This article was written by Brad Hepler, who has graciously allowed me to post it here. It contains alot of very valuable information and I hope it as useful to others as it has been to me.
"I'm not an expert on this stuff, but I've compiled some tips over the years. There are three (3) possible adjustments on the front and rear suspension of a motorcycle: spring preload, compression damping, rebound damping. The spring preload sets the ride height of the bike and determines how much of the total travel will be available for compression and how much will be available for extension. Damping keeps the bike from behaving like an old Cadillac - i.e., still bouncing 10 seconds after hitting a bump. Compression damping slows the shock when it is being compressed. Rebound damping slows the shock when it is rebounding.
Set the preload on each end so that the bike settles 1/3 of its total travel -- look in your owner's manual for total suspension travel. This ("sag") is measured between (a) the wheel suspended off the ground, and (b) you on the bike with all applicable gear, luggage, passenger, etc. Use less sag for smooth roads and fast riding (e.g. CLASS), say 25%. On shock absorbers: get the sag/preload right before messing with the damping. Start at the shock manufacturer's recommendations. Only change one setting at a time, and don't change damping more than 2 clicks at a time. For compression damping, increase until expansion joints or sharp bumps are jarring, then back off one click. Don't forget that when you increase preload, you have to increase rebound damping and vice versa for decreasing preload.
Front Suspension Symptoms - Likely Causes & Possible Solutions
Symptoms: Not using full travel, feels harsh, poor traction in bumpy corners
Causes: Overly stiff springing or compression damping
Solutions: Lower air pressure; reduce compression damping; softer coil springsSymptoms: Bottoms, soft throughout travel
Causes: Spring rate too low throughout travel, or too little compression damping
Solutions: More air pressure; increase compression damping; stiffer coilspringsSymptoms: Bottoms, but compliant over small bumps
Causes: Spring rate not progressive enough
Solutions: Raise oil levelSymptoms: Harsh over large bumps, but good over small ones
Causes: Spring rate too progressive
Solutions: Lower oil levelSymptoms: Excessive sack, feels soft initially but doesn't bottom
Causes: Initial spring rate or preload too low; springing too progressive
Solutions: Add air pressure *and* lower oil level; increase spring preloadSymptoms: Harsh over small bumps but uses full travel
Causes: Initial spring rate or preload too high, springing not progressive enough, or too much compression damping
Solutions: Raise oil level *and* lower air pressure or install softer springs; reduce compression damping; reduce spring preloadSymptoms: Takes first bump in a series well but harsh over later bumps, poor traction in washboard corners
Causes: Too much rebound damping
Solutions: Use thinner fork oil (or decrease rebound damping if adjustable)Symptoms: Front end springs back too quickly after bumps, poor traction in bumpy corners
Causes: Not enough rebound damping
Solutions: Use thicker fork oil (or increase rebound damping if adjustable)
Rear Suspension Symptoms & Causes
Symptom: Not using full travel, feels harsh, poor traction accelerating out of bumpy corners
Cause: Overly stiff springing or compression damping, possibly too much preloadSymptom: Bottoms, feels soft throughout travel
Cause: Spring rate or compression damping too lowSymptom: Bottoms, feels harsh, sinks far into travel with rider board
Cause: Too little preload, perhaps combined with too soft springSymptom: Wheel chatters over small bumps during braking or downhills, doesn't hook up accelerating out of washboard turns
Cause: Too much preload (perhaps because of soft springs) causing suspension to top out; possibly too much compression dampingSymptom: Kicks over large square edge bumps, but not over large rolling bumps
Cause: Too much compression dampingSymptom: Kicks over rolling bumps and square edge bumps
Cause: Too little rebound damping.Symptom: Rear end takes first bump in a series but is harsh on later bumps. Poor traction out of bumpy turns or braking on washboard
Cause: Too much rebound damping.Symptom: Back end extremely springy and shock doesn't respond to adjustments
Cause: Damping is gone because of low nitrogen pressure, tired oil, or damaged internal components.Some of this material came from either Cycle or Dirt Rider magazine originally intended for dirt but mostly applicable to street."
