While not exclusive to road bikes, bicycles with derailleurs are equipped with sprockets.

When choosing a sprocket, you'll encounter values like "12-28T."

If you ask about these numbers without understanding what they mean, you might end up asking something like, "Is a higher or lower sprocket number better?"

To someone knowledgeable, this would seem like a "strange question."

Here, we'll explain the meaning of sprocket numbers and provide tips on how to choose the right one.

Is it a Gear or a Sprocket?

Many people refer to derailleurs as "gears."

However, some also call the individual components "sprockets."

Is "gear" the common term, and "sprocket" the more technical one?

That's not quite right.

In the mechanical industry, "gear" and "sprocket" are distinctly different terms.

A "gear" meshes with another gear, while a "sprocket" meshes with a chain.

Considering the mechanism of a bicycle, "gear" is technically incorrect; "sprocket" is the accurate term.

However, in daily life, you rarely see gears directly meshing with each other.

This is because Japan prioritizes safety, and any place where gears mesh is always covered to prevent accidents.

Since "gear" is a familiar term and its meaning is generally understood, the author believes it's acceptable.

For example, if someone asks to borrow a "stapler" and you constantly correct them by saying, "It's a stapler, not a Hotchkiss (brand name)," you won't build smooth human relationships.

As long as the familiar term conveys the meaning without misunderstanding, I think it's fine.

To be precise, some websites explain that the front gear is called a "crank" and the rear gear is called a "sprocket," but a sprocket is simply a toothed component that meshes with a chain.

A "crank" refers to a mechanism that bends in two or more places, as shown in the figure below.

Since it's the sprocket on the crank side, it might be referred to as a "crank," but those unfamiliar with the terminology seem to mistakenly remember the front gear as a "crank" and the rear as a "sprocket."

The author believes that the fact that some crank-side sprockets are sold integrated with the crank arm further contributes to this misunderstanding.

Meaning of Sprocket Numbers

Sprockets have numerical values.

For example, "12-25T" or "11-34T."

"T" stands for "Teeth," indicating the number of teeth.

25T means 25 teeth, and 34T means 34 teeth.

Once you understand it, it's quite simple.

On the crank side, the larger diameter sprocket on the outside is called the "outer," and the smaller diameter one closer to the frame is called the "inner."

"12-25T" means the inner has 12 teeth, and the outer has 25 teeth.

For example, let's say you see the following description:

"SHIMANO 105 FC-R7000"

"FC-R7000" indicates that it's part of the "SHIMANO 105 R7000 Series."

Looking at the chainring combinations, there are three types: "50-34T," "52-36T," and "53-39T."

The larger the number of teeth on the gear, the "heavier" it is. Comparing "50-34T" and "53-39T," "53-39T" is heavier. (Assuming the rear sprocket remains unchanged.)

Generally, you can assume there are only three patterns: "50-34T," "52-36T," and "53-39T."

Since sprockets can be purchased individually, some people might want to choose the widest possible range, such as "20-39T." However, Shimano front derailleurs (shifters) only support a difference of up to 16 teeth.

In the case of "20-39T," the difference is 39-20 = 19 teeth, which exceeds the capacity (shiftable range).

Conversely, if you wanted something like "39-50T," the difference is 50-39 = 11 teeth, which is within capacity.

However, since these are not the basic three options, it's difficult to adjust effectively.

Furthermore, if you use a different number of teeth, the chain and teeth simply won't mesh properly.

Gears and sprockets need to have a "coprime" relationship.

Simply put, if some teeth are used frequently and others less so, the wear on the gear or sprocket becomes uneven, preventing smooth rotation.

Sprocket Selection

When choosing a crank-side sprocket, we've learned that you can select from three patterns: "50-34T," "52-36T," and "53-39T." Now you need to consider which one best suits your needs.

If you use a smaller front gear and a larger rear gear, the resistance to pedaling will be less, making it feel lighter.

This is suitable for climbing hills.

This is commonly referred to as "cadence."

Cadence originally means "revolutions per minute," so it doesn't hold meaning on its own, but in this context, it refers to reducing pedal resistance to rotate as quickly as possible to climb hills.

In that case, you would choose the "50-34T" with the smallest 34T.

Conversely, if you use a larger front gear and a smaller rear gear, the pedal resistance will be maximized, but the rotation speed of the rear wheel per pedal revolution will also be maximized.

This is ideal for environments like cycling paths where high-speed riding is possible, and there are no traffic lights or pedestrians suddenly appearing.

You would use the 53T of the "53-39T."

Extreme conditions like constantly climbing hills or always riding on a cycling path are rare.

Therefore, options arise such as choosing the intermediate "52-36T," choosing "50-34T" if your usual route has many hills, or choosing "53-39T" if you often ride in the countryside.

Please choose according to your specific use case.

▲ICAN Aero Road Bike A9

External Writer: Koichi Okuno