Why does a bike not fall over when it's in motion?

[FreakJoe]

Hello there,
This is my first post on here, so I hope that I've posted in the appropriate sub-forum. My question might seem simple to some, but I haven't been able to find a satisfactory / understandable answer to this question.

Thanks in advance for taking the time to respond.

 

[ZapperZ]

We have had A LOT of such question. Please browse several of our existing threads:

https://www.physicsforums.com/showthread.php?t=741211
https://www.physicsforums.com/showthread.php?t=712892
https://www.physicsforums.com/showthread.php?t=704937

Zz.

 

[adjacent]

See http://sciencequestionswithchris.wordpress.com/2013/04/18/what-keeps-a-bicycle-balanced/ and this.
The first article would be the most useful.

 

[FreakJoe]

We have had A LOT of such question. Please browse several of our existing threads:

https://www.physicsforums.com/showthread.php?t=741211
https://www.physicsforums.com/showthread.php?t=712892
https://www.physicsforums.com/showthread.php?t=704937

Zz.

Looks like I haven't searched well enough. Thank you :)

See http://sciencequestionswithchris.wordpress.com/2013/04/18/what-keeps-a-bicycle-balanced/ and this.
The first article would be the most useful.

Thanks, I'll read through those when I get a chance.

 

[Mesafarmer]

You need not bother looking at earlier postings because they all end up citing research that is highly flawed. The answer to why a bicycle does not fall over when moving can be found by simply studying the tire forces when the bicycle tips. Those in physics will not do this because it will give them the answer and end that area of study. The current explanation leads to such absurd conclusions that the whole lot should be discarded. If you are interested in this, start by doing a force diagram on a stationary bike that is falling over. Now ask yourself, what changes when it is moving?

 

[ZapperZ]

You need not bother looking at earlier postings because they all end up citing research that is highly flawed. The answer to why a bicycle does not fall over when moving can be found by simply studying the tire forces when the bicycle tips. Those in physics will not do this because it will give them the answer and end that area of study. The current explanation leads to such absurd conclusions that the whole lot should be discarded. If you are interested in this, start by doing a force diagram on a stationary bike that is falling over. Now ask yourself, what changes when it is moving?

Sorry, but studying the physics of bicycle is an "area of study"? Since when? Who got research grant money to do that?

Furthermore, just because you said that something is flawed doesn't mean it is, especially when you have shown no valid argument to counter that. If these published research are flawed, then write a rebuttal! Otherwise, talk is cheap. Who is to say that your own argument is not flawed? If you think it is valid, then PUBLISH IT. Or are you not able to stand up to the scrutiny that every published ideas is subjected to?

Zz.

 

[A.T.]

Those in physics will not do this because it will give them the answer and end that area of study.

Obviously a conspiracy to keep those billions flowing into bicycle research. I bet we could have had those flying BMX bikes from E.T. years ago!

 

[sophiecentaur]

You need not bother looking at earlier postings because they all end up citing research that is highly flawed. The answer to why a bicycle does not fall over when moving can be found by simply studying the tire forces when the bicycle tips. Those in physics will not do this because it will give them the answer and end that area of study. The current explanation leads to such absurd conclusions that the whole lot should be discarded. If you are interested in this, start by doing a force diagram on a stationary bike that is falling over. Now ask yourself, what changes when it is moving?

Without anything more substantial that this, to offer, how can you expect anyone to take you seriously?

Many people have studied the forces when I bicycle tips - but there's more to it than"simply" studying. What form of study did you have in mind? It would be appreciated if you were to translate your ideas into conventional terms and use conventional Maths of your message may not get across.

I ask myself what changes when a bicycle is moving and I can see several things - where does that get us?

 

[Craig Shave]

Hello there,
This is my first post on here, so I hope that I've posted in the appropriate sub-forum. My question might seem simple to some, but I haven't been able to find a satisfactory / understandable answer to this question.

Thanks in advance for taking the time to respond.

Gyroscopic's and momentum relative to gravity. As long as something has enough kinetic potential it can counteract the forces of gravity.

 

[rcgldr]

Gyroscopic's and momentum relative to gravity. As long as something has enough kinetic potential it can counteract the forces of gravity.

As mentioned in some of the threads linked to in the first reply, gyroscopic forces are reactions to a change in lean angle, as opposed to the lean angle itself. So gyroscopic forces may dampen the rate of lean, but they don't correct a lean. The steering geomery for a self-stable bike will turn the front wheel into the direction of the lean sufficient enough to return the bike to a vertical orientation (but the direction may have changed) within a range of speed. Normally this is done via trail, where the contact patch is behind the extended pivot steering axis of the front wheel. Some studies have implemented an alternate method that relies on a weight positioned ahead and above the front wheel, so that a lean produces an yawing torque on the bike frame that causes the front wheel to turn into the direction of the lean.

 

[A.T.]

momentum relative to gravity.

What is that?

As long as something has enough kinetic potential it can counteract the forces of gravity.

How does horizontal movement counter gravity?

 

[sophiecentaur]

Gyroscopic's and momentum relative to gravity. As long as something has enough kinetic potential it can counteract the forces of gravity.

This is shooting fish in a barrel: What is "kinetic potential"? What you write could suggest possibilities of levitation and a reactionless force.

 

[Doug Huffman]

I am a bicyclist. I stopped counting miles at 50K. I am an advocate of John Forester's Effective Cycling (from the eponymous book).

The gyroscope as a model of bicycle stability has been debunked; as in Bicycling Science by Whitt and Wilson (Ch 9 'Balancing and steering'). Others built bicycle analogues, with counter-rotating front wheels, to demonstrate their rideability.

A bicycle rider steers to fall in the desired direction and then steers his CG over the line of travel to stop the fall.

The most common bicycle related incident is the slow speed fall, when there is not enough motion rate to get the CG over the line of travel. I cannot tolerate falls, as a senior, and so ride a recumbent trike (Inspired Cycle Engineering Sprint 26" Falmouth, Cornwall)

 

[A.T.]

The gyroscope as a model of bicycle stability has been debunked; as in Bicycling Science by Whitt and Wilson (Ch 9 'Balancing and steering'). Others built bicycle analogues, with counter-rotating front wheels, to demonstrate their rideability.

Gyroscopic precession is a factor steering the front wheel, when you let the handles free. But it is not a necessary factor for self stability, which can be achieved in different ways.

A bicycle rider steers to fall in the desired direction and then steers his CG over the line of travel to stop the fall.

Bicycles are self stable even without a rider. Which, by your logic from above, debunks your rider steering model of bicycle stability. Of course it doesn't really. It merely shows that rider steering is just one of multiple mechanisms which can contribute to stability, just like gyroscopic precession of the front wheel.

 

[rcgldr]

Gyroscopic precession is a factor steering the front wheel, when you let the handles free. But it is not a necessary factor for self stability, which can be achieved in different ways.

Except in rare combinations of speed and angular momentum of the front wheel, gyroscopic precession steers less than what is needed to correct a lean, but steering geometry like trail can overcome this dampening of the steering correction to result in self-stability within a speed range. Once at or above some critical speed, (capsize mode) gyroscopic forces in both the front and rear tires combine to result in a bike that mathematically falls inwards at an extremely slow rate, slow enough that the perceived reaction on a bike at high speed is that it tends to hold the current lean angle unless the rider applies countersteering torque to the handlebars to change the lean angle.

 

[Mesafarmer]

A bicycle is made up of two components, a vane and a trailer. The front frame is a vane, it tracks the direction of travel. What is the direction of travel? If the bike is upright, the direction vector is aligned with the rear frame. When the bike tips, the center of mass develops motion to the side so imagine a vector that points mostly straight ahead but swings to either side when the bike tips. This vector is tracked by the front wheel which is forced into line with the direction of travel by the slip angle and the trail. Understand that this is not steering the bike but that the bike is steering the wheel. Steering is usually thought of as turning a wheel at some small angle to the direction of motion producing a slip angle and a subsequent lateral force. This lateral force steers the vehicle by creating a yawing moment and changing its direction. In the case of a bicycle we have a direction vector that changes to create the slip angle.
Consider a stationary bike that falls over. The lateral force that is generated at the ground is equal to the weight of the bike and the tangent of the tip angle. Without this force the center of mass would fall straight down and the wheels would slide to the side. This camber force is also present when the bike is moving and tests with motorcycles demonstrate the interaction of turn radius, camber force and lean angle.
The first difference between the stationary and moving bike is the freedom of motion. When a bike moves forward it can also be pushed sideways by the camber force. The bike tips and the camber force pushes it in that direction. The center of mass moves closer to being over its contact line. Camber forces will push the bike sideways and this creates slip angles because the wheels are still aligned with the frame but the motion is not. The slip angle forces oppose the camber forces at both wheels. At the front the trail and slip angle cause that wheel to align with the direction vector until the slip angle is zero. The back wheel is slow to respond and a slip angle persists until the frame and direction align. Thus there is a yawing moment equal to the rear slip angle force and the distance to the center of mass that exists to steer the bike into a fall whenever the bike tips.
The steering that is manifested by this is through the camber forces being unbalanced. The rear wheel is held between the camber force and slip angle force while the camber force at the front is unopposed. The front wheel can be steered by the rider to increase or decrease this action but under self stability, the bike does not steer by increasing slip angle but by eliminating it. The TMS bike acts the same way except it uses a two mass system in place of the trail system.
Steering becomes less important at speed because the side motions become small compared to forward motion making slip angles small so the slip angle forces no longer oppose the camber force and a bike at speed is held up mostly by camber.