It's an excellent question, but if you look around at the whole motorcycle industry...and not just Kawasaki...you'll find all kinds of warts apparent on many otherwise excellent motorcycles and ask the same question.

It seems inconceiveable that Kawasaki didn't make the holes in their compression piston in both the fork and the shock to flow oil more effectively, but the answer is "they didn't". Every suspension expert will tell you the same thing about the KLX specifically and about a surprising number of other models generally...many brands. Race Tech became famous for their Gold Valve setup, and that's not just because of Kawasaki forks. Many more modern bikes have gotten better about their main piston design after these many, many years and often only need shim changes to achieve ideal performance. But even some new ones benefit from a piston change.

The short answer might be that Kawasaki was trying to meet a middle-of-the-road appeal with the main valve design and shim stack, but in reality they missed the mark. If you don't have the main compression piston where it will flow enough oil, then the fork does an almost hydraulic lock as the oil slows almost to a stop under a hard, fast compression stroke resulting in a jarring feedback. This is a compression spike. The exact term probably results from what it looks like on a graph of a suspension dyno...a spike. When the main compression piston won't flow enough oil at a certain and critical point in the compression stroke, it doesn't matter what shim stack you have. It can't do its job if the oil flow pretty well stops or slows down to a point where the shims are ineffective. Now...if you can flow enough oil through the piston, then the shims can handle the rest of the damping for the compression stroke. I noticed that when I took the stock piston and shim stack out, there were very few shims in there...perhaps an effort to minimize the poor flowing piston? My Race Tech setup has 3 separate compression shim stacks, so obviously the fork oil is flowing now. Yes, you can have too much oil flow through an inappropriately design piston, and this would probably overwhelm the shim stack. It's a balancing act, but one that professional suspension tuners and engineers can achieve.

Race Tech, MotoPro, Pro Circuit, and many high end suspension tuners do this for a living and have the luxury of time and money to get it right. A mass produced fork and shock...and especially ones of the era of the KLX...gets what it gets from the factory, and they usually never look back unless they replace the fork or shock with a new model or design. I'll bet some/many of Kawasaki's more modern bikes need little if any valving mods outside of a shim stack tweak for a really personalized setup. That said, I notice in the latest test of the popular Yamaha WR250R dual purpose bike in Dirt Rider involved a complete Race Tech Gold Valve setup front and rear. They stated that the fork in particular was harsh and exhibited compression spiking until the mod. Go figure.

 

I will vouch for the MotoPro setup. I'm running their UMAX valves in the forks and shock and it's a huge improvement over stock. I still know very little about suspsension, but learned a lot just working with John.

I one case, I didn't have enough of a particular shim thicknesses and diameter for Supermoto setting and when he told me what to substitute I was a bit confused when the total thickness of the stack wasn't the same....that's when he explained to me that the shims actually flex various amounts based upon diameter and thickness. Yep, it's still black magic to me, but I can tell you it definitely works.

 

Thanks for the detailed and very logical explanations. I've been trying to find drawings of fork inside on the net, so as to better figure out the role of piston, valves, shims, ... but not luck. I guess disassembling one myself inside is the best way to understand their working, may be when I need new fork seals.

 

Race Tech isn't the only source of high quality pistons and shim kits. There are lots of good suspension outfits out there. But, if you're going to do it yourself, RT may have the best written and video instructions to walk a first-timer through the process. I have a good bit of mountainbike suspension experience with fork and shock internals, and in reality the best ones are mini-motorcycle units. I had never done a shim or piston tweak on any of my dirt motor units, but the RT video that comes with the kit was extremely good to walk me through the whole process. It also is a good primer for suspension design and tuning in general. You probably need an impact wrench, and you have to buy, borrow, or make a special tool that will hold the damper stationary while you ring off the nut at the foot of the damper. I made mine out of an aluminum bicycle seatpost that was long enough to work.