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Author Topic: Why do bicycles have such big wheels?  (Read 28058 times)

Offline Geezer

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Why do bicycles have such big wheels?
« on: 05/09/2009 17:05:52 »
Well, most bicycles anyway. Is it a macho thing? "Hey, look at the size of my wheels!" or do big wheels serve a useful purpose?

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Offline RD

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Why do bicycles have such big wheels?
« Reply #1 on: 05/09/2009 17:25:40 »
Not all do ...


http://www.treehugger.com/files/2007/11/marking_the_spo.php

Above smaller wheels have been used for portability.

The smaller tyres on smaller wheels will wear out quicker than larger tyres on larger wheels.
(ditto for hubs and spindles).
« Last Edit: 05/09/2009 17:36:30 by RD »
 

Offline syhprum

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Why do bicycles have such big wheels?
« Reply #2 on: 05/09/2009 17:29:20 »
Cyclists like to operate in a near vertical position so big wheels are needed to accommodate their legs also they have less rolling resistance and ride the bumps better.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #3 on: 05/09/2009 17:33:26 »
Not all do ...


http://www.treehugger.com/files/2007/11/marking_the_spo.php

Above smaller wheels have been used for portability.

The smaller tyres on smaller wheels will wear out quicker than larger on larger wheels.

Wow! I'd like to see one of them in the Tour de France.
 

Offline RD

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Why do bicycles have such big wheels?
« Reply #4 on: 05/09/2009 17:52:42 »
The bigger the diameter of the wheel the less the cyclists energy is lost to friction,
 so for maximum speed/efficiency the bigger the better.
 

Offline John Chapman

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Why do bicycles have such big wheels?
« Reply #5 on: 06/09/2009 02:45:13 »
Presumably the frictional benefits are in the spindles. Larger wheels spin at slower RPM to cover the same distance.

There are also tranmission benefits to having a large driving wheel. Otherwise the front (pedal) sprocket would have to be enormous to gain the same benefit.
 
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #6 on: 06/09/2009 03:00:29 »
Presumably the frictional benefits are in the spindles. Larger wheels spin at slower RPM to cover the same distance.

There are also tranmission benefits to having a large driving wheel. Otherwise the front (pedal) sprocket would have to be enormous to gain the same benefit.
 
Also, the equilibrium on the bicycle is more difficult with little wheels because of little gyroscopic effect.
 

Offline syhprum

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Why do bicycles have such big wheels?
« Reply #7 on: 06/09/2009 11:59:31 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.
 

lyner

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Why do bicycles have such big wheels?
« Reply #8 on: 06/09/2009 12:48:30 »
The Moment of Inertia of a bicycle wheel is pretty small (particularly a light weight racing wheel)  so is it really likely to have much effect?

Surely the biggest advantage of big wheels is that the footprint is longer for a given tyre pressure. This will mean that the distortion , and hence the losses / internal friction will be less. The better efficiency of the transmission with big wheels will also be a factor, natch, but tyre friction is sure to be a major factor.

btw, does anyone know of any work done cycling on rails / steel track, with steel wheels. (Obviously, stabilisers could be needed). There may be a world record out there for someone.

There is also the issue of ironing out bumps in the road but neither this nor any (possible?) balancing help would not be relevant for competitive cycling on high quality cycle tracks. Pros would cycle on 10cm wheels if there was a speed advantage.
 

Offline LeeE

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Why do bicycles have such big wheels?
« Reply #9 on: 06/09/2009 17:55:24 »
The bigger the diameter of the wheel the less the cyclists energy is lost to friction,
 so for maximum speed/efficiency the bigger the better.

Hmm... on a hard surface, a smaller wheel will have a smaller area of contact than a larger wheel, so a smaller wheel should offer less rolling resistance than a larger wheel.

A smaller wheel would impose a higher pressure upon the surface it's traveling across though, so if you're traveling across an unmetalled surface you'll sink further into it with a smaller wheel than with a larger wheel, which will raise the rolling resistance.  The larger wheel, with it's larger contact area, will sink less on a soft surface, so it's rolling resistance will be more constant across hard and soft surfaces.  This was an issue when the first bicycles were invented, as many 'roads' were unmetalled.

Smaller wheels are also more susceptible to bumps, irregularities and even stones on the surface.  Obviously, a wheel cannot roll over an obstacle unless the height of the obstacle is less than the radius of the wheel, so larger wheels can roll over larger obstacles than smaller wheels, but even when the obstacle is small enough that both large and small wheels can roll over it, the larger wheel, with its reduced curve, will intercept the obstacle sooner than a small wheel and result in a lower vertical acceleration as the wheel passes over it; in short, a larger wheel is more comfortable than a smaller wheel.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #10 on: 06/09/2009 18:02:44 »
btw, does anyone know of any work done cycling on rails / steel track, with steel wheels. (Obviously, stabilisers could be needed). There may be a world record out there for someone.

No data. But these guys might have some. (Note very small front wheel). There are several kits available to convert a regular bike into a rail bike.

http://www.railbike.com/buying.htm
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #11 on: 06/09/2009 18:25:08 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.
I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?
 

Offline LeeE

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Why do bicycles have such big wheels?
« Reply #12 on: 06/09/2009 19:06:44 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.
I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?

When a bicycle is stationary you can only keep it in balance by moving your weight from side to side to keep your CoG over the wheels.  This isn't an ideal solution because you're moving the larger of the two masses instead of the smaller to maintain balance.  Moving the larger of the two masses also requires more force to be used.  When the bicycle is moving though, this is reversed and instead you steer the smaller mass of the bicycle to keep it beneath the larger mass of yourself, with consequently less force being required to do so.

The steering process is also more progressive than than simply moving your body from side to side because the bicycle follows the sum of the forward and steering vectors, instead of just the sideways vector, giving a finer degree of control.
 

Offline John Chapman

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Why do bicycles have such big wheels?
« Reply #13 on: 06/09/2009 19:36:48 »
That was well explained LeeE.

You'd make a good teacher  :)
 

lyner

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Why do bicycles have such big wheels?
« Reply #14 on: 06/09/2009 20:14:50 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.

I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?
I thought we'd been here before. You can make a bike which is impossible to ride by having the front forks so that the line of the top bracket hitting the ground behind the point of contact with the wheels. Gyroscopic action is the same in both cases. It has to be the castor action which steers the front wheel to produce a moment to push you upright. I have done diagrams which satisfy me but they wouldn't work on a post here cos I need to wave arms at the same time! ;)
 

Offline syhprum

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Why do bicycles have such big wheels?
« Reply #15 on: 06/09/2009 20:37:49 »
Research has also been done on this matter, bicycles fitted with gearing that causes the forks to rotate in the reverse manner to the handlebar's can be ridden with practice but it is not as instinctive as riding a normal machine.
With a zero castor angle the steering would be heavy but could be learnt.
As a boy I practiced riding a bicycle with ropes tied to the handlebar's like reins, by crossing the ropes you could produce the same effect as the reverse geared steering.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #16 on: 06/09/2009 20:48:58 »
Wasn't there something called the Moulton bicycle produced in the 60's? It had much smaller wheels. I think Raleigh produced something similar, but it had wider tires. As I seem to recall, they actually worked quite well, certainly on well paved surfaces.

(I just Googled Moulton - still seems to be going strong.)

http://www.moultonbicycles.co.uk/index.html
 

lyner

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Why do bicycles have such big wheels?
« Reply #17 on: 06/09/2009 22:17:07 »
syphrum
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Offline that mad man

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Why do bicycles have such big wheels?
« Reply #18 on: 06/09/2009 22:24:00 »
Sir Clive Sinclair developed a portable folding bike with small wheels called the A-Bike and was described by some as a bit "wobbly" because of the size of the wheels.

I remember a science lesson at school when we had several sized bicycle wheels each mounted on a small axle that we could hold in outstretched arms. We sat on a desk type chair (one on wheels) and held the bicycle wheel out in front of us. Someone then spun the wheel and then we were asked to tilt the wheel from the vertical and also simulate it turning like a bikes front wheel.

The outcome was the chair, with you in it, spun round a bit. The different sized wheels produced different sized effects with the larger wheels producing greater effects. Unfortunately that was years ago and I cant remember exactly what that lesson was about but I think it was something to do with the gyroscope effect. [:I]

 

Offline Turveyd

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Why do bicycles have such big wheels?
« Reply #19 on: 06/09/2009 23:40:33 »

As a Mountain Biker who's messed with wheel sizes and has been running a 29" wheel on the front of a standard normal 26" wheel for 2years,  I can safely say the main advantage is it takes longer from contact to highest spot therefore the peak bump force is lower,  therefore they ride smoother over rough ground which makes them more efficent as they stall less,  which leads to going over the bars and pain and maybe even a broken kneck!!

There is a slight drop in rolling resistance,  the foot print becomes slightly longer but narrorow and the narrower part helps in short,  for the same air pressure though the overall size is unchanged.

 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #20 on: 06/09/2009 23:59:24 »
29"? Holy crap! That's bigger than the tires on my truck.

Have you considered getting a penny-farthing? It would probably be the ultimate mountain bike.
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #21 on: 07/09/2009 12:42:08 »
I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?
When a bicycle is stationary you can only keep it in balance by moving your weight from side to side to keep your CoG over the wheels.  This isn't an ideal solution because you're moving the larger of the two masses instead of the smaller to maintain balance.  Moving the larger of the two masses also requires more force to be used.  When the bicycle is moving though, this is reversed and instead you steer the smaller mass of the bicycle to keep it beneath the larger mass of yourself, with consequently less force being required to do so.

The steering process is also more progressive than than simply moving your body from side to side because the bicycle follows the sum of the forward and steering vectors, instead of just the sideways vector, giving a finer degree of control.
And you cannot steer when the bicycle is not moving?
With *not moving* I intended that you are staionary in the same point of terrain, not that you can't move the bicycle at all. Even if you steer, it's more difficult to stay in equilibrium, in comparison to when you are going at a minimum speed. Why?
« Last Edit: 07/09/2009 12:47:37 by lightarrow »
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #22 on: 07/09/2009 12:49:50 »
Sir Clive Sinclair developed a portable folding bike with small wheels called the A-Bike and was described by some as a bit "wobbly" because of the size of the wheels.

I remember a science lesson at school when we had several sized bicycle wheels each mounted on a small axle that we could hold in outstretched arms. We sat on a desk type chair (one on wheels) and held the bicycle wheel out in front of us. Someone then spun the wheel and then we were asked to tilt the wheel from the vertical and also simulate it turning like a bikes front wheel.

The outcome was the chair, with you in it, spun round a bit. The different sized wheels produced different sized effects with the larger wheels producing greater effects. Unfortunately that was years ago and I cant remember exactly what that lesson was about but I think it was something to do with the gyroscope effect. [:I]
Yes, and about angular momentum conservation.
 

Offline LeeE

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Why do bicycles have such big wheels?
« Reply #23 on: 07/09/2009 15:38:48 »
Quote
And you cannot steer when the bicycle is not moving?

Well, you can waggle the handlebars about and, because of the castor angle of the front wheel, the front of the frame can pivot around the rear wheel but to keep balance you need to keep your CoG over the wheel/ground contact points.  It doesn't matter where the frame is or whether the frame is tilted, or not.  So if the bicycle is not moving the wheel/ground contact points won't change and you won't have steered anywhere.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #24 on: 07/09/2009 18:41:16 »
With *not moving* I intended that you are stationary in the same point of terrain, not that you can't move the bicycle at all. Even if you steer, it's more difficult to stay in equilibrium, in comparison to when you are going at a minimum speed. Why?
I think the balancing effect is similar to balancing something like an inverted broom/brush. Put the broom handle on your palm with the brush end vertically above it. It is remarkably easy to balance the broom by only making small adjustments in the position of your hand. The large mass at the other end of the broom handle helps, because it has some inertia, so it takes a large hand movement to cause it to accelerate in any direction.

When a bicycle is moving, it's easy to continuously adjust the steering and make fine adjustments to the lateral position of the tires to maintain equilibrium against the other forces (similar to the hand movements with an inverted broom). When the bicycle is stationary the rider can only maintain equilibrium by transferring weight from side to side. I could never do it, but I knew people who could, and they did it by continually adjusting the steering angle (if I remember correctly).

I suppose they were taking advantage of the castor angle to make very small adjustments to their center of mass relative to the line joining the contact points of the two tires, but that might be another thread entirely  :)

EDIT: I think LeeE was perhaps making the same point re. balancing a stationary bike.
« Last Edit: 07/09/2009 19:37:23 by Geezer »
 

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Why do bicycles have such big wheels?
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