Tech Center
Suspension Setup
Setting Race Sag
1) Make a small mark in the center of the rear fender. Then measure the distance from there to the center of the axle block. (Bike is on the stand with both wheels off the ground.) This is your unloaded measurement—call this A.
2) Now measure the same two spots with the rider (with full gear) on the motorcycle with both feet on the pegs. (You should have two people for this for it to be accurate). This is your loaded measurement—call this B.
3) The race sag is the difference between the unloaded (A) and loaded (B) measurement.
Example:
A - B = Race Sag
125cc - 450cc = 95 - 115mm Race Sag
85cc = 85 - 90mm Race Sag
65cc = 70 - 75mm Race Sag
Checking Free Sag
Measure the motorcycle with both wheels on ground (weight of bike itself on level ground). Call this C. Now take A (from race sag) and subtract C (from free sag).
Example:
A - C = Free Sag
20mm or less Free Sag requires stiffer SPG rate.
35mm or more Free Sag requires softer SPG rate.
Adjusting Spring Preload
Adjusting spring preload is necessary to obtain the correct handling results. Remember that more preload improves the turning of the machine because it lifts the rear end up, putting more weight on the front end, but makes the machine a little less stable on high speed straights. Less preload makes the bike handle better on straights but gives up turning. This lowers the rear of the bike taking weight off the front end.
Adjusting the fork height also affects handling by raising and lowering the front of the machine. The higher your forks are in the triple clamps will improve the turning, but will lose the stability on high speed straights. The lower your forks are in the triple clamps, the better the machine will handle on straights but won't turn as well in corners. Always use an accurate measure to set fork heights instead of just eyeing them (it does make a difference).
Remember that properly adjusted suspension should bottom out occasionally while riding. If not, then adjustment or spring rates should be considered.
Valves
Checking the Valves
I think everyone knows that metal expands as it gets hot and contracts as it cools. Because of this, there needs to be some clearance for the metal to expand into.
Valves lose most of their heat through the valve seat. If there is too little clearance, the valve doesn't fully seat and it will start to get hotter and hotter. Sooner or later the valve starts to melt or burn. Not good! If the clearance is too much, the valves will be hammered every time the valve opens and closes. This is better than running hot, but still it is not good. Titanium valves are a good choice for performance because they use a lighter valve spring and they are very strong and yet light weight (less reciprocating mass equals more horsepower), but when they get hot (from valves not being adjusted properly), they become soft and begin to tulip on the valve face. A stainless steel valve requires a heavier valve spring but robs the motor of horsepower (more reciprocating mass equals less horsepower). This is something to remember when building a motor.
If an engine is running right, the valve will slowly wear its way into the engine, taking up clearance until eventually the valve is held open and burns. Checking the valves regularly will reduce valve damage due to the clearances being too tight or to loose. This could be the difference between needing to have the valve seats cut with new valves installed or just shimming them. Check the valves regularly and keep your oil changed. Dirty oil will reduce engine performance and life of the engine.