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The only accurate means to determine the fastest clutch is through
testing. Forget the fancy advertising, internet forums and racetrack gossip
for they are arguments to improve the financial position of kart shops and
to boost egos of some kart racers.
Factual and useful information on clutch performance is nearly non
existent. The size and weight of a clutch may be no more meaningful
than its color.
Lap times do not count. There are too many variables such as tires,
handling, racing line, track condition, traffic and driver skill that muddle
the data. For example, the new clutch you are testing may be a tenth faster
than your old clutch but a cloud passed in front of the sun, cools the track
and the tires are three tenths slower. The stop watch indicates a lap time
two tenths slower (-0.1 for the clutch, + 0.3 for the tires = +0.2 total). The assumption is the clutch is two tenths slower when in fact
it is one tenth faster.
Drag racing eliminates much of the confusion. Nearly
all the variables that affect time are eliminated if only we drive for a
short distance in a straight line. Keep it simple. Accelerate for a fixed distance and
record the time.
Change clutch. Repeat. The clutch with the least elapsed time wins.
The drag race is elementary.
Assuming a common four-stroke racing engine, it’s a 100 yard dash against the clock. Paint two lines
300 feet apart on a
smooth, flat surface. Allow some additional length to slow down after the
second paint mark. The testing procedure is easy too. Set the kart on the
first line and start the engine. Hold both petals down for a couple of
seconds then release the brake. Assuming tires with a 36 inch circumference
and 16/68 gear ratio, the kart will reach a speed of approximately 30 miles
per hour in five to six seconds. Reapply the brake after passing the second
paint line. Stop the engine and record the data.
Timing should be accomplished with a data acquisition system. Stop watches
are inadequate. The response time of the human thumb is inconsistent and we’re looking for differences in time that are small.
Pertinent data include elapsed time, clutch parameters, gear ratio, tire
sizes, tire pressures, pipe, jetting, temperature, atmospheric pressure,
wind speed and direction. If the clutch can be adjusted or replaced quickly,
weather conditions (and jetting) are not important. But if the tests are too
far apart, comparing elapsed times get complicated because correction
factors apply. Weather affects engine horsepower. Horsepower affects elapsed
time. Plan your testing and execute quickly. But allow time for the clutches
to cool between runs. Hot clutches are usually slower than cold clutches.
Test each clutch at various engagement rpms. In theory, the maximum
acceleration occurs if the clutch engages the engine at its peak torque rpm.
In practice, many clutches exhibit moderate to severe hysteresis (his-ta-'ree-sis)
and will accelerate better at another rpm.
Hysteresis, sometimes described as engine bogging, must be eliminated for
maximum acceleration. Less hysteresis equates to less elapsed time. It is a
function of the centrifugal weight configuration, spring characteristics and
lining materials. It is barely detectable with a tachometer. With a good
data acquisition system it will be evident from the shapes of the rpm vs.
distance and rpm vs. time curves.
Hysteresis has a greater affect on acceleration than clutch weight or
inertia. It is the main difference among clutch designs. Yet most
manufacturers do not know what hysteresis is, its significance or how to
control it. Instead they dwell on insignificant data such as weight and disc
diameter. Consequently kart racers must derive useful clutch data for
themselves.
In summary - the size and weight of a clutch may be no more meaningful than
its color. The true indicator of clutch performance is the clock and you
have to measure it yourself.
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