Fork re-design

11 replies to this topic
  • DaveJ

Posted May 21, 2001 - 05:47 PM


Another long one.

For the last year and half I’m been attempting to resolve two key design flaws that are inherent with the 43mm production forks used in the 125 through 426 displacement YZ production bikes.

Some users on this forum have contributed to this effort with either recommendations or with similar complaints. I’m not about to say I can resolve arm pump and blisters, or keep you from getting broken up when you case a 100 foot triple, but perhaps I can offer an alternative for those that are suspecting that things could a little better up front.

More specifically, these forks suffer from two inherent design flaws. 1. An inadequate bottoming system, and 2. Overly aggressive high compression damping.

I ride a 00 YZ 426. I weight 167 lbs. My riding style is probably very similar to the majority. I spend a lot of time riding trails but I would never consider myself a trail rider. The difference? Speed. I also spend a lot time riding tracks, many of which are the same ones the pros ride. I clear the doubles and triples, and when I don’t, my day is quickly done. I can keep up or pass about 80% of the weekend riders, but the best put me to the shame. Most importantly, my track days on the road machine taught me the simple principle that speed is the direct result of being smooth.

Since day one, this bike has rattled the front wheel with an agitation that puts blisters on my hands and sores on my kidneys. It felt like the springs were just too stiff, and the hit of every rock and bump transmitted through the bars with enough vibration that I often wondered if the forks were moving at all. Turns out they were.

To make matters worse, my not so perfect landings slammed the internals of the forks so hard that my wrists found new limits in movement. I was taking the abuse that the bike should have been.

So onward.

If I have counted correctly, there are 15 factors in the hydraulic design of these forks that effect damping. 15!

All of which can be modified to change the handling. And I think I can safely say that in my efforts I have modified at least half of these options in several ways.

Last weekend, after concluding about 100 hours of work and who knows how many hours of testing (keep in mind I took last summer off to ride) I may have concluded my work. The day ended in fork nirvana, so much so that I finished the day without a kidney belt, the true test of all tests. I have never smiled and laughed so much during a hard ride. And I’ll spare you the obvious glee from the day’s jumping efforts.

So here’s the deal. The harshness is caused by a thing called the “mid-valve”. It’s a stack of shim plates on the opposing side of the rebound piston. It is either causing too much high-speed compression, or it’s failing altogether resulting in total loss of all compression, (something that you may not even be aware of). I tried nearly every modification in the book, including recommendations from 4 of what would be considered the top 10 suspension shops. Some did the trick, but all failed the longevity test.

As for bottoming, the fork uses a very short hydraulic stopper in the seal head and of course traditional rubber bumpers in the fork caps. This means you go from free movement to no movement in the space of about 10 to 12mm.

The fix.

To my surprise, I reverted back to some of the first efforts that took place. In the beginning, I installed a Race Tech Gold Valve kit. The conclusion. The valve is not as superior as the factory unit. Sorry loyal fans. However, they are the only ones that came up with the genius idea of removing the mid-valve and running a restrictor plate.

11 months later, I finally got hold of one of the many Japanese engineers at KYB only to have him tell me to do the same thing. “Yea yea, you take out mid-valve and be happy”.

So to rid the harshness, it’s out with the mid-valve stack and on with a 27mm check plate. Run one or two springs behind it. To get to it, you’ll need to remove the seal head from the cartridge tube. That’s a story in itself.

However, remove the mid-valve and you’ll get excessive oil flow passed the seal head since the shim stack built into the seal head is meant to be used with a mid-valve installed. More on this later.

Now for the good news. To provide yourself with the best bottoming system you’ll ever case a jump on, there seems to be two companies in the world that have done a very effective job at keeping their products a complete secret from those that need it the most. I still don’t know who the other one is. C-Cycles makes a replacement for the seal head that offers about 2.5 inches of hydraulic bottoming pleasure. However, you have to rip out the bushing from the factory seal heads, which usually destroys them just enough to destroy your compression rods, which you won’t learn about until four rides and $400 later. In other words, purchase the cones and the bushings. And as I found out, most suspensions shops carry the bushings but don’t even know what a seal head is. That bushing must also be used in shocks or something.

Now it gets silly, since the bottoming cones offer to much flow-by, (they too are meant to be used with a mid-valve) you have to run a restrictor plate, made available by the guys at Race-Tech. However, the design of the restrictor plate was meant to be used with the factory seal heads and not the bottoming cones. So too much oil flow is a bad thing, no oil flow is even worse. Therefore a spacer has to be created, in your very workshop, that allows the breather hole in the restrictor plate to reach the center of the bottoming cone bushing. It should stand .16 inches in height (double check my work) and about 27 to 31mm in diameter. You’ll also need to bond it to the top of the restrictor plate so that it doesn’t rotate during assembly.

Lastly, for those that are aware of removing the peens that hold the factory seal head, it turns out those holes you drilled leak. This effects the compression cycle of the fork, so get yourself some epoxy filler. Tricks of the trade.

Lastly, one little comment about oil. I don’t want to say you get what you pay for because I have tried many expensive oils that I never really liked. I’m not sure what to say about ATF, other than you may just want to dip your fingers into some real fork oil and feel the difference for yourself. Some things work. Other things work better.

Since I pulled the forks apart on a regular basis, I had my fair share of oil tests. I can’t say I tried them all, but I can say that nothing compares to the performance level of what I am running now. Do I have your attention? It’s KYB 01 suspension fluid. $15 a quart. Another secret kept. Enzo Racing and White Brothers can get it for you. If you’re serious, use nothing else.

As for details of other forks tuning goodies such as shims stacks in the base valve assembly and the use of ultra-adjusters, I’ll save that for later. But at least this is perhaps enough to let you know that you’ll never know what the best feels like until you have ridden on it.

Questions and comments are welcomed.


  • YZ400Court

Posted May 21, 2001 - 06:03 PM


Fascinating Dave.
Please, buy a KTM and fix the spongey front end, then post about it.

  • dirtdad

Posted May 22, 2001 - 03:36 AM


Yes, fascinating! DaveJ, you keep demonstrating to me that you must be an engineer (or just think like one). I'll make sure I never ask you something as simple as what time it is! I'm affraid I'll get instructions on how to best build a clock! LOL, just kidding. Very good post.

00 YZ426F
01 TT-R125L (my son's)
91 CR125
83 YZ490
74 Hodaka Super Combat(gone but not forgotten!)

  • Scott_F

Posted May 22, 2001 - 04:53 AM


Dave, a couple things...I made a tool for pressing out the seal head bushings without damaging them. Also, Pro-Circuit offers bottoming cones, but the price is high because they won't sell them unless they install them.

  • flyinguitars

Posted May 22, 2001 - 05:24 AM


Why dont you grab a video camera and make a "how to" tape? I'd buy it! I bet at least 10 other guys here would buy it also. Sometimes its easier to see the procedure than to follow written directions.

[This message has been edited by flyinguitars (edited 05-22-2001).]

  • SUnruh

Posted May 22, 2001 - 07:41 AM


very interesting read.
my suspension guy Jeff Howe -> and Rob Mann -> have had this very setup for a couple of years now. however, Jeff has the brains to actually MAKE the mid-valve work the way it was intended to. i have the bottoming cones in my bike and talked John Walker (EMARacing, 426) into the same setup. if you find us in person you will not get us to shutup about our suspensions. i've had Factory Connection (don't compare). john's had several and says they don't compare either. we both agree (violently i might add) that the bottoming cones are the best darn thing for forks. i've taken my bike from arenacross to harescamble and only changed the compression 2 clicks.

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  • shaun

Posted May 22, 2001 - 09:20 PM


Thanks for the good info. I agree with flyinguitars. I would pay good money to see exactly what you are talking about.

I recall reading something last year about installing new bottoming cones to help with the bottoming harshness. It sounds like you can't just replace the cones. I assumed the spring rate of the stock cone was to high after reading this article. They didn't mention anything about the hydraulic effect of the cone.

  • Scott_F

Posted May 23, 2001 - 05:01 AM


Shaun, bottoming cones are different than bottoming bumpers. They are purely hydraulic and have no "spring rate".

  • DaveJ

Posted May 23, 2001 - 09:59 PM


Scott - Does FactoryConnection make their own cones or do they OEM them?

And I just found out that Race-Tech is now selling a "mid-valve removal kit". $99.


  • SUnruh

Posted May 23, 2001 - 11:19 AM


dave j,
by removing your mid-valve you are taking away 1/3 (or more) of the ability of your forks. just because yamaha has kyb ship them all screwed up does NOT mean that it needs to be removed, just fixed. my '98 had a full Factory Connection setup (mid-valve removed). my '01yz250f (done by jeff howe) HAS the mid-valve and is a superior setup in ALL situations. in a back to back comparison, i'll take the setup EVERY time!

  • DaveJ

Posted May 23, 2001 - 03:28 PM



There are many variables in running or not running a mid-valve. I don't know what additional work they have or have not done on the bike with the mid removed, so don't jump to any conclusion too quick.

I had also tried a collection of designs with the mid-valve installed, some with very good results, but none that could reliability hold up to long-term abuse.

Additionally, when I spoke about this with the boys at KYB, they made it sound like removing the mid-valve was as much of an option as swapping springs. Their only disappointment was that they didn't have a collection of rebound pistons to offer me for the 32mm cartridge.

In either case, suspension work never ends. It can always be better. I'll be the first to attempt that I would never consider one design over the other, knowing that each has something to offer.

It's too bad you're not in the area. We could meet up and swap bikes for a session.


  • Scott_F

Posted May 25, 2001 - 04:39 AM


Dave, I don't remember if Factory Connection makes their own cones. I think Tom Morgan makes them also.

As for mid-valves, my general opinion is that removing the shims and replacing them with a check plate is better for off road or enduro type riding. It can provide a super plush feel, but can blow through the travel too quickly for serious MX riding. Of course the effect can vary depending on the rest of the fork setup.

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