Specifying the Brake

Alcon Advantage Extreme Brake Kit

Alcon's approach to supplying Brake equipment aims at adding to our customer's competitive advantage, increasing their opportunities of Winning. Whilst we cannot accompany every customer to their events, these notes might help to avoid the more common issues which are brake related.

With similar cars, overtaking opportunities occur immediately before, during or just after, braking. Therefore, taking time to get brake performance optimised will produce results.

Brake Basics

The task of the brake is to convert Kinetic Energy into Heat Energy, and this process is expressed by the formula: K = 1/2MV2, - and then dissipate it. From which it can be seen that the speed (V) from which braking is commenced has a greater significance on the energy (K) to be dissipated than does the weight (M) of the vehicle

 

 

Sizing the Brake

Generally, the disc diameter is limited by the available space within the wheel and the depth of the caliper bridge section. In general terms, the front disc is usually of the largest diameter that will allow the caliper to clear the inside of the wheel. The rear brake may not follow the same rules - particularly where front-engined cars are concerned. In such a case, the weight distribution is biased towards the front and the Centre of Gravity is higher than with a single seat car. Therefore, due to weight transfer under braking, the front brake has more energy to absorb than the rear. So the rear brake may be smaller and lighter. Sizing of Master Cylinders is determined by considering the vehicle's weight and weight distribution, C of G height, down-force, brake and wheel sizes and pedal ratio and if in any doubt, expert advice should be sought.

 

 

Weight Issues

The brake needs to be as light as possible and is determined by the amount of energy that has to be absorbed and dispersed by the disc and also by the pad area and volume needed. Hence, calipers for motor sport are invariably made from an aluminium alloy for long distance events, will use larger pads and therefore, be larger than those used in 'sprint' events.

 

 

Instructions

Calipers should be mounted vertically - unless of a special design intended for off-vertical location. Vertical mounting prevents air entrapment within the calipers. It is essential that calipers are located securely on the steering knuckles or uprights and that the mating areas are rigid and suitable to resist maximum braking torque. If the caliper twists under braking due to mounting weakness, brake performance will be affected and pad wear may be distorted. Disc mountings may be of the rigid mounting type where the disc is bolted directly to the mounting bell - in which case it is essential that the fixings are safely locked. When mounted on the hub, the disc 'run-out' should be measured, close to its outside diameter and be less than 30microns. This is important because if disc run-out is greater than this figure, excessive brake pedal travel may result.

Where 'floating' disc and bell arrangements are used, again, fixings must be secure and run-out should be checked. When fitting a new brake installation - or wheel type - always carefully check that the wheel is well clear of the caliper. Bear in mind that wheels may deflect under cornering loads. Always check the brake caliper and inner surfaces of the wheel for any signs of contact after the first run following any change to brakes or wheels. Similarly, check the inside surfaces of the calipers for any signs of contact with the disc. Master cylinders should be mounted rigidly on bulkhead or pedal box ensuring that no deflection occurs under heavy pedal loads. Similarly, the brake pedal must be located securely by pivot bracket to the floor of the car or to a pedal box - again ensuring that whilst free to travel in the operating range, no deflection occurs. For pedal set-up - see Brake Balance. Hydraulic pipes and hoses are important and should be securely fixed, laid out using the shortest length practical and should be clear of hot areas e.g. exhaust pipes and areas where they could be scuffed or damaged. When determining the flexible hoses feeding the calipers, it must be ensured that each hose is clear of its wheel in all extremes of travel: lock to lock and full bump and droop. Stainless steel braided hose is generally used for the flexible hoses feeding fluid to the calipers - normally '-3' size but '-4' is advisable for clutch lines. Stainless steel brake pipe is preferred for the non-flexible pipework. It is important to avoid vertical loops in brake (or clutch) hydraulic lines to avoid air entrapment - see also BLEEDING.

 

 

Cooling

Cooling of the brakes is essential - particularly on 'heavy braking' circuits or events. The heat generated during braking is stored in the discs and must be dissipated by air-flow through and around the disc. Failure to do this will result in reducing brake efficiency and in the extreme, increasing brake pedal travel and loss of retardation. Brake heat is removed by using ducted air directed into the 'eye' of the disc, through the internal vanes - and across the braking faces. It is important to ensure that the heated air escapes from the wheel and wheel-arch - otherwise air-flow is restricted and temperatures will increase. This is very much vehicle specific, there are no general rules. Thermal paints are available which, applied to the outside diameter, are used to determine the maximum temperatures reached by the disc. Whilst there cannot be a general rule - because of varying circumstances - with the commonly available and used paints (Pink, Orange & Green) these are some pointers. If the Pink paint has turned white, this indicates that the disc has exceeded 610°C and it is probably too hot, so action is needed to correct the situation. If the orange has changed colour, it has reached 560°C, an acceptable running temperature and the green will change colour at 430°C which, generally, is below the optimum temperature range for racing pads - this could be so for a number of different reasons. Similarly, adhesive temperature indicator strips (THS0080x087 showing 121 -280°C) are available to indicate the maximum temperatures reached by the calipers.

Caliper temperatures must be kept within the capability of the brake fluid being used, otherwise, ultimately, vaporisation will occur with the result that the brake pedal travel will increase to the point that no hydraulic pressure can be generated and therefore, no braking force. Again and assuming that fresh, good quality Racing Brake Fluid is being used, if the caliper temperature indicators show a temperature of 200°C or more, this is cause for concern. It should be noted that although Racing Brake Fluids advertise a vaporisation temperature of 600°F (321°C) this is not totally representative of their performance in a vehicle, where inevitably there are some small amounts of air and moisture present both of which reduce vaporisation temperatures. So, new Racing Brake Fluid before each event should be the rule. In wet conditions, where tyre grip is reduced, cooling ducts should be partially closed to keep disc and pad temperatures up to their optimum. There is a point worth observing here: if, under normal dry conditions, brake temperatures are always cool, it may be that smaller, lighter brakes could be specified.

 

 

Bleeding

BLEEDING Whilst there is some 'mystery' about this, it is basically a simple operation. The objective is to ensure that all air is purged out of the system(s) so that only good, fresh brake fluid is in the calipers, master cylinders, pipe-work and hoses. Air must be removed from the hydraulic system because it is compressible whereas brake fluid is not. The absence of compressibility gives the firm pedal that we require. In order to achieve that 'good', firm pedal, it is advisable to check the brake pipes & hoses to ensure that there are no vertical loops in which air could be trapped. For the same reason this is why, with racing calipers, the bridge pipe connecting the two sides of a caliper, is always at the bottom with the bleed screws at the top. On race or rally cars with dual systems (front & rear), it is essential that both systems are bled simultaneously. When bleeding is complete, ensure that bleed screws are 'torqued' to 14 Nm when the system is cold - or 18Nm if hot (at the track). Remove any signs of fluid from around the bleed screws and calipers.

 

 

Brake Balance

BRAKE BALANCE Safety regulations require that racing cars, other than veteran or vintage, must have two separate brake systems so that in the case of a hydraulic failure, one system will remain operative - giving the driver some chance of reducing the intensity of the impending impact! There is an additional advantage in this, as it allows the driver to adjust the front/rear brake balance to suit conditions and driving style. For example, if the track becomes wet during an event, the driver can move the brake balance to the rear; thus reducing the braking on the front wheels which, due to lack of grip in the wet, will tend to lock since there will be reduced weight transfer. There is also the opportunity to adjust the brake balance to suit the driver's preference. It should be noted that, in dry conditions, although brake balance biased to the front tends to make the car feel stable under braking, brake balance should be set as far to the rear as the driver can tolerate as this will give the shortest stopping distance. This ensures that each tyre is giving the maximum braking effort that it can. The balance bar system should be set up so that when the brake pedal is depressed to the position it adopts under heavy braking, it and the balance bar are at right angles to both push-rods - in both planes. This is achieved by adjusting the push-rod lengths - care is needed to ensure that adequate push-rod thread length remains in each clevis. This gives maximum efficiency of the mechanical operating system. Diagram*****. The fact that in this condition and with the brake released, the brake pedal in its 'off brake' position, the balance bar does not make right angles to the push-rods, is quite acceptable. This occurs because, generally speaking, the front calipers are larger than the rears and therefore more fluid is used to generate the required braking force, hence there is greater master cylinder travel at the front.

 

 

Durability

Racing brakes, unlike those fitted and used on Road Cars are subject to continually high temperatures and stress and therefore must be serviced correctly if they are to give continuing good performance. The main items to be looked at very regularly are the discs and pads - both of which wear out. Inspection of these should be a regular part of car preparation - with worn items being replaced. Whilst inspecting calipers, it is advisable to look for any sign of moisture around the pistons - especially if the calipers have been very hot (see COOLING above) and the fluid connections. If there is any sign of fluid leakage from calipers, they should be serviced and fitted with new seals. In long distance races it may be necessary to fit replacement pads - and possibly discs - during the event. This procedure should be properly practised (with HOT brakes) prior to the event to avoid risk, confusion and time wastage. It is important that replacement pads (and discs if being replaced) are pre-bedded so that the driver needs only to warm up the brakes prior to achieving an acceptable, but not at this stage, optimum, retardation. Discs should be regularly inspected for cracks and must be changed if necessary. During routine maintenance, clean calipers particularly the outside diameter of pistons - with clean soft cloth or paper towel - and inspect for any signs of damage or leakage, check fixing bolts for correct tightness.

 

 

Advantage Extreme Brake Kits benefit from our experience in top level motorsport to provide enhanced brake performance in a specially designed direct replacement package.

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