- Mike Sachs said it VERY well. SO, to jump on the bandwagon (happens there is a strong thread on this same topic on the Speed Triple list currently, too), I'll add mu tuppence:
- PRELOAD is the amount the spring is compressed in the fully-etended fork. A function of fork "depth," spring length, spacer length, and fork cap intrusion into the tube. Primarily controlled by spacer length (if one installs caps with preload adjustment, then it's fine tuned by the intrusion of the cap's adjuster into the tube). PRELOAD is used to adjust (control) only one fork variable - static sag. As Mike said, it should NOT be used to compensate for too soft springs. My Trophy is right with 3/4" of preload - which happens to be the protrusion of the fork caps, so when I assemble the forks the top of the steel washer atop the PVC spacer is even with the top of the fork tube. That's right for me - 210 lbs in gear, and my non-stock springs.
- STATIC SAG is the amount of fork compression when the fully-clothed rider is atop the fully gassed and oiled bike, straight upright, sitting still.
Measure the fork length (from a moving part to a static part - for example, from the top of the axle to the bottom of the triple clamp when FULLY EXTENDED - wheel in the air), then sit on the bike, full weight on the seat, and have an assistand measure between the same points. Compress and extend the suspension, take same measurement again. Repeat as necessary and take the average (all the data points should be within 1/4" of each other, or there's excessive stiction in the forks). The difference between the fully extended length and the average weight-on length is STATIC SAG -the target is about 25% of compression for track, and 33% for street. Adjust PRELOAD to get the sag RIGHT.
- OIL WEIGHT or viscosity controls the damping rate - compression and rebound. On non-adjustable forks, this is a balancing act. Too much rebond damping and the forks "pack down" over a series of bumps - that is, the forks compress on the first bump, then cannot extend before the compress again for the second - basically, the fork ratchets down, and is less effective. Too little rebound damping and the fork springs back up past the static point - like a pogo stick. This is the advantage of the RACE-TECH GOLD VALVE CARTRIDGE EMULATORS - you change your fork oil to achieve the proper compression damping, then adjust the emulator to achieve the proper rebound damping. (I may have these reversed - forgive me. It's been a while since I installed emulators). Compression damping controls the suddenness of dive under braking and how harsh bumps feel. Oil weight is given as 5wt, 10wt, 15wt, 7.5wt, etc - and is NOT THE SAME AS ENGINE OIL WEIGHT. USE FORK OIL. PERIOD.
- SPRING RATE is how stiff a spring is - and controls total dive when braking. Stiff spring rate tries to overcome rebound damping. SPrings are rated by force (since we're all on earth, actually they're rated by mass) per distance - usually kg / mm. A 1.0 kg/mm spring compresses 1 mm with a 1 kg "weight" vertically atop it, 2 mm with a 2 kg "weight", etc. Race-tech springs are straight-rate, and quite good. Progressive springs are designed - by having coils close on one end and further apart at the other - to behave like a light spring near extension, and progress to a heavier spring rate (using the further-spaced coils) as they compress, so it feels like a 0.8 kg/mm in the beginning, and maybe a 1.2 kg/mm at the end. Liked them on my cruiser, not on my sportbike. The weight of the bike and rider, riding style, etc. determine the required spring rate. Stock Trophy springs are soft - I think 0.63 kg/mm? - and I'm using 0.85 kg/mm now (that's a 50% increase over stock!) and like it. Others using 0.95 or 1.0 - enjoy it. You have my support and respect. Neither of us is wrong.
- AIR GAP - the air above the oil level, measured from the top of the fork tube to the top of the oil when the fork is compressed, the springs and spacers removed. Triumph says 125-133mm. Note that when you change springs (less coils displace less oil, so will raise the oil less when you install them, so will leave more air,...) your required air gap may change. PVC pipe takes up more volume than steel tube spacers, too. Anyway, the air gap controls how progressive the fork compression is - the air is trapped, and while springs compress linearly with force, air compresses inverse exponentially. Bottom line - less air (higher oil level) makes the forks feel more progressive, less air makes them feel more linear. Start with stock and experiment.
OPINION - suspension is the MOST important tuning we can do to our bikes. While it's fun to mess with the engine and get more power, they usually already have plenty. THe best way to go fast is to not go slow - and that comes from being able to control the bike over bumps, around corners, etc. A well suspended bike is more comfortable, faster, and SAFER than a poorly suspended bike. Suspension starts with the right tires, at the right pressures, properly balanced and aligned, and continues to the frame.. Invest the little money, medium effort, and mostly TIME to understand how your suspension works and to dial it in to meet your needs.
Neil M. Christie