Modern automotive disc brake designs share common features. The primary components are a rotor and caliper. The rotor attaches to the wheel hub and rotates at the same speed as the wheel and tire assembly. Machined flat and true, the rotor's parallel faces are the braking surfaces for lined brake pads.
Whether your truck is a stock restoration project or a modern street truck build, performance and safety always include brakes and steering. Last month, we covered drum brakes and the hydraulic system. In this month's lesson, we'll explore disc brake basics and upgrades.
The caliper is a clamping mechanism that straddles the rotor and squeezes the two brake pads inward toward the rotor when the brakes apply. On a single-piston caliper, the piston presses outward against one brake pad back. This transfers force across the bridge of the caliper and pulls the opposite pad toward the rotor. At this stage, both pads squeeze against the parallel rotor faces. Pad friction slows and stops the rotor.
Single-piston calipers are...
Single-piston calipers are popular original equipment. Older calipers can be pitted, worn, or scored. Vehicle manufacturers recommend discarding a housing that requires more than light touchup with crocus cloth.
Some calipers have multiple pistons. On some designs, the pistons push toward the rotor faces from opposite sides. As a rule, high-performance calipers have multiple, opposing pistons. Popular racing calipers have as many as six pistons.
For the modern OEM or custom disc brake system, stopping power equates to the clamping force of the caliper, the diameter of the rotor, the surface area of the pad, and the pad and rotor material. There is no self-energizing effect* with modern disc brakes, just clamping pressure against the rotor. Power boosters are common on disc brake systems.
*1949-54 Chrysler-Crown Imperial models used twin-disc, internal expanding disc brakes. This Ausco-Lambert design featured self adjusters and ramping ball bearings between the discs. The spreading, self-energizing action of the ball bearings increased braking force-without additional driver input. Chrysler tested these brakes on Dodge military Power Wagons.
By 1967, disc front brakes...
By 1967, disc front brakes were original equipment on most domestic passenger cars. A majority of the braking effort is at the front brakes. Rear drum brakes reduce production cost and solve the issue of how to incorporate an emergency brake. Disc front with drum rear brakes became a standard.
When the brakes apply, a vehicle pitches forward. Depending upon suspension, speed, height, and chassis weight distribution front to rear, a brake "bias" exists. Bias relates to the transfer of weight; all vehicles pitch forward under braking. In pickup trucks, where manufacturers grapple with the dynamics of a tall vehicle that must carry a load, brake bias is a challenging issue.
An engineer sizes and matches up brakes to bring a loaded vehicle safely to a stop. The pickup truck must have brakes capable of safely stopping a load at the vehicle's Gross Vehicle Weight Rating (GVWR). Yes, classic trucks were once beasts of burden.
Now let's unload the truck. If that same vehicle must make a sudden stop, and if brake design and bias are for a vehicle with much more weight over the rear axle, we now have a problem; the percentage of front weight is greater, which increases the pitch forward. The frame lifts at the rear of the truck, lightening weight on the rear tires. Rear-wheel lockup and skidding can occur, risking the loss of vehicle control.
The goal is to balance out the braking under a variety of driving conditions and loads. Can the brake hydraulic system adjust for weight and load transfer? Yes, hydraulic controls can compensate for weight bias changes. Trucks have benefitted from devices like a proportioning valve at the rear axle. Here, the hydraulic valve has mechanical levers that fit between the frame and axle. As the light load causes the rear frame to lift under hard braking, the levers move the valve. The valve reduces brake apply pressure to help avoid wheel lockup.
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