Hold a currency coin between a U-shape that you create with your thumb and index finger. Now imagine this illustration as a disc brake assembly, where the currency coin is acted upon by an imaginary force and rotates like the rotor or disc which is attached to the wheel of a motorcycle, the U-shape created by your digits is the caliper, the part of your fingers holding the coin are the brake pads, and the muscle force used to apply pressure on the coin is the hydraulic force in a braking system which makes the whole thing work. Since we now have an overview of how a disc brake works, let’s take a look at the different types of rotors and calipers which are used in such a system.
Brake calipers and its types
The brake caliper is the assembly which hovers over the disc. It contains the brake pads which are pushed towards the disc by pistons, as a result of hydraulic forces acting upon them, every time you squeeze the lever and push oil from the reservoir towards the assembly. Depending on the braking forces required, pistons can be made of plastic, aluminium or chrome-plated steel. A caliper which is cast out of a single piece of metal is known as a ‘Monobloc’. Such calipers are stronger but expensive to make. On the other hand, 2-piece brake calipers are made in two halves which are then bolted together. Although it is less resistant to flex under load, a 2-piece caliper is inexpensive to make and easier to mass produce.
Calipers are of two types, floating or fixed. If we go back to our imaginary illustration, a floating type caliper will have a piston on just one side. For example, if the piston is on the side of your thumb, it will be the thumb which will press against the coin first, followed by the index finger. The caliper moves laterally upon application of brakes to allow the two pads to come closer.
A fixed caliper does not move relative to the disc and is thus less tolerant of disc imperfections. In such a setup, the entire caliper is solidly mounted and pistons push the brake pads onto both sides of the disc. A fixed caliper allows the use of multiple pistons in pairs, with two, four and six being the usual number of pistons as opposed to the one or two pistons in the floating type. A fixed caliper allows more squeezing power and distributes braking forces on both sides in an even manner simultaneously. In our example, both your fingers will press against the coin at once and with equal force.
Fixed calipers are said to provide better feel at the pedal which is something many racers appreciate. But such a setup is also more expensive as the pistons are precision machined and there are always more than one. For that reason, a fixed caliper setup is generally fitted on a high-performance motorcycle.
A floating caliper, on the other hand, is solidly mounted too, but the outer frame is allowed to slide left and right via pins and bushings on the assembly. When a hydraulic force is applied, it acts upon a piston on the inner side of the disc which pushes the pad towards the disc as if to move it. But since the disc is fixed and cannot slide, the hydraulic force is pushed back and it then pulls the sliding frame which is holding another brake pad on the other side of the disc. The advantages of the floating caliper are that it is inexpensive compared to a fixed caliper, is lighter, uses fewer parts and is more compact.
Calipers which are mounted to the fork via bolts that are parallel to the face of the brake disc are called Radial calipers. On the other hand, bolts which are perpendicular to the disc and hold the caliper together form an Axial system. A radially mounted setup offers better caliper-to-disc alignment and is also a more rigid setup, as calipers are secured and supported at both ends. It allows the pads to have better contact with the disc under maximum braking. For an everyday rider though, there is hardly any difference to be felt between the two.
Like calipers, the same principle is also true for master cylinders which hold the brake fluid next to the lever. If the master cylinder piston is parallel to the handlebar, it’s a radial master cylinder. If the setup is perpendicular to the bars, it’s an axial master cylinder. When you pull the brake lever, force is transferred directly to a piston when a master cylinder is radially positioned since it is not routed through a fulcrum. This results in better feel and feedback at the lever. In the case of an axially positioned master cylinder, the force applied to the lever is redirected by a pivot, which takes away the desired feel and feedback at the lever. But again, this behaviour is mostly experienced by racers who are scrubbing the pads and making the discs glow on the track, lap after lap.
Types of rotors or discs
A disc or rotor spins along with the wheel and when the brakes are applied, the brake pads grab the rotor to stop the wheel from spinning. Most modern motorcycles come equipped with drilled brake rotors. They can either be fixed or of the floating type, where the latter rules out disc warping and reduces heat transfer to the wheel hub. Floating rotors rest on buttons or rivets instead of solid mounts. These buttons are layered with washers, which allow the rotor to flex at varying degrees when the pads press into it, thus providing a constant point of contact which results in a stronger bite.
Wavy or petal discs place the mass closer to the axle for reduced inertia; allow better heat dissipation and lower weight. Originally developed for Motorcross bikes, they are now widely used on road-going motorcycles. Brake rotors are generally made out of stainless steel these days as it is rust-resistant and lasts longer compared to Iron. Since braking action is performed on the outer rim of the brake rotor, the rest of the brake rotor is predominantly a support for this outer rim.
The greater the mass of the brake rotor is, the greater is its resistance to heat. However, the larger mass adds to the unsprung weight of the bike. This has resulted in a wider adoption of the wave pattern rotor as it stimulates air movement for better cooling. Holes and slits are cut into the rotors to help with cooling and also to reduce the rotor’s mass. Heat management is a major consideration in brake rotor design. Since it reduces the effectiveness of the brake pads, it is important to eliminate heat as much as possible. Motorcycles have an advantage over cars in this regard, as the front brake rotors are exposed to air which provides natural cooling as the motorcycle moves forward. However, repeated heavy use can heat the system and lead to brake fade.
Apart from managing heat, brake rotors must also be efficient while shedding water in order to remain effective during wet conditions. Holes and grooves are both added to brake disks with this purpose in mind. But drilling holes isn’t as effective as grooves since they provide a channel for water to leave the contact area. Grooves are more expensive to cut and reduce the life of the brake pads. For this reason, they are not as common for use in bikes meant for everyday conditions. Race-spec bikes these days make use of carbon brakes which offer absolute performance and minimal fade. But their ideal operating temperature is much higher than steel brakes, which is also the reason why most MotoGP bikes switch to steel brakes when it begins to pour.