Brake system

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Components[edit]

Pedal[edit]

The brake pedal is the first part of the brake system, and is simply a lever that multiplies the force input by the driver's foot. The pedal ratio is the ratio of distance between the driver's foot and the pivot, and the distance between the pivot and the point where the pedal pushes on the master cylinders. A larger pedal ratio will enable the driver to apply more force to the brakes, but will also increase pedal travel. A good pedal ratio will be a middle ground between developing the necessary braking forces and using a reasonable amount of travel to do so. Lastly, the pedal must be strong enough to withstand the forces on it.

Master Cylinders[edit]

The master cylinder is essentially a piston pushing on a fluid-filled region. It converts the force input to it by the pedal to pressure in the brake lines. The pressure developed in the brake lines increases with increasing force (obviously), and with decreasing master cylinder bore (a bit counterintuitive).

Calipers[edit]

The brake calipers convert pressure in the brake lines into a clamping force. The force created by a caliper increases with increasing pressure and increasing piston area. Calipers come in two types. In floating calipers, part of the caliper body moves when the brakes are applied, and in fixed calipers, the entire caliper is rigid and only the pistons move.

Rotors[edit]

The brake rotors are (indirectly) rigidly mounted to the wheels (actually to the hubs in front and the output shaft in the rear) and are what the calipers clamp on. The clamping force applied on the rotor creates a torque that slows down the rotor and therefore the wheel(s). This braking torque increases with increasing clamping force and increasing rotor diameter. The function of the rotor is essentially to convert the car's kinetic energy into heat. Rotors are usually cast iron and are often drilled or slotted for better cooling. However, this does make the rotors slightly weaker and more susceptible to damage.

Bias Bar[edit]

The bias bar distributes the force from the brake pedal between multiple master cylinders to split the brake power between the front and rear wheels (and/or left and right in a cutting brake design). Often a bias bar uses a whiffletree mechanism with an adjustable pivot point to change the percentage of pedal force applied to each master cylinder. In addition to adjusting the front-rear brake bias, the bar allows for flexibility different stroke from both the master cylinders. For example, if the front brakes lock first, the bias bar allows the master cylinder for the rear brakes to continue applying.

References[edit]

Use these references to learn more about brake system design:

Brake Bleeding[edit]

Bleeding the brakes is the process of removing air from the brake system. Since the brake lines are pressurized, air will compress if it is present; this leads to worse brake performance. To bleed the brakes:

1. Have someone sit in the car and pump the brakes 5-7 times, then keep the pedal pressed all the way in.

2. Open the bleeding nipple on the caliper. To avoid messes, attach flexible tubing to the bleeding nipple and let the fluid drain into a bottle.

3. Observe what comes out of the bleeding nipple, then close it. The person in the car can release the pedal when the nipple is closed.

4. Repeat steps 1-3 until no air bubbles escape from the caliper, only brake fluid.

5. Keep an eye on the level of brake fluid in the master cylinder or reservoir: refill it if it gets low. Do NOT let the master cylinder run dry, or air will get into the system and you're starting over.

6. Repeat steps 1-5 for both front calipers or both halves of the rear caliper (there are 2 bleeding nipples on the rear caliper, you need to bleed both sides).

Practical Advice[edit]

  • Do NOT EVER weld on parts that have brake fluid on them. This creates phosgene gas, which was used in World War I as a chemical weapon.
  • If possible, have external reservoirs on the master cylinders to make refilling and bleeding far easier.
  • As a general design philosophy, if everything else on the car breaks, the brakes should be the last thing still working.
  • Make SURE that the brake fluid you're using is compatible with the components you have. DOT 3 and 4 and 5.1 are interchangeable; DOT 5 is NOT. DOT 5 fluid should never be mixed with DOT 3/4/5.1 fluid or used in a system that has been exposed to those fluids.
  • DOT 3/4/5.1 brake fluid absorbs water from the atmosphere, which makes it perform worse. Consider using an unopened bottle of brake fluid at competitions.
  • When installing brake fittings, be careful not to strip or bend them. Fittings aren't as strong as they look.
  • Always use Teflon tape when installing brake fittings, and wrap the tape so it tightens as the fitting is tightened.
  • The mounting bolts for the front calipers (Honda TRX300EX) are 12mm, don't use a standard wrench.

Troubleshooting[edit]

If your brakes break, try the following (in order):

1. Bleed the brakes.

2. Check for leaks in the brake lines.

3. Check the brake pads and spacing of pads from the rotors.