Budget SP-2 Fork Upgrades - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 86 (permalink) Old 01-14-2017, 06:52 AM Thread Starter
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Budget SP-2 Fork Upgrades

I have a separate project in the works for the new bike upgrading a spare pair of stock Showa forks featuring some expensive surface refinishing but in the meantime I thought I'd overhaul the existing units and see what's possible re-using the stock valves. I have to thank Way2slooow for his help both machining some spacers and for suggestions for the valving. This thread is not a 'how to', just some notes on how I went about it.

Some people remove the hydraulic anti bottom-out pistons from the cartridges altogether, I'm keeping them. If I knew for sure what valving I was going to settle with I'd probably disassemble the cartridges by driving the anti bottom-outs off the rods, do the re-valving and then re-install them. But as I might have to access the valves a few times to tweak the shim stacks I thought it'd be better to drill out the peening on the outside of the cartridges with a 4 mm bit and unscrew the bottom-out cups to get in there:





A trick Hord passed onto me a while back is drilling a 1 mm bleed in the side of the bottom-out cup approx 1/3 distance from the bottom (measuring from the inside). My bit is a hair finer, about .9 mm. Here it's shown after reassembly but I did the work with the rod removed from the cartridge for obvious reasons. The bleed helps the forks feel less like they've hit a brick wall in the travel when the anti bottom-out piston enters the cup on compression. The position of the bleed hole still gives some resistance to bottoming out if hitting a sharp bump hard on the brakes on the street or the ripple strip on the track, it also assists the cup to fill with oil behind the piston on rebound.

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post #2 of 86 (permalink) Old 01-14-2017, 07:09 AM Thread Starter
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At 75 kgs the stock mainsprings work for me. I have a pair of DK modified top-out springs (cut down + delrin spacer) and considered fitting them but decided for various reasons to keep the longer originals. One thing I really like about the Kyle mod is the spacer which prevents the top-out springs distorting on compression inside the cartridge like the OEM set-up potentially can:



Way2slooow machined some spacers for me which ensure the stock top-out springs stay true:



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post #3 of 86 (permalink) Old 01-14-2017, 07:48 AM Thread Starter
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Stock the SP-2 main springs have 8 mm installed preload with the adjuster backed right out, I prefer about 10 mm. A simple 4 mm spacer installed between the stock spacer and the main spring increases the installed preload to 9 mm, close enough. This together with the top-out spring mod significantly reduces the potential for the main spring to unseat from the top of the cartridge at full extension.

Stock:



Modified:

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post #4 of 86 (permalink) Old 01-14-2017, 07:57 AM Thread Starter
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The stock valves look identical to those I've seen on other Hondas going back as far as '94. I have quite a bit of experience replacing these with Ohlins and RaceTech components but none with re-valving them. Here's the stock shim stacks laid out - Rebound above, Comp. below:



And here's the upgraded shim stacks featuring Mid Valves as well as revised Comp. and Rebound:

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post #5 of 86 (permalink) Old 01-14-2017, 08:40 AM
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Cool thread. I have a set of stock forks I want to upgrade with Elka valves. I think I might want to ripoff some of your mods. I've never rebuilt, or even taken apart, forks before. This looks like fun.

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post #6 of 86 (permalink) Old 01-14-2017, 02:39 PM Thread Starter
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Originally Posted by BadToTheBown View Post
Cool thread. I have a set of stock forks I want to upgrade with Elka valves. I think I might want to ripoff some of your mods. I've never rebuilt, or even taken apart, forks before. This looks like fun.
It is fun. It's nice to have a fork spring compressor for working on these but if you can get away without one if you have someone to hold down the spacer so you can access the jam nut under the caps. A cartridge holding tool is handy too but it's easy enough to make out of al. or mild steel tube (I'm happy to do a drawing if you need one). The only essential tool I can think of is a torque wrench that'll measure inch-pounds for re-installing the valve retaining nuts.

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post #7 of 86 (permalink) Old 01-14-2017, 02:52 PM Thread Starter
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Oh, and a shaft holding tool.
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post #8 of 86 (permalink) Old 01-14-2017, 06:16 PM Thread Starter
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Many Hondas have a stepped landing for the fork inner bush which allows it to pivot a little reducing stiction under side loads. The SP-2 inner bush landing are flat:



I had them machined at a 1 degree angle:




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post #9 of 86 (permalink) Old 01-14-2017, 06:42 PM Thread Starter
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And the outside of the tubes are buffed in a cross-hatch pattern with 400 grit paper. This is something I've been doing to chrome tubes for years now, it can radically reduce stiction in the front forks.

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post #10 of 86 (permalink) Old 01-17-2017, 03:39 AM Thread Starter
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After a few days riding the bike I'd have to say the forks are miles from where they were - obviously less stiction (I measure it now @ 7 mm which is pretty damn good), much plusher ride and the damping is nice. The rebound is pretty much where I want it @ 1 turn out with no preload added - plenty of scope to dial in more if adding extra preload for the track etc. The compression is much, much better than standard - very responsive on small, sharp bumps and with a really nice feeling on the brakes. All in all a dramatic improvement for minimal outlay - I'm into this for the cost of the bushes, seals and oil at this stage and I'm very happy to ride it as it is for a while.

Sitting a little higher in the travel than it was :



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