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| BRAKE
PAD TERMS |
-
COEFFICIENT
OF FRICTION
- TRANSFER LAYER
- TEMPERATURE CONTINUUM
- THERMAL STRESS / SHOCK
- RESINS
- BRAKE FADE
- PAD TAPER
- BEDDING YOUR ROTORS AND PADS
COEFFICIENT OF FRICTION
The scientific definition for coefficient of friction is the ratio of force required to move or stop one surface, with a given perpendicular force applied to it, as it drags across another surface. The layman can think of coefficient as the slippage of the pad as it clamps down on the rotor. The coefficient measurement ranges from zero to infinity, but most racing pad's range from .15 to .65. The mathematical formula for coefficient of friction is:
Mue = F/W
Where:
Mue = coefficient of friction
F = force required to move one surface over the other
W = perpendicular force
TRANSFER LAYER
This is what determines the stability and effectiveness of the pad's coefficient of friction. It is the simultaneous transfer of pad material between the rotor and the pad. Pad material is constantly moving between these two surfaces during brake engagement. Every pad leaves a transfer layer. This is one reason why a new pad will not work optimally against a rotor that had previously run a different type of pad.
- TEMPERATURE CONTINUUM
This is a brake pads' comfort zone and signifies the range in which the pads are more effective. Some pads are designed to perform optimally at cooler temperatures while others do not start performing well until they are hot. The expanse of the temperature continuum is dependent upon the materials (powder, fibers, and resins) used to manufacture the pads. The thermal stability of these materials determines their combined effectiveness. Different materials have different properties at different temperatures. Very few brake pads are designed to retain performance while transitioning along the temperature continuum. The key to a great brake pad is to have one material transition in while another material is fading away; all without the driver feeling any difference in the brake pedal.
- THERMAL STRESS / SHOCK
This happens when the rotor is heated up and cools down rapidly causing an uneven heat distribution throughout the rotor. The result is the expansion of the rotor material in one area, while not expanding in another area. Heat checks in the rotor are a symptom of thermal stress. Be careful not to confuse small cracks in the transfer layer laid on the rotor by the pads as thermal shock. Bedding of a new rotor will help reduce the possibility of thermal stress/shock.
- RESINS
Resins are organic materials used to bind and reinforce the different components of a brake pad. The resins act like a thermoset plastic, which holds the components together like glue. This glue forms the matrix of the brake pad. When heated, resins tend to shrink and crack, and at very high temperatures they may turn to carbon. Carbonized resins cause weakening of the matrix and accelerated pad wear.
BRAKE FADE
There are three types of brake fade. The first is the conventional burning off of resins as the pads are being broken (bedded) in. This is known as green fade and occurs when gases from burnt resins are trapped between the pads and rotors. When this happens the pedal feels firm but the car will not stop. The second type of fade is when the pads are forced to work outside their temperature continuum. This is the point at which the resins burn off rapidly, and the pad has accelerated wear. The third type of fade is glazing. This is when the resins, which soften during active braking, then cool and solidify on the face of the pads. This glazed surface is hard and slick, and will not give the same coefficient of friction as an unglazed or new pad.
- PAD TAPER
This is a rather disputed point among drivers, mechanics and manufactures of calipers and brake pads. The following are a few theories as to why pads taper.
- One type of pad taper is leading edge to trailing edge taper. One theory as to why this occurs is that the leading edge (front of pad) is hotter than the trailing edge (rear of pad) and thus the front part will wear faster. Another theory for leading to trailing edge taper is as the pad material smears against the rotor it flows from the front to the rear of the pad and accumulates there, giving the pad a tapered face. A third theory is that pads themselves are not made to the proper density, and thus have uneven wear.
- The second type of pad taper occurs in the form of top to bottom taper. In this case the top radius of the pad wears at a faster rate than the bottom radius of the pad. The caliper flexing under hard braking conditions may cause this. This problem is very common in racing where original equipment calipers are used. In many cases the original equipment calipers were not designed to withstand the high heat and high line pressure of a racing environment. Under these strenuous conditions the original equipment caliper bridge bends and flexes.
- BEDDING YOUR ROTORS AND PADS
First, slowly build heat into the pads by making slow to medium speed stops. Then make a series of very hard high-speed stops. Some fade may be experienced when doing the high-speed stops. The brakes should not be dragged during this procedure. Let the brakes cool down for twenty minutes or more and they are then ready to race. Never use new pads to bed new rotors. It is best to bed new pads on old rotors that had run the same type of pads. It is best to bed new rotors with old pads that are the type that will be used in the race. A bedded rotor should have a uniform, polished appearance with no cracks or grooves.
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