[Geezer (OP)]

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?

(Lights blue touchpaper and retires.)

[RD]

Not all do ...

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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).

[syhprum]

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.

[Geezer (OP)]

Not all do ...

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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.

[RD]

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.

[John Chapman]

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.
 

[lightarrow]

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.

[syhprum]

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]

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.

[LeeE]

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.

[Geezer (OP)]

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

[lightarrow]

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?

[LeeE]

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.

[John Chapman]

That was well explained LeeE.

You'd make a good teacher  []

[lyner]

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! []

[syhprum]

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.

[Geezer (OP)]

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]

syphrum
Did you run away and join the circus?

[that mad man]

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]

[Turveyd]

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.