The Science Behind Hydraulic Disc Brakes

Hydraulic disc brakes are the most commonly used type of brake system; mechanical and rim are increasingly rare. Here’s a quick explanation about the hydraulic brake system and specifications depending on how the bike is used.

When you pull the brake lever on a hydraulic system, brake fluid is pushed down the hose towards the calliper. The pistons are pushed out towards the rotor and press the pads against it, stopping the bike. The type of brake fluid, piston size and rotor size are therefore the main features of the system and differences in these specifications can aid performance.

Brake Fluid

Brake fluid can be either DOT fluid or mineral oil. Mineral oil can be a lower maintenance option. It’s best used for riders who don’t ride competitively and have long periods of time between rides where their bike isn’t being used. DOT fluid is more commonly used and supports faster, harder riding as it performs across a range of temperature. It does require a higher level of maintenance as, over time, DOT fluid absorbs water so it needs to be changed every so often. DOT fluid and mineral oil also require different seal materials and the seals that can be used with DOT fluid are better performing. This is because the rubbers they are made from have a broad temperature operating window. This means extremes of temperature can be better withstood by DOT mineral brakes when compared with mineral oil.


Two piston callipers have a piston on each side of the calliper and four piston systems have two pistons on each side. More pistons means a larger surface area to press the pad against the rotor. Larger diameter pistons work in exactly the same way. This means a bigger pad is used and more pad material is available to create friction. The more friction, the more stopping power.


A larger rotor has a larger brake surface area and can keep brakes running at a more optimal temperature. However, simply using the largest rotor possible isn’t necessarily going to increase brake performance. Variables such as braking intensity, braking duration and rider weight build heat between the pad and the rotor. Too hot or not hot enough could lead to a reduction in the control the rider has over braking power. Brakes that get too hot during use could be helped by an increase in size of rotor. Brakes that don’t heat up enough to create optimal friction could benefit from a smaller rotor. The colour of the spokes of the rotor shows the effect of heat dissipating down the material. When it looks brown, heat is generally being dissipated well. If it is purple, this shows a scorching effect.

What this all means is the specification for your bike brakes really comes down to several main variables- your weight, the way you ride it and how aggressively you use the brakes, the type of terrain and conditions you like to ride in and how often you are expecting to use your bike.

Our other article, Brake Tuning and Setup, details how the brake system you have is set up for the individual rider and covers brake pad options – all of which you will find available to buy in our wide range of products in the online shop.