This is the final installment of our long-running disc brake guide. Now it's time to choose your rotors—the discs that generate friction against the brake pads. When selecting rotors, the two most important factors are the mounting method and the size.
For information regarding mounting methods that depend on your hub type, please refer to our previous article, "[PIT TIPS] About those plates on the wheels you see lately – Disc Rotors." In this installment, we will take a deeper look at rotor sizes—which we touched on briefly last time—and consider why differences in size exist, along with their respective strengths and weaknesses.
For example, Shimano offers four types of rotor sizes. From largest to smallest, they are 203mm, 180mm, 160mm, and 140mm.
Here is a comparison of the largest and smallest options.
These size differences exist to provide varying levels of braking power. Since bicycle brakes use the principle of leverage to generate stopping power, a larger rotor—which places the caliper (the point of force) further from the hub axle (the fulcrum)—will provide greater stopping power.
You might be tempted to think, "So bigger is better!" However, if you blindly install a rotor that is too large, you may find yourself in a situation where the brakes are "too effective," which causes its own set of problems.
What does "too effective" mean?
In the context of bicycle brakes, "too effective" means the force used to stop the wheel is too strong relative to the tire's grip, causing the wheel to lock up immediately. This prevents the brakes from fulfilling their primary purpose: controlling your speed.
This state is extremely dangerous and can lead to situations like the one in the photo below, so it must be avoided at all costs.
Rotor Size Differences and Characteristics
To help you avoid the "too effective" scenario, here are the general characteristics of each rotor size.
・140mm: Primarily for road bikes. It is commonly used with flat-mount calipers and is also featured on some gravel bike models.
・160mm: The first disc rotor size developed by Shimano. Used on a wide range of bikes, including MTBs, cyclocross, and gravel bikes.
・180mm: The standard size for modern MTBs. It is generally not used on road or gravel bikes.
・203mm: Almost exclusively used on bikes specialized for descending, such as downhill MTBs.
While these sizes may be limited by your frame's compatibility, the general usage guidelines are as described above.
So, when should you change your rotor size to adjust braking power?
The reason for changing the size is, naturally, to increase stopping power. It is common, for example, to upgrade from a 160mm rotor to a 180mm rotor on an MTB if you want a bit more bite from your brakes.
However, just because you want better performance, you shouldn't install an oversized rotor like a 203mm on a road bike. As mentioned earlier, it doesn't just look out of place; it can make your brakes "too effective," making it impossible to control your speed. You must always consider your tire width and how you intend to use the bike.
That said, as long as you understand these basics and the mounting requirements, you shouldn't run into any issues when swapping out your rotors. Please choose your rotors carefully, keeping your intended use, the aesthetics, and the materials in mind.
Now, the path to disc brakes lies open before you. Please take a look through the catalog and choose your new brakes. I am sure you will discover a whole new riding sensation. If you are ever unsure, feel free to consult our staff as always.
We are always here to help.
Related Disc Brake Articles
【PIT TIPS】About those plates on the wheels you see lately – Disc Rotors –
【PIT TIPS】Disc Brake Guide Vol. 1 – Pros and Cons of Disc Brakes –
【PIT TIPS】Disc Brake Guide Vol. 2 – Preparation for Switching to Disc Brakes –
【PIT TIPS】Disc Brake Guide Vol. 3 – Choosing Levers and Calipers –