Minimum force on the lever blade with maximum effect! How does a hydraulic disc brake work, and what creates the high braking force?

Hydraulic disc brakes have dominated the bicycle brake market for many years because of their multiplied and almost loss-free power transmission from the lever blade to the brake caliper. Only a minimal amount of manual force can generate high pressure and powerful braking performance thanks to the hydraulic transmission ratio. This safe performance has become essential, especially in the case of today’s e-bikes and MTBs!

The basic principle of a hydraulic disc brake

If you take a look at the cross-section of a MAGURA MT, you’ll see that the diameter of the master piston in the brake master is smaller than the diameter of the brake caliper pistons. Due to the ratio of the piston surfaces, the force at the slave piston (the force with which the brake pads press against the disc) is many times higher than the force at the master piston generated by the lever blade.

In addition to the hydraulic transmission ratio, the lever blade provides a mechanical transmission ratio, amplifying the applied manual force per the law of leverage. The choice of a lever blade and its length can significantly change the braking force and modulation. This is one reason why MAGURA offers several lever shapes for individual brake tuning.

However, the total transmission ratio of a brake cannot be increased arbitrarily. On the one hand, the pressure point feeling changes depending on the transmission ratio and the resulting braking force. The lever travel also changes with different lengths of the lever blade.

The art of a perfect brake is to use the hydraulic and mechanical transmission ratio as effectively as possible – this will ensure perfect braking performance.

Resetting pads and compensating for wear

The piston is retracted inside the brake caliper by quad rings to ensure that the rotor can rotate freely again after the brake is applied and is not permanently blocked by the piston and brake pad. With each movement of the slave piston, these quad rings are deformed in a defined groove that determines the retraction of the pistons. The quad ring returns to its original shape and position as soon as the brake pressure is released, retracting the piston. A spring presses the master piston back at the same time.

When a brake pad wears, the slave piston of the brake caliper must be able to move closer to the rotor to compensate for the wear. The piston can move on the sealing ring to achieve this movement. At the same time, oil flows in from the expansion reservoir through the so-called sniffer hole to adjust the quantity of oil in the system. This oil flow can already occur while the master piston is sliding back, so no pressure builds up in the expansion reservoir during braking. The bleed screw on the expansion reservoir is not under pressure during the braking process – it only has a sealing function.

The expansion reservoir also provides more space for the oil, which expands as the system heats up. Without this compensation, the brake could lock in extreme cases.

Air in the system?

The principle of the hydraulic brake is based on the incompressibility of oil. Unlike air, oil cannot be compressed. If air does enter the system over time through seals or a defect, the expected braking effect can only be reduced since air is also compressed during actuation. The force is not amplified between the master and slave pistons as it usually is. In this case, the remedy is to bleed the air from the brake.

The brake pad is more than just a wear part!

The importance of the brake pad for brake performance is often underestimated. Choosing the ideal pad can increase braking strength by more than 20% and minimize noise. The same applies to the rotor and its diameter. A 203 mm rotor generates 10% more than a 180 mm rotor, and a 220 mm rotor generates 10% more – for the same manual force. A larger rotor is not always the better choice. The two decisive factors here are total weight and the usage scenario.

Which MAGURA brake is the most powerful?

We analysed all the factors that affect braking force, and we found that an MT7 Pro with 220 mm MDR-P rotors, Race brake pads and long, 2-finger brake levers (which are not standard on the MT7 Pro) generates the most braking force. However, braking force is not the be-all and end-all! A brake that’s too powerful is harder to modulate and impairs your control over the bike – so you should always choose a brake that’s ideal for your total weight and your usage scenario.

You’ll find more about individual brake setup at

https://magura.com.tw/product.php?i=8