Bike disc brakes come in many different shapes and sizes, however, whilst there are many different manufacturers and products to choose from, the majority of brake discs have holes in them.
At first, the holes may not stand out as a key design feature of a brake disc, but there are a few very good reasons that manufacturers machine holes into disc brakes.
Why are there Holes in Bike Disc Brakes?
The holes in a disc brake provide ventilation for cooling, enabling the brake to dissipate heat and perform optimally as a result. The holes also aid in removing water and dirt from between the disc and brake pads, increasing braking effectiveness while reducing wear and the weight of the brakes.
There are a couple of practical reasons for the holes in disc brakes. The holes help to keep the disc cool, free from debris and water, and help reduce the weight of the disc.
Let’s have a look at each of these benefits in more detail:
Keeping the Brake Disc Cool
When the brakes are applied to a wheel, the calipers clamp together and apply a force onto the brake rotor which in turn slows the rotor, and therefore the wheel, bringing the bike to a stop.
In this scenario, it’s the friction force between the brake pads and the rotor that slows the wheel down as it acts in the opposite direction of the forward motion of the wheel.
The friction between the disc and the brake pads generates heat. If the brake disc itself gets too hot it can begin to wear as the metal the disc is made from becomes softer and more malleable when it heats up.
The holes in disc brakes provide cooling to the disc in two ways. Firstly, the holes in the disc provide ventilation, allowing cooling air to pass through the disc as opposed to just around the disc as would be the case for a disc with no holes.
Secondly, just by reducing the amount of material in the disc, it will naturally cool more quickly. This is because less material (less steel in the case of a brake disc) means less thermal mass and therefore quicker cooling.
You can think of thermal mass in terms of water. If you fill up a cup with hot water and a bath with water of the same temperature, the cup of water will cool down at a much faster rate than the bath.
There’s far more water in the bath so there’s a lot more stored energy, meaning the bath stays hotter for longer. That’s the same for disc brakes with no holes – but unlike a bath, we want our brakes to cool down as quickly as possible.
Holes Keep the Disc Clean & Dry
The holes also provide an escape route for any water, mud, or dirt that may have found its way to the brakes.
Instead of getting stuck between a solid disc and the brake pads, any particles of debris that found their way into your brakes will quickly make their way out through the holes.
This ensures your brakes perform well even in bad weather as mud and water can’t stay trapped and cause your brakes to slip, whilst also reducing wear on your discs and brake pads.
Without the holes dirt would build up in your brakes and act as an abrasive, reducing the lifespan of your brakes significantly.
Disc Brake Holes Reduce the Weight of the Disc
A bonus of machining holes in disc brakes is simply reducing their weight.
Brake discs are made from very strong materials, whether they be aluminum alloys, steel, or ceramics. Because they’re made from tough materials, there’s no need for the part to be solid in order to be strong enough to brake effectively without being damaged.
Having a solid brake disc with no holes would be over-engineered and stronger than it would ever need to be. This is quite wasteful, so it’s in everyone’s best interest to put holes in the disc.
Less material means less money spent on materials for the brake manufacturer, less weight on your bike, better performing brakes, and will have less of an impact on the environment as less material is used and lighter parts mean less fuel used in shipping.
Are Brake Discs with Holes Better for Bikes?
Brake discs with holes are better on bikes as they offer improved performance vs. discs with no holes. The holes provide ventilation for cooling as well as keeping the brakes free from water or dirt that could reduce the brake’s effectiveness. Additionally, discs with holes are up to 50% lighter.
There are a number of applications in the automotive industry where solid discs, or at least grooved discs with no holes are used.
Bike brakes don’t need to be as robust as a car’s though, so provided the discs have been designed properly and are plenty strong enough to cope with the demands of braking, the more holes the better!
I’m not suggesting you select a new pair of brake rotors based on how many holes they have, but a rotor with a larger diameter (and larger diameter equals more stopping power) could have the same weight as a smaller, less effective brake if the engineers designing them are creative when removing material from the disc.
Is there any Benefit to Having a Disc with More or Fewer Holes?
The main benefit of having a disc with more holes/bigger holes is that the diameter of the disc can be increased for the same weight.
For example, if two rotors both weigh 200g but one is an inch wider in diameter, the larger diameter rotor would provide more stopping power as it has more leverage than the smaller rotor. The two could only be comparable in weight if the larger rotor has more material removed than the smaller one, assuming they’re the same thickness.