Brake disc

The brake disc is one of the main components of disc brakes. It is most often shaped like a plate, the edge of this plate forms the main frictional surface captured by the brake pads. The brake disc is firmly connected to the wheel hub and therefore rotates with the whole wheel. The friction segments in the form of brake pads sit on the annular friction surface of the brake disc. The brake pads are located in the brake caliper, which does not rotate with the wheel and is firmly connected to the wheel hub. The clamping of the brake disc generates a frictional force which converts the motion energy of the moving vehicle into thermal energy.

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Brake disc materials:

The ideal brake disc material has low density, which affects the resulting brake weight, high thermal conductivity and a minimum coefficient of thermal expansion. For an idea, see the materials comparison table:

MATERIAL	DENSITY [kg.m-3]	THERMAL CONDUCTIVITY [W.m-1.K-1]	THERMAL DISSOLUTION SUMMARY [x106.K-1]
Grey cast iron (3,4% C)	7800	62	10-13
Al-MMC	2700	182	18-25
Ceramic + carbon composite	1700	300	0,1-1,5

In practice, brake discs are most often made of tempered cast iron or steel cast iron. Cast iron is used because of its better metallurgical stability, ease of manufacture and lower production costs. To improve material properties, alloying elements such as molybdenum, copper, chromium or titanium are used. With the help of these elements, the resulting alloy achieves a more optimal homogeneous structure and better temperature parameters. Conventional steel discs lose their effectiveness from around 800 °C, after which the brakes begin to fade due to overheating. Overheating of the brakes also has a negative effect on excessive brake wear and can cause brake discs to warp. Another common problem with steel discs is rapid surface corrosion of the friction surfaces when the vehicle is stopped.

 

At the other pole of the market are composite discs for sports and luxury cars made of steel ceramic or better still carbon ceramic. Such brake discs have excellent thermal and mechanical properties – they do not fade. The ceramic component ensures above-average durability and heat resistance. In combination with the carbon matrix, such discs have very little thermal expansion and a very low specific weight (the brake disc is usually part of the unsprung masses, so low weight is important). The high thermal conductivity makes ceramic discs very resistant to overheating. Unfortunately, their main disadvantage is their considerably high price and their tendency to squeak when cold. This is because this type of brake disc was originally developed for racing purposes and achieves optimum braking performance at operating temperatures of up to around 200 °C, which is unusable for normal driving. The lifetime of carbon ceramic brakes is many times longer compared to conventional brakes, but not for life as we often hear. When you factor in the purchase price, their use is more suited to circuit riding.

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Brake disc cooling:

Brake discs can be divided into unvented and vented. Venting of the discs is provided by axial holes, grooves or internal fins, see pictures above. Good cooling of the brakes affects their efficiency and resistance to fading under high loads.

Solid discs are standard equipment on most vehicles. They are the simplest to manufacture and therefore the cheapest. They have the maximum contact area for the brake pads, but on the other hand they are prone to overheating (baking) and brake fade.

Vented – drilled discs are characterised by a large number of holes passing through the braking surface. The idea of drilling holes in the discs is to provide a path for faster heat dissipation. The gas accumulated during braking is able to escape through the drilled holes and is not trapped between the surface of the discs and the pads. This plays a vital role in the possible heating of the pads and brake fade at high loads. Drilled discs are also slightly lighter. On the other hand, the holes weaken the disc structure, which can lead to the introduction of cracks. Drilled discs act partly as a grinder, so they wear the brake pads a little more.

Vented – grooved discs are characterised by grooves on the friction surface. The grooves remove dust from the brake pads and prevent them from sintering. Grooved discs tend to be noisier when braking. The grooves also serve as an indicator of disc wear.

Vented – internally cooled discs are characterised by a spiral labyrinth that seems to connect the pair of discs. However, the disc forms a single unit. As the brake disc rotates, the spiral-shaped internal channels cause air to flow, effectively removing heat from the brake discs. A further advantage is that internally cooled discs are wider by design, have a higher bending strength and are therefore less susceptible to possible shape distortion.

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Brake disc design:

One piece brake rotors – are made from one piece, most commonly used brake discs fall into this category. The advantage of one-piece construction is the ease of assembly and manufacturing, i.e. the resulting low cost. The disadvantages include the effect of thermal expansion, which in the case of a solid disc causes internal stresses throughout the disc. Thermal stress can result in permanent twisting of the disc. Another disadvantage is the transfer of heat from the disc to the wheel hub and its bearings, which reduces their service life. This can be partially avoided by using a pot-shaped disc, which extends the heat path to the centre of the disc.

 

Two piece brake rotors – although more complex in design, they have one major advantage, they can combine two different materials as they are split into two parts – the crown and the boot. For example, the crown can be made of tempered high carbon cast iron and the cup forged from a special aluminium alloy. Thanks to the appropriate combination of construction materials, the resulting brda disc is not only lighter, but also, and most importantly, allows for much better heat dissipation.

 

The wreath and rotor can be connected in a floating way. The cast iron wreath can move slightly in the axial direction (outwards) with respect to the rotor. Thanks to this solution, the two-piece discs are less likely to warp or crack due to excessive heat.

Another advantage is the cost savings if you are only replacing a worn cast iron wreath.

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Temperature stress on brake discs:

Brake thermal stress is one of the main issues in brake design. For commonly used materials such as grey cast iron, temperatures as low as near 700°C are critical. It is referred to as the red heat temperature. At this temperature, the friction surfaces of heated brake discs glow bright red in the surrounding area. This temperature is not a problem for grey cast iron from a strength point of view, as the melting temperature is up to 1150 °C. The problem is the structural changes in the material and the associated formation of localised areas with different properties. Brakes damaged in this way lose efficiency and service life and vibrate during braking.

 

A computer simulation shows the difference between unventilated discs and discs with cooling.

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Did you know that…?

Discs with integrated parking brake – the parking brake is usually mechanically operated in most cases. The handbrake lever acts via a rod on the brake shoes to create a braking effect. In the case of disc brakes, such a method cannot be used, or much greater control forces would be required. Therefore, some cars use a combination of disc and drum brakes. The brake disc thus has a friction surface for both the brake pads and the drum shoes (shoes), see illustration.