There are three main factors which decide how well brakes will perform.
- The pressure applied to the brakes - unless the car is under 2 years old the chances are that the braking efficiency has deteriorated to some degree. Most brake fluids are hygroscopic (absorbs water) and systems should be recharged with fresh fluid at least every 2 years. Always use a good quality brand name fluid (DOT4 or 5.1) or, if you are racing, the special high temperature fluids available. The standard rubber hoses do deteriorate over time so a set of braided steel hoses are always a good investment because they prevent pipe expansion and eliminate spongy pedal syndrome. Lastly, the wheel cylinders or caliper pistons should be operating efficiently. The piston should move freely within the bore and there should be no visible wear. Piston seizure is quite common and can sometimes go unnoticed, especially on rear discs.
- The ability to dissipate heat - even with a super efficient hydraulic system, if the pad’s maximum working temperature is exceeded, the car will give up stopping no matter how hard the pedal is pressed. High performance pads will provide braking at higher temperatures, typically 450°C for road spec., and these items should be high on your shopping list. Ventilated front discs (also known as rotors) are a standard fitment on most modern cars, but replacing these with grooved or cross-drilled items will aid braking efficiency for two reasons. Firstly, pad dust and road dirt are removed more effectively by the action of the grooves or holes sweeping across the pad surface and secondly, they will reduce the build up of hot gases forming between the pad and disc surfaces (NB. This condition increases as pad sizes get larger). One last but perhaps pretty obvious point, hot brakes require airflow to dissipate heat, (unless you have some very special water cooled calipers as fitted to some racing formulas, etc.). So any improvement to the airflow around the brakes will pay dividends e.g. cool air ducting.
- The brake surface area - to improve braking effectiveness by a considerable factor the disc size and pad area has to be increased. Unfortunately this is not just a case of fitting a pair of larger discs and a bigger set of pads. Bigger pads need bigger calipers and larger discs invariably require larger inside wheel sizes! Most modern car braking systems use a single piston floating or sliding caliper arrangement that is economic to produce but not 100% effective. Two pot fixed calipers are an improvement but, with only one piston each side, the pad size is very restricted. 4 and 6 pot calipers allow for much larger pad areas to be employed with greater control over pad distortion (a major problem with the floating caliper system).
Finally, a few answers to some popular questions we are asked about brakes:
Q: ‘Are grooved and cross-drilled rotors noisier than plain discs?’
A: Yes and no! There is more than one factor to consider here - the number of grooves or holes, the pad material and the type and size of the disc. Generally speaking they will be a bit noisier, however, a few manufacturers claim that for some applications they are quieter.
Q: ‘I have just fitted a new set of pads - how do you advise bedding
A: Again, not a simple straight forward answer. For many motorists bedding in is the process of knocking off any high spots on the pad faces until full contact is made with the disc. But with high performance pads, the correct stabilisation of the friction material can be paramount in their ultimate performance. Some pads are supplied pre-stabilised but many others will require this process to be carried out on the car. This usually involves bringing the pads up to their operating temperature and allowing them to cool naturally but may involve more complex procedures for race applications.
Q: ‘Most upgrades concentrate on the front end, but what about the rear?’
A: Some rear disc conversions are available and high performance pads are available for most cars. However, for normal road use, the rear brakes on rear wheel drive cars supply only 25% of the total braking effort and on front wheel drive cars it is only 10%.
Q: I have been told that high performance pads will wear my discs out?
A: To improve the high temperature efficiency of brake pads the organic material commonly used has a metallic content added to it. These metallic particles will be slightly more abrasive but the major factor in disc and pad wear will be how hard and how often the brakes are applied. And REMEMBER: DO check your discs at regular intervals if you do a lot of heavy braking. Persistent thermal stress will eventually create fatigue cracks. ALWAYS replace pads and discs on BOTH sides.
Brake juddering is a fairly common complaint on all cars, whether standard or high performance. In most cases, the brake discs will have warped and a replacement set is the obvious cure for the problem. However, the cause of the problem can often be found elsewhere and the replacement discs will soon warp as well. It is not only essential to fit new brake discs, but to also rectify the cause of the problem in the first place, otherwise the juddering is likely to return.
From our own experience, we have found that the following causes are likely to result
in a warped brake disc:
Seized calipers - either a seized caliper piston or a seized caliper sliding pin will result in uneven forces being applied to each side of the brake disc. Suspension bushes - tired TCA or anti-roll bar bushes.
Poor quality pads - these can overheat quickly, particularly if the brakes are used often and hard. The excessive heat from the pads can cause the discs to overheat, resulting in the discs warping.
Hubs - although rare, it is possible for the hubs to warp. Bolting a disc to a warped hub will always result in brake vibration. The same will also happen if any rust from the hub face is not removed before fitting the disc. After fitting a disc, we always recommend checking for disc run-out using a dial gauge. If the run-out is out of tolerance, then the disc should be re-fitted in an alternative position (i.e. turned through 90o) until run-out is within tolerance.
Problems with bleeding brake systems can be frustrating when the system has been bleed several times to no avail. Under such circumstances, a systematic approach is needed to tracing the problem:
- Visually check the system and rectify any obvious leaks.
- Begin the diagnostic procedure with the master cylinder. Remove one pipe and blank off with a spare bleed screw. If the pedal is good, the problem is further down the system, otherwise the problem lies with the master cylinder.
- Reconnect the pipe and move to the next junction in the system (e.g. front to rear split). Remove one pipe from the junction and blank off with a bleed screw.
- If the pedal is good, the problem is in the part of the system that has been disconnected. Otherwise the problem is in part of the system still connected.
- Continue this procedure throughout the system until the problem has been isolated and identified.
- Do not jump ahead of the systematic process since this can cause confusion.
- Remember that vehicles fitted with rear bias brake valves will not bleed properly when the rear wheels are hanging freely.