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being turned fully clockwise. (A click is approximately
1
⁄
8
turn) See photo III.62. The compression circuit determines the rear wheels
resistance to moving quickly upwards, such as when hitting a high speed bump or landing from a jump. Stiffer compression settings
will help resist bottoming. However, too stiff settings will make the bike harsh and the rear wheel tend to wash out. Large jumps will
require stiffer settings. In the absence of unusual obstacles, the suspension would ideally be set so it just bottomed lightly on the hard-
est bump or landing on the track so the full range of suspension travel would be utilized. Tuning the suspension to work well for 80%
of the track will probably yield better lap times than tuning for the extreme 20% portion. Tire pressures will also affect the rear while
cornering and accelerating, tire pressure can be adjusted as needed as long as minimum pressure is maintained.
Rebound dampening is adjusted by turning the large rubber adjuster at the bottom of the shock spring retainer. Turning it counter-
clockwise as viewed from the top will increase the rebound dampening. The rebound dampening knob has 16 “click” positions total
and the standard setting is 8 clicks out from being turned fully clockwise. See photo III.61. The rebound dampening controls the speed
at which the wheel pushes downward after being pushed up by a bump. If the rebound dampening is set too light, the bike will tend to
bounce after a landing. The rear wheel will kick up when hitting bumps and leaving jump faces. If the rebound dampening is too
heavy, the wheel will not be able to return to its normal position quickly enough, and the bike will tend to run low with little suspen-
sion travel left after hitting the first bump in a series. Try decreasing the dampening setting 1 click at a time until the bike begins to
bounce or kick up a little, then increase the rebound setting slightly. Increasing shock spring preload will probably require increased
rebound dampening to compensate.
In general, the compression adjuster will probably need more fine tuning than the rebound adjuster once it is properly set. If avail-
able, use a video camera to film the bike on various sections of track. Play the recording back at slow speed and determine if the tires
seem to be bouncing too much to maintain contact with the ground, or if the bike seems
to ride too high or too low. If the tires can’t react quickly enough to maintain contact
with the ground, reduce rebound and/or compression dampening. Adjust spring preload
to obtain proper level ride height.
Curnutt Shock (X3 model)
The shock “sag” must be adjusted for rider weight and ability. To measure sag, put
motorcycle on stand with rear wheel off the ground. Measure from center of rear axle
to side panel screw near rear of seat. Have rider with all riding gear sit on bike and
take measurement again. Sag is the difference between the two measurements. Set sag
to 2˝ minimum, 3˝ maximum by adjusting the spring preload as described below.
Optional stiffer and softer springs are available for different rider weights and abili-
ties. Faster, heavier riders will likely need the stiffer spring, while lighter beginning
riders may need the softer spring. If the sag must be set to 2˝ or less to prevent bottom-
ing, purchase the stiffer spring. If using 3˝ of sag or more to make suspension soft
enough and bottoming is not a problem, the softer spring is indicated.
Adjusting Sag
- To adjust the sag, first clean and lubricate the shock body and threads.
Then, loosen the set screw on the spring seat
1
⁄
4
turn using a
1
⁄
8
” allen wrench. Adjust the
spring seat using a
XXXX
tool. Turning the spring seat clockwise will preload the
spring more and decrease sag, counterclockwise will increase the amount of sag. Once
sag is set properly, tighten the set screw and apply a dab of silicone sealer to the set
screw to seal out dirt and water. Do not overtighten the set screw or damage will result.
The Curnutt shock is further tuned by utilizing air pressure in the shock reservoir. To
prevent damage never use less than 75 psi or more than 150 psi! Within this range, lower
pressures will yield softer dampening and higher pressure will yield firmer dampening. If
150 psi is required to prevent bottoming out, a stiffer spring may be necessary. If the sus-
pension still seems too stiff with 75 psi of pressure, a softer spring is required.
The supplied air pump can be used to adjust the air pressure in the shock. Begin with
changes of approximately 25 psi, then fine tune by 10 psi increments.
Adjusting Air Pressure
- Adjust air pressure when shock is cold and rear wheel is off
ground (shock is fully extended). Remove the cap from the valve on the shock reservoir
bottom. See photo III-64. Thread the pump onto the valve approximately 5 turns, or until
you feel it touch the top of the fitting. Once it touches, tighten 1 further turn (do not
overtighten to prevent damage). When pump is properly fitted, the shock pressure will
register on the pump gage. Stroke the pump a few times, the shock pressure should
increase slowly. If the pressure increases rapidly, the valve may not be open. In this
event, tighten the pump cautiously until valve opens. Pump air into shock to reached
desired setting (pressure must be between 75 and 150 psi). Remove pump and replace
valve cap.
Preload adjuster on Curnutt shock
Air reservoir fitting - Curnutt shock
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