Rolling down the road, a truck's tires generate significant inertia when the weighty rolling stock get to spinning. To bring the vehicle down from speed, a brake system is employed. In this article we'll review some of the technical aspects of disc brake systems, along with select upgrades and troubleshooting considerations.
For the last 30 years or so, 4WD vehicles have been equipped with front disc brakes. The front discs are often complemented by rear drum brakes, but increasing numbers of new vehicles are factory-equipped with the superior four-wheel disc system.
Disc brakes offer several advantages over drum brakes. Since the friction surface (rotor) is exposed, disc brakes dissipate heat more efficiently than drum brakes. There is also no problem with the brake's friction surface deflecting, as there is a pad pushing on each side of the rotor. A drum brake can deflect under extreme pressure, delivering less braking force and erratic stopping. Disc brakes do not retain water as do drum brakes. Disc brakes are also lighter in weight, easier to service, and brake more consistently than drum brakes.
There is one disadvantage to disc brakes when compared with drum brakes: Self-energizing drum brakes are actually designed to pull the drum's brake shoes into the drum when the brake system is activated. This offers the driver's foot a certain degree of mechanical advantage over the brake's function. Disc brake pads are flat and run parallel to the rotor's surface, so there is no self-energizing action with disc brakes. To compensate for the lack of mechanical leverage, large calipers and a bigger master-cylinder booster are designed into the system.
There are three varieties of disc brake calipers: fixed calipers, floating calipers, and sliding calipers. A fixed caliper has a piston, or 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. Most fixed calipers are assembled from two castings that are bolted together. Passages within the two halves allow brake fluid to flow to pistons on both sides. This style of caliper often offers the best braking performance, but is also a more expensive option due to the greater complexity and higher parts count.
In contrast, a floating caliper has a single large piston (or two smaller pistons in some cases) located on the inboard side of the caliper. Since force must be applied equally to both sides of the rotor, the floating caliper must able to actuate a brake pad on both the inner and outer rotor surface. 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 backwards 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. One advantage of a floating caliper is that the moving caliper design can better tolerate the movement of a warped rotor than a fixed caliper.
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 calipers are inherently self-adjusting by design. Each piston is built to fit with close tolerance to its bore. An O-ring seal and dust boot are also fitted to each piston. The action of the O-ring is such that it allows the piston to slide through it as the pads wear so that the brake pedal remains firm throughout the life of the pads. However, the O-ring also provides a slight retraction movement to keep the pads pulled just off the surface of the rotor. In this way, the seal is constantly keeping the caliper adjusted as the pads wear.
Disc Brake Upgrades
Front disc/rear drum systems offer better braking performance than four-wheel drum systems, and disc/disc performance surpasses that of disc/drum systems. When starting with an older vehicle that came from the factory with drum front brakes, you can often swap to more modern factory disc omponents. You can do the research and hit the junkyard and parts store to assemble all the OEM parts, or you can order a complete kit from an aftermarket supplier.
The friction produced when the pads contact the rotor is what slows down those big tires you're spinning. However, all that rotational energy gets converted into heat. Front disc upgrade kits are available that offer higher-performance calipers and larger specialty rotors. Increases in caliper piston area, pad area, and rotor size all translate to greater stopping power. Other rotor features, such as vented rotors or drilled rotors, can also help to dissipate heat faster so that repeated, frequent stops remain firm and consistent.
More often than not, rear brakes on a 4WD come from the factory in drum form. While many modern, self-energizing drum brakes are quite efficient, rear disc brakes remain desirable for several reasons. Rear disc brakes require no adjustment, are more consistent, and are much more immune to mud and sand retention problems compared with drum brakes. For many vehicles, it is possible to convert from rear drum brakes to rear discs. Kits are available for a wide variety of applications and very often allow use of calipers that retain the stock emergency brake function.
Disc Brake Troubleshooting
Like any hydraulic-fluid component, a caliper may eventually develop a fluid leak at the piston seals. This can result in a visible loss of brake fluid and the ingress of air into the caliper.
A common problem with aging floating calipers occurs when the inboard pad wears significantly faster than the outboard pad. This typically signals that the caliper 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.
You may also experience uneven wear on a fixed caliper. This is generally due to a piston sticking in its bore. This can prevent the piston from fully retracting into the caliper, causing the pad to drag on the rotor and resulting in premature pad wear.
Disc Brake Maintenance and Repair
Aside from the normal wear and replacement of brake pads, there is not much maintenance needed for brake calipers. You should periodically check the pads for even wear and pad life, leaks, and hose condition.
Should a caliper develop a leak, it may be possible to rebuild it by replacing the internal seals, or you can purchase a new or rebuilt one. Whenever you rebuild a caliper, it may be necessary to lightly hone the bore to remove any residual buildup and provide a fresh, even surface on which the piston seals can slide.
While servicing your disc brakes, the rotor surface should also be inspected. Deep grooves worn into the rotors will require you to remove them and have an automotive machine shop turn them smooth again. Also, rotors warped from excessive heat or over-tightening of lug nuts may cause erratic disc-brake performance. Disc brakes are reasonably easy to understand, troubleshoot, and repair. This article should have provided a good overview of their function. Good brakes make for reliable stops and peace of mind.