Saturday , 25 January 2020

Tech Counsel: The low-down on hydraulic disc brakes

If you’re relatively new to mountain biking, you’re unlikely to have a significant appreciation for having hydraulic disc brakes on your bike. Some of our TREAD team reckon that hydraulic disc brakes did more to change the mountain biking ride experience than tubeless tyres and suspension. Hydraulic disc brakes are virtually specced on every mountain bike sold now days. They offer consistent, predictable braking which really do enable all of us to ride with more control. Better brakes allow us to ultimately ride faster. Here’s the low-down… By Ruan Deyzel

Photo: Dave Trumpore/Yeti Bikes

Photo: Dave Trumpore/Yeti Bikes


A bit of history

It’s hard to imagine that there was a time that mountain bikers relied on soft pieces of rubber rubbing on the outside of their rims to (hopefully) bring them to a halt. This all worked relatively well until you were faced with any amount of moisture, rendering your purple anodised cantilever brakes absolutely useless!

It wasn’t until the late 90s that this all started changing. There were many small companies starting to experiment with closed and open hydraulic brakes, but one company stood head and shoulders above the rest.

Hayes, a company making brakes for tractors, trucks and motorcycles, saw the opportunity in the emerging mountain bike market and started development of their Hayes MAG hydraulic disc brake. They used all their design expertise to hit the nail on the head with a powerful, reliable package in the form of the Hayes MAG. Things got off to a very slow start for Hayes but it all started changing when Trek and Gary Fisher started specking the Hayes MAG on a couple of their longer travel 8900 and X-Calibre hardtails. This shot the then novelty disc brake into mainstream mountain biking! Hayes was so far ahead when they launched the MAG that they ran with it relatively unchanged for almost eight years.

How they work

The fundamental principles of hydraulic brakes are the same for cars, motorcycles and bicycles. It’s simply a force that is applied at one point (the lever) that gets transmitted by an incompressible fluid to another point (the calliper). The input force can also be multiplied so that the output force is much greater than the initial input force, this is achieved by making the bore size of the master cylinder smaller than the bore size of the slave cylinder or calliper. There is no correlation between the amount of pistons in the caliper and the power of the brake, it merely comes down to the ratio in surface area between the lever and the caliper pistons.


LEVER: Takes the mechanical force of your hand and converts it with the help of a small master cylinder inside the lever to hydraulic pressure that pushes the brake fluid down the brake line to close the pistons in the caliper and clamp the disc between two brake pads.

BRAKE LINE: Brake lines have the ability to transmit pressurised hydraulic fluid to the caliper. Brake lines are made from a combination of materials that resist stretching under high pressures so that no energy is lost.

BRAKE FLUID: The two types of brake fluid commonly used for disc brakes are DOT fluid and mineral oil. It’s very important to note that these two should never be mixed. The systems that they are designed to be used in are specific and mixing them will cause permanent damage.

CALIPER: Houses a single or multiple slave pistons that is/are moved by the force applied at the lever to clamp the disc between two brake pads.

Photo: Dave Trumpore/Yeti Bikes

Photo: Dave Trumpore/Yeti Bikes


Most common problems

In order for a hydraulic brake to function properly all the fundamental components that make up the system need to function as intended. The following are the most common problems experienced:

  • Air in the system
  • Leaks in the hose, calliper and lever
  • Squealing
  • Improper adjustment
  • Touching brake pads

It’s worthwhile mentioning that bleeding a brake should be considered the last resort to any problem. Often, making adjustments to the lever reach and pad contact (dead stroke) will solve most problems. It’s also useful to reset the caliper – it’s a bit like rebooting your computer as it helps the system to start fresh (use a pad-spreading tool to gently push the slave cylinders back into the caliper.

If proper adjustment and resetting of the caliper doesn’t solve the problem, then it’s time to bleed the brake. Bleeding a brake differs between brands and models so take some time to find the instructions for your specific brake and do it exactly the way they describe.

The most important thing is to make 100% sure that you use the hydraulic fluid that the manufacturer specifies and get the appropriate bleed kit for your specific brake.

Bleeding brakes isn’t the easiest thing to do so get an expert to help if you don’t feel comfortable.

Squealing is usually caused by glazed or oil-contaminated pads, also make sure that your degreaser or bike wash is bike friendly. A light sanding of the disc and pads will normally take care of glazing, but if you have a set of oil-contaminated pads, replacement is your only option.

If you note any leaking on your disc brake system, take it to an authorised service centre; they will inspect, replace or repair the brake so that it’s safe to ride again.

Proper alignment of the caliper and disc is need to stop rubbing, start by resetting the caliper and then adjust the caliper on the mounting bolts until there is an equal gap on both sides of the disc.

Brake pads      

There are basically three main types of brake pads. Each one has different properties and it’s often not a case of better and worse but rather your preference in feel and the typical riding conditions that your local trails offer.

ORGANIC: Organic pads are made from organic fibres bonded under very high pressure with a special heat resistant resin. Organic pads have a strong initial bite but lack power at very high temperatures. They are less noisy compared to other pad types but do not last very long in wet, gritty conditions.

METAL: Metal-sintered pads are made from bonded metallic ingredients, they have a poor initial bite but very good end power when the pad has time to heat up. Metal pads are great for wet-weather riding and work very well for heavier or gravity riders. Unfortunately there are a couple of drawbacks, metal pads produce a rough feel and can be noisy.

CERAMIC: Ceramic pads are starting to become more popular as an alternative upgrade pad. They are less noisy than metal pads and will maintain braking performance under extreme heat. They last longer than most other pad compounds and are less abrasive on discs than metal pads.


TREAD Magazine is sold throughout South Africa and can be found in: Spar, CNA, Exclusive Books, Discerning bike shops and on Zinio

*Originally published in TREAD  Issue 37, Dec 2015 – All rights reserved



Tech Counsel: The low-down on hydraulic disc brakes Reviewed by on . If you’re relatively new to mountain biking, you’re unlikely to have a significant appreciation for having hydraulic disc brakes on your bike. Some of our TREAD If you’re relatively new to mountain biking, you’re unlikely to have a significant appreciation for having hydraulic disc brakes on your bike. Some of our TREAD Rating: 0

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