lightweight flywheel

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Crankshaft Torsion and Dampers

4. Other Rotating Group Parts
• Heavy vs Light Flywheels – Light flywheels lower the whole system inertia, move the high
torsion stress point a little farther forward (away from the flywheel), raise the first mode frequency
a bit, and sometimes allow a little smaller damper. They also allow more crank lugging motion
which usually shows up as gear rattle in the transmission. (This is called “rigid body motion” in
the torsion trade since the crank is just seeing rpm variation without twist)
• Light vs. Heavy Pulleys – Heavy pulleys are just more parasitic inertia at the light end of the crank
and make the dampersΆ job more difficult. They lower the frequency a bit but make the crank seem
larger to the damper; ie. a larger damper ring needed.
• Blower Drives – Mechanical blower drives for Roots type blowers can actually help the damper
by acting as more damping at the front of the crank. Mechanically driven blown engines generally
are no worse torsionally than unblown. Turbos usually are worse.
• Dry Sump Drives – Not much effect other than some torque required to drive the pump.
• Cam Chains – Cam chains can take a beating from the torsion peaks. Same for cam gear drives.
The vibration motion tries to make the cam drive do the same motion and jerks the chain back and
forth. If this motion gets through to the cam, it doesnΆt help cam dynamics and can also show up in
the spark scatter with cam driven distributors.
• Spark Scatter – If the spark is taken off the front of the crank, the torsion motion is in the trigger
wheel as well. The best place for the trigger wheel is near the rear of the crank where torsion
motion is smallest. (guess where the OEMΆs are putting their sensors now)
• Transmission Gear Rattle – This is mostly caused by low frequency torque pulses (“rigid body
motion”) in the crank at lower speeds when flywheel energy is low, although this motion usually
persists out as far as 2000-3000 rpm. Aluminum flywheels make it worse for the obvious reason.
These frequencies are way lower than the damper is designed for and the damper has virtually no
effect on the gear rattle problem. Dual mass flywheels go after this problem.