The simple physics of going straight or around corners
Mon, July 23, 2018 Laws of physics tell us a moving object will travel in a straight line until some other force changes its direction. Throw a rock or a ball, or shoot an arrow from a bow and it will fly straight, until it bounces off some other object, or a side wind pushes it to the left or right.
I realize this is all basic stuff that we learned in grade school, but sometimes we have to remind ourselves of the simple stuff in order to understand the more complicated stuff, because through it all, the laws of physics remain the same.
Spinning wheels have a tendency to stay upright, but have more effect on steering than balancing. Get up to speed on a bike and in theory we are no different from a cannon ball fired from a gun. The laws of physics keep us going straight. In other words, our momentum. The faster we go the easier it becomes to ride straight and stay upright.
Riding slowly at a walking pace, staying upright becomes a balancing act as simple as balancing an upturned broom on the palm of our hand. We move the palm of our hand to keep it directly under the broom’s center of mass. (Its head.) Or we steer the bicycle left or right to keep the wheels directly under the body’s mass.
It is easier to balance an upturned broom than a lightweight stick, because its center of mass is high. Similarly, a bike may weigh 30 lbs. or less, and a body 100 lbs. or more. The bike is the broom handle, and our body the broom head, some four feet above the tires in contact with the ground.
Let’s talk about centrifugal forces. If we tie a rock to a piece of string and swing over our head the rock will travel in a circle and the string will remain taunt. We call this pull on the string centrifugal force. Actually the rock is a moving object and would travel in a straight line, but it can’t because it is tied to a piece of string. If the string breaks the rock would fly off in a straight line.
Let’s say we are traveling at speed and momentum is keeping us going straight. Now we want to turn left. If we simply turned the handlebars to the left, the bike would turn left, but our body would continue to go straight, and a horrible crash would ensue.
Instead we lean to the left. A spinning wheel will turn in the direction it leans. That is another law of physics. So we don’t actually steer the bike, it steers itself. Don’t forget the rear wheel too, even though it is fixed within the frame, it too is leaning and so assists turning.
Because we are leaning to the left, if we were not in motion gravity would cause us to fall over. But the force of momentum that would normally keep us going straight, is counter balanced by the force of gravity pulling us over. We are the rock being twirled on a string, and gravity is the string.
Momentum or centrifugal force is pulling on the string. Too much speed (Momentum.) and the string breaks, and we fly off the road. Lean too much, or wheels lose traction and the bike slides out from under us.
We quickly learn these skills, and do all this naturally without even thinking about it. That is why the bicycle is probably one of human kind’s greatest inventions. It is simply a mechanical extension of the human body.
Read more about the physics of spinning wheels and steering in previous articles: “The Mechanics of Steering.”
Also: “Head Angles and Steering.”
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Reader Comments (3)
And if you want to turn left, really fast, turn your bars to the right slightly, while you are leaning left. Sometimes you have to do this if you go into a turn too hot.
I remember years ago when I was first riding seriously, a more-experienced rider showed me how to take the corner with my outside foot down and with my full bodyweight on that pedal, and virtually no weight on the saddle. In theory, it lowers center of gravity and expands the turning radius (slightly). I'm always a bit nervous following riders who go in with the inside pedal down or with cranks horizontal and all weight on the saddle. And those riders aren't always as receptive if you try to show them how to corner more safely...
:-) great as always! thanks! ;-)