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Tech Guide: Brake Discs

Tech Guide: Brake Discs. Improve your stopping power with our in-depth guide. Words: Gerry Speechley. Photos: Tarox. This month we begin to examine what makes up your braking system, starting with brake discs.

Gerry Speechley Written by Monday, 03 April 2017 15:14
Tech Guide: Brake Discs Tech Guide: Brake Discs Photos: Tarox
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AN INTRO TO BRAKE DISCS

Brake discs are just one of the major components in the overall braking system for cars. In combination with the brake calipers, pads and actuating systems, they are wholly responsible for the system of stopping the vehicle.

Brake discs, or brake “rotors” as they are known in many parts of the world, come in many different sizes, designs and materials with the most common being the grey cast iron discs seen on the vast majority of production cars. Grey cast iron has a graphitic structure with the addition of carbon and silicon and an excellent thermal conductivity and specific heat capacity. Other common materials used are the cast iron discs on the more sporting cars and the exotic carbon or carbon ceramic discs found on the highest performance vehicles.

The history of the disc brake goes back in time almost to the first road automobiles. Originally invented in 1902 in England, following the development of disc brakes on aircraft and military vehicles (German Tiger Tank) during WW2, they found their way onto production cars in the 1950s, replacing the age old drum brake used exclusively until then.

Following a win at the 1953 Le Mans 24 Hour race by a C-Type Jaguar using a disc brake set-up, it was clear that the system had significant benefits over the drum brake system with much of the success being credited to the superiority of the car’s braking power. The first production car to have disc brakes was the Citroen DS in 1955 and used the inboard disc mounting system, rarely seen today.

The disc system has the advantages over the drum system of better heat dissipation, reduced weight, some self cleaning effect and braking force proportional to pedal effort, unlike the drum system which has an inherent self-servo effect.


HOW DISCS WORK

The function of the brake disc is to convert the motion energy of the vehicle (kinetic energy) into heat energy by creating friction between the surface of the disc and the brake pads, operated by the caliper, squeezing against it. This friction is then used to retard the rotation of the disc, and hence the wheel (or driveshaft) it is attached to, reducing the vehicle speed, or holding it in a stationary position.

To perform the braking function, the brake discs are usually mounted at the wheel, usually between the wheel rim and hub. The disc can be bolted directly to the hub, be integral with the hub, or more commonly, sandwiched between the wheel rim and hub, held in position by the clamping force of the wheel fixing nuts/bolts. In this system, the disc naturally rotates at wheel speed and so any reduction in the disc speed from the action of the pads’ clamping force on the disc will reduce vehicle speed.

Initially, the manufacturers installed discs on the front of the vehicles and retained the drums at the rear because, as around 80% of the braking force is handled by the front wheels, the rears did not require substantial upgrading. The convenience of the drum brake handling the parking brake system (handbrake) also gave the rear drum brake a few more years of life. However, with the design of using the inside of the disc bell as a drum facilitating the handbrake, and later, handbrake mechanisms integrated with the rear calipers, the drum brake’s days were numbered.


DISC DESIGNS

In general, there are three main designs of brake disc. The first is the “solid” disc, which uses two friction surfaces, parallel to each other on either side of the brake disc. This type of disc is used on lower performance vehicles because the limited surface area reduces the ability for the disc to dissipate the heat produced, and increased braking loads will cause the disc to overheat causing brake fade and possible disc distortion. Disc diameter can be increased to improve heat dissipation but the wheel size can be the limiting factor. The next performance progression is the ventilated, “vented”, disc. In this arrangement, the disc still has the two friction surfaces, but with the addition of an air gap between the two faces. The two faces are joined by fi ns or cast-in webs and cooling air can flow through these fins, between the friction faces, to greatly increase the cooling capability. On many designs the fins, or webs, are shaped to actively create a draught flow between the faces and in these performance discs, often seen on M cars, the discs are actually left- and right-handed.

The third main design of brake disc is actually an enhancement to the vented design and is the floating disc. The floating disc design, whether semi or fully-floating, isolates the friction section of the disc to the mounting section. In this design, the mounting section is known as the disc “bell” and this is attached to the disc braking section either by pins (semi floating such as the E34 M5) or by sliding clamps (fully floating). The alloy bell design not only significantly reduces the disc weight, (which when mounted outboard is a significant addition to the vehicle’s unsprung weight), by eliminating the large cast iron bell section, it also isolates the hub and wheel areas from heat transfer from the disc. In addition to this it also allows thermal expansion and contraction of the disc without stressing the mountings, which could otherwise lead to disc distortion.

This, then, brings us to the current highest level of brake disc technology: the carbon/carbon and carbon/ceramic discs. The carbon/carbon arrangement (derived from the design used on Concorde) is currently limited to race vehicles due to the high wear rates and the fact they need to be hot to work efficiently, but carbon/ceramic discs are already found on a large number of very high performance cars either as standard or as an option. These discs are always of the floating design, with an alumunium bell mounting and vented disc design.

The carbon ceramic disc, known on BMWs as the BMW M carbon ceramic, is currently the ultimate in high performance braking systems. The discs themselves, a siliconised carbon fibre, are a fraction of the weight of the cast iron discs they replace, significantly reducing unsprung weight, have a very high thermal capability, produce less brake dust, have a longer operating service life (60x the equivalent iron disc, it’s claimed) and are highly corrosion resistant with a super-tough silicon carbide finish. So why do car manufacturers not start using these on all production vehicles if they are better than the cast iron discs in seemingly all respects? The simple answer here is cost. With the price of a CCM brake upgrade for an M3/M4 costing as much as a brand-new Ford Fiesta and a pad set at around £800 fitted, it’s not the majority of owners’ idea of a reasonable ownership cost.


DRILLED AND SLOTTED DISCS

Looking through the wheels of a performance car and seeing drilled or slotted discs is not only pleasing to the eye, it just gives the impression of speed and performance, so just what is the function of these?

As both are widely offered in the aftermarket as performance upgrades, there are clearly different ideas as to which is the best and why, so lets take a neutral look at both.

Discs with multiple holes in them may not necessarily be drilled. Some manufacturers actually cast the holes in the discs, but for the vast majority of discs on offer, they are drilled. The function of the drilling offers a number of advantages. The holes increase the surface area of the disc, increasing its ability to dissipate heat. The holes also break up any surface water, and allow the pads to push any remaining water quickly through the holes to improve wet weather braking. The holes allow any gasses trapped between the pads and the disc surface to escape through them and there is also the weight saving from the removal of material from the disc, reducing unsprung weight. Finally, the holes tend to reduce the likelihood of brake squeal and clean the surface of the pad during use.

 In slotted discs, the channels allow a route for any trapped gasses between the pads and disc to escape and the slots continually deglaze and clean the pad surface. The slots also dissipate surface water on the disc.


BRAKE DISC MAINTENANCE:

LOOKING AFTER YOUR DISCS SCORING

Brake disc scoring is generally attributed to excessive wear, overheating due to excessively worn pads or contamination. Most brakes have some sort of low brake warning from an electrical circuit that is broken, producing a warning light when the pads are worn low, to a basic metal tab on the pad that fouls the disc, warning of impending pad replacement by screeching or grinding on the disc surface. Despite these systems, pads are very often worn completely through to the metal backing, or so low that the pad can no longer dissipate the heat and the result is scoring of the disc. Mild scoring can be ignored if the brakes were operating effectively as the new replacement pads will wear to the existing discs and operate just as efficiently once bedded-in but excessive scoring will require disc replacement.


CORROSION

Brake discs will self-clean when in regular use due to the action of the pads rubbing the surface of the disc but cars that are used infrequently may experience noisy brakes when first used after wet weather or a lack of use for a week or more. Corrosion can, however, become a significant issue on vehicles stored for long periods. Corrosion on the disc surface can reduce braking efficiency for long periods whilst the rust is worn off, or the differential running temperatures between clean and rusted areas can cause disc warping. Finally, where vented discs are installed, corrosion within the disc, between the ventilation slots, can cause weakening of the disc structure and in these cases, a replacement disc is inevitable.


FRACTURING

There are two main causes of disc fracture and they are very often in combination. The main cause of disc fracture is the use of a disc that has worn below its minimum design thickness. Thermal loads and fatigue are the cause, but this can be made worse by the use of drilled discs. When wearing thin the drilled disc experiences an uneven thermal expansion rate around the edge adjacent to the holes and cracking occurs. This effect is reduced by chamfering the holes to reduce the surface stress and by drilling the holes small enough and far enough apart to eliminate a build-up of surface stress between them. The golden rule for drilled discs is to replace them before they exceed the minimum recommended thickness and some manufacturers drill a dimple in the disc face so that as soon as the dimple can no longer be seen, the maximum wear limit has been reached and the disc should be replaced.


BRAKE JUDDER

By far the most commonly experienced brake disc problem is brake judder. This is when a low frequency vibration can be felt through the vehicle during braking. There is a whole range of causes for this judder and warped discs are often cited as the cause before any proper checks have been carried out, which often leads to a misdiagnosis of the issue. Disc run-out is the diagnosis of a warped disc but this may not be a disc fault. Using a magnetic stand and dial gauge on a fixed part of the suspension against the disc, you will be able to measure the lateral run-out of the disc and anything over 0.002”/0.05mm will be felt through the vehicle. This may not be caused by the disc, though, and separation of the disc from the hub often reveals corrosion between them, forcing the disc to run off from the hub. Because of this, the hub and disc should always be thoroughly cleaned before installing a new disc, with a little copper grease for corrosion protection. There is also a possibility the hub itself is not running true and this should be checked before pointing blame at the disc.

The next cause of judder could be a thickness variation of the disc causing pulsation of the pads in the caliper. Typical causes of this are hard spots on the discs from localised overheating. The disc thickness variation should be checked for this and if found, a replacement disc should be fitted.

A major cause of brake judder is the incorrect installation of the wheels. As the wheel usually holds the disc in position, tightening the wheel nuts in a circular pattern can warp the discs and so wheels should always be installed with the wheel nuts tightened progressively and in a criss-cross pattern, with the final torque correct applied with a torque wrench.

Other causes of judder, both of which are surprisingly common, are pad imprinting and pad deposits unevenly distributed around the disc. Pad imprinting is caused in a couple of ways. If the car has been driven enthusiastically and the discs have become very hot, and then the car has been stationary with the brake pedal depressed or the handbrake applied, the pads will often leave an area of overheated pad material on the surface of the disc. Another cause can be a wet car parked up and when returning a few days later, the discs have corroded and the pads have stuck to the disc. These deposits then cause vibration as the pads sweep over the area, giving the impression of a warped disc. Pad deposits are caused by uneven pad deposits on the disc, often left by improper bedding-in procedure.


Gerry Speechley

Drive-My editor technical and engineers for the automotive industry. Our expert of engines and units of the BMW

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