by Charles Owens on 01/05/2011
The majority of off-road motorcycles have poor overall chassis setup.
One of the major differences between a beginnerís bike and a factory riderís bike is the experience & knowledge of the person performing the setup. Unfortunately many riders just ride their bikes with no thought of chassis setup, and only do the bare minimum required maintenance. This is unfortunate as not only will the bikeís performance and your enjoyment of the sport suffer, but having the proper setup offers enormous advantages in SAFETY!!
There are a lot of handling problems associated with incorrect chassis setup. Having the proper spring rates, race sag, head bearing tension, tire pressures, and freshly blead forks, are the first steps in getting your bike to handle better. ALL of these steps must be done prior to performing suspension adjustments, and certainly before attempting revalving.
The most common chassis setup problem is incorrect race sag/static sag. The proper amount of race sag for a full sized Japanese motocross bike is 95-105mm (unless you have a track record using a different setting). The proper amount of free or static sag is 25-40mm. KTMís can run as low as 110mm race/40mm static sag. Contrary to popular belief, 2mm difference in race sag makes a dramatic difference in overall handling!
Step 1: Put bike on a stand with both wheels off the ground.
Step 2: Take a tape measure and measure the FULLY EXTENDED length of the rear suspension. This is done easiest by measuring from the rear axle to a spot on the rear fender that is directly above the rear axle. Put a small piece of tape on the fender and leave it there as a consistent reference point. DO NOT MEASURE FROM THE REAR AXLE TO THE SEAT BOLT, AS THIS USUALLY ISNíT EXACTLY VERTICAL. Write down this measurement.
Step 3: Put on all of your riding gear that you would normally race with. Take the bike off the stand. Have someone stand in front of the bike, holding it upright. Sit on the bike in the attack position, with your feet on the pegs.
Step 4: Have a friend pull down slightly on the rear of the bike, then SLOWLY let it come back up on its own. Measure the exact distance between the same reference points. Write down this measurement.
Step 5: Now have a friend lift up slightly on the rear of the bike, SLOWLY letting it settle. Measure between the reference points again. Write down this measurement. The average of these two measurements is the TRUE RACE SAG. If the two measurements are more than 5mm different, the rear suspension linkage assembly is sticking and must be serviced. If the race sag isnít between the 95-105mm range, it can be adjusted by tightening or loosening the shock spring preload with the springís retainers.
Step 6: Stand beside the bike and hold the the bike straight up by the end of the handlebar. Have a friend slightly compress the rear suspension, then SLOWLY let it extend on its own. Measure between your reference points. Write down this measurement.
Step 7: Now slightly pull up on the rear of the bike, then SLOWLY let the rear end settle under its own weight. Once again, Write down this measurement. The average of these two numbers is the TRUE STATIC SAG. As with the race sag, if the two numbers are any more than 5mm different, your linkage needs service. If the free sag isnít between 25-40mm, WITH THE CORRECT RACE SAG, you will need a different rear spring rate. If there isnít enough static sag, your spring is too soft. If there is too much static sag, your spring is too stiff.
IF YOU DO NOT CHECK YOUR SAG USING THIS METHOD, YOUR CHASSIS SETUP & SAG WILL BE INCORRECT. THIS IS ABSOLUTELY THE BEST, MOST ACCURATE PROCEDURE FOR SETTING SAG HANDS DOWN.
Incorrect Motorcycle Tire Pressure:
This is an extremely important yet often overlooked chassis setup as 90% of riders do not set their motorcycle tire pressures properly! The majority of riders just set their tire pressures somewhere between 12-15 psi, and never really give it much thought. Too much motorcycle tire pressure will tend to make the suspension feel stiff, and too little will make the bike wallow and push.
There is only one method to deciding the proper motorcycle tire pressure and it is called RIM CLEAN. There must be up to a 4-5mm maximum clean strip on the outside of the rim where it contacts the tire. During use the tire rolls over the rim somewhat, keeping this part of the rim clean. ALL off-road motorcycle tires are designed to work this way, front and rear. ďDĒ shaped rims will require less rim clean for obvious reasons.
The motorcycle tire pressure is adjusted so that the proper amount of rim clean is visible. The tire pressure will differ dramatically from tire to tire, tube to tube, bike to bike, rider to rider, and track to track. A soft carcass tire with a stock tube may require 16 psi in order to have the proper rim clean. A hard carcass tire with a heavy duty tube, may only require 6psi.
It is very easy to run the wrong tire pressure, but most people donít know how to calibrate it. If you have the proper amount of rim clean, and your buddy has the exact same bike/tires/tubes but he has no rim clean, you theoretically have say 10% MORE TRACTION! Tire pressure (psi) is only a number, and that number is used to calibrate the rim clean.
Steering Head Bearing Tension:
Steering head bearing tension is another important chassis setup that is often overlooked. Place the bike on the stand, with BOTH wheels the same distance off the ground. Slowly turn the bars from side to side. There must NOT be any ďcrunchyĒ feeling spots throughout the sweep. If there is, the head bearings must be replaced. With the proper amount of bearing tension (and a well serviced bearing set), the wheel will stay 1?-2? OFF OF CENTER BEFORE IT FALLS TO THE STOP. If the wheel wonít stay centered on its own, the bearings are too loose. If the wheel stays more than 2? off of center, without falling to the stop, the head bearings are too tight. This is a general guideline. Most riders prefer it this way. There is absolutely no bad handling traits associated with the head bearing tension, if they are set using this procedure. Slightly loose bearings tend to make the bike want to head-shake more while braking hard, and it makes the bike feel sloppy. Slightly tight bearings tend to make the steering darty, especially on the face of a jump. Both increase arm-pump!
Front Forks Air Pressure Build-Up:
Many riders arenít aware of the fact that your forks need to have the excess air blead from them for proper performance and chassis setup. Many riders that are aware of it do not perform the procedure properly. Start off by placing your bike on a stand, so that at least the front wheel is off the ground. Take a small screwdriver, and crack the bleeder screw loose from the top of each fork leg. DO NOT try and turn the damping adjuster. The bleeder screw is located to the side of the fork cap, NOT in the center. Leave this screw loose for a few seconds, until you hear no further air escaping, then re-install it. DO NOT DO THIS WITHOUT THE BIKE BEING ON THE STAND!!! (front wheel off the ground). It is best to do this before and after every ride as it will make the suspensionsí performance more consistent.
It is very important that your front axle not bind in the fork legs for proper chassis setup and performance. Most modern front axles accept a 19mm Allen socket. The proper method of installing the front wheel is to use a wrench to hold the nut, and turn the axle using the 19mm Allen socket. Once the axle has been tightened to the proper torque, the fork dropout pinch bolts can then be tightened. By spinning the axle in, you guarantee the axle is not bound up in the fork dropouts.
Rear Axle Position:
The rear axle block on most off-road motorcycles have over 20mm of adjustment. This is a very important area in proper chassis setup as depending upon where you place your rear wheel will alter the wheelbase, front/rear weight distribution, and cause a very different reaction on the performance of the rear shock. When the rear wheel is all the way forward, there is less leverage on the rear shock and will make the rear motocross suspension feel stiffer. This position will also reduce the overall wheelbase of the bike, and distribute less weight on the front tire and more weight on the rear. Mounting the rear wheel farther back will have the effect of softening the rear shock. This will lengthen the wheelbase and tend to distribute more weight on the front tire.
This artcle was found, if anyone knows the publisher let me know and I will add credit.