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Fork Damping - Honda 3-Port Valves

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    Fork Damping - Honda 3-Port Valves

    Hello,

    The good folks at VFRWorld pointed me this way in search of some Honda 3 port fork valve advice and general suspension guru wisdom.

    I’m rebuilding the front end on my 1990 VFR750 and aim to address the damping issues by delving into the world of re-shimming the 3-port Honda valves which I know isn't strictly Hawk related but I believe they are similar / the same as those from certain CBR600’s of a similar age and a common upgrade for the NT650.

    My current, underdamped, setup is as follows:

    Rebound :
    9 x 17 x 6 x 0.1mm
    1 x 8.5 x 6 x 0.2mm

    Compression :
    5 x 17 x 6 x 0.1mm
    1 x 8.5 x 6 x 0.2mm

    My main issue with this setup is the lack of low / mid speed compression and rebound damping. I’ve gone up to 15wt oil, can’t remember brand for cSt rating, with not much improvement. My goal is to improve the aforementioned damping issue without creating an overly harsh ride.

    As a starting point I’d like to confirm if my current setup is indeed standard. The rest of the fork internals were standard when I got the bike and the damper rod was neatly mushroomed over the rebound valve retaining nut, as if from factory, so I’m guessing that it is standard but any knowledge would be useful.

    I like the look of the setup with the pre-load shim described in the thread : Interchangeability of Hawk/F2/F3 forks. It seems to address the low-speed damping issue without creating high speed harshness which I think will be well suited to my less that perfect road conditions and riding preference. I am tempted to start with a slightly lighter version of this but with a more typical ≈ 7.5wt / 30cSt oil. Can anyone comment on this setup or similar?

    Lastly does anyone have experience with porting valves to reduce turbulence? What is the specific goal here, is it to reduce overall friction losses in the ports (i.e. maximise effective port size) or is it to limit the onset of turbulence over a greater velocity range for a more consistent damping effect (i.e. avoid damping ramping up on high speed hits)? This is probably getting beyond the point where I’d notice any real-world difference but I like to play around with the theory. Since maximum fluid velocity occurs during high-speed compression events it make sense to focus on any active ports during the compression stroke, I see no reason not to chamfer the inlet-side of the compression ports and both-sides of the rebound by-pass ports, the added advantage of the later being to reduce the impact of the rebound valve on compression damping and potentially reduce the occurrence of cavitation.

    Any help or advice on the above would be greatly appreciated. I plan to document this as I go so will keep posted with results for future reference.

    Cheers, Lewis
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    #2
    JanM is your man.
    Don't spend money and buy, spend time and learn.

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      #3
      Hello Lewis,

      I don't know if your listed stacks are stock VFR, but they do look soft. For 3 port Showa pistons I usually use the preloaded stacks you mention, as well as several other (mahonkin).

      I just did a Suzuki GSXR 750 SRAD with similar pistons and used preloaded stacks Traxxion style. If you do your own work, it isn't a problem to try different stacks, so I would suggest to start on the softer side, but still stiffer than your current stacks. Something like 2 to 3 0.15x17 with 0.1 preload and a 9mm clamp for compression and 4x 0.15x17 with 0.05 preload and a 9mm clamp on rebound and start from there. These stacks are without faceshim or the preload and centering rings, so these needs to be added. You can vary number of 0.15x17shims, clamp sizes and preload to change the damping performance. Use something like 7.5W (viscosity and W is another topic...). Also get some linear springs.

      Oil viscosity has less impact in a cartridge setup using thinner viscosities compared to damperrod setups using thicker viscosities.

      Turbulence is topic that comes up from time to time - some factories add a chamfer to the inlet ports, others don't and argue that it isn't necessary or even beneficial. I do think that the inlet port area is important and so is return flow. If you look at the Independent Bike Suspension Forum there are many threads on similar topics,

      BR, Jan.

      Comment


        #4
        Thank you Jan, one happy customer!

        I had to stray from your settings a little bit to make do with what I had but I’m happy to report a significant improvement – 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.

        On the compression I ended up running 0.13mm pre-load as I had to get the ring shims made at work from 0.009” stock, so compensated with a slightly softer stack after the pre-load shims. I also added a tapered end to the stack to avoid bottoming on the clamp and because I had the shims available… I decided not to run any pre-load on the rebound. I finished the whole thing off 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 about 90mm travel so with the correct sag I think I should be close, I'll measure and record everything properly once I've adjusted the spacers.

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

        Compression:
        1 x 17x6x0.15
        1 x 17x14.1x0.229 (Ring)
        1 x 14x6x0.1 (Centre)
        2 x 17x6x0.15
        1 x 17x6x0.1
        1 x 15x6x0.1
        1 x 13x6x0.1
        1 x 8.5x6x0.2

        Rebound:
        3 x 17x6x0.15
        2 x 17x6x0.1
        1 x 15x6x0.1
        1 x 13x6x0.1
        1 x 8.5x6x0.2
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          #5
          Hi Lewis, thanks for the detailed feedback. BR, Jan.

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            #6
            Thanks Lewis. Keep us updated.

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