Bike Disc Brakes: The basics
This entry was posted on May 13, 2013.
Disc brakes have become widely used in the cycling industry, and many types of bikes are being designed to utilize their advantages over rim brakes. Most recently, there has been a push for disc-equipped road and cyclocross bikes. With all these different uses come different models, compatibility concerns and a huge selection of features. We’re going to try and simplify things here.
How do disc brakes work?
Before digging too far into the broad spectrum of bike disc brakes, we’ll quickly discuss how they work. The most basic explanation is this: two pads squeeze a brake rotor -- shaped like a disc -- which turns mechanical energy (forward motion) into thermal energy (heat).
In rim brake systems, the wheel acts like the brake rotor. Disc brake rotors are much smaller than wheels, which means more force must be applied from the brake pad to the rotor in order to create a sufficient amount of stopping power. This allows for better control of the braking forces because you have a broader range of force that can be applied to the disc rotor, creating a smoother transition from no braking to some braking to hard braking to locking up the wheels.
So many options! Which do I choose?
Because disc brakes have so many uses, there is a large number of options available. The first thing anyone should do when looking into new components is decide what they plan to use the component for, and what they may be used for in the future. With that in mind, let's start to categorize things a little bit to help funnel down to what you may be looking for.
Mechanical or hydraulic
This is the most basic breakdown of disc brakes. Mechanical disc brakes use cables to actuate the brakes and apply the stopping forces, like most rim brakes. Hydraulic brakes use fluid (brake fluid or mineral oil). While similar at first glance, this creates some very different capabilities for the brakes themselves. To keep from digging too deeply into the details, here’s a quick guide to the differences.
- Generally cheaper.
- Often considered entry level.
- Can be applied to more types of bikes.
- Easier to install and initially set up.
- Trail/roadside repairs can be finagled to get you home.
- Distance between brake rotor and brake pads can easily be adjusted (helpful if the rotor is not true).
- Only one brake pad moves when the brake lever is pulled.
- Generally have better stopping power, especially in higher-tiered models.
- Have better feel and modulation (subjective).
- Requires the system to be bled anytime air is introduced to the brake system.
- Often more expensive.
- Less likely to be affected by dirty conditions.
- Huge selection available.
Disc brake rotor size and design can vary substantially. The most common sizes are 140mm and 160mm: 140mm for commuter bikes and sometimes the rear on mountain bikes, and 160mm for most other uses -- especially for the front brakes.
180mm rotors are also available and are used on the front of more aggressive mountain bikes like all-mountain and trail bikes. For very aggressive riding, brake rotors can be sized up to 200mm. There are some other sizes available, but this covers the vast majority. Be cautious when changing rotor sizes; frames and forks are designed around a certain size, so adaptors may be required to change.
The shape and design of the rotors really serve one main purpose: to turn mechanical energy into heat efficiently. Many top model rotors are great at turning that mechanical energy into heat and then getting rid of it. If the rotor gets too hot, it can warp or cause the fluid to boil.
Brake rotors are one place where weight savings, at the cost of braking power, isn’t advised; there won’t be enough surface area to slow you down or to spread the heat out. Very lightweight rotors can often warp and/or bend easier because there is simply less material. Two-piece rotors would be the exception, as they use a lighter mounting piece with a regular braking surface to reduce overall weight.
The business end of disc brakes, calipers vary some depending on intended use. Mechanical calipers are basically the same in design amongst themselves with some variations on extra features. They have one piston (the piston is what pushes the brake pads against the rotor) and only on one side.
Mechanical brakes work by having one pad push against the rotor, and that squeezes the rotor between the two brake pads. It’s not uncommon for a brake rotor to flex some with mechanical brakes in order for the pads to squeeze the brake rotor.
Calipers on hydraulic brakes vary a good deal more. Most calipers still have one piston, but now both sides have them, so both brake pads move towards the rotor to create the clamping forces. This helps create a more even braking action which generally increase stopping ability. Some calipers intended for very aggressive riding will have two pistons per side (4-piston calipers), which creates an even more uniform spread of clamping force across the surface area of the pads and rotor.
Disc brake pads are a much simpler piece of the puzzle. There are two types: sintered (also called metallic) pads and resin (also called organic) pads. Sintered pads have small pieces of metal bonded together, and resin pads are made up of organic fibers (like kevlar) and materials bonded together.
Sintered pads have the better stopping power and are used more often for their increased performance. Organic pads are used because they are cleaner, quieter, and have no break-in period.
At first glance, disc brake levers are mostly the same, but there are some differences that you should be aware of. Many entry level levers are not adjustable, which could be a big deal if you are riding for a long time. It’s usually a good idea to have the option to adjust the levers -- usually reach and free-play (the amount of lever movement before the brakes start stopping) -- so that you can fine-tune the feel of the brakes to your hand.
What are you paying for?
This is going to be a big question when you start looking into higher-end models and as many start looking at upgrading. Since this is cycling, there’s almost always a price increase when things start to shed grams. This is because more exotic materials can be used and/or more machining is required, which comes at increased manufacturing costs.
You may also find yourself paying for better parts, such as rotors and brake pads, than what come with a cheaper brake system. There are many circumstances that involve the same brake caliper and levers but with different rotors and/or brake pads to give you a better overall system. This also means you can upgrade a lower-end system some simply by upgrading these parts.
Some brake calipers, especially ones designed for aggressive riding, are designed with cooling fins that will most likely add to the overall cost as well.