The smoothness of the rotary in a street car is in part a function of
its 35 pound flywheel, or its torque converter, and always its nearly 20
pounds of rotors.

When the light weight flywheel is substituted, a normal 650 RPM idle
becomes harsh and noisy. When the flywheel is removed and a flex-plate
is substituted along with a 5 1/4" Tilton clutch assembly, and lightened
rotors, should the idle be set too slow, the engine can shake the
transmission hard enough to frighten children a mile away.

The late Mr Graham used a Mazda transmission locked in 2nd gear as a
reduction unit. One day with the idle perhaps too low, and fortunately
with the plane parked on the ground, one of the gears lost its teeth.
His rotors were not lightened.

Typical race car idle is 1,800 to 2,200 RPM. An unhappy situation for an
airplane.

Lynn E. Hanover


This is quite true for a spring clutch plate. However it is not that unhappy for
an airplane as you are looking at a 2.85:1 gear reduction. 1,800 RPM is then 632
prop RPM which is about 100 to 200 RPM lower than my O-470 engine idle in my C182.

It all depends on the torsional resonant frequency of the spring clutch plate.
The CenterForce clutch I now have in my RX8 has a slightly different resonant
frequency than the stock RX8 clutch plate. Under certain rare circumstances of
engine speed and low road speed I can detect a very slight judder. Under most
situations you go through this period very quickly. Also when the RX8 is started
up from cold it idles around 2000 RPM under control of the computer anyway.

Tracy Crook maintains however that his latest rubber bushing torque isolator
plate will allow a 750 RPM engine idle with the prop ticking over so slowly you
can count the blades :) 263 RPM. I have no reason to doubt him as one can soften
the rubber bushings to almost any durometer and lower the frequency. One way is
drill holes in them.

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Also I might add rubber bushings have inherent damping while a single steel spring does not. The CenterForce clutch disk has five single steel springs while the stock RX8 clutch disk has four sets of dual coaxial steel springs. Eight springs in all. The inner coaxial spring is wound anti clockwise to the outer spring. If the two springs rub together that provides some friction damping.
Don't get me wrong. I love the CenterForce clutch but it could be improved.

Paul Lamar