Just a thought I had today while biking home from work and I don’t know enough physics to answer it
In: 48
The rotating front wheel acts like a gyroscope to help keep the bike upright. The effect is so strong on a motorcycle that it’s difficult to turn and you have to counter steer by turning the handlebar in the opposite direction to get the bike leaned over in the direction you want to go.
On a moving bike, you can steer a little to the left or right if you’re toppling to the left or right. It becomes a thing you learn, though most people fall off a few times before they master it.
On a perfectly stationary bike, you’re stuck. Either your center of mass is above the patch of ground between the patches covered by your tires, or … it isn’t.
On a not-quite stationary bike, you may be able to do a “track stand”. Turn your front wheel, and move your bike a little forward or backward, thus moving your front wheel a little to the left or right. For this, you either need a track bike, which can be pedaled backwards, or a gentle slope which’ll push your front wheel backwards if you ease up on the pedal.
Bicycles and motorcycles balance themselves when they are in motion. It’s more noticeable in motorcycles though. You probably have seen videos of motorcycles riding away after the rider fell.
When the bike starts slightly leaning to one side two main effects counter this leaning by turning the bike towards the side it’s leaning:
– The contact surface of the tires with the ground now is curved, because of the shape of the tire.
– Due to geometry of the front assembly (angle and relative position of fork, axle, pivoting point and contact patch), the handlebar turns to the leaning side.
There’s also a gyroscopic effect of the wheels turning, but it gets quite complicated.
When these effects make the bike turn to the leaning side, you can look at the effect in two ways:
– The mass of the bike was going straight, and by the magic of inertia it wants to keep going straight. So let’s say it was leaning to the right, it started making a curve to the right, so “straight” is now to the left of the bike, so now there’s a force towards left, with the equivalent point of application at the bike’s center of mass, reacting to the force to the right, applied by the ground at the tires’ point of contact. This torques the bike to rotate (roll) to the left, neutralizing the right lean.
or
– A “centrifugal” force (which is the reaction to the force required to make a moving mass change direction) appears pushing the bike at its center of mass towards the outside of the curve, which is opposite to the leaning, straightening the bike up.
This effect in reverse can be easily tested by countersteering: You slightly turn the handlebar to the left, the bike “tries” to turn to that direction, but inertia (or centrifugal force) leans the bike to the right, making it change direction to the right.
Edit: typo and botched auto list numbering
and reference:
https://en.m.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics
>long-standing hypotheses and claims that any single effect, such as gyroscopic or trail, is solely responsible for the stabilizing force have been discredited.
I find [stationary](https://www.dickssportinggoods.com/p/proform-500-spx-exercise-bike-22pfmaprfrm500spxbks/22pfmaprfrm500spxbks?sku=23444320&srsltid=AR57-fDXWlwyMBzWcFT79_X-8_WgllL0KFiPhhxcymLYekewdubFVjm7A54) bikes the only kind I can balance on. Lol
when wheels spin they act like gyroscope
that helps hold you up. like this https://www.youtube.com/watch?v=8H98BgRzpOM
Just an FYI OP, the answers in this thread seem compelling but aren’t entirely accurate. Truth is no one actually knows what part(s) of bicycle physics are responsible. We have pieces of the puzzle, but there’s an odd amount of mystery as to why it works.
Source:
https://www.cbc.ca/amp/1.3699012
As long as your center of gravity is above the line between the contact point of your two tyres, you stay upright. The bike doesn’t know whether to fall left or right.
This is not stable. As soon as your center of gravity is a little bit more to one side, the bike will tend to fall that way.
The rider senses this. If the bike is rolling, the rider can, with a minute movement of the handlebar, move the line between the wheel-contact-points so that it is under the center of gravity again.
If you prevent the rider from being able to move the handlebar, by locking it into place, the bike falls to the side just as quickly as it would when not rolling.
If a bike is rolling slowly, you need to move the handlebar more to move the line between the two wheel-contacts.
If you’re stationary. The line won’t move at all.
PS: A common myth is that the spinning of the wheels is what keeps the bike uptight. If that were true, you could stay on a bike that handlebar is locked into place. But you can’t. You can see this claim on other replies.
PPS: it does contribute, but to a negligible amount. Other factors also contribute a little bit, such as having the front wheel axle ahead of the fork by a little bit. This allows bikes to remain upright when rolling without a rider. The minute handlebar adjustments happen by themselves. This also allows riders to ride without holding the handlebar, steering with hip movements. Cool shit. It’s still not the spinning of the wheels that does it.
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Lots of effects working together for that to happen, I’m going to explain one but it is not the only factor.
Angular (or rotational) momentum. Basically, since all of the mass in a bike wheel is at the edge, when it spins there is a lot of force and momentum along that edge (due to the mass spinning in a circle quickly). This causes the wheel to want to stay on that axis and resist any attempts to deviate.
I had a physics prof in college who brought in a bike wheel with handles at the axle. He’d have someone hold the handles, then spin the wheel and tell them to try to tilt it. It is much harder to turn while spinning. If you have an old fidget spinner, you can test it yourself by trying to rotate your hand while it’s spinning vs not spinning. This is also the core concept behind tops and gyroscopes.
So when you are riding your bike and you have some speed built up, your tires have a lot of angular momentum and they will resist the forces that would normally cause them to fall over. This effect works in synergy with other effects people have mentioned here and you are able to keep your bike upright.
When you sit stationary on a bike the only leverage you have to counteract the falling to one side or the other is to shift your body.
When you are moving forward, you have the **additional means of preventing a fall by turning the handlebar toward the fall** — a slight turn of the handlebars while moving, gracefully and easily, stops the fall. We do this without thinking about it once we learn to ride a bike.
Both things together make it simpler to stay upright than one alone.
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