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  • Bleeding procedure - motorsport calipers

    1. Connect a bleed bottle and tube to each caliper bleed screw and fill the reservoir, leaving the reservoir cap off. Open the bleed screws of each caliper in turn to allow the system to gravity fill, until clean fluid can be seen in each bleed tube. Check that the fluid level in the reservoir does not fall below the outlet opening. Close all bleed screws.

    2. Where dual master cylinders are used, bleed one front and one rear caliper together. For calipers with two bleed screws, bleed the outer side of the caliper first, followed by the inner side.

    3. Never bleed the system by pumping the pedal until it is firm followed by opening the bleed screws. If there is air in the system, this procedure will aerate the fluid, making removal more difficult.

    4. Air in the master cylinder primary and secondary chambers should escape to the reservoir via the feed line when the brake is off. If there are any restrictions in the feed line or reservoir connection that prevents air from escaping, air that remains in the feed line will be drawn back into the cylinder on the recuperation stroke. To minimise the restriction, dash 4 hose and fittings should be used for the feed line, particularly if the reservoir outlet is close to the cylinder inlet.

    5. Open the outer bleed screw of a front and rear caliper and slowly depress the pedal to avoid fluid aeration, using the full master cylinder stroke. Close the bleed screws and let the pedal return fully to its original position to allow the master cylinder to recuperate fresh fluid from the reservoir. Do not allow the pedal to snap back, use a controlled rate of return. Rest for 5 seconds to allow the master cylinder to re-fill. Top up the reservoir as required. Repeat until no air is visible in the bleed tube. Depending on brake hose runs, a clear tube should be achieved within 3-5 strokes.

    6. Repeat section 5 for the inner bleed screws of the front and rear caliper until no air is visible in the bleed tube.

    7. Repeat sections 5 & 6 on the other side of the car.

    8. Repeat sections 5, 6 & 7 if pedal travel is not satisfactory.

    9. If the pedal is not firm after repeating the procedure, there must still be air in the system and an alternative procedure, backbleeding, is recommended. Using this method, a large volume of fluid and any air that is trapped in the system is returned to the reservoir via the master cylinder inlet port.

    10. Fit thin pads, or preferably just pad backplates, to each caliper and slowly pump the pedal so that caliper pistons move forward to contact the pads. Working on one caliper at a time, squeeze the pistons back into the caliper, displacing fluid to the reservoir. The reservoir will fill with displaced fluid so it must be emptied to prevent it from overflowing. Repeat the procedure for each caliper and re-fit the original pads before pressurising the system with the brake pedal.

    11. After bleeding, check the complete system for leaks before driving the car.

    12. Recommended bleed screw torque (do not over-tighten bleed screws)

    13. The aim when bleeding is to achieve a firm pedal that holds its position under a sustained pedal load. Re-bleeding the brakes after some running can further improve the pedal.

    14. IMPORTANT – When the system is fully bled, the threaded rod of the balance bar should be at right angles to the master cylinder push rods when the normal maximum pedal load is applied.

  • Temperature effects

    Calipers must be regularly inspected for leaks and damage. Temperatures must be monitored at all times to prevent overheating.

    Ideally, on-track caliper temperatures, using thermocouples that measure fluid temperature, should be kept below 180ºC (356ºF) by effective use of ducted air. Surface temperature of the caliper housing recorded with thermal stickers is the result of heat soak from the discs and pads after the car has stopped and is typically 30-40ºC (86- 104ºF) higher than on-track temperature. To minimise heat soak, the driver should be encouraged to back off on the in-lap to allow the brakes to cool.

    Excessive temperature will also affect other components used in the caliper. If caliper temperature exceeds 210ºC (410ºF), the hardness of the caliper housing should be checked to ascertain if the temperature has permanently affected the tensile properties of the material. The recommended method is Rockwell B Scale using 1/16” Ball and load of 100 Kg (see figure 1).

    Recommended time at temperature before seals must be changed and hardness of caliper housing is checked is shown below (figure 2).

    The elastomers used for brake caliper seals begin to deteriorate when exposed to temperatures above 150ºC (300ºF). However, the degree of deterioration is time dependent and seals can withstand exposure to temperatures up to 240ºC (464ºF) for a short time. Whilst seals will withstand such high temperatures for a short time without leaking, deterioration of the seal will affect caliper performance: Compression set causes a reduction in squeeze force and hence friction between the seal and bore, leading to an increase in pedal travel due to knock-off, a condition that is not recoverable without changing seals. Seal extrusion occurs after prolonged use at elevated temperature. Brake line pressure causes the seal to be extruded between the piston and bore. Under close examination, the seal edge will appear ‘nibbled, at the inside diameter. Severe degradation of the seals will eventually occur if calipers are continually used at extreme temperature. The combination of high temperature and brake line pressure causes tearing at the inner diameter of the seal and detachment of material. This can lead to fluid leakage and loss of brakes.

  • Pad changing

    Thoroughly clean the protruding pistons with brake cleaner before pushing the pistons back to fit new pads. Scotchbrite or similar abrasives should not be used to clean pistons as the coating may be removed from the piston.

    The pistons in Alcon calipers are ground to achieve close dimensional tolerance, roundness and surface finish. Friction between the piston and seal is lower with lubricated seals than with dry seals. Pistons and seals in Alcon calipers are assembled with a lubricant that evaporates at around 100ºC (212ºF), therefore piston retraction will increase as caliper temperature rises during normal use.

    The level of friction also helps to resist piston displacement due to disc run-out or suspension/ hub deflection (knockback). Typically, pistons will recover from displacement into the caliper by up to 0.5mm (0.02”), preventing increased lost travel at the pedal and maintaining a constant pedal position under all conditions. This feature means that a higher than normal force may be required to push pistons back into the caliper during a pad change.

    Note the importance of cleaning pistons during a pad change, to prevent debris being deposited in the seal/ piston interface as pistons are pushed back. Debris will reduce seal to piston friction, and have an adverse effect on piston retraction.

  • Disc / pad bedding and running in

    Pre-bedding at Alcon

    Most discs and pads supplied by Alcon are pre-bedded to deposit an even transfer layer of friction material on to the surface of the disc and to thermally condition the disc. After the discs have been pre-bedded and allowed to cool, the disc and pads can be bolted on to the car and should be ready for competition use providing that the discs and pads are “run-in” correctly.

    Use of pre-bedded discs in competition

    Care needs to be taken during “running-in” to obtain the best performance and life from pre-bedded discs and pads. Lightweight discs are particularly sensitive to potential problems during use, failure to correctly run-in the discs and pads can result in problems including Long pedal, poor feel and modulation, vibration, premature wear and disc cracking. Heavier-weight discs are more stable and less prone to these problems, due to the increased structural rigidity gained from 48 and 72 vane design and generally increased flange thickness. It is still advisable to run-in the discs carefully as per instructions to follow.

    To prevent these problems, an appropriate and proven “running-in” procedure needs to be followed during rallies, races and tests. We suggest that the following procedures are employed:
    • 5 brake applies from slow speed and light pedal pressure to complete system check.
    • 15 brake applies from 80 to 40 kph, light to moderate pedal pressure. (2.5 – 3.0 seconds, line pressure 20 bar).
    • 15 brake applies from 120 to 60kph, light to moderate pedal pressure. (4.0 seconds, line pressure 20 bar).

  • Disc warming

    Irrespective of which friction materials are being used, pads and discs used in all forms of motorsport require a period of bedding-in before being used.
    Discs and pads that are supplied as ‘pre-bedded’ as well as those that have been run before, must be brought up to temperature on the car before being used in test, qualifying or race conditions.

    A suggested brake warming routine from cold is to carry out 3 stops from 120km/h to 40km/h followed by 6 or 7 stops from 160km/h to 40km/h, both at around at 0.5g. Brake ducts can be left fully open. During this procedure, disc surface temperature will rise gradually to around 370°C. Note that this is a disc warming procedure, to be repeated each time the car runs with cold brakes. The actual procedure will vary depending on the circuit layout and can often begin exiting the pit lane on the out lap. A correctly warmed disc will have an even, grey layer of friction material with no sign of spots or blotches of friction material.

    Within a few additional laps the brake pedal should become firm and consistent as a transfer layer of pad material develops on each brake disc. Failure to carry out a procedure to warm the brakes may lead to uneven deposition or spotting of friction material on the disc surfaces, causing vibration under braking. Uneven deposition also leads to uneven temperature distribution, which may cause the disc to permanently distort, and brake pedal travel to increase, particularly after a long run without the brakes being used.

    Alternatively, the disc may crack prematurely due to an unequal distribution of thermal stresses around the disc.

  • Sintered, cerametallic and carbon clutches

    The Alcon range consists of three friction materials:

    Sintered clutches are lightweight and have low inertia. They are generally used in lightweight circuit applications such as touring car or lower duty single seater. They also benefit from having a lower clutch height for the same number plates than the other friction materials.

    Cerametallic or “paddle” clutches have a greater temperature resistance than sintered. They are generally used in rally applications or circuit applications with numerous standing stars. As a trade-off between inertia and temperature resistance the number pads can vary, e.g. 4 or 6.

    Carbon clutches are used in high end applications e.g. Rallycross, Formula 1, Endurance racing, etc. They have very high temperature resistance and offer a significant reduction in weight and inertia when compared to metallic clutches. By using pressure plate “shims” in increasing thickness to compensate for carbon pack wear, the clutch life can also be several times that of a metallic clutch. Alcon offer a recondition service for carbon clutches

  • Clutches installation

    Before installation onto the vehicle ensure:

    • The clutch fits the flywheel correctly i.e. pot or step location, bolt PCD and diameter.
    • The mounting bolts or studs are of the correct length.
    • All parts are present and are fitted to the clutch in the correct order (see below).
    • The carbon driven plates are free to move on the hub.
    • The pressure plate and carbon floater plates are free to move on the cover legs.

    The carbon plates must be installed in the clutch in the same position and orientation as when the clutch was originally built. One of the clutch legs is marked with a serial no and a triangular orientation mark “Λ“. The floater plates are marked as “Λ”, “ΛΛ”, “ΛΛΛ” etc. Floater plate “Λ” is installed into the cover first next to the pressure plate and with its marking next to the marked cover leg. It must also in line with the orientation mark. The other plates are fitted in numerical order either side of the driven plates with the highest number plate against the flywheel.

    The driven plates are marked in the same way and must be fitted in the same sequence, i.e. “Λ” assembled into the cover first. Before fitting the last driven plate the hub must be fitted. The hub will have a “web” between the teeth to maintain hub engagement with the carbon pack. This “web” must be fitted towards the flywheel. When fitting the clutch to the flywheel, a dummy input shaft should be used to centralise the clutch hub spline with the
    flywheel bearing.

    When mounting the clutch onto the flywheel and inserting the mounting bolts/studs, ensure the bottom floater plate is not allowed to become trapped between the cover legs and the flywheel. As the clutch will be under load, tightening should be carried out half a turn at a time in a star like pattern. Recommended tightening torque for M8 and 5/16” is 22Nm (16 lbft).

    When removing the dummy input shaft ensure that it moves freely before attempting to fit the gearbox. When assembling the gearbox to the engine ensure the gearbox is not allowed to exert a bending load on the clutch hub as this could damage both the hub and the carbon plates. When the clutch is tightened down on the flywheel to the correct torque, the diaphragm fingers should be almost flat. If the fingers are not flat the flywheel may be incorrect for the clutch e.g. pot instead of flat or an incorrect pressure plate thickness may have been used.

  • Clutches maintenance

    Regular checks should be carried out for damage, excessive wear or contamination of the friction material by e.g. oil:

    Firstly clear out all dust from the clutch components using a vacuum cleaner and a brush.
    Carefully check the tightness of the spring retainer fixings but DO NOT break the Loctite.
    Check each carbon plate for damage and ensure they are all free to travel along the cover or hub.
    Carbon plate drive face wear should also be checked using feeler gauges. With the carbon floater plates in the clutch cover, measure the gap between the drive face and the clutch cover leg.
    With the carbon driven plates on the hub measure the gap between the drive face and the hub, (figure 4). These gaps should be no more than 1mm.
    The diaphragm spring should be checked for “blueing” that would indicate excessive temperatures have been experienced. A diaphragm spring exposed to excessive temperatures can lose clamp load and should be returned to Alcon for Inspection.
    The diaphragm spring fingers should also be inspected for wear from the release bearing. It is normal to have some wear over the life of the clutch. If the wear is uneven or there are signs of localised heat then check the release unit / bearing for problems. Spin the release bearing, if it feels dry or has more resistance than normal replace it.

    Check the hub spline for wear. Worn spline teeth can be a result of a misalignment between the input shaft and the crankshaft. This could include a worn flywheel bearing or even the bell housing flexing during use. Having minimal spline engagement for high torque applications can also result in excessive spline teeth wear.

    Carbon stack wear: Additional pressure plate “shims” can be purchased to compensate for wear of the carbon plates and restore the original torque capacity of the clutch. Using a micrometer, measure the thickness of each carbon plate in the centre of the friction area in 3 places 120° apart and calculate the mean value for each plate. These figures can then be added to the build sheet and then subtracted from the original as new figures to determine the carbon stack wear. As a general rule, the next thickness pressure plate should be used.

    Important: Do not fit a thicker pressure plate than appropriate for the carbon stack height as this will cause the clutch to malfunction.

    Ensure the carbon plates are reinstalled into the clutch in their original positions. Do not swap complete carbon packs between clutches.