How Does A Bike Work?

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invisifly2
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How Does A Bike Work?

Postby invisifly2 » Sat Sep 24, 2011 11:01 pm UTC

How exactly does a bike work? And yes, I know that turning the peadles spins the chain rotating a wheel which pushes you forward. How excactly is it that I don't fall over whenever I try to ride one?

Essentially, how does a bike stay up?

I thought it would just be because when you are riding it that you're constanly shifting your weight to balance things out, but that does not explain how I can ghost ride my bike though (go really fast, jump off, and the bike keeps going, like it was being ridden by a ghost). Is it just the forward motion? If so that doesn't make sense though. Say I pushed a thin sheet of plywood forward on it's edge. It'll start to fall as soon as I let go of it, but if I do the same thing to my bike, it just like, "Go forward? Ok sure!" even if I have it leaning at a 45 degree angle it'll straighten out (to be perpindicular to the ground) if I push it right.

My theory is it is the work of ghosts.

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Re: How Does A Bike Work?

Postby poxic » Sat Sep 24, 2011 11:21 pm UTC

Gyroscopic ghosts. With longitudinal forces. Or something like that. (Note: I didn't quite find the answer to your question in that link, but that's probably because I only skimmed over it.)
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Re: How Does A Bike Work?

Postby Macbi » Sat Sep 24, 2011 11:35 pm UTC

I think it's something to do with how the shaft from the handle bars to the front wheel goes at a forward angle. This (for some reason I don't understand) means that when the bike goes forward it corrects itself. If the shaft was tilted backward instead I think you'd fall of straight away.

EDIT: poxic's link is teh awesome.
Last edited by Macbi on Sat Sep 24, 2011 11:37 pm UTC, edited 2 times in total.
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Re: How Does A Bike Work?

Postby gorcee » Sat Sep 24, 2011 11:36 pm UTC

It's actually an open problem. People used to think it was gyroscopic forces (and to some extent, in practical bikes it probably is). But then they built a bike with net zero gyroscopic forces, and it worked.

Then they thought it had to do with hinge location relative to the contact point of the wheel. And then the built a bike where that was reversed, and it worked.

So yeah, no one really knows for sure.

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Re: How Does A Bike Work?

Postby Charlie! » Sun Sep 25, 2011 12:17 am UTC

Wheels of finite width and curvature help a bike stay up too. When the bike tilts, the inside goes farther than the outside, so it helps turn the bike into the tilt and keep it upright.
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Re: How Does A Bike Work?

Postby Swivelguy » Fri Sep 30, 2011 3:02 pm UTC

When you're riding a bicycle and the bike starts to unintentionally tip to the right, you steer right, which brings the bike back beneath you. When you aren't riding the bike, it does this for itself.

Stand next to your bicycle, at rest, holding the saddle, with the bike vertical and the front wheel straight. Now tip the bike to the right. You'll notice that the front wheel turns to the right on its own. This happens even with no rider, so it keeps the bike upright when you hop off. The amount that it turns is called the trail of the bicycle and is a function of both the head angle (the angle of the steering axis wrt vertical) and the rake of the fork (the forward-sweep of the fork blades). More rake actually gives less trail.
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Re: How Does A Bike Work?

Postby york_hunt » Sat Oct 01, 2011 8:08 am UTC

The way that we've studied them in physics class was to look at them from an angular momentum standing. The definition of Angular Momentum is this (for those of you who don't already know it :) ):
The quantity of rotation of a body, which is the product of its moment of inertia and its angular velocity

The front wheel spins with a certain angular velocity and has an inertia and thus has an angular momentum. The momentum will stay constant as long as the velocity and moment of inertia stay constant. When the bike begins to fall over there is this new angular momentum that is developed from no where and it has to be cancelled in some way, the bike does this by turning the handle bars around their axis thus stopping the bike from falling over. At lower speeds the force of gravity must overcome the little bit of angular momentum left and so the bike falls over.

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Re: How Does A Bike Work?

Postby Gigano » Sat Oct 01, 2011 8:43 am UTC

Try to ride your bicycle as slowly as possible, and you'll see instantly why it can stand up: you steer. Also try watching children who learn how to ride a bicycle. They don't steer properly the first few times, hence they often fall over. Another factor that is involved is intuitively shifting your weight to maintain balance. I thought that this had become common knowledge, to be honest.
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Re: How Does A Bike Work?

Postby yurell » Sat Oct 01, 2011 9:37 am UTC

The problem with that, York, is that they've constructed zero net angular momentum bikes (by using counter-masses spinning in the opposite direction) and they still work.
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Re: How Does A Bike Work?

Postby dainbramage » Sat Oct 01, 2011 11:39 am UTC

yurell wrote:The problem with that, York, is that they've constructed zero net angular momentum bikes (by using counter-masses spinning in the opposite direction) and they still work.

Is the fact that the net angular momentum is 0 actually relevant? Each component still has angular momentum and will resist the change in direction. What you're implying here is that if the wheels resist a change in motion, the flywheel will then aid a change in motion.

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Re: How Does A Bike Work?

Postby pietertje » Sat Oct 01, 2011 6:06 pm UTC


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Re: How Does A Bike Work?

Postby Winter Man » Sat Oct 01, 2011 9:18 pm UTC

You can't push a bike down a hill and expect it to stay upright. It falls over. The rider's what keeps it upright.
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Re: How Does A Bike Work?

Postby york_hunt » Sun Oct 02, 2011 6:38 am UTC

dainbramage wrote:
yurell wrote:The problem with that, York, is that they've constructed zero net angular momentum bikes (by using counter-masses spinning in the opposite direction) and they still work.

Is the fact that the net angular momentum is 0 actually relevant? Each component still has angular momentum and will resist the change in direction. What you're implying here is that if the wheels resist a change in motion, the flywheel will then aid a change in motion.


A net angular momentum of zero should still resist the change a dainbramage pointed out. I wonder where the flywheel was located on the bike as if it we located on the steering axis like the wheel then I'd expect a different reaction to the change in momentum than one which is mounted, say, directly onto the frame. One which is directly mounted onto the frame would only have the the axis of the bike on the road surface (roll) which had different consequences to the wheel on the road surface and the steering axis (I guess the yaw would be the best descriptor)

Winter Man wrote:You can't push a bike down a hill and expect it to stay upright. It falls over. The rider's what keeps it upright.


Winter Man, I rather confused by your statement. The whole nature of this discussion is why a bike stays upright even without a rider.

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Re: How Does A Bike Work?

Postby Gigano » Sun Oct 02, 2011 7:48 am UTC

york_hunt wrote:
Winter Man wrote:You can't push a bike down a hill and expect it to stay upright. It falls over. The rider's what keeps it upright.


Winter Man, I [am] rather confused by your statement. The whole nature of this discussion is why a bike stays upright even without a rider.


Fixed that for you.

Also, his point is that the bike will not state upright for long or forever without a rider: it will eventually fall over. Which is what I said a couple of posts ago though no-one seemed to notice. If someone does ride the bicycle it will tend not to fall over. Hence, the rider is influencing the balance of the bike by shifting weight and steering. Therein lies an answer to the question why the bicycle can stay upright for at least sometime without being ridden: it has proper balance for a small duration to stay upright as given by a rider who jumps off the bicycle at the last moment. After that, no more corrections are made by a rider so the bicycle will eventually fall over when the balance is no longer sufficient to keep it upright.
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Re: How Does A Bike Work?

Postby Jplus » Sun Oct 02, 2011 8:01 am UTC

But the reason that at some point, the bike falls over is that it loses speed. We have seen above that as long as a bike is going fast enough, it will stay up out of itself.

So the driver is only needed to maintain speed, not to maintain stability (although drivers will probably typically also help stability).
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Re: How Does A Bike Work?

Postby dainbramage » Sun Oct 02, 2011 8:04 am UTC

Gigano wrote:
york_hunt wrote:
Winter Man wrote:You can't push a bike down a hill and expect it to stay upright. It falls over. The rider's what keeps it upright.


Winter Man, I [am] rather confused by your statement. The whole nature of this discussion is why a bike stays upright even without a rider.


Fixed that for you.

Also, his point is that the bike will not state upright for long or forever without a rider: it will eventually fall over. Which is what I said a couple of posts ago though no-one seemed to notice. If someone does ride the bicycle it will tend not to fall over. Hence, the rider is influencing the balance of the bike by shifting weight and steering. Therein lies an answer to the question why the bicycle can stay upright for at least sometime without being ridden: it has proper balance for a small duration to stay upright as given by a rider who jumps off the bicycle at the last moment. After that, no more corrections are made by a rider so the bicycle will eventually fall over when the balance is no longer sufficient to keep it upright.


Yes, corrections from the rider allow the unstable equilibrium of a bike to stay upright. But I'm sure anyone who's ever ridden a bike can attest that staying balanced on a moving bike is much easier than on a stationary bike. Which is where the gyroscopic inertia comes in to play, giving the rider more time to make small corrections and not fall over. Also with wider tyres, it's possible to achieve a stable equilibrium without needing corrections from a rider, and the gyroscopic inertia here servers to make a knock to the bike smaller, reducing the chance of pushing it out of the region where it's stable.

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Re: How Does A Bike Work?

Postby Gigano » Sun Oct 02, 2011 8:34 am UTC

dainbramage wrote:Yes, corrections from the rider allow the unstable equilibrium of a bike to stay upright. But I'm sure anyone who's ever ridden a bike can attest that staying balanced on a moving bike is much easier than on a stationary bike. Which is where the gyroscopic inertia comes in to play, giving the rider more time to make small corrections and not fall over. Also with wider tyres, it's possible to achieve a stable equilibrium without needing corrections from a rider, and the gyroscopic inertia here servers to make a knock to the bike smaller, reducing the chance of pushing it out of the region where it's stable.


I would reckon then that it's a combination of the rider maintaining balance by steering and shifting weight together with the gyroscopic effects as you have described.

Jplus wrote:But the reason that at some point, the bike falls over is that it loses speed. We have seen above that as long as a bike is going fast enough, it will stay up out of itself.

So the driver is only needed to maintain speed, not to maintain stability (although drivers will probably typically also help stability).


Have you ever tried to drive a bicycle as slowly as possible? Speed is not a required factor for keeping the bike upright, but as dainbramage described it produces a gyroscopic effect which helps to maintain balance. So speed helps but isn't necessary. In the case of a bicycle moving on its own, I do not think speed alone is going to help much if the bicycle's balance and angle relative to the road are already such that it cannot stay upright for long.
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Re: How Does A Bike Work?

Postby yurell » Sun Oct 02, 2011 9:07 am UTC

Here's the Science journal article I was looking for on countering the gyroscopic motion of a bike (I couldn't find it again after first year physics, but fortunately wikipedia had a link)

For those not interested in reading the paper, here's the abstract:
Spoiler:
A Bicycle Can Be Self-Stable Without
Gyroscopic or Caster Effects


A riderless bicycle can automatically steer itself so as to recover from falls. The common view
is that this self-steering is caused by gyroscopic precession of the front wheel, or by the wheel
contact trailing like a caster behind the steer axis. We show that neither effect is necessary for
self-stability. Using linearized stability calculations as a guide, we built a bicycle with extra
counter-rotating wheels (canceling the wheel spin angular momentum) and with its front-wheel
ground-contact forward of the steer axis (making the trailing distance negative). When laterally
disturbed from rolling straight, this bicycle automatically recovers to upright travel. Our results
show that various design variables, like the front mass location and the steer axis tilt, contribute to
stability in complex interacting ways
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Re: How Does A Bike Work?

Postby york_hunt » Sun Oct 02, 2011 10:33 am UTC

A small aside but this [http://www.youtube.com/watch?v=IXhRPnh1JSU] is the scene from the 1949 film Jour de Fête which shows the self-stability in action, as quoted in the above mentioned journal article.

This has gotten me thinking about other non-motorised systems, similar to a bicycle, such as a scooter:
Image

Which will not balance by itself and yet it has all the same components that defines a bicycle:
A bicycle is defined as a three-dimensional mechanism (Fig. 1A) made up of four rigid objects (the rear frame with rider body B, the handlebar assembly H, and two rolling wheels R and F) connected by three hinge.


So the matter seems to go down to where the centre of mass is located. The centre of mass of this object is not near the centre of this object, which would make it less stable and more susceptible to falling over.

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Re: How Does A Bike Work?

Postby jmorgan3 » Sun Oct 02, 2011 6:57 pm UTC

dainbramage wrote:Is the fact that the net angular momentum is 0 actually relevant? Each component still has angular momentum and will resist the change in direction. What you're implying here is that if the wheels resist a change in motion, the flywheel will then aid a change in motion.
york_hunt wrote:A net angular momentum of zero should still resist the change a dainbramage pointed out.

No, it won't. Imagine one of those bicycle-wheel-on-handles things that are used for physics class demonstrations. They resist torques because they move at angle perpendicular to applied torque (and also perpendicular to the angular momentum, with sign determined by a cross product), e.g. if you are holding the handles with both hands and the wheel is spinning with the top moving away from you, then pushing forward with your left hand will result in your left hand moving downward. If the wheel was spinning the other way, your left hand would move upward. Putting two wheels on the same shaft with net zero angular momentum will cause those two effects to cancel out.

dainbramage wrote:But I'm sure anyone who's ever ridden a bike can attest that staying balanced on a moving bike is much easier than on a stationary bike. Which is where the gyroscopic inertia comes in to play, giving the rider more time to make small corrections and not fall over.

But there is also a stabilizing effect from centrifugal force, which scales as velocity squared and which also vanishes as velocity goes to zero. It is impossible to judge from just riding a bike which of those effects is dominant.

I believe that the centrifugal force effects of small corrections in direction are dominant. At the very least, they are sufficient for stability, as ice skaters can glide on one skate as long as they are moving, but cannot stand on one skate. Ice skaters obviously have no spinning wheel to help keep them upright, but can take advantage of centrifugal force.
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Re: How Does A Bike Work?

Postby Sockmonkey » Fri Oct 07, 2011 11:17 am UTC

IIRC an experiment was done with a specially made bike that experienced no gyroscopic forces and it was perfectly ridable.
Now, for a riderless bike the wheels are a significant part of it's overall mass and thus exert enough force to keep it upright, but for one with a rider, the faster you go, the faster you can make corrections to the balance with steering, just like with ice skates.
And those foreward raking forks actually make the bike less stable than straight ones would for better manuverability.


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