Friday, May 2, 2008

Motorcycle physics

Some one I know has a motorcycle, and he told me that it's much more difficult to drive than a car. Furthermore, in order to turn right, you push the right handlebar. Counterintuitive, huh? I was puzzling over this for about a month, trying to figure out how this was physically possible. After all, shouldn't the wheel turn to the left if you push right?

Well, I've finally got my answer! The wheel is precessing!

To visualize this, imagine that you are sitting on a motorcycle. The motorcycle must be moving very quickly, much faster than a bicycle. If it were moving at the same speed as a regular bike, then pushing right would indeed cause the wheel to turn left. But if the wheel is spinning very quickly, there will be too much energy invested in rotation for the wheel to simply turn like a bicycle's wheel.

The wheel of the motorcycle is spinning forward. This corresponds to an angular momentum going to your left. When you push on the right handle bar, you apply torque in the upwards direction. The angular momentum changes in the direction of the torque, tilting upwards. As a result, the entire spinning wheel will lean to the right. Thus, you turn right by pushing right.

Precession: not just for physics demonstrations!

1 comment:

Paul Simmons said...

Gyroscopic precession may play a part, but the motorcycle's steering geometry is more important - you need to consider the motorcycle's trail. When you push on the right handgrip, the wheel deflects to the left of center; the motorcycle's center of gravity is now to the right of its base, so it falls to the right, initiating a right turn.
You can test this with a bicycle, standing still: turn the bars to the left by pushing on the right handgrip and it will fall to the right. If you drew a line between the contact patches of the tires and hung a plumb bob from the CG, you could easily see why this happens.
On motorcycles with a steep steering head angle (sport bikes), you are right when you say that the gyroscopic moment induced when you turn the bars will help steering - it will induce a rolling motion that results in a lean into the turn. On a bike with a long front end (an extremely long chopper, motorcycle or bicycle), the steering head angle is closer to horizontal and the resultant torque will not help to lean the bike over.