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TKART magazine Tech Talk | All about brake calipers
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11 January 2018
The brake caliper is a concentrate of engineering and technology that is essential for any go-kart. We analyse in detail how it is constructed, what characteristics it can have and how it interacts with the other components in the system
A kart braking system mainly consists of a brake disc, a master cylinder and a caliper (except in classes, such as the KZ, in which front brakes are permitted. Its schematic operating system is simple: by pressing the brake pedal, the driver operates the master cylinder. The master cylinder then pushes the oil into the pipes under pressure up to the caliper, which, in turn, exerts force on the disc through the pads, controlled by one or more pistons.

Therefore, the actual braking is performed by the brake caliper, the characteristics of which have to meet different needs.
First of all, a good caliper must have high braking power related to
a reduced brake pedal stroke; it must be as modular as possible; it must be light, but at the same time very rigid.

All characteristics that, in order to be observed, require careful design: dimensions, materials, manufacturing systems... everything must be accurately assessed according to the product you wish build and, above all, the performance you wish to achieve once out on the track.

So, without going into too many specialised and complicated details, but also without neglecting anything, let's see all the fundamental aspects required to construct a karting caliper.
Examples of rear brake calipers for karts: CRG, Parolin, Righetti Ridolfi and IPK
An example of a "racing" brake caliper and all its components
An FEM simulation in which the deformed shape is only visibly amplified, in order to observe more clearly happens under stress
Three alternative machining operations performed on the brake discs: axial drilling helps to disperse the gas generated by the pad due to heat, making braking more aggressive. Helical milling disperses the powder from wear of the pads while keeping the braking surface clean. The "daisy" profile revives the surface of the pad which, in high temperatures, can tend to crystallise
The first fundamental aspects of a brake caliper, such as its shape and the materials for the pads, depend on ... the discs. Therefore, it is necessary to briefly mention this component of the system, although a more in-depth and detailed analysis will be the subject of a future article. Mainly, brake discs can be made of cast iron or martensitic stainless steel series "AISI 400" (AISI 410, AISI 420, etc.), which lends itself to being enriched with carbon and heat treated, i.e. hardened to give it greater hardness (over 50 HRC). Usually, cast iron is suited to pads made of organic material (so-called "lining", black in colour, so to speak); steel is suited to pads made of sintered material (with a mixture of copper, iron, etc., the "recipes" for which are jealously guarded by "sinterers"). Other differences between the two materials are the specific weight (7.3 Kg/dm3 for the cast iron, almost 8 Kg/dm3 for the steel); and the fact that steel, unlike cast iron, does not rust, with the possible exception of a light patina that is "self-removing".
Other materials may include ceramics and carbon, but their use is more complex (and in some cases prohibited) and, for this reason, please see the future article that has already been mentioned.
The size of the disc and, in particular, of its braking surface, which is based on the energy that needs to be dissipated during braking, is crucial. Given a minimum surface area, calculated on the pad that is in contact with the disc, there can be advantages to choosing a "low-end" disc, which weighs less and offers a greater "medium braking momentum" than a high-end disc. The average momentum is understood as force multiplied by medium radius (i.e. the distance between the centre of the disc and the central midpoint of the width of the actual braking surface). However, excessively reducing the braking surface can "over-stress" the disc and make it malfunction.

Another important factor is the ratio between the diameters of the caliper pistons and the master cylinder: a small diameter master cylinder can provide greater braking power on the caliper, but it will create a greater stroke on the pedal lever (which is not appreciated by drivers). Vice versa in the case of larger diameter master cylinders.
Once the correct size of the discs, pads and master cylinder has been established, the diameter of the pressure surfaces on the pads is defined, i.e. the sum of the total diameters of the pistons which, multiplied by the quantity of oil pressure pumped in the circuit, gives the value of the braking force exerted on the pads and, consequently, on the disc.

This allows us to understand another important aspect in the design of a brake caliper: the decision regarding the number of pistons (1, 2 or 3) and the relative diameters. In the case of a multi-piston caliper, in order to balance the force exerted on the pad and have more uniform wear, the use of a small piston together with a larger piston is preferred. In fact, for both physical and natural reasons, it's the first part of the pad that the disc "sees" when it begins to rotate that is subjected to the most stress under braking. Therefore, you can decide to reduce the effort on the first part of the pad by opting for a smaller first piston, and then exert more force using the second, larger piston.
According to the Pascal F1/A1=F2/A2 principle, a small piston in the master cylinder is the same as a big piston on the caliper (greater braking power, but longer pedal stroke) and vice versa
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