Big Brake Upgrades
|BIG BRAKE UPGRADES|
THE BIG BRAKE APPLICATION
|When it comes to the overall customization of your vehicle, brake kits are one of the most important and also the most overlooked of all the upgrades to be made. Sure, they look great when they match your new wheels, but they do so much more like stopping your vehicle in all sorts of conditions.
When one makes a brake upgrade, they usually choose to go with a bigger brake disc, a caliper with increased clamping force, as well as either vented or slotted brake discs. But why do that? The following will explain the benefits the benefits of making such modifications to your vehicles brake system.
BIG BRAKE BENEFITS
|Most current cars are able to make a single stop from maximum velocity. The braking systems on the majority of cars, sports cars, and light truck are not designed for hard driving and towing. Stock brakes do not usually have the ability to absorb and transfer the heat by conduction, convection, and radiation into the air during driving that is needed. Stock calipers and their mountings are usually not stiff enough at higher line pressures and the resultant higher clamping loads either.
Your stock brakes have the torque required to lock the front wheels at highways speeds, caliper flex at the increased system pressure required to stop the car from high velocity may prevent wheel lock up. Stock brake pads are not built for extreme use since new car buyers are really looking for cold stopping ability as well as silent operation.
HEAT AND ROTATIONAL MASS
|Here are some basic facts that must always be kept in mind when you think about brake systems:
Tires stop the vehicle, not the brakes.
The brakes simply slow the rate of rotation of the wheels and the tires. Meaning that braking distance is dependant on the stopping ability of your tires, which may not be the ones, designed for your car if you have already made the move to aftermarket wheels.
Brakes work by converting the kinetic energy of the car into thermal energy during the car?s deceleration. This produces an excessive amount of heat, which is then released into the surroundings as steam.
When you think about the heat produced, you have to about it in the context of the brake system?s rate of work, or power. One side of a front brake assembly produces power of about 172 Hp to stop a 3500lb. vehicle going 100 mph in 8 seconds. The disc will dissipate about 80 percent of the energy produced by this stop. The rate of heat transfer among the three mechanisms is totally dependant on the temperature of the system itself. The biggest difference is the ever increasing contribution of radiation as the temperature of the disc increases.
The amount the conductive mechanism contributes is dependant on the mass of the disc and the attachment designs, with the discs used for race cars having a typically mass and being fixed by mechanism that are restrictive to conduction. At 1000 degrees F, the ratios on a racing two-piece annular disc design are usually 10 percent conductive, 45 percent convective, and 45 percent radiation. On a high grade street one-piece design, the ratios are as follows; 25 percent conductive, 25 percent convective, and 50 percent radiation.
MORE THAN JUST BRAKES
|Braking performance is more than just brakes. Tires, suspension, and driving techniques play major roles in making a brake system truly effective. For maximum brake potential, vehicles can benefit from proper corner weight balance, a lower center of gravity, a longer wheel base, more rear weight , and an increased aerodynamic down force at the vehicle?s rear.
The following is a glossary of key terms you may need to know when it comes to knowing the physics of braking.
Mechanical Pedal Ratio
Since it is impossible to step directly onto the brake master cylinder hard enough to stop the vehicle, the pedal you do step on is built to multiply your efforts. The distance from the pedal?s pivot point to the effective center of the footpad is divided by the distance from the pivot point to the master cylinder push rod is the mechanical pedal ratio. You?ll find that the ratios range from 4:1 to 9:1.
Brake Line Pressure
This is the hydraulic force that starts the braking system when the pedal is pushed. It is measured in pounds per square inch, which broken down is the force applied to the break pedal in pounds multiplied by the pedal ratio divided by the area of the master cylinder in square inches. This calculation implies that the smaller the master cylinder, the higher the brake line pressure. A typical brake line has a pressure during a stop range from under 800 psi in normal conditions to nearly 2000 psi at maximum effort.
The clamping force of a caliper is the force exerted on the disc by the caliper pistons. Measured in pounds, it is the brake line pressure, measured in psi, multiplied by the total area of the piston in square inches. This applies to both cases of the caliper being either fixed or floating. If you choose to increase the pad area, you will not gain a larger clamping force.
This is the most discussed aspect when it comes braking. Braking torque is measured in the units of pounds-feet. To calculate braking torque on a single wheel, take the effective disc radius in inches multiplied by the clamping force multiplied by the coefficient of friction of the pad against the disc. That figure is then taken and divided by 12. The maximum braking torque on a single front wheel will normally exceed the entire torque output of an engine.
Drilled vs. Slotted Rotors
For years, drilled rotors were widely used in the racing world. The reasons for this were two-fold; the edges of the holes gave the brakes more bite and it also gave the fire band boundary layer of gasses and particulate matter a path to escape.
There was a downside to drilled rotors that they later discovered. The holes would reduce the thermal capacity of the discs and they served as a very effective way to elevate the stress on a disc leading to lower disc life. The evolution of friction materials have made drilled rotors obsolete. Most racing rotors now are slotted, because the series of tangential slots serve the same purposes as the drilled rotors had without any of the disadvantages.
Pad Area Brake torque is important since it is proportional to the piston area, the coefficient of friction, the effective radii, and the system pressure. But it doesn?t affect the pad area, so many think that pad area is irrelevant. Here are a few points about pad area and its geometry that make it important.
The life of the pad. Since the pad?s material is consumed over time, an increase in the pad area will result ultimately in a longer life of the pad. Stock pads are designed to produce a quieter brake system, so they usually sacrifice pad area to accomplish that.
The dispersion and dissipation of heat over a larger surface and greater mass. In the case of a larger pad, where the pad masks a large part of the rotor face, which absorbs more of the radiant energy and shields the area from cooling that may cancel out any gain from doing so.
The geometry of the pad. The rubbing speed between the disc and the pad is greater at the periphery of the disc; the pad?s geometry will sometimes reduce the area towards the center of the disc. This done because of the effort to produce an even distribution of temperature and pressure across the pad?s face.
When metal is heated, it expands. The diameter of cast iron will increase up to 2 mm at high temperatures. When the disc is restrained from expanding, the friction plates are forced into a coned shape as the temperature rises, which in turn effects the distribution of temperature and pressure within the pads and the feel of the pedal.
High-performance discs are usually mounted on separate hats that are typically made of aluminum. Their fastening system is built to allow radial growth and to minimize the axial float resulting from a mechanically stable system. The hats should be constructed from 7075 or 2024 heat-treated aluminum billets that are pre-stressed and relieved, not from the 6061.