James R. DavisPeer Review:
Posted - 01/03/2006 : 9:52 AM
The Third Gyroscope
By: James R. Davis
There are three, not two, sources of gyroscopic forces on your motorcycle: the spinning front wheel, the spinning rear wheel, and your engine.
Within the engine the spinning crankshaft and flywheel create rather strong gyroscopic forces that are NOT dependant upon how fast your motorcycle is moving!
Indeed, a motorcycle that is stopped is far more stable with the engine running than it is with that engine off (this is known as 'sur place' stability) because of the gyroscopic force created by those spinning components.
Gyroscopic forces are generated as a function of the mass involved, where that mass is located relative to the axis of spin, and how fast that mass is spinning.
On a medium sized bike (about 600 cc's) the mass of the crankshaft and flywheel is roughly equivalent to that of your front wheel, tire included.
Because a wheel with tire has the vast majority of its mass located toward the outside (relative to the hub), it generates substantial rotational momentum when it spins.
Clearly a wheel spinning at 100 RPM generates a larger amount of gyroscopic force than a similar mass in the form of a crankshaft and flywheel that are also spinning at 100 RPM.
But no wheel on a motorcycle ever spins at 7,000 or 8,000 RPM like a revved up engine and that is when that spinning crankshaft and flywheel generate the majority of their gyroscopic forces.
Yep, the faster your engine turns, the more stable your bike will be, regardless of how fast that bike is traveling.
So what? How does that help you with your riding?
Well, as you know, it takes virtually ZERO balance on your part to keep a moving motorcycle from falling over. That is, if the bike is moving faster than you can run.
But when that bike is moving more slowly then you need to contribute balancing effort to keep the bike upright. Some people find riding very slowly to be almost beyond their skill level - or beyond their comfort level - or, because of inner ear problems, beyond their capability.
Ever watch a 'slow race'? That is where a group of motorcycles try to cover a marked distance in the longest possible time without ever touching the ground with their feet.
It takes great skill to be really good at such a race. Some have learned that riding only the rear brake helps in a slow race because that tends to lower the motorcycle and lengthen its wheelbase.
But very few have thought about simply revving up their engines while riding slowly to add stability. The audience of such a slow race might not understand what a knowledgeable rider is doing if he races his engine during a slow race, but his hidden gyroscope would be working to help him win that race.
Now let's be clear about something here. MOST motorcycle engines are mounted in such a way as to have their crankshafts and flywheels spinning on the same axis as their wheels. Some, however, have engines mounted where the crankshaft and flywheels are spinning at a 90 degree angle relative to the wheels. In THOSE motorcycles, when you rev the engine there will be a noticeable (sometimes severe) tendency of the bike being 'pushed' into a lean to the right or left and that is NOT stability, it is instability in evidence.
Big bikes, like the Honda Goldwing, have crankshafts and flywheels that are mounted at a 90 degree angle relative to the wheels but they are purpose built to have the crankshafts and other components (like the alternator and possibly the clutch) spin in opposite directions to self-cancel their gyroscopic effects. They, as a result, neither tend to lean to the left or right when you rev them up, nor do they add stability.
But MOST engines, as I said earlier, have crankshafts and flywheels spinning along the same axis as the wheels and THOSE engines add stability when you rev them.
Moving slowly and not too sure if you can keep that bike upright without touching a foot to the ground? Try revving the engine. (Be CERTAIN to have a good grip and control of your clutch lever, of course.)
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)