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Brake Information & Upgrades - Stop Technology

Posted in How To: Suspension Brakes on March 25, 2015
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Brakes convert kinetic energy of motion into heat to slow or bring our vehicles to a stop. More clearly, the mechanical movement of your brake pedal actuates the hydraulic brake master cylinder, pushing fluid to calipers or brake cylinders at each wheel to apply friction and slow the turning wheels.

The master cylinder itself may be constructed from either cast iron or aluminum. It bolts to the booster or firewall via a flange. A master cylinder has two top ports where the brake fluid enters from the attached reservoir. There are two outlet ports on the side: one for each half of a split system.

The brake pedal pivots near the firewall and uses leverage to apply a strong mechanical force to the brake master cylinder piston. This is the first mechanical advantage in the brake system. The goal is to build tremendous pressure at the brake calipers or wheel cylinders without requiring too much legwork. A vacuum assist (sometimes hydraulic) booster is typically used to further multiply the force available using engine vacuum as an assisting force.

Drum Brakes
Drum brakes are fast disappearing from new vehicles, but many older vehicles had drums on the rear and sometimes on the front. A drum brake system consists of four major components on the axle: the supporting backing plate, hydraulic wheel cylinder, brake shoes, and the brake drum itself.

If you spend a lot of time in mud and water, a rear disc brake conversion may be high on your mods list. Disc brakes do not retain water as do drum brakes, and dry much quicker. Fine gravel and sand can also get stuck inside drums and wreak havoc on the brake shoes. For many vehicles, it’s possible to convert from rear drum brakes to rear discs using kits that are available.

There is one advantage of drum brakes over disc brakes. Self-energizing drum brakes are actually designed to pull the drum brake shoe into the drum when the brakes are applied. This offers the driver further mechanical advantage. Power assist master cylinders are largely used for disc brakes that don’t offer this type of mechanical assistance.

Aside from leaking wheel cylinders, drum brake performance can suffer from worn shoes or drums that have been worn to an out-of-spec inside diameter. Chattering or surging may be due to drums that are old and have been worn out-of-round. Sticking or dragging rear brakes, or brakes that seem to get excessively hot may suffer from sticky wheel cylinders or broken/damaged retracting springs. Modern drum brakes are self-adjusting through a star-wheel screw mechanism at one end of the brake shoes. However, over time, the mechanism may start to stick and cease its adjustment function.

On a drum brake, the wheel cylinder mounted to the brake backing plate converts the hydraulic line pressure from the master cylinder into a mechanical movement and force onto the brake shoes in each drum assembly. There are several styles of wheel cylinders, with some pushing directly on the brake shoes and some using pushrods to transfer the force to the shoes. Often, only one wheel cylinder is used per wheel, but some designs will use two wheel cylinders, one for each brake shoe per wheel.

Disc Brakes
Disc brakes were first tried on vehicles over 100 years ago with limited success, then started to appear on post-WWII cars as a result of designs that were used on airplanes. Discs then replaced many front drum brakes in the ‘70s, and in the last decade or so discs have become common on rear axles.

Disc brakes offer several advantages over drum brakes. Since the friction surface (rotor) is exposed, disc brakes cool much quicker than drum brakes. Disc brakes are usually lighter in weight, easier to service, and brake more consistently, so are less likely to pull to one side as is sometimes a problem with drum brakes. Disc brake calipers are also inherently self-adjusting by design.

There are three varieties of disc brake calipers: fixed calipers, floating calipers, and sliding calipers. A fixed caliper has one or more pistons, on each side of the rotor. The caliper is solidly bolted to the steering knuckle or other caliper mount. Pistons move equally from each side of the rotor to apply pressure directly on both sides of the rotor.

In contrast, a floating caliper has a single large piston (or two smaller ones) located on the inboard side of the caliper. Since force must be applied equally to both sides of the rotor, the floating caliper must be able to actuate both pads. The caliper is bolted to the mount with special pins, and the caliper slides back and forth on sleeves or bushings. The inboard pad sits directly on the piston. The outboard pad sits within the caliper frame. When the brakes are applied, the piston moves outward pushing the inboard pad into the rotor. Simultaneously, the force exerted backward from the piston onto the caliper frame causes the frame to move inboard, bringing the outboard pad into contact with the rotor. In this way, both pads press on the rotor.

A sliding caliper operates in much the same way as a floating caliper. Instead of using a set of pins, a sliding caliper rides in a set of machined guides. The guides allow for the lateral movement of the caliper.

Disc Brake Upgrades
Since disc brakes offer significant advantages over drum brakes, upgrade swaps to discs are common. Older vehicles that came from the factory with drum front brakes can often be upgraded using newer-model factory disc components. Rear drum brakes can often be changed over to disc replacements using aftermarket kits. If you already have discs and want increased braking to stop larger tires, the aftermarket offers higher-quality brake pads, vented or drilled rotors for improved cooling, or larger replacement rotors and calipers to increase brake surface area and stopping power.

Bigger axles with bigger brakes require more fluid movement, usually provided by a larger bore master cylinder. On the downside, any increase in master cylinder bore diminishes your mechanical advantage by an amount equal to the gained fluid flow. The fluid flow and mechanical advantage offered are directly dependent on the bore size (surface area) of the master cylinder piston.

Disc Brake Troubleshooting
Disc brake calipers can leak, requiring rebuild or replacement. Another failure mode of disc calipers occurs when a sticking caliper piston does not retract a bit once the brake pedal is released. This leaves one or both pads in contact with the rotor where they continue to drag on the rotor. This results in excessive rotor heat and premature pad wear. Uneven pad wear may also be the result of a caliper that is not floating freely on its pins or slides. When this occurs, most of the braking force is coming from just the piston side of the caliper. The other pad is doing little work, and braking performance suffers. Cleaning and lubricating the pins or slides should cure the problem. Uneven pad wear on a fixed caliper could also signal that a piston is sticking in its bore.

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Master cylinder Upgrades
Swapping to larger axles and/or brakes can be a good way to increase your stopping power when building a rig. However, the hydraulic components of a braking system need to be reasonably matched to offer proper performance. Larger calipers or wheel cylinders require greater fluid flow for the same pad travel at the wheel. It’s often necessary to swap the master cylinder for one with a larger bore to mate with the larger calipers or cylinders. Otherwise, the smaller bore master will require too long of a pedal stroke to fully engage the larger brakes.

On the downside, any increase in master cylinder bore diminishes your mechanical advantage by an amount equal to the gained fluid flow. The fluid flow and mechanical advantage offered are directly dependent on the bore size (surface area) of the master cylinder piston. When deciding on a new master cylinder bore size, consider the increase in surface area of the calipers or cylinders, and match the master cylinder increase accordingly. This will help ensure you don’t have too long of pedal travel, or too short of travel and a great loss of mechanical advantage.

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