I found this on one of the overseas sites a while ago. I haven't read it for a while but hope it is useful.
Rear Shock Tuning
Contributed by:
Jeff Bertrand #25
Ventura, California, USA
The first thing to do after you install your new shock is to set the ride height. This is accomplished by adjusting the preload on the spring. You will need a helper to take measurements while you do this. First, with the bike on the center stand, measure the distance from the rear axle to some fixed point on the frame. Next sit on the bike and while balancing as well as you can have your assistant measure again from the axle to the same fixed point on the frame. Subtract the first measurement from the second one. If the difference, called sag, is more than about 1.5" dial in more spring preload until your sag measurement is between 1.25 and 1.5 inches. Once you have the preload correctly adjusted, start riding the bike to get an idea where to go with the damping adjustments. These adjustments are much more subjective than the preload and you will have to ride the bike under a variety of conditions to get an all around idea of which way to go. Compression damping controls how quickly the shock can be compressed when the wheel encounters a bump. Note the bike's behavior on sharp-edged bumps such as broken pavement and patched pot holes. If it seems to bottom too easily, add more compression damping. Go back over the same piece of road so that you can compare. If the rear end feels too harsh and you get the sense that the suspension isn't moving very much, try backing off the compression damping. My personal preference is to use as little compression damping as possible while still preventing excessive bottoming. Rebound damping controls how fast the shock can extend after being compressed. Again observe the bike's behavior under a variety of road surfaces, throw in a few twisty roads as well. Try to sense the behavior of the rear end after traversing rolling bumps as opposed to the sharp edged ones described above. What you are looking for is rises and dips that create a large amount of suspension travel. Try to determine if the rear end settles quickly or does it bounce a couple times? If you sense any "floating" or "bouncyness" increase the rebound damping. This may come across as a wallow or weave in fast cornering. If you dial in too much rebound damping the shock may not be able to extend from one bump in time to handle the next one. In the case of several closely spaced bumps the rear end can actually pump down. Check this by running over a railroad track (or some other series of closely spaced bumps) a few times at different speeds. I have a perfect spot not far from my house where there are two tracks in parallel. If after traversing a series of closely spaced bumps you have a sense that the rear gets lower after each bump or if the last bump in the series feels much harsher than the first you may have too much rebound dialed in. Back it off and try again. You won't be able to set it up all in one day, more likely you will gradually dial it in over a period of several weeks. It's strictly a trial and error process but when you get it dialed to your satisfaction, you'll know it. Keep notes so that you don't end up going in circles and remember as a general rule, if you increase your spring preload, such as when loading the bike up or carrying a passenger (remember you should try to maintain that 1.25 - 1.5" sag between unloaded and fully loaded) you should also increase the rebound to control the additional preload. Hope this helps.
