Flywheel Effect

Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.

Kevin Cameron's Top Dead Center banner
Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

Normally when we think of flywheel effect, we are thinking of how the weight of a crankshaft can affect idle or acceleration. Typically, single-cylinder engines have had quite massive flywheels for the purpose of storing enough kinetic energy at idle rpm to push the piston through its next compression stroke, thereby keeping the engine running. For this reason the crank in my old 1952 AJS 500 single weighed roughly 30 pounds. Conversely, when you divide up an engine’s displacement among four cylinders, it takes a lot less flywheel mass to push one 187-cc cylinder through compression at idle than it would a single 750-cc cylinder.

The basic physics of flywheels says that the flywheel effect is proportional to the square of the radius at which the mass is located. This means that one ounce of flywheel material located at the rim has four times the flywheel effect of that same one ounce, located halfway between rim and center. In a flywheel pair for a 1916 Indian 'Powerplus' engine that I examined, as much mass as possible was therefore located at the largest possible radius – in the rim. As little material as possible was used elsewhere, resulting in a reasonably light crank with flywheel mass adequate to smooth idle and other operation.

There are special circumstances that can require use of a crank lighter than this;

  1. So-called "lightweight four-strokes" could not have been very light with 30-lb cranks
  2. Engines with pressed-together roller cranks need to be light to prevent vibrations from slipping their joints (examples; Honda 250-six RC-166, and Honda NR500).

In both cases, the unfortunate result of having to use such light cranks was fairly frequent stalling in corners. Those of us who have attended Supermoto events have seen the comical stalling that occurs when riders let the rpm of their light flywheeled big singles drop too low.

Flywheel effect is also important in dirt-track racing, where its ability to smooth out engine torque can improve hook-up and acceleration on slippery tracks. I’m told that turning the rear wheel into a big flywheel is something tried so often in US flat-track that AMA Pro Racing has now set a maximum wheel weight. In response, teams do what they can to lighten the low-radius part of the weight so they can locate more weight at large radius. Does that mean running an inner tube? Only Superman knows for sure.