Why can’t bikes stand up by themselves? Because they are two-tired! But what’s the actual physics behind the joke? Let us find out.
Q: Why can’t bikes stand up by themselves?A: Because they are two tired.
If you have read this oft-quoted joke on online forums and from your cyclist friends, you might have had a chuckle about it.
But the fact is that it is quite perplexing why a bicycle can very well move forward without needing a cycle stand or anything else, but as soon as it comes to rest, a bicycle cannot stand up by itself.
If you want to know the real reason why bikes cannot stand up by themselves, read on.
In this article, I will try to explain:
- Why do bikes fall when they are not moving
- Why do bikes stay up when they are moving
Why Can’t Bikes Stand Up by Themselves?
Two forces act on a bike, momentum and gravity. Let me explain.
If you have a bike and start pedaling, what do you feel? You feel the movement of your bike as you move forward. What keeps the bike moving forward? It is just because of momentum.
If I talk about a stationary bike and push it without a rider, then the pushed bike will move for some distance and then become slower, and ultimately it will fall because of gravity.
In the absence of momentum, the bike has only two points of contact with the ground, i.e., its two wheels. To keep your bike standing, the center of gravity of your bike should be within the points of contact with the ground.
But unfortunately, this is not possible, because there are only two points of contact. The lack of touchpoints is why a standing bike will fall unless you provide it with a third touchpoint like a kickstand.
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How Do Moving Bikes Stay Upright?
Your bike easily stays in an upright position when a rider pedals it and your bike starts moving. A few decades back, scientists thought that the bike’s upright position was because of the gyroscopic effect. After that, some researchers denied that a caster effect was responsible for maintaining stability in bikes.
Let us discuss these two effects in detail.
Your bicycle remains in the upright position because of the gyroscopic effect.
A gyroscope is a device for measuring orientation and angular velocity. In other words, you can say the gyroscope is a spinning wheel whose spinning is free to assume any axis.
I think the above explanation would have served to confuse you even further! So let me illustrate the effect with the movement of a spinning top, which explains the properties of the gyroscopic effect.
Imagine the spinning top that is not in motion, and you keep it on the ground; it falls instantly. But if you provide motion to your spinning top and leave it on a horizontal surface, it will rotate on its axis, called rigidity.
Now make the horizontal surface slightly inclined or slightly tilted. The spinning top will align its axis of rotation with the horizontal surface without falling, as long as it is moving fast enough. This effect is technically known as gyroscopic inertia.
The concept of a spinning top applies to the wheel of your bike. The wheel of your bike holds all the properties of a gyroscope, just like a spinning top.
Now let’s discuss precession which is another essential property of the gyroscope device. For this, you imagine an axis in which a wheel rotates continuously.
For example, you can take the spinning wheel of your bike, and If you apply some force on the rotating disk to change its plane of rotation, then that force will not act where you have applied it. It will shift 90 degrees in the direction of rotation.
This force is called precessional force, and the property is called precession. The below video illustrates this property very effectively. Notice how the wheel stays upright because of the precessional force instead of falling towards the ground.
When you spin the wheel by pedaling continuously, you generate this force. The greater the force, the greater will be angular acceleration produced. This force keeps the bike in an upright position while riding.
But if the wheel is stationary, there is only one force acting on it which is gravity. Like everything else in the world, gravity pulls down the wheel, and it falls without a third stabilizing point to hold the bicycle.
However, the researcher proved that the bike would stay upright even to cancel the gyroscopic effects. The rear spinning wheel of your bike cancels the gyroscopic effect.
This observation helped evolve a new theory known as the Caster effect.
#2. Caster Effect
Have you ever noticed Caster wheels on a chair or a shopping cart? Casters align themselves in the direction in which you push the chair or cart, even though you are not applying a direct force on them. You only applied force to the chair/cart. How does this happen?
Notice how the front wheel of your bike is connected to the frame with a fork angled away.
The steering axis passes through the wheel’s point of contact inclined along with the fork. The touchpoint of this imaginary axis would be in front of the wheel’s actual touchpoint on the ground.
The caster angle is angle between the steering axis of the fork and the actual axis of the wheel due to its contact with the ground. This angle plays a vital role in maintaining the directional stability of the bike.
If you look closely to observe your bike, you will see that the steering axis of the front wheel is made so that the front wheel of your bike moves faster compared to the back wheel, whose fork is inclined in the reverse direction.
The front wheel of your bike touches the ground slightly behind the steering axis. When you pull on the bike, the wheel is “following” the bike instead of going along with it. In effect, you are pushing your front wheel in the direction you want it to go, which keeps the wheel stable.
The video below explains the Caster effect very clearly.
Your bike is stable only when ridden by a rider. Otherwise, you need to prop against a wall to prevent falling. As per the scientists, centrifugal force and angular momentum play an essential role in maintaining the bike upright.
Other researchers say that riding a bike remains stable just because of the rider. While riding a bike in a straight line, if you desire to move towards the right, you steer right and then again steer back to remain in a straight line. So, according to them, maintaining the upright position depends on the rider’s brain.
A bicycle is a small and simple vehicle with just two tires, and you need to pedal to move forward. But the mechanism of staying upright and falling when they stand themselves is still a mystery.
I hope you liked this article and got some ideas regarding some theories that help the bicycle stay upright while riding. Please do share your suggestions for this article.