Friday, December 23, 2005

The Mechanics of Steering.

(Or is it the Physics of Steering?)

Roll a wheel or for that matter any round flat object even something as small as a coin on a flat surface and it will roll in a circle. It will continue rolling in ever decreasing circles until it finally falls and settles in one spot. This is a demonstration of the law of gyroscopics. That a spinning wheel will remain upright as long as it keeps spinning.

Most people know that this is what keeps a bicycle upright while in motion and if you stop you fall over as all cyclists do at least once in their lifetime. Also a spinning wheel (Or rolling coin.) as it looses momentum and starts to fall it will turn in the direction it is falling, which is why it rolls in a circle.

This law of physics gives a bicycle a simple built-in self steering capability. You can demonstrate this to yourself by holding a wheel in both hands by the spindle and spinning it. The first thing you will notice is that the wheel wants to stay upright in the same plane, demonstrating the first law mentioned in the opening paragraph.

If you forcibly move the top of the wheel to the left or right as it is spinning it will also turn in the direction you are leaning it. Just as a rolling coin will turn in the direction it is falling. So as you lean a bicycle into a corner it will steer itself around the corner. Let’s not forget the rear wheel. Although it is in a fixed position within the frame and cannot turn, it is still spinning and leaning therefore assisting in steering the bike around the corner.

This also explains the importance of a frame being straight with both wheels in the exact same plane. If a frame is twisted one wheel is always leaning and therefore always steering the bike in that direction. In normal riding conditions you may not even notice this as you will automatically and subconsciously correct this by steering in the opposite direction. But try riding hands off and the bike will pull to the left or right. Make sure you are on a level surface as a camber in the road will also cause you to move in that direction.

There are some who argue if it is the gyroscopic motion of a spinning wheel that keeps a bicycle upright, how come it is possible to ride at a slow walking pace? The answer is that it is not all gyroscopics, it is balance. The law in balancing anything is that the center of gravity of the object be directly over the point of contact with whatever the object is balancing on. In theory it should be possible to balance a golf ball on a pin head.

Someone walking on a high wire, as they start to fall to the left will shift their body, therefore their center of gravity to the right. Balancing a broom on your hand is a relatively easy trick because you simply move your hand in the direction the broom starts to fall keeping your hand directly under the broom’s center of gravity.

Have you noticed also that it is easier to balance a broom on your hand with the head of the broom up, than it is to balance a lightweight stick on your hand? The center of gravity of the lightweight stick is somewhere in its center, whereas the upturned broom has a center of gravity near the top where the head is.

A bicycle and rider has a high center of gravity; the bike can be twenty pounds or less while the rider is a hundred pounds or more. Center of gravity is somewhere in the middle of the rider, three feet or more from the ground. And just like the high center of gravity of the upturned broom, this works in favor of the bicycle and rider when it comes to balancing.

When riding very slowly, as the bike and rider fall to the left the bike turns to the left. The rider corrects this by steering to the right, then back again as the rider starts to fall in the opposite direction.  What the rider is doing is moving the bike from left to right under them just like moving your hand under the upturned broom. And just like the simple broom balancing trick there is no conscious thought process to this, it is automatic.

Steve Barner said...

Nice analysis, but I think there is an error in the last paragraph. Just as with the balanced broom, as the rider starts to fall to the left, he or she must steer the bicycle to the left, not the right, in order to stay upright. With the broom, the object is to move the balance point back under the center of gravity. The same is true of the slow moving bike rider. The rider needs to move the bike back underneath the center of gravity. The rider is relocating the contact point rather than the center of gravity.

The truly stationary rider, doing a track stand, is like the tightrope walker, moving the center of gravity itself, since the contact point doesn't change. (Those of us who are not all that good at this technique use a combination of rocking the bike and adjusting the center of gravity, usually to some comic effect for those observing.) On the other hand, a rider on a set of rollers moves the contact point to one side or the other, in the same way a moving rider would, in spite of the fact that the rider is not moving forward in reference to the surroundings.

Dave Moulton said...

Thank you Steve; you are right in the case of riding slowly, however a true track stand is done on a fixed wheel bike. The front wheel is turned almost 90 degrees but not quite. By applying pressure to the pedals forward and backward the bike swings sideways back and forth (Because the front wheel is turned.) thus moving the bike under the rider. It is possible to find the exact point of balance and stand completely motionless. Most people who are able to stand still on a bike with a freewheel (Road bike.) are not doing a true track stand as I have just described, but are balancing as you suggest by shifting their weight rather than moving the bike under them.

Anonymous said...

Hi, Dave -- just intro'd to your blog by The Howard...

Interesting point about balance -- the broomstick experiment is a good one. As a recumbent rider (Stratus XP), people always ask me if it's harder to balance on a 'bent. I didn't have significant issues myself when I started, but the typical center of gravity is definitely lower, which would contribute. In addition, you can't "adjust" with your legs as much due to the different body position. So I suppose this would be mirrored by a "long broom vs. short broom" comparison. (no jokes about long bus vs. short bus, please :^) )
However, I somewhat agree with the previous poster about steering in a turn. I don't feel that I steer right during a left turn -- it's almost a feeling of steering FURTHER left to create an outward force (centrifugal? centripetal? centri-pedal?)that rights my balance against the perceived fall to the left...
Best healing wishes, as well as legal ones.

Paul (FlyingLaZBoy) / Dallas Tx