Hi Craig,
I hesitate to answer the because I don't have a definitive "that's why" Kawasaki type response, but I'll attempt to give you a design type answer.
Let's say Kawasaki design builds a motorcycle with 120 hp, and no one else has done this,so there's not a lot of past experience (tried and true) type info out there. So the engineers do what they do and come up with a clutch pack that will handle the load and still be able to be disengaged with a standard clutch lever by an average Joe. Now lets say Kawasaki gets some warranty type feedback that there are clutch slipping issues. We need an engineering change.
Logically, we know we have a certain area (in square inches) of engagement based on 9 plates and 8 friction discs and we have a spring force of a certain pressure times 5 springs, so the easy fix would be to put some heavier springs in the clutch pack to apply more pressure, but the consequence might be that "Average Joe" may not be able to pull that new pressure to disengage the clutch. Well if they can't increase the spring then the only way they can increase the pressure per square inch on the plates would be to reduce the number of plates.
Now I know your about to say there's more plates in the later models than in the first models but we must remember that the steel plates that are up against each other are not applying a squeeze between the steel plates which are engaged to the inner hub and the friction plates engaged to the outer hub, hence half of the steel plates are nothing more than shims keeping the preload on the springs when the clutch pack is compressed. Also by placing them in the pattern they specified, we now have 1 friction plate between 2 steel plates, so when the clutch is pulled and the plates are free the friction plate is now sandwiched between 2 steel plates and will dispense it's frictional heat between 2 plates rather than 2 friction plates possibly heating up 1 steel plate in the original setup (1 friction plate 1 steel plate, 1 friction plate etc.).
The same principal is common (for us North Americans) in the design of snow tires. A drag racer will put the biggest fattest slicks on his drag car because the hot rubber will stick like glue to the hot pavement and the more rubber in contact with the pavement, the less slippage in the takeoff. Take that same big fat tire and now put some snow on the ground and you barely have to touch the throttle and the car is spinning. Now the snow tires. The tread on a snow tire is roughly half of the contact rubber (area) of a summer tire and the pressure (weight of the car) didn't change but now we have traction between the road and the tire, just like the clutch, same pressure but applied over less area.
As for your clutch drag issue. 12 steel plates warped enough such that you measured 2 mm. of compression. That 2mm would probably eat up about 2/3's of the free movement in your clutch pack and there wouldn't be a lot of room for oil to get in there to minimize the friction between the plates. Reducing the number of warped steel plates would definitely help with that problem.
As far as steel plates warping- This is why it's so important to pay attention to that "I can't get it into neutral" thing. As the friction material wears off of the friction plates, it eats up the free play in the clutch cable. Then you can't get it into neutral at the stop light so you keep the clutch pulled in and allow the friction plates to ride on the steel plates, heating up the plates and heating up the engine oil. Not good. What's worse, you're warping the steel plates even more, increasing the problem. The consequences are heavy clunking when engaging gears, which is hard on the gear dogs, more clutch friction material floating around in the oil until it gets to the oil filter (should I go on?)
I prefer the old '79-'80 set up and would upgrade the spring pressure and then to overcome the heavy pull, go for the upgraded clutch release mechanism mentioned in the FAQ section. I bought one and I'm impressed with the design.
Hope these thoughts answer your question.
Z13 UK
