Chainwheels and Derailleurs

The Front Wheel Role in Gearing

Modern cycles create speed by using gearing to turn the rear wheel faster than pedals. However, the front wheel is not connected in any way to the gearing, and is now “along for the ride,” and simply following the lead of the rear wheel.

How Gearing Works

Gearing is easy to understand. The key is the chain which provides a hard connection between the front and rear chainwheels. That means there is a physical connection between how many teeth pass by the front of the chain due to pedaling and how many teeth pass by the rear. This in turn determines how many times the rear wheel rotates.

Chainwheel Teeth Counts

Modern road bikes have two or three front wheel chainwheels, bonded together. The front derailleur moves the chain between the chainwheels based on the cyclist’s selection at the handlebar, and the chainwheel pulls the chain forward at a rate determined by the cyclist’s cadence.

This makes it important to know how many links are pulled per pedal cycle or equivalently how many teeth there are per chainwheel. Chainwheels are standardized for manufactured bikes, so for a three ring configuration, the teeth count are (30, 39, 53) while for a two-ring configuration, the counts are (34, 50).

The standard configuration of rear wheel cassettes is ten different chainwheels. The standard number of teeth is (12, 13, 14, 15, 16, 17, 19, 21, 23, 25).

Teeth Size

When looking at a derailleur, it may appear the teeth have different sizes. However, when you think about the chain needing to work with all of the various front and rear combinations, you quickly conclude the teeth must be all of the same size and specifically equal to the chain link width which for a standard chain is 0.5 inches.

Gearing Ratios and Rear Tire Rotation

Gearing fixes the rear wheel rotation rate.  If your front chainwheel is set at 34 and your rear at 17, every pedal turn pulls 34 teeth producing two full turns of the rear wheel.

In fact, for each gearing, you can determine the rear wheel rotation rate per pedaling cycle by taking the ratio of the front/rear gearing counts. In the above, it was 2. The implications of this are discussed in the next page.

Next Topic:   Cadence, Gearing, and Speed