KX500 USD Report

[kawagumby]

Kind of a long post, but....

I just got back from the shake-down cruise on my KDX220 with KX500 USD's. Since I did a search and couldn't find any info on 500 fork conversions I'll give the run-down here.

First off - they work very well geometry-wise, as they had been modified by a previous owner who had them shortened by about 1.5 inches. I used forks from a 2003 that were like new, and used triple clamps from a 1999 YZ250. The geometry (hole-center relationship) of the upper and lower YZ clamps was identical to the original KDX clamps.
Second - The modified 500 forks are SHORTER than the KDX forks by about 5/8 of an inch. I ran them with the caps flush to the clamp and the ride height was perfect for me. The 500 forks come with 40 kg springs so they will work for most folks.

A short run-down on the job:
The YZ triple clamp hole was the same as most KX's meaning it was about .001 larger than the KDX stem diameter. I knurled the KDX stem .005 and it was almost too much...I'd recommend no more than .003 press fit to do the job.
The YZ handlebar clamp location was about 3/16 further back than the KDX which I thought would be a problem for me... but rotating the bars foward was the trick. I still might opt for an adjustable upper clamp someday.


I used an antique KX wheel probably a 90 or 91 that I had to drill and tap for the larger disk rotor bolts that are current for my KDX rotor, but it worked fine.
I used a KX500 axle that I had to straighten - ebay ugh.
The KX500 axle floats at both ends in pinch clamps so allignment is a snap.

Brake-wise I used the KDX unit and zip-tied the line to the fork guard with the guide outside brackets removed. I ended up using a YZ fork guard guide/clamp because it had inside guides that would provide support without interfering with the brake line. I had to reshape the inside of the YZ guide/clamp with a dremel to fit the KX fork. No big deal.

I also had to trim the very top of the left fork guard to keep it from hitting under full travel the brake cable guide attached to the lower triple clamp.

The exposed top of the triple stem is perfect height above the triple clamp. No adjustment spacer needed. A stem "shim" is needed to take up slop in the top hole (although I left it out for today's ride, so it started clunking a little toward the end of the ride).

The ride:
I went on tight, badly worn single track, rough fire roads with small jumps. The forks made the bike steer much more precisely in all conditions and made it more stable in rough chop. I ended up running compression damping nearly all the way out so it needs to be revalved. The springs feel very firm (too much so when you just sit on the bike static) , but seem to work well in all riding conditions. I'm running a 4.8 rear spring so I thought it might be imbalanced but it sails level off of jumps and doesn't tend to rock around gas'n in the chop. No compression spikes in the travel.

The only part that was rough was down hill stutter-bumps, my huge (heh) biceps were flopping around like water ballons and itched like heck. Definitely a valving issue I'll be working on. It looks like I was using full travel or very close to it at times so the 40's might be the ticket after all.

[m0rie]

Good deal. With some tuning they will only get better!

[canyncarvr]

I hope you do post valve info..current and change-to.

Just from the standpoint of interesting reading and maybe even learning something!

The KX500 is still KYB I presume?

What is the tube diameter on them?

I figure I'll change to a YZ type fork guard guide/clamp somewhere along the line....

Thanks for the input! Good deal!!

[kawagumby]

The forks are KYB and are 46mm's.
The upper housing outer dia at the clamp location is just under 54mm.
The lower housing outer dia at the clamp location is just under 59mm.


I think most japanese companies went from 43mm forks to 46's in the late nineties. I have a set of 96 yamaha forks that are 43mm's and would not work with the 99 YZ triple clamp I used for this project.

Race Tech shows the fork slider sizes; 46's, 43's etc., on their site for the different year/brands of bikes which may help out in buying decisions. You don't need an access code to get that info.

[kawagumby]

I revalved the forks yesterday and rode it again today. The purists among you will no doubt cringe at what I did to improve the ride...but I stuck in the compression valve bodies from an O3 YZ250F that I had on a shelf. The reason I did so was that when I compared the valve setups side by side between the KX and the YZF units, the YZF clearly had more flow capability due to its larger valve body design. It is a single stage stack including (10) 24mm shims from which I removed 3.



It now works better and the stutter-bump issue previously encountered is gone.

Now if my back will only heal after all this pounding and testing all will be good.

[m0rie]

Doesn't bother me in the least. Any chance that you could post a listing of the shims in both the KX500 valving and the YZF250F valves? Would be nice to have from a comparison stand point.

[canyncarvr]

So, you're running SEVEN 24s....at what thickness? .1 or .15mm? (.004-.006")

Seven is a whole lot.

How about maybe THREE (if they're .15s). Or two........

But...you had full travel..with ALL of them in there? Maybe those .40s are .040s.....

[kawagumby]

They're all 004's.
This is a single stage stack - as a matter of fact, the original bike they came from, the YZ250F had one of the most plush suspensions I've ever ridden on...and it had 43's in the forks and the full compliment of shims (it was light too, about 217 dry as I recall).
Now get this; the KX500 stack had only (4) 24's @.004 and 5 taper-down shims (also setup as single stage) and was much more harsh all the way to mid-stroke. Also, the KX stack was not the stock setup. So, shims are not the only factor in the overall performance...if they were, you wouldn't see such a variety of valve body designs and race tech wouldn't be selling gold valves. The valve bodies, even the threaded bases, used for the YZ50F and the WR250F's for the same year are completely different designs, for example. That's why I had an extra set of YZF units to play with - after fiddling with WR units for woods riding.

At any rate (no pun intended), the springs in the KX500 forks are 40's all right, and a little too stiff for me, but I'll live with it. I'm coming close to bottoming only when nosing off of jumps and hitting square edges hard... within an inch or so. These forks have shorter travel than the 250/125's - I'd say about 10" effective wheel travel which gives a lot of control steering-wise on rough turns as it turns out (again, no pun intended). heh

I've been thinking about experimenting with converting to dual-stage with trying different stack setups, but that'll be later when I get more riding time in. I'm no expert at this but it's kind of fun to tinker when you have the time.

Man, this thing steers like it's laser-guided now.

[IdahoCharley]

One of the things you'll likely see is that the valve bodies will be different for different brand (same year) OEM bike even though KYB is suppling the forks. Cartridge rod diameter size, fixed orifice sizes, free space between the inner and outer tubes also play a role in sizing the valve body and piston.

I don't really know why it is so, but it is likely so that one manufacture can make claims for the improved suspension which is vastly superior (according to most manufactures) year to year. One of those -build it to my specs Mr KYB (me thinks); therefore shim stacks can vary widely. Also due to running mid-valves or not, bladders or not, etc.

Edit - sure go post while I'm writting and say it better than me. Good Job. I should dig out the original OEM specs on my 2001 CR125 forks something like 13 or 14 24X.1 on the BV: And a mid-valve with more shims than I've ever seen on it - I'm thinking about 20 shims counting the reverse taper stack.

[kawagumby]

Hey, what can I say, great minds working together! LOL...

[kawagumby]

Additional info : I just bought a 98 KX125 wheel for a spare. It came with the axle and it is identical, spacers and all, to the KX500 axle. So that era 125 wheels/axles will fit nicely with kx500 forks.