These two tech articles are a contribution from Takashi for all people who want to redo their suspention, he has done an excellent job in describing the possibility's and problems involved in upgrading your suspention, these articles will help you make your decision more thourough.
Are you sitting comfortably?
Then, I'll begin...
I've been asked to tell you about setting up the suspension on our old bikes. Most likely you bought your bike as a used one and the suspension system is well past its sell by date. Most people don't even bother to change fork oil (unless the seals blow up) A well set up suspension not only will allow you to go faster(or, you have more safety margin for the same given speed) ,but also increse the pleasure of riding. Everybody knows high -grip tires make a big difference in handling and your confidence. However, nothing works particularly well unless the suspension is sorted.
So, I guess we'd start from the front end. The first thing we need to do is to find out if the spring rate is correct. Whether you keep the stock forks or switch to different forks, this is the first thing to do. If you don't have a correct spring rate, you will be wasting your time for all the adjustment you make here and there. So, how do you figure this out? Springs have to be able to keep the suspension from bottoming out under the worst condition it meets, but, at the same time, they have to be soft enough to keep the front tire on the ground during hard acceleration.(meaning front is very lightly loaded.More about this later.)
I guess most of you already figured out...yes, "right " spring rate is different from person to person, depending on rider weight and riding style.(The main reason I don't like to hear an advice like "fit progressive springs, and pour heavier fork oils " ...)
My *personal* choice is 1.0 kg springs(straight rate), they are the stock ZX-9R(B2)springs, and set them with a little less preload than stock set up. Those ZX-9 fork's travel is 110mm, 20mm less than stock GPz forks. And relatively short and heavy springs allow you mad braking from the speed. Set-up like this really makes a difference in last 20mm of fork travel.
So why use stiffer springs and less preload, not softer springs and more preload? If you do the later, the result is the suspension that feels firmer than Pamela Anderson's tits in intial movement and too soft through the rest of travel... You want exactly the opposite way. Also, to control front end dive under the braking, let the springs take care of business. In other words, don't try to compensate by the compression damping, or the preload. If you want less dive, you need stronger springs, period.
You can go to Race tech web site (www.race-tech.com) to find out the guide line. (Do valving serch. Or you can go to Traxxion Dynamics web site www.traxxion.com and do the same thing from the graph) You can also use cable tie on the fork tube to determine how much travel is used, to see if the springs are hard enough. (And you will need this cable tie for measureing sag.)
So you have chosen a set of springs. Now it's time to measure "sag".
You can put a cable-tie around on a fork tube, then , unload the front end (lift the font end by the frame or engine) ,with the forks fully topped out, slide the cable-tie against the lip of the fork seal. Put the bike down, sit on the bike with your normal riding gear (and a full tank of gas). The cable-tie will slide to the new positon. Top the fork again, and measure the distance between the fork seal and cable-tie. This is what's called "race sag "or "street sag "by some. (This means the sag with rider onboard.) There's also "static sag" or "free sag", (means without the rider, the bike sits with only its own weight.)
There's a rule of thumb, that says one-quater to one-third of the usable suspension travel should be used up by race sag. This much of sag is necessary because the suspension has to extend after the bumps that it must compress over. Also, for turbo bikes, which are powerful enough to squat the rear end under accerelation, this is somewhat important as the front tire is not going to stay in contact with the ground if there isn't enough sag.
If you have dropped front end by pulling the stock forks through the triple-tree, you are already taking up part of suspension travel, so you need to remember about this when adjusting sag...
Increasing preload (screwing down the adjuster, or adding more spacer) means reducing sag, therefore you are setting ride height higher.
Like I said earlier, if the suspension has to carry more load, you need to increase spring rate, not to increase preload more and more. Softer springs with a lot of preload create two potential problems. One, as they are still "soft " springs, you can still get them bottomed out easily. Two, during the rebound stroke of the suspension, the heavyly loaded spring will push itself up at near fully extended position. This will upset the bike's handling, pushing the bike toward the outside.
After finding out the starting point of the front end, next step is to set the rear end up.We do this in pretty much the same way as front end. You lift the rear end (unload the rear end), then measure the distance between a fixed point on the tail section and rear axle. Then, let the bike sit on its weight, measure it again. The difference is "static sag(rear)". Now sit on a bike, with your usual riding gear(this is all the same as front end , oh, and preferbly put both feet on the pegs and put both hands on the bars ) measure the distance, subtract this measurement from the first measurement(suspension topped out), this is your "race sag(rear)"
Just like the front end, one-third of the suspension travel is a good starting point. (Most suspension systems are designed to work best when one-third of the travel is used up by the rider. This gives room for the suspension to load and unload. Almost all shocks are designed to work best from a given point in the linkage; setting preload correctly puts your spring in that range. )
Adjust the preload so that you can set right amount of sag.... well, only if you have already replaced the stock shock unit...
Err, yes, the stock shock has to go , I'm afraid. This is one of the worst offending items (another one is anti-dive system on the front) that you really have to get rid off.
Brand new Works Performance shock cost about $400 to $500(? I thought.... Anyone knows the current price?)You can try to find a used Fox or Works. ( you may need to replace spring, as there's no guarantee your weight etc will match to the spring on the used shock) You can also use shocks from other bikes, which is much cheaper option. Chris is runningZZR1100 shock, and Chip has GSX-R shock on his bike.
That is, about Unit-Trac GPz 750s and Turbos. What about those twin shock bikes? Well, the same logic applies , the difference is it doesn't have linkage. So the suspension stroke=shock unit stroke.