Motorcycle Handling and Shaft Drives

Does the orientation of a motorcycle engine’s crankshaft and other rotating parts (eg, gears, clutch, driveshaft) affect the motorcycle’s handling?

Reviews of motorcycles often mention how “light” certain bikes feel when being ridden, even for very heavy motorcycles. BMW is one brand often cited; Moto Guzzi riders are fanatically attached to their bikes; and Honda Gold Wing has lots of adherents also.

What makes these motorcycles different? Besides shaft drive, one sees engines mounted with a longitudinal crankshaft (ie, the crankshaft axis is in line with the front-to-back axis of the bike). In contrast, most motorcycles have transversely mounted engines, with crankshafts at right angles to the bike’s longitudinal axis. Not only the crankshaft, but other rotating parts including the clutch, gears, transmission shafts, camshafts, water and oil pumps, and alternator will have the same longitudinal orientation. Does this make a difference in the handling of the motorcycle?

I believe it does. Rotating mass functions like a gyroscope, and as you will remember from playing with tops or toy gyroscopes as a child, rotating mass tends to resist tilting of its axis of spin. The bigger the mass, the further that mass is from the axis of spin, and the higher the spin speed, the more pronounced this effect is. Besides keeping the top upright while spinning, this gyroscope effect has found its way into various designs for reducing rolling in ships (stabilizers). Gyroscopes have even been proposed for increasing motorcycle stability!

In what follows, I will refer to the motorcycle’s motions in 3 dimensions by the terms roll, pitch, and yaw.

For transversely mounted engines, the gyroscope effect will tend to resist a motorcycle rider’s actions to lean the motorcycle over on its side for making turns (ie will resist roll). This resistance to leaning might be described as “heavy” handling. There will also be gyroscopic precession forces which will tend to cause yawing, but this will probably be counteracted by tire friction (unless there are circumstances which reduce the friction between the tire and the road surface).

Conversely, for longitudinally mounted engines, the gyroscope effect will tend to resist pitch motions, which may make the motorcycle more comfortable to ride especially for the passenger, as less pitch means less up-and-down motion near the ends of the longitudinal axis. But leaning the bike for making turns will not be resisted by the gyroscope effect, making the bike handling seem “lighter”. On the other hand, accelerating and decelerating the rotating masses will tend to roll the bike (see “reaction wheel”), although this effect can be reduced by having some of the parts rotating in the opposite direction (“counter-rotating”).

One might speculate that pulling “wheelies” will be more difficult with longitudinally mounted engines, as the precession force from changing the pitch attitude will tend to yaw the bike.

Please note that the gyroscope effects I am discussing have nothing to do with the well-known phenomenon encountered in shaft-drive motorcycles known as shaft jacking, which manufacturers deal with successfully using suspension modifications.

How important are these gyroscope effects? I don’t know; so far, I have not found any research on this, nor even any discussion of the topic. But there is reason to believe that it may become more important as motorcycles become bigger and more powerful. As engine displacement increases, so does rotating mass. Other trends also increase rotating mass: more gearbox speeds means more pinions; higher power outputs mean bigger and heavier components including clutch and driveshaft, the extra electrical energy required for various modern accessories including heated grips and heated clothing means bigger alternators. Finally, with the higher rpm’s developed by modern short-stroke engines, angular momentum increases. Thus over the years, for both transversely and longitudinally mounted engines, flywheel and gyroscopic effects have likely become more pronounced and more important.

Are there researchers in motorcycle dynamics and handling looking at this topic?

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