Electric bicycle comparisons: chain vs. friction

Written by david weinberg
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Electric bicycle comparisons: chain vs. friction
An example of a chain drive system. (chaine de moto image by Jerome Dancette from Fotolia.com)

Electric bicycles are two-wheeled vehicles powered by a combination of human and electric power. They provide a compromise between a bicycle and a motorcycle, using a small electric motor to supplement or replacing the rider's pedalling power. You can purchase an electric bicycle premade or add an electric motor to a standard bicycle using a kit. Electric bicycle motors work in two ways: chain drive and friction drive.

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Friction Drive

A friction-drive electric bicycle connects power directly from the motor to the bicycle's tires. The electric motor rotates a short cylindrical shaft that presses against the outside surface of one of the bicycle's tires. Friction between the motor's rotating shaft and the outer edge of the tire causes the wheel to turn and the bike to move.

Friction Drive Advantages

Simplicity of construction is the greatest advantage of a friction-drive system. Installing a friction-drive motor is simple: You merely attach it to the bicycle's frame, above the rear tire, in such a way that the motor's driveshaft makes contact with the tire. This layout also makes it simple to remove the motor or the bike tires whenever you need to do maintenance or change a flat.

Friction Drive Disadvantages

Friction-drive bicycles suffer from loss of power in poor conditions. Anything that reduces friction between the driveshaft and the tire will result in "slippage," wherein the driveshaft spins but the tire does not. This translates into wasted power. Riding through puddles of water or oil can cause slippage, as can normal wear on the tyre's tread.

Chain Drive

A chain-drive system functions in a similar way to bicycle pedals. A sprocket (the gear that drives the chain) mounted on the motor's driveshaft connects by a chain to another sprocket on one of the bicycle's wheels. When the motor turns, the chain turns the sprocket on the wheel, and the wheel rotates. On most bicycles, a chain-drive set-up requires the installation of a new sprocket on the wheel and a separate chain for the motor to use. Using the existing rear sprocket would cause the bicycle's foot pedals to turn with the motor, making for a very uncomfortable ride.

Chain Drive Advantages

The chain between the motor and the bicycle tire provides better acceleration and a steadier top speed than you'll get with a friction drive. The chain connection ensures that when the motor turns, the wheel will turn as well, avoiding slippage between the tire and the motor. The use of a chain also means that you can place the motor anywhere on your bike, as long as it's within reach of the chain.

Chain Drive Disadvantages

The addition of extra moving parts means that a chain drive is more likely to break down than a friction drive. The chain connection is also considerably more complicated to install and maintain. In addition to adding a sprocket to one of the tires for the motor to utilise, you may have to adjust the chain length to fit your bicycle and add a chain tensioner to keep the drive chain from slipping off its sprockets.

Comparison

If you are purchasing your electric bicycle or if you are comfortable with tools, a chain-drive bicycle will provide better performance in a wider range of conditions. If you are building your electric bicycle from scratch and are not comfortable with tools, or if you want to ensure that maintenance of your bicycle will be simple, a friction-drive bicycle will make an excellent do-it-yourself project.

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