Have New Fork Springs, Top out mod, Why? - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 10 (permalink) Old 07-26-2017, 03:51 PM Thread Starter
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Have New Fork Springs, Top out mod, Why?

Hey guys. I am re-springing my forks for my weight on my daily driver and making it a "for now" track day bike as I have crashed and totaled my track prepped, GSXR750 after 4 years of tracking. Searched and tried to understand where to start on RC51 SP2 forks. My forks are soft for my weight and bottom under medium braking, as most do. I do understand step one is always to spring the bike for you weight. I cannot do any valve upgrades until winter but would like to do what I can for now while changing springs.

Soooooo the question is,
What is the top out spring mod? Should I do it at this point? Do I need to do it? If so, does someone know a link to share on a thread about the process?

Thank you guys very much for your time. Rich

Last edited by Richard7676; 07-26-2017 at 07:06 PM.
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post #2 of 10 (permalink) Old 07-26-2017, 05:07 PM
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SP-1 or SP-2?
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post #3 of 10 (permalink) Old 07-26-2017, 07:05 PM Thread Starter
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Ahh yes. SP2
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post #4 of 10 (permalink) Old 07-26-2017, 11:10 PM
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Stock SP-2 cartridge in the centre, SP-1 on the right, modified SP-2 on the left.



As you can see the SP-2 has long, straight rate top-out springs. These work pretty well for lighter riders however given the weight of the bikes and the relatively soft rate of the top-out springs the forks tend to sit low in the travel. Frustratingly as you dial in pre-load the soft top-out springs will compress relatively more than the main springs and for heavier riders on the track the forks will still bottom out regardless of how much preload is added (guess you know this).

Stock SP-2 top-out spring above, my cut down top-out spring + spacer in the middle, Dan Kyle modified top-out spring + spacer at the bottom.



Reducing the length of the top-out spring increases it's spring rate. The forks will sit higher in the travel for a given amount of preload and adding preload will also act on the mainsprings relatively more than the stock setup. Adding a spacer will help keep the top-out spring true when it compresses. Happy days.

Obviously my modified top-out springs will be softer than DK's (I didn't actually measure them) but as I'm pretty light they work for me on the street with the stock mainsprings. Once you start looking at changing the rate of the main springs it becomes a more complex story as depending on the length of the replacements the spring spacers will have to be modified or replaced to achieve the correct installed preload.
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post #5 of 10 (permalink) Old 07-26-2017, 11:36 PM
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To answer your question I think it's pointless just replacing the main springs without re-valving (the stock rebound damping is inadequate for the stock mainspring as it is) and as the top-out springs cannot be accessed without removing the rebound valving in any case it makes much more sense to do it all at once.

So at this point I'd consult a tuner. Dan Kyle has many years experience modifying the stock forks with many options for re-valving including re-using the stock pistons.

If you want to do the work yourself I'd start by calculating the spring rate you need, the RaceTech calculator may assist:

RT - Digital Product Search
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post #6 of 10 (permalink) Old 07-27-2017, 01:28 PM
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The real issue is the brake dive is controlled by the valving not the springs. If you put enough spring rate in the fork to keep the front end from diving under braking during normal riding the front would ride so high that your geometry will never be correct. The valving is the key especially with the already front biased RC Piggy One.

Basically I agree 100% with JB's assessment about doing it all at once. Go big or go home

That being said if you go the Dan Kyle route I am doing 24-48 hour turn around on RC51 forks currently... They come in and go right back out the next day in most cases.
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post #7 of 10 (permalink) Old 07-27-2017, 03:41 PM
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Originally Posted by Richard7676 View Post
Hey guys. Thank you guys very much for your time. Rich
A number of us here (many of us?) have Kyle-reworked forks, and I can tell you that they're like buttah. Buttah! The beauty of it is that once you give LDH your info, like rider weight with gear, riding style, racing intentions, etc, when you get the forks back you bolt them in and and go ride. No twiddling of comp and rebound knobs, and no real need to adjust sag (but you should check it). Bolt 'em in and go, knowing you have the experience of zillions of previous Kyle RC forks to rely on. [My sag/preload was dead-nuts on, based on the rider weight I provided, FYI]

I realize it's a bit more spendy than buying $150 worth of springs and oil, but because you're a racer you'll fully understand what you're getting for your money. Buttah!

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post #8 of 10 (permalink) Old 07-28-2017, 03:08 AM
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I can attest to Kyle Racing and their fork rebuilds. I got the whole package from them. I am finally getting to ride my bike more this year and I'm finding that now that the suspension is awesome and I'm riding a lot smoother and faster that my brakes are now the weak link! Lol
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post #9 of 10 (permalink) Old 07-28-2017, 12:58 PM Thread Starter
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Many Thanks

I hear the wise words and you got me convinced. I have been spoiled to good suspension up to now and I know I will never be happy without it. My GSXR was an AK-20 cartridge setup in front with an Ohlins out back so I do value money well spent on what matters. I will send off my forks once my better half gets back to work.

Thanks to all for the effort to respond. It is well appreciated. Rich
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post #10 of 10 (permalink) Old 07-29-2017, 11:15 AM
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I hear the wise words and you got me convinced. Thanks to all for the effort to respond. It is well appreciated. Rich
The Force is strong with this one.....


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