How Brakes Work

Recall that the brake’s hydraulic system must supply movement and force. The movement must be enough to take up all slop, clearance, and deflection of parts as well as move the caliper pistons sufficiently to bring brake pads into firm contact with the rotors. The force must be enough to create enough friction between pad and rotor to stop the trailer.

It is the piston of the master cylinder that provides both the movement and the force, and the brake fluid that transmits both to the calipers.

The goal is to have a system that provides maximum force with small movement – i.e. we want to be able to brake hard without excessive travel.

Force applied to the MC piston creates pressure in the brake fluid. The pressure is the force applied, pIDED by the area of the piston. Therefore, the smaller the master cylinder piston, the greater the pressure created.

Recalling that the pressure created is a direct factor in how much clamping force and therefore brake torque is developed, it may seem that for the most powerful brakes, we would want to use a small MC piston.

But, the trade-off is the other component required of the brake system – namely movement. Because the fluid is incompressible, any movement in the MC piston translates into movement of the caliper pistons (excluding expansion of hoses and lines, which should be minimal in a properly working system). This movement in a hydraulic system is known as displacement and is calculated as the area of the MC piston multiplied by its stroke. Displacement is a volume, measured in cubic inches. Therefore, the smaller the master cylinder piston, the less displacement created.

The trade-off between force and movement in selecting a MC piston size. The smaller the piston, the greater the pressure created but the less displacement produced (and therefore greater pedal travel required.)

So far, we have considered only the effect of the size of the MC piston, but of course in a brake system there are two pistons – the MC piston and the caliper piston (for calculations, the total area of all pistons in a multi-piston caliper act the same as a single piston of equivalent area in a single-piston caliper.)

There is, of course, a relationship between the pistons in the system that affects both force and movement.

Because the brake hydraulic system is a closed, sealed system, and brake fluid cannot be compressed, there is a law of hydraulics that we make use of to multiply force – that is, to apply more force at the calipers than the driver applies to the MC piston. It is quite simple, and quite possibly the most important concept in this is:

In a closed hydraulic system, pressure is equal over all surfaces of the containing system.

In our discussion of brake systems we will refer to the MC piston as the “input” piston and the caliper piston as the “output” piston.

The above law means that whatever pressure is created by the input piston is applied equally to the output piston. Because the output (caliper) piston is of much larger area than the input (MC) piston, this has the effect of multiplying force in the brake system.

The amount of force-multiplication thus achieved is known as the brake’s “Hydraulic Ratio” Hydraulic Ratio can be calculated or expressed a number of ways. It is the ratio of fluid displacement by the master cylinder to fluid displaced in the caliper pistons. It is also equal to the ratio of force applied to the MC piston to the force generated by the caliper pistons. The stiffer the caliper and the stiffer the pad, the higher the hydraulic ratio that can be employed.

For example, suppose we apply 100 lbs of force to a ½” diameter MC piston, we develop approx. 500 psi. This 500 psi acts evenly on all other surfaces in the system, including the caliper pistons. Suppose the caliper piston has a diameter of 3 inches. Multiplying our 500psi by the area of the caliper piston (~ 7 sq. in), we develop nearly 3500 pounds of clamping force at the brake pads.

The clamping force that a caliper exerts, measured in pounds, is the hydraulic pressure (in psi) multiplied by the total piston area of the caliper (in a fixed caliper) or two times the total piston area (in a floating caliper), in square inches. To increase the clamping force it is necessary to either increase the hydraulic pressure or the caliper piston area. Increasing the coefficient of friction will not increase clamping force.

The coefficient of friction between pad & rotor is an indication of the amount of friction between the two surfaces. The higher the coefficient, the greater the friction. Typical passenger car pad coefficients are in the neighborhood of 0.3 to 0.4. Racing pads are in the 0.5 to 0.6 range. “Hard” pads have a lower coefficient but wear less; “soft” pads have a higher coefficient but can wear quickly. With most pads, the coefficient is temperature sensitive – which is why sometimes racers need to “warm up” the brakes before they work well, and also why most brakes will “fade” when they overheat – the coefficient of friction is reduced as the temperature rises. For more info in coefficient of friction, see section on pads.

The brake rotors must be capable of absorbing the heat generated by the brakes as they convert the moving car’s kinetic energy into heat. The amount of kinetic energy a trailer has (and, therefore, the amount of heat the rotors must be able to absorb) depends on the weight of the trailer and the square of the speed of the trailer. The rotor’s ability to absorb this heat depends on its mass (weight), and on how well it cools. Exposed as they are to cooling airflow, this is one area where discs are superior to drums.

Recall that brakes need to be able to do 3 things:

1. Develop enough clamping force to create enough friction between pad and rotor to convert trailers kinetic energy to heat.
2. Develop enough brake torque to reach the limit of traction (lockup the tires) in all conditions, and
3. Have enough mass to absorb the conversion of the trailers kinetic energy to heat without boiling the fluid, warping the rotors, cooking seals, etc. And they must do both with good and consistent feel!

In order to apply the brakes the driver must be able to both move and pressurize the hydraulic fluid. The master cylinder piston has the job of moving the brake fluid through the lines in order to bring the brake pads into contact with the rotor, and of pressurizing that fluid creates clamping force.

The master cylinder is the heart of the brake system. Actuated by surge or pump, its piston provides the force and the movement required to apply the brakes. When the pedal is released, an internal return spring returns the piston to its resting position.

Initially the piston moves forward and fluid volume is displaced, taking up all clearances in the system. This fluid movement actuates the caliper pistons which extend and bring the brake pads into contact with the rotors. Because the fluid is incompressible, once the pads are in contact with the rotor, fluid movement stops and pressure rises.

The critical specs of a master cylinder are its bore (diameter of the piston) and stroke (how far the piston can travel – and therefore how much fluid it can displace when applied).

Reservoir Size
Disc brake reservoirs are larger than those for drum brakes. You will often see two reasons given for this:

First, because the pistons in a disc brake caliper are MUCH larger than the tiny pistons in a drum brake wheel cylinder, disc brakes require more fluid volume to be displaced than drum brakes – requiring a larger reserve of fluid for operation.
Secondly, as disc brake pads wear, disc brake calipers are self adjusting. That is, the calipers only retract the piston just enough to prevent pad-to-rotor contact. Now, imagine you start with disc brake pads with ½” thick linings and you have a 4” diameter caliper piston. Every time you apply the brakes and the pads wear a little bit, the caliper retracts just a tiny bit less. By the time the pads wear to 25%, or 1/8” thick, the piston at rest will be .75” further out in its stroke than it was when the pads were new. That .75” behind the piston must be taken up by additional fluid – and in the case of a 4” diameter piston, the additional volume required is given by (pi [d/2] ^2 * 0.75) or about 8 cubic inches. Multiplied by two calipers (one for each wheel) and that’s 16 cubic inches of extra fluid reserve required to compensate for pad wear. That’s much higher than the amount required to compensate for drum brakes shoe wear. Therefore, disc brake reservoirs are larger than for drums because OEM designers must design a reservoir for disc brakes large enough that the brakes will still function even if Joe Public doesn’t check the fluid or add a drop between new pads and completely worn out pads.
Piston Size
It is true that disc brakes require both more pressure and more movement (volume) to operate than drum brakes.

Built-in Valve
This one can be a deal-killer. If the MC in question was designed for drum brakes and has a built-in residual pressure valve, it will not be suitable for disc/disc brakes. See section below on valve for description of residual pressure valves. For now, the point is, know the MC in question and whether or not it has built-in valve. If it does, you would have to either modify it by removing the residual pressure valve to make is suitable for use with disc brakes, or choose a different master cylinder.

Summary
The differences between disc and drum master cylinders are as follows:

Disc brake MC’s normally have a longer stroke and larger reservoir than those for drums
Drum brake MC’s have a built-in residual pressure valves.
Valve
Residual Pressure Valve
A residual pressure valve is a simple, one-way, spring-loaded valve installed either in the master cylinder. They operate by keeping a pre-determined amount of pressure in the brake lines, even with the brakes released. The internal spring determines the amount of residual pressure kept in the brake lines – normally 10 PSI. Here the use of a residual pressure valve:

10 PSI: Drum brakes only. Because drum brakes don’t use calipers and are therefore not self-adjusting there are springs installed to retract the brake shoes away from the drum. A 10 PSI residual pressure valve is used in drum brakes to keep a little pressure in the lines to balance the return-spring force so that the shoes are maintained in close proximity to the drums. Without the residual pressure valve, the return springs would retract the shoes so far from the drums that excessive pedal travel would be required before the brakes are applied.

When converting from drums to discs, you will need to remove any RPV in the master cylinder.

Brake torque in in-lbs (for each wheel) is the effective rotor radius in inches times clamping force times the coefficient of friction of the pad against the rotor. Brake torque is the force that actually decelerates the wheel and tire. There are two components – how hard the pads clamp the rotor (clamping force) and how far that clamping takes place from the center of the wheel hub. The larger the effective rotor radius, the further the clamping takes place from the wheel center, and the more torque generated by this longer “lever effect”. This is very similar to the manner in which a longer handle on a ratchet generates more torque than a short handle (for the same input). To increase brake torque it is necessary to increase the hydraulic pressure, the caliper piston area, the coefficient of friction between pad & rotor, or the effective rotor diameter.

DEEMAXX Floating
Calipers are “floating” design. A floating caliper, common on production vehicles, has a piston or pistons only on the inboard side of the caliper. The floating caliper is mounted on pins or slides so that when the piston extends and presses the inboard pad against the rotor, the whole body of the caliper slides on its pins or guides in the opposite direction, bringing the outboard pad into contact with the rotor.

Here is the advantage of a floating caliper:

Floating calipers are smaller, lighter, and easier to package. They are also cheap, readily available, and easy to mount.
Floating calipers cool better as the fluid and piston are only on the inboard side of the rotor.
Floating calipers have fewer moving parts and seals to leak or wear out.
Floating caliper design more easily incorporates a cable-operated parking brake.

Operation of a Floating Caliper
When brakes are applied, piston on outboard side extends in direction of red arrow until outboard pad contacts rotor. At that point, caliper slides on pins in direction of blue arrows until inboard side of caliper presses inboard pad against rotor.

DEEMAXX Vented –rotors are made with radial cooling passages in them to act as an air pump to circulate air from the rotor center to the outside of the rotor.

Caliper pistons are normally sealed with an O-ring that has a square cross-section. This O-ring stretches when the brakes are applied and the piston extends towards the rotor. When the pedal is released, the O-ring relaxes and retracts the piston. Because the rotor and brake pad surfaces are flat and aligned it takes very little movement to obtain pad-to-rotor clearance.

Mounting
Apart from issues of clearance, protection from debris and damage, and ease of bleeding, there is no reason you can’t mount a brake caliper at any position around the circumference of the rotor. The 3 ‘o’ clock and 9 ‘o’ clock positions are the most common (i.e. in front of, or behind the rotor), as this affords good protection and allows the caliper to be mounted with the bleed-screw up (highly recommended), for ease of bleeding and a better proportional brake system. This is easily achieved with the DEEMAXX EXCLUSIVE 120 degree bleed screw mounting. Mounting at 6 ‘o’clock would almost certainly make the caliper far too vulnerable, rob ground clearance, and imparts no particular advantage. However, there’s no reason you can’t mount the caliper at 12 ‘o’ clock, provided you are prepared to remove the caliper (to orient the bleeder screw up) to bleed the brakes. If you do, be sure to insert a block of wood or other spacer in the caliper while bleeding.

Pads need to be clean, even, and have a high coefficient of friction against the rotor for maximum braking performance. Disc brake pads are available in an array of compounds – all claiming certain benefits. One thing that is consistent is that the pad’s compound will have a different coefficient of friction depending on whether it is cold or hot. As a general rule of thumb, the following compounds exhibit the following coefficients of friction:

Organic – cold 0.44, hot 0.48
Semi-metallic – cold 0.38, hot 0.40<
Metallic – cold 0.25, hot 0.35
Ceramic– cold 0.38, hot 0.45

The higher the coefficient of friction, the “softer” the pad is said to be. Keep in mind that there are other factors to consider when selecting pads, such as noise and wear. The softer the pad is, the more rapidly it will wear.

The first letter of the code, sometimes known as the “cold” code, represents the normal friction coefficient. This is defined as the average of four test data points measured at 200, 250, 300 and 400 degrees Fahrenheit.

The second letter of the code, sometimes known as the “hot” code, represents the hot friction coefficient based on a fade and recovery test. Recovery is basically the period where the brakes are gradually cooling off. The hot friction coefficient is defined as the average of multiple data points: 450, 500, 550, 600 and 650°F on the first recovery cycle of the pad; and 500, 400, and 300°F on the second recovery cycle.

The ranges of friction coefficients assigned to each code letter are as follows: C = less than 0.15. D= 0.15 to 0.25. E= 0.25 to 0.35. F= 0.35 to 0.45. G= 0.45 to 0.55, and H= over 0.55.

Each letter grade spans a range of coefficients, but the combination of the two letters and the order in which they come can be a useful indicator of pad performance as it demonstrates the change in coefficient of friction for that pad from cold to hot (and vice versa). For example, an FE pad will grab better when cold (i.e. tends to fade when hot) whereas an EF pad would not grab well when cold (i.e. would need to be warmed up for max performance). Note that the coefficient of friction of steel on steel is 0.25, so EE pads grab only marginally better than no pads at all! FF pads are usually considered the minimum for a high-performance brake pad.

When new brake pads are installed, they should be “bedded-in”. Bedding-in brake pads is a process of breaking them in before severe use, similar to the way an engine or set of gears must be broken in.

To perform the “break-in”, follow the steps listed below:
Step 1: Make 10 stops from 30 mph (50 kph) down to about 10 mph (15 kph) using moderate braking pressure and allowing approximately 30 seconds between stops for cooling. Do not drag your pads during these stops. After the 10th stop, allow 15 minutes for your braking system to cool down.
Step 2: Make 5 consecutive stops from 50 mph (80 kph) down to 10 mph (15 kph). After the 5th stop, allow your braking system to cool for approximately 30 minutes.
This completes the “break-in” of the pads to the rotor surface. Full seating of new brake pads normally occurs within 1000 miles.

Wear Limit
The brakes effectiveness can be limited by worn components. If pads or rotors are worn excessively, or if any seal, tube, hose, bracket, or fitting is damaged, worn, or leaking, a reduction in performance, or worse, brake failure, can result. The likelihood of reaching the wear limit can be reduced by using different (higher quality) pads and/or rotors, or increasing brake cooling. Good inspection and maintenance practices can prevent reaching the wear limit.

“Brake fade” generally refers to any loss of braking caused by overheating. In fact, there are three very distinct forms of “brake fade”, and it is useful to distinguish between them, as their symptoms and solutions are entirely different.

Pad Fade
Pad Fade is caused by the temperature of the brakes exceeding the maximum temperature limit of the brake pad friction material. When the maximum temperature limit is reached (and even before, as it is approached), brake pads can expel gases when heated, gasses that act as a lubricant between pad and rotor. When pad fade occurs, the brakes will feel “normal” (high and firm) but there will be very little stopping power.

Fluid Boil
When the temperature of the caliper exceeds the boiling point of the brake fluid, tiny bubbles are formed in the brake fluid. As a result, the pedal goes soft and perhaps even to the floor as the fluid is no longer incompressible. Once this has happened, the fluid must be replaced. Over time brake fluid can absorb water vapor and the more water vapor in the fluid, the greater its susceptibility to fluid boil. Solutions to fluid boil include: flush and fill with new brake fluid, use a brake fluid with a higher boiling point, improve cooling of the caliper.