[Geezer (OP)]

Why is it that (most) bicylces still use chains to transmit power, rather than gears? Why has the chain not been replaced by a system of gears?

[CliffordK]

Certainly not all bicycles have chains.



If you think of your classic 10-speed, it may have 13x52 gearing, or allow the wheel to turn about 4 times for every rotation of the pedals, and you would travel about π * 27" x 4 = 339" or about 28 feet.

The big wheeled bicycles above had wheels up to about 60", but no gearing, giving a maximum distance per pedal of about π * 60" = 188" or about 15 feet.

Placing a geared bottom bracket through the middle of the front hub would be complicated, but perhaps possible.  Turning radius might be significantly compromised, but one could imagine building a recumbent bicycle with such a configuration.

To connect the two pedals together, traditional bicycles have a solid shaft through the bottom bracket in front of the tires.  With moderate sized wheels, this leads to a fairly long distance between the bottom bracket and the hub, and would require several gears if one was to make it gear driven.  Assuming one would wish to pedal in the conventional direction, one would need at least an odd number of gears (3, 5, etc).  A chain of gears would add complexity, weight, and perhaps additional friction, although there are good composites available now.

There are new Kevlar Bicycle Belts that are available now.  Perhaps they will eventually replace the old chains, but it will take some time for them to come into common usage.

http://cycledrive.com/


Some of the new small-wheeled bicycles might not have the same distance limitations that their larger cousins have, and might be better suited to a gear drive, but apparently all use either a chain or belt drive system.




There is a lot of progress on internally geared hubs which are better than the old 3-speed bicycles, and allow the usage of tight chains without necessitating the use of derailleurs.  In fact, I believe the kevlar chains above were being used internally shifted hubs.

[Geezer (OP)]

Yes, but why are chains the preferred method?

[CliffordK]

With something like the little A-Bike, you could probably get away with 3 gears.  I suppose if you used teflon or a composite, you wouldn't need to lubricate the gears themselves, but they would wear over time.  Steel, of course, would be heavy. 

A larger bicycle with 26" or 700c wheels would likely need a total of 5 gears, and would be a nightmare.

The belt above likely works much in the same way as a timing belt.  As far as I can tell, they don't work with derailleurs, and thus require heavier, more expensive internally geared hubs.

Here you go.
Shaft driven bicycles.

http://en.wikipedia.org/wiki/Shaft-driven_bicycle


There is, however, cost.
The cheapest bicycles would have their gears cut out of stamped steel.  A replacement chain is about $10, but no doubt the manufacturers pay less. 

This shaft drive would be proprietary, from the frame, bottom bracket, to the hubs...  all proprietary, and thus expensive.

I don't see the racing circuits picking them up either due to the additional weight, and lack of flexibility. 

One of the more unique bicycle innovations is the NuVinci continuously variable planetary (CVP) hub.

http://www.gizmag.com/go/6132/

It looks pretty sweet, but perhaps a bit over priced.

Perhaps these are some of the reasons then.
People are comfortable with the current bicycle design.  There are slow improvements such as the "mountain bike", then "suspension".  New Brakes, etc.  But, there is still a debate about the difference between hub and disk brakes.

But you might just as well ask why everyone doesn't ride a recumbent bicycle.

Something like the shaft drive above would hit a very narrow niche of expensive commuter bicycles, and miss out on the masses of the low-end commuter bikes, and the high-end racing bikes.  It may also have troubles with the newer mountain bicycle suspension systems.

Well, I have to take that back.
Here is a full suspension, shaft-drive, folding mountain bike.
http://cgi.ebay.com/SAFE-CONVENIENT-ALLOY-CHAINLESS-FOLDING-BIKE-26-INCH-/170629851597?pt=LH_DefaultDomain_0&hash=item27ba54e9cd

[Don_1]

Right, for what its worth, here's my penny farthing two pennies worth.

Using gears to transfer the power from the pedals to the rear wheel hub would surly involve a loss of power, since it would need to change direction of the power, while a chain directly transfers the power without the need to change direction, therefore less power loss in the transfer.

[peppercorn]

Because derailleurs are cheaper to make and maintain than hub gears.
I suspect belts couldn't take the prolonged stress of twisting along their length and 'jumping' between gears.
A shaft-drive can only work with a hub-gear or something similar.
A train of gears would need many bearings (all with races) - expensive and impractical to maintain.

[imatfaal]

Shaft-driven motorbikes (basically BMW's) have a pretty bad rep in the hard core biking community

Note below on this very theme is very funny - but rude, puerile and very dodgy

[Geezer (OP)]

Thanks for the nice answers everyone, but nobody seems to have come up with the fundamental reason for the success of the bicycle chain.

[Geezer (OP)]

Note below on this very theme is very funny - but rude, puerile and very dodgy

Ah yes! The Bayerische Mist Wagen 

[CliffordK]

Shaft-driven motorbikes (basically BMW's) have a pretty bad rep in the hard core biking community

I'm not a motorcycle rider, but as I understand it, the problem with a shaft-drive motorcycle isn't as much the shaft drive as the engine orientation.  When you spin up the engine, it would naturally try to toss the motorcycle to the side.

It wouldn't be a problem with a shaft drive bicycle, unless you mounted the rider sideways.... which would be a pretty unique way to ride a bicycle, but doing so you would have to contend with the shaft connection to the cranks.

From the examples I've posted, there aren't any insurmountable problems with a belt drive or shaft drive, or even gear driven system.  However, it has to be designed into the bicycle.  For example, a typical chain crosses the frame.  The belt drive, as above, has to be designed so as not to cross the frame, or the frame has to be designed to allow a gap to be made for mounting the belt.  No matter what, one still needs an easy way to replace tires.

[Geezer (OP)]

BTW, they are referred to as "roller chains", and for a good reason.

[Atomic-S]

The way I see it, every alternative to the chain is more cumbersome and expensive.

[SeanB]

Chain is used as it is both cheap, has very low losses and can transmit a lot of power for it's mass. There are only 2 places where a chain loses power, at each end, and it can accommodate a lot of misalignment and movement of the ends, which the other systems cannot do. A shaft needs 2 direction changes to move power, and both have quite high losses. Gears have losses in each stage, from teeth running on each other and from the losses in bearings for each gear, which adds up to a lot.

[Geezer (OP)]

Yeah! I think that's it Sean. The rollers in the chain act like wheels rotating on an axle, so there is not much friction at all. Chain drives are very efficient, but gears are better for handling large amounts of torque.

I suppose the friction could be reduced even more by incorporating rolling bearings of some sort to support each roller, but I suspect any improvemnet would be marginal. 

[qazibasit]

those that have chains converts ur mechanical energy from the body to the rear tyre of the bicycle or it will just be an aerobic stationary cycle.