Car Brakes

A Guide to Upgrading, Repair and Maintenance

Jon Lawes

• Publisher: Crowood
• Year: 2014
• ISBN:
  9781847976758
• Topic: Technologie et ingénierie
• Subtopic: Transport et ingénierie de l'automobile

Modern car braking systems are designed to a very high standard, but the need for the home mechanic to know how to maintain their braking system is as important as ever. Whether upgrading your brakes at home or for the race track, Car Brakes offers guidance on upgrading, repairing and maintaining car braking systems. With step-by-step instructions, the book covers the key principles of braking systems, both drum and disc; stripping and rebuilding disc and drum brakes, and the replacement of brake pads and callipers; rebuilding and maintaining handbrakes and how to install a hydraulic handbrake; replacing and repairing brake lights; upgrading your brakes and finally, fault-finding and safety tips. Fully illustrated with 121 colour photographs and step-by-step instructions.

CHAPTER ONE

BRAKES: AN OVERVIEW

Braking, although based on firmly set scientific principles, seems to be shrouded in confusion and misleading information. Any highly regarded professional in the field, if asked whether there are any specific techniques one should use for setting up a braking system, tends to answer, ‘suck it and see’. There are so many factors that affect how the brakes work: the effectiveness of your tyre compound, air temperature, ride height, damping rate, even just moving a component in the car can change the braking to a significant degree. Because of this complex interaction, getting an effective braking system to work at its best can, to a certain degree, sometimes be more of an art than a science. We can give ourselves the best possible chance of getting it right first time, however, by understanding the principles and how they make a difference to the way the brakes decelerate the car.

Brakes have an unusual potential for being ‘sexy’ a number of times I have seen brake calipers fitted to vehicles purely for their aesthetic appeal, being completely unsuited to the application in hand. Proceed with caution: upgrades should be carried out only to the level that suits your needs. A vast increase in brake size will usually result in an increase in unsprung weight, with all the handling penalties that this incurs. Braking systems can also be costly, not only to install but to maintain. Consider carefully just how much braking you actually require. As a guideline, if you are not suffering brake fade and can easily lock the wheels, then you probably have enough braking for your needs.

Whether it is a friction block applied directly to the wheels or road, a six piston competition caliper on a ceramic disc or an air-brake, any system used to decelerate a vehicle is considered a braking system. The lineage of even the most modern systems can be traced quite clearly to the same systems fitted to the earliest automobiles, and this is because the basic principle was robust and simple in its application. Braking can be considered an energy conversion process: the forward motion of the vehicle is turned, via friction, into heat. Heat is the primary limiting factor with most systems. The ability to dissipate heat is often what dictates just how much braking the vehicle can do. However, the factor of traction on the road surface is too important to ignore; the most powerful brakes are useless if they lock instantly without gripping to the road.

PADS

It is almost too obvious to suggest that pads should be the first port of call when upgrading a system, but when modern road cars make the transition to track cars, just changing the pads is sometimes the only modification required. Compounds designed for higher temperatures are available to suit almost every vehicle on the market and the compromises that high specification pads used to impose on the driver have almost entirely been eliminated. High temperature pad materials used to work only when the disc and pad was up to temperature, but most high spec friction materials now work very adequately from cold. Selecting a pad material depends on the weight of the car and the type of driving intended. Certain circuits, for example, are very heavy on the brakes, requiring frequent heavy braking and few opportunities to allow the brake assembly to cool. In this scenario a pad that operates at a high temperature is best, although it can be difficult to determine exactly what these temperatures can be. During testing it is possible to return to the pits and apply an infrared thermometer to the discs, but this only tells you what the temperature is once the car has already started to cool.

A more reliable technique is temperature sensitive paint, which is applied to the edge of the disc and changes colour depending on the temperature reached. A typical paint kit, for example, might give temperature ranges from 300 to 650°C. The paints would come in three or four different colours, depending on the temperature they represent, and would be applied in small strips around 1cm long. After a test session the paint is checked, and any colour that has bleached to white represents a heat level exceeded: for example, a disc painted with red, yellow and green paint returning to the pits with white, white and green paint remaining has exceeded the temperatures represented by red and yellow, which could be 400 and 500°C respectively. The unchanged green paint could represent 600°C, meaning you would need to find a pad material capable of resisting fade up to a maximum of 600°C. Some manufacturers actually incorporate temperature sensitive paint into their pad production process, coating the metal backplate in a paint that discolours when the peak operating temperature has been exceeded. Bear in mind that ambient temperature, track temperature, humidity and other variables may affect the readings; just because a pad material suits one circuit, it does not mean it will be suitable for others.

Pad wear rates vary drastically depending on the compound and use. For this reason, if you are using a pad with which you are unfamiliar, it is prudent to monitor the wear more closely than usual.

Excessive wear due to the use of an incorrect pad compound, resulting in a melted piston and warped backing plates. ERIC JACOBSEN

Pads come in a confusing array of compounds and materials, with different levels of bite, wear resistance, heat resistance, dust production and noise level. A few of the materials used are listed here. The pad material is probably the most rapidly evolving part of the braking industry, so this list reflects only a very small section of a very complex area, and each manufacturer’s pad is likely to consist of a complex blend of these products:

• Asbestos was used in brake pads owing to its excellent resistance to heat. It is rarely used today because of its carcinogenic properties, but can still be found on older vehicles.

• Aramid is a contraction of the term Aromatic Polyamide, a synthetic fibre used as an alternative to asbestos. The Nomex material used in race-suits is an Aramid fibre, as is Kevlar.

• Ceramic pads are perceived as quieter than their rivals since the frequency at which they squeal is usually beyond the frequency range of the human ear. They are generally more expensive than metallic pads, but kinder to the brake disc. They provide aggressive stopping power, good fade resistance and are lighter than metallic pads, making them ideal for competition. They tend to produce less dust, and the dust that is produced is lighter in colour and generally less noticeable. Because the temperatures generated are usually higher, and the pad does not conduct the heat away very effectively, a good quality disc should be used as it will be subjected to severe heat if used in competition.

• Metallic brake pads tend to be slightly noisier than the other types due to the metals used in their construction. They are more aggressive to the disc than organic pads, relatively heavy, and often used as standard equipment due to their lower cost.

• Organic is a term commonly used to designate a pad as being made of something other than metallic or ceramic compounds. Kevlar pads are considered to be organic. They are usually softer and less hardwearing than their ceramic or metallic equivalents.

• Polyacrylonitrile is a synthetic polymer resin, and yet another modern alternative to asbestos.

• Sintered pads are a variation on the metallic pad where the metal compounds are fused together during the manufacturing process at a temperature below the melting point of the metals used. Other materials may be added to sintered pads that are not employed with metallic pads. These include compounds or materials added to alter their wear and abrasion characteristics. Sintered pads tend to sit above organic and metallic pads, but below ceramic pads in their braking power and fade resistance. They usually use copper as the base material bonding the others together, contributing to their increased cost over organic pads.

Some calipers use more than one pad, meaning different compounds of friction material can be used together to achieve a hybrid stopping ability tailored to the type of competition you are engaging in.

If you notice squealing or poor performance, especially from new pads, it is possible that the disc and pad may have glazed. Disc and pad glazing can occur when the resins binding the pad materials together collect on the surface of the friction material and crystallize, forming a smooth surface that does not grip the disc effectively. It is for this reason that most pads have a bedding-in period, which allows the excess resin to be boiled off. Manufacturers tend to have different bedding-in procedures depending on the intended use. A road car application, for example, might simply suggest that the brakes are only used gently for the first few hundred miles. A racing pad manufacturer might suggest a few high speed stops separated by long cooling periods, allowing the resins to be heated and then gaseously dissipate through the gap between the pad and disc before they get a chance to crystallize. No matter which procedure you use, the basic principle is designed to avoid constant application of heat and pressure, which will cause the resins to glaze. In the event that the pads do glaze up, it is sometimes possible to remedy the situation by gently removing the glaze from the surface with a mild abrasive paper. Do not apply any liquid solution to the pad other than brake cleaning solvent and lightly abrade without causing damage to the surface. In the event that the pad starts to break up or crumble, it should be discarded along with the rest of the pads in the set. The rotor should also be very gently abraded using a fine grit paper evenly across its surface, as the resin will have transferred to the metal. Do not use an aggressive machine for this; hand pressure with an even effort across the whole surface is the best technique. Rinse with brake cleaner to remove any contamination once you have finished.

Brake pads and other friction materials are assigned a code called the WVA number. The WVA number, an acronym of Waren Vertriebs Artikel (Merchandize Sales Numbering System), is generated by the Verband der Reibbelagindustrie (VRI, Federation of Friction Industries) and is used to provide a unique identification for the exact material and dimensions of the pad you are installing. Although it applies to all forms of friction material, the numbers relating to brakes are:

10000	to	14999	Car Drum Brake Linings
15000	to	19999	Commercial Vehicle Drum Brake Linings
20000	to	25999	Car Disc Brake Pads
26000	to	27999	Shoe Assemblies
28000	to	29999	Commercial Vehicle Disc Brake Pads

The intention is to remove the risk of human error resulting in the installation of an incorrect component to a critical braking system.

DISCS

The disc is one of the simplest components in the system and yet the manufacturing process is actually more complicated than you might expect. High quality discs are often dynamically balanced, and a great deal of design effort goes in to ensuring that the internal venting is efficient while maintaining a high level of structural integrity, something that requires a certain amount of compromise. Usually a disc brake is manufactured from cast iron (due to the ability of grey iron to conduct heat and withstand abrasive pad wear) and then machined to the correct tolerances. Often the only evidence of the original casting process is visible down between the vanes of the rotors. Any additional drilling or slotting will take place after the disc has been cast and machined, followed optionally by the dynamic balancing process.

This BTCC car utilizes a floating disc. Note the temperature sensitive paint to denote when temperature thresholds have been breached. SIMON CROSSE

The outside diameter of the disc will affect the amount of braking force applied for a given amount of pad pressure. The further from the centre of the hub that the pad is applied, the more leverage the pad and disc will be able to apply to the rotating mass, giving more stopping power, even if the total pad area has not increased. Therefore one upgrade that can be considered is to increase the disc diameter and move the caliper further away from the hub centre, using a custom bracket. The design of the bracket needs careful thought to ensure that it will not allow flex or breakage under heavy braking. Ideally a high grade steel should be used, although aluminium can be used if weight is an issue, provided that the inherent weakness of aluminium is taken into account. The primary advantage of this modification is that the bulk of the braking system remains unchanged: there are no hydraulic modifications as the hydraulic system remains the same. Don’t forget that improving the front brakes may throw the system out of balance, and that after moving the caliper the flexible pipe may be too short or foul something critical. The custom caliper bracket can be manufactured as a template out of MDF or a similar easily worked material: this allows you to test fit the wheel to ensure it fits over your new assembly and to verify that nothing is fouling. Under no circumstances, however, should you try driving the car using this template. The template can be used for both sides of the car as the bracket should be interchangeable.

For maximum efficiency the swept area of the pad should extend right to the very edge of the disc. Finding a disc to fit your car is often a matter of taking your original disc to a scrapyard or parts shop and physically comparing it with a few samples off the shelf. The new disc will need to have the same number of wheel studs, pitch circle diameter (PCD) and centre bore size, although this can be modif...