DirtRider . Net MX, SX, Arena Cross, Off-Road Community
Dirt Rider . Net Text Version Home
Dirt Bike Dirt Bike Dirt Bike Dirt Bike

This is the text version of DirtRider.Net
Click Here for the Full Version


Pages: 1

KDX250 Fork Spring Source

(Click here to view the original thread with full colors/images)

Posted by: ssaulnier---------------------

I want to improve the ride on my 1991 KDX-250. I read that the stock fork springs are too soft (0.35 kg/mm) and the damping is not ideal. Race Tech makes a Gold Valve kit that is supposed to help the damping and they recommend 0.455 kg/mm springs for my 210 pound weight.

Where can I buy 0.455 kg/mm springs for this 43 mm KYB fork?

Although this is an old bike it is still in good condition and is fun to ride. I swapped bikes with another over 50 guy and rode his new Yamaha 250 4 stroke. I did not really feel much difference except the KDX250 has a lot more low end & midrange but you really have to rev the 4 stroke to get much power. He got off my old KDX with a big smile.

Thanks in advance!


Posted by: dirt bike dave---------------------

I had a '91 KDX-250, with gold valves in the front forks. I put in 0.40 kg/mm fork springs and weighed about 190 when I had the bike.

IMO, the Race Tech calculator recommends too stiff a spring for your intended use (trail riding). I would suggest more like 0.41 - 0.42 kg/mm for your weight. I think 0.455 will be too stiff.

The KDX250 has slow steering geometry. So to improve cornering in the tight woods, you want a little bit of fork dive when braking into corners. Overly stiff springs will hurt this. On my KDX250, I set it up with no spring pre-load. You can also grind the steering stop tabs on the frame slightly to allow for a tighter turning, without hitting the forks on the gas tank. If you go down 1 tooth on your countershaft sprocket from stock, you can take two links off the chain and move the rear wheel all the way forward in the adjustment range. The bike will still be plenty stable at speed (especially if you have a steering damper).

You can probably order the springs directly from Race Tech. You may have to call, as many companies do not include the older, more obscure bikes on their websites.

If you like your KDX 250 now, you will like it a bunch more when you fix the forks! It's a really decent bike underneath, but Kawasaki did not do a good job on the forks and the jetting, as well as a few other set-up related items.

The bike has excellent low end power for a 250, and you can get even more with an '88 KX250 thin head gasket. There is a lot more info floating around out there on this site for your bike, but you might have to dig for it.


Posted by: glad2ride---------------------

You can get the springs for a 1993 - 1995 KX125 / KX250 to work. You will need to grind the white, plastic spring guides that go on the damper rod. The inner diameter of the springs increases as the rate goes up, and at some point they will rub on the spring guides. You will need a spacer to take up a little bit of difference in the spring length. Eibach used to make a 997 series spring for the 1991 - 1994 KDX250, but they discontinued it several years ago.


Posted by: ssaulnier---------------------

Thanks for the advice! I will go with the 0.41 or 0.42 kg/mm springs if I can find any.

I can't find any on line sources for 93-95 KX springs from Race Tech, Eibach or Progressive. They all list 89/91-92 KX springs though.

I'll call Race Tech when I get back home from business trip. I sent them an email last week with no response.


Posted by: Matt90GT---------------------

If I remember correctly, we use to use the stock XR600 or XR400 fork springs on the KDX250s. Do some research there, but that is my memory.

But you are way better off swaping forks for the KX of the same vintage 91-95. Better valving and rebound adjustment to boot.


Posted by: glad2ride---------------------

RT # 3949

I don't think .40 or .41 will be stiff enough. With the combined weight of the bike an rider, some .44 or .46 springs may be in order.


Posted by: ssaulnier---------------------

Thanks for the advice. I think installing a set of 91-95 KX forks makes the best financial sense.


Posted by: ssaulnier---------------------

I bought a set of 1993 KX250 forks off the on-line auction site. They will need to be rebuilt (seals, orings, bushings etc) and the blue anodize coating is a bit worn off on spots with a little surface corrosion (my KDX did the same). When I take them apart for rebuild I can sand off the corrosion and get them anozided again.

Is there another coating that would be better than anodize?

I have always bought factory replacement oil and dust seals, bushings, etc. Is any of the aftermarket stuff longer lived or much less expensive or should I stick with Kawasaki parts?

I am looking for a more plush ride and less front wheel deflection in the woods over rocks and roots in 2nd and 3rd gear. I also want to start practicing a little bit of jumping (nothing extreme just single jumps or table top stuff). I am 51 now and do not like crashing.

Also is the stock damping ok for trail riding or should I modify it? Are there adjustment that I can make while I am rebuilding them without buying a RaceTech Gold Valve kit?

The RaceTech web site lists the stock KX250 springs at 0.377 kg/mm, so they are a little stiffer than the stock KDX250 springs (RaceTech site says 0.34 kg/mm). I guess I will try the KX springs first and see how it feels for my 210 pounds (no gear).


Posted by: dirt bike dave---------------------

As a general rule, you want less high speed compression damping for rocks and roots. Most MX bikes have too much high speed compression in the forks to be optimum on the trails.

Even the KDX 200 and 250 have too much high speed compression for very sharp hits, and Kawasaki makes it livable with too soft springs, which are then horrible on jumps and big whoops.

BTW, a free way to slightly stiffen a spring is to cut a few coils off (make sure you don't cut too much off or you will get coil bind before full travel). You will need to lengthen your pre-load spacer if you do this.


Posted by: mudpack---------------------

Quote:
Originally Posted by ssaulnier
Is there another coating that would be better than anodize?
Better in what way?
Anodizing is one of the hardest of all the aluminum coatings, but it is also costly. It will also allow the dissipation of heat better than paint or powdercoating.


Posted by: glad2ride---------------------

For the KX250 forks, I would stick with OEM parts and modify the base valve stack for less high speed compression.


Posted by: jeffcooper---------------------

I replaced mine with a set of KX '94 forks, and of course a new wheel and axle to go with them, but even without setting the new ones up, they're a different world to the original!


Posted by: ssaulnier---------------------

Thanks for the advice about modifying the stock valve stack. How can I get some guidance on how to perform this modification to reduce the high speed compression damping and where to get the valve shims? Looking at the OEM parts diagrams on the Babbits.com page just shows a "valve assembly".


Posted by: glad2ride---------------------

You might want to search through the suspension forum for posts on others doing it. You might be able to get away with just removing a few shims instead of replacing them. Less shims = less damping (slowing down the oil)



Text Version Home





vBulletin Copyright ©2000 - 2009, Jelsoft Enterprises Limited.
vB Easy Archive Final ©2000 - 2009 - Created by Stefan "Xenon" Kaeser