theory (although MX biased) on Restackor and from Peter Verdone for anyone that’s interested.
     
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  5. Lovie

    Lovie New Member

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    Happy to report that the re-shimmed valves are in and have made a significant improvement to the bike – there is definitely less front end-dive and pogoing on the brakes and through the rough stuff which was my biggest issue with the front. It is nicely plush, so damping is probably still a little bit on the light side, but it’s a VFR not a CBR… I’m going to leave it for now, I’ve made a few other changes to the front/rear this winter so want to get used to the new baseline before going any further.

    I took some advice from a suspension guru on another Honda forum and went with a pre-loaded compression stack, the full details are below. I had to make do with what I had and ended up making the pre-load shims at work so it's probably not ideal but I'm very happy with the result.

    As well as doing the shims I also replaced the bushing and seals which no doubt has helped to breathe some fresh life into the forks. I went with Silkolene Pro RSF 7.5wt which is more like a 10wt in terms of viscosity which I hoped would compensate for the softer damping, topped it off to 170mm air-gap, and slipped the forks 7mm through the top yoke. I made longer spacers so I could use the full range of pre-load adjustment for one-up vs two-up riding but went a little too far as I could only get 29mm rider sag with them fully out, but that’s not a big job to fix. After a quick test run avoiding any really big holes I had used 90mm travel so with the correct sag I think I should be close.

    This upgrade along with the rear eccentric flip have transformed the bike, I would recommended it to anyone looking to improve the stock suspension.

    Thanks again for the support, happy to share any details with anyone that’s interested.

    Compression:
    1 x Ø17xØ6x0.15
    1 x Ø17xØ14.1x0.229 (Ring)
    1 x Ø14xØ6x0.1 (Centre)
    2 x Ø17xØ6x0.15
    1 x Ø17xØ6x0.1
    1 x Ø15xØ6x0.1
    1 x Ø13xØ6x0.1
    1 x Ø8.5xØ6x0.2

    Rebound:
    3 x Ø17xØ6x0.15
    2 x Ø17xØ6x0.1
    1 x Ø15xØ6x0.1
    1 x Ø13xØ6x0.1
    1 x Ø8.5xØ6x0.2
     
    Last edited: Apr 18, 2022
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