Below are some guidelines and instructions to insure that you are able to get the maximum life and performance out of your clutch.

Spring and weight combinations may be interchanged to suit the individual application.  As you make your selections be aware that the heavier the weights and the stiffer the spring, the more positive the engagement becomes. However, with stiff springs and heavy weights, the clutch will have to drop well below its engagement RPM before the clutch will once again release.  Stiffer springs are springs with a larger wire diameter and they will give higher stall speeds.  

Click Here for a chart with the common weight and spring combinations that we recommend on the “Bully III” clutch.

When installing the “Bully III” clutch, the drive sprocket should be installed facing toward the engine.  Even though the clutch will have equal performance installed in either direction, the life of the aluminum clutch basket will be much shorter if the clutch is installed with the sprocket outward.  Additionally, the clutch adjustment springs will be more accessible when the clutch is installed correctly.

Stall speed is the reading shown on the tach as the kart is accelerating from a slow speed before the clutch has become a locked couple.

As the springs are adjusted clockwise, the tension on the springs is increased and the stall speed is raised while the inverse is true of turning the springs counter clockwise, as this will lower the stall speed.

Many theories exist on what the ideal stall speed for a given engine will be.  Some say that the ideal point is at the torque peak of the engine and others say it is at the BHP peak of the engine.  We have found from extensive testing with a computerized accelerometer that the greatest rate of acceleration will occur when the clutch engages at the torque peak of the motor.  However, it also should be pointed out that the addition of the clutch to an engine may change at what RPM your engine produces its torque peak.  (usually it moves up 100 to 200 RPM)  Also, a given clutch set up will be most efficient with a given amount of torque and engagement RPM.  Figure 3 provides some information for anyone looking to deviate from the above settings.

When setting stall speed we suggest making your adjustments in 1/4 turn increments at a time.  When testing for your stall speed, use extreme caution as to not over heat the clutch.  Next to contamination of the friction disks, heat is the clutch's biggest enemy so be sure to let the clutch cool down completely between stall speed tests.

Clutch Disk Run-in
The clutch discs require a period of run-in in order to achieve maximum clutch performance. During this time the outer surface of the linings becomes hot and the microscopic pores of the lining material will burnish themselves to a stable condition.  The friction material used in the manufacture of the Bully clutch discs will cause the discs to have more bite as the clutch gets hot.  The bite from the disc will also continue to increase throughout the life of the disc. When a chatter is present the transfer of power becomes very inefficient.

Air Gap
The air gap is the clearance between the friction disks.  It is measured with a common feeler gauge and is stated in thousandths of an inch.  When new, the “Bully III” clutch is pre set between .040” and .045”.  As the friction disks wear, the air gap may increase to the wear limit of .060”.  When this dimension is reached the floater disk should be changed to a thicker unit to restore the air gap back to the original dimension.  Alternately, there are also .005” shims available that will fit underneath the activator plate to further fine tune the air gap. See Figure 1.

Spring Height
As mentioned earlier, spring height will effect the engagement RPM of the clutch.  We suggest making equal adjustments to all 6 springs of 1/4 turn increments at a time. The spring diameter, preload, weight package, engines power and several other variables all will effect how much of an RPM change is found with a 1/4 turn change but an average change will be approximately 100 RPM.  Turn the spring adjustment clockwise for more slip and counter clockwise for less slip.  The range of adjustment should be between .165” and .285”.  This is the distance between the bottom of the spring retaining washer and the top of the activator plate.  See Figure 2.

Theory Of Operation And Dialing In The Bully Clutch
For theory's sake lets suppose that the outer clutch basket is constrained and not allowed to rotate such as would occur if the brake was held on tight.  When the throttle is opened, the engine will come up to a certain R.P.M., the clutch will engage and a point of equilibrium will be found......at this point of equilibrium, or stall speed, the entire amount of torque being produced by the engine is being transferred to into the clutch. What R.P.M. this occurs at is controlled by 4 variables. Namely the weight of the levers, applied tension of the springs, the coefficient of friction between the friction disks and the amount of torque the engine is producing. A change of each of these variables has it’s own unique effect on the engagement quality of the clutch. As a guideline, lighter levers and less spring tension performs best on motors with lower peak torque values and on tracks where the clutch is continually cycled through its slip range. Heavy levers and high spring tensions will perform best on motors with high torque values and on tracks that require a good launch on the start.  If levers are too light for the torque available, the clutches point of equilibrium will be inconsistent and the transfer of power will be inefficient.

Back to "Buller Clutches" Main Page