What are Bike Rotors? (Rotors & Disc Brakes Explained)

If you ride a mountain or hybrid bike, chances are you use disc brakes. They’re even becoming a popular choice for many road cyclists.

Bike rotors are a key component of a disc braking system. In this article, we’ll take a look at what bike rotors are, why they’re important, and why they vary from bike to bike.

What are Bike Rotors?

Bike rotors, or discs, are circular metal discs that are clamped between the brake pads when force is applied to the brake lever. The friction force between the brake pads and the rotor opposes the forward motion of the wheel, bringing the bike to a stop as a result.

What is a Bike Rotor?

The term ‘bike rotor’ simply refers to the disc brake rotor, or disc, in a disc braking system. The terms ‘brake disc’, ‘bike disc’, ‘brake rotor’, and ‘brake disc’ are used interchangeably to refer to the same part.

When pressure is applied to the brake lever, the brake pads are squeezed together to apply a force to the disc (or rotor) which acts against the movement of the wheel.

The brake rotors are attached to the wheels, so as the friction applied to the rotor through the brake pads slows the rotor down, the wheels are slowed down as a result.

Bike rotors come in a variety of shapes and sizes. Typically speaking, a larger brake rotor will provide more stopping power at the expense of added weight. If you’re interested in understanding what effect rotor size has on braking effectiveness, check out the article we wrote here.

What is a Disc Brake on a Bike?

Disc brakes are simply a type of brake that clamps a disc attached to the wheel as opposed to the wheel itself.

Before disc brakes became the norm, most bikes used V-brakes. This type of brake clamps the wheel directly instead of a disc.

Whilst lighter and cheaper than disc brakes, the performance of V-brakes deteriorates quickly in wet or muddy conditions. This is particularly bad for mountain biking, so disc brakes were adopted.

Disc brakes are designed to allow mud and water to run off the disc, resulting in more consistent braking performance in all weather conditions.

If you’ve taken a look at a bike disc brake in the past you will have noticed holes in the discs. These holes play an important role in the functionality of your brakes, check out the article we wrote here that explains why bike rotors have holes in them.

How do Disc Brakes Work on a Bike?

Disc brakes are composed of 4 main parts: The brake lever, the cable (for mechanical brakes) or hydraulic hose (for hydraulic disc brakes), the rotor, and the brake calipers.

There are subtle differences between mechanical disc brakes and hydraulic disc brakes. Here’s a brief overview of each:

Mechanical Disc Brakes

The brake lever is connected to a steel cable which is connected to the brake calipers (the part of the brake that clamps). When the brake lever is pulled, this pulls the cable which in turn clamps the brakes together.

The brakes clamp the moving rotor providing a force that opposes the wheel spinning and as a result, the wheel slows down.

Hydraulic Disc Brakes

Hydraulic disc brakes use a hydraulic hose instead of a steel cable. When the brake lever is pulled, the pressure in the hydraulic hose increases and causes the calipers to clamp together.

Do Bike Rotors Matter?

Bike rotors are extremely important as they are the link between the brake lever and the wheels. Without a brake rotor or a properly functioning brake rotor, the force applied to the brakes will not be effectively transferred to slow the wheels. In other words – the brakes won’t work.

Now we know rotors are a key component of any disc braking system, it’s worth checking that your discs are fit for purpose.

To make sure you’re using rotors that are going to be reliable and allow you to stop effectively, there are 2 main points to check:

Are your brake rotors in good condition?

Brake rotors are generally very robust components that are less likely to require maintenance than most other parts on your bike. That said, they can be bent out of shape during crashes or striking obstacles on the trail.

A bent brake rotor will reduce the effectiveness of braking as a bent disc can’t be clamped effectively between the brake pads.

On top of that, a bent disc will inevitably rub on the brake pads even when you’re not pressing the brakes. This reduces your riding efficiency and makes riding that much harder. If your rotors are bent it’s time for a replacement.

Are your brake discs big enough?

For most riders, there’s a sweet spot when it comes to rotor sizes. Discs need to be big enough to provide enough stopping power but not too big as bigger brake discs mean more weight.

A cross-country rider won’t need brakes as large as a downhill rider. The downhill rider will face more extreme downhill sections, tackling steeper trails at higher speeds, therefor they need bigger brakes.

Likewise, a cross-country rider doesn’t require the same stopping power as a downhill rider, therefor smaller brake discs can provide more than enough stopping power all whilst saving those precious extra grams in weight.

Are Bigger Brake Discs Better?

Bigger brake discs provide more stopping power than smaller brake discs when an equal force is applied. In other words, if you squeeze two different brakes with the same amount of force, the brake with the bigger disc will provide more stopping power.

This is because a larger disc has a greater distance between the center of the brake disc and the outer edge of the disc where the braking force is applied.

Stopping power is a moment, i.e. a force applied at a distance. The greater the distance between the force of the brakes and the center of the disc, the greater the moment is and therefore the greater the stopping power.

It’s the same principle as using a wrench with a long handle to provide greater leverage.

However, whilst larger brake rotors mean more stopping power, they come with the drawback of adding weight.

Brake discs should therefore be big enough to provide all the stopping power you need, whilst not being so big as to provide more braking power than you will ever need and weighing your bike down simultaneously.