MASCOT March - April 2005

How Does it Work? - No 12 - The Brakes
Part 1 - Mechanically Operated Systems

The Tool Chest

The majority of vehicles up to the 1960s have rotating drum type brakes, where operation of the brake pedal causes friction material on the brake shoes to be forced into contact with the drums, thus converting the energy in the rotating drum into heat and slowing down or stopping the vehicle. From the mid 1960s, disc type brakes were introduced, first on the front brakes only, but later on all four wheels.

The internal expanding shoe brake is the most common on historic vehicles, see Fig 1. Two `Tee' section shoes are anchored to a back-plate, which is itself bolted to the axle casing or stub axle carrier. Friction linings are riveted or bonded to each shoe, and these are expanded to contact the rotating steel/cast iron drum. From Fig 2 you can see that the friction between shoe and drum will push one of the shoes harder into contact with the drum, whilst the other is pushed back

against the cam towards the off position. This self-application action is called the `Self-Servo Effect', and the affected shoe is known as the `Leading Shoe', being the first shoe after the expander (in the direction of rotation). The other shoe, known as the `Trailing Shoe', only contributes about a quarter of the braking effort because the drum rotation is acting against it. When travelling in reverse, the Trailing shoe becomes the Leading shoe and vice-versa.

The two main types of brake operating system on motor cars are Mechanical, ie, Rods and Cables, and Hydraulic.

A cam or wedge (cone) is used to expand the shoes in the majority of mechanical systems. Fig I shows a simple cam type of expander. A return spring is fitted between the shoes to pull them away from the drums after application of the brakes. A series of rods, or cables, and levers connect the cams to the brake pedal. A simple rod layout is at Fig 3. The foot and handbrake controls are connected to a cross-shaft which rotates in bearings mounted on the chassis. The connecting rods have elongated holes at their cross-shaft ends that enable each to be operated individually. Four adjustable rods connect the cross-shaft to the brake cam operating levers. Care is needed when adjusting these rods to ensure the same amount of effort is applied to each brake otherwise the brakes will pull to one side. This is best achieved by placing the vehicle on axle stands and having someone press the pedal so that the resistance at each wheel can be felt, the rods then being adjusted until the resistance at each wheel is as near the same as possible. The pedal should then be applied harder to check that all the wheels lock up fully, and that when released, all rotate freely without binding.

To obviate the need for such meticulous adjustment, a compensator device was introduced see Fig 4. The rods to the rear, or front, brakes are connected to each end of a floating link which has a centre pivot connecting it to the cross-shaft. If one pair of brake shoes contacts the drum before the other, the link will pivot until the other brake shoes make contact so that further pressure on the pedal will be applied equally to each wheel.

As they provide most of the braking effort, the linings of the leading shoes wear more rapidly than those on the trailing shoes. The resulting greater shoe-drum clearance at the leading shoe means that the cam is applying the major share of the pedal force to the trailing shoe, with consequent loss of efficiency. To overcome this problem, a floating expander unit was introduced, but whilst this improves the braking effort, it does result in much higher wear on the leading shoe.

A typical Girling floating expander brake is at Fig 5. The cone and roller expander unit is free to move a small amount radially on the back-plate to allow the shoes to centralize themselves. To ensure constant leverage and minimum travel of the expander, an adjuster is fitted in place of the shoe anchor. This consists of a screwed wedge and two plungers. Flats machined on the wedge act as a locking device and give some feel during adjustment to enable the correct shoe clearance.

The disadvantages of these early arrangements can be overcome by making both shoes in the brake into Leading Shoes. A diagram of a two leading shoe, mechanically operated brake, is at Fig 6. Fixed expander and adjuster housings provide the shoe anchors. Operation of the expander forces out the tip of the first shoe and the top and bottom of the second shoe. Forward rotation of the drum causes the second shoe to rotate slightly until the shoe contacts expander housing, in which position both shoes are `leading'. When the drum is rotated in the reverse direction, the first shoe becomes a trailing

shoe, and the second shoe becomes the leading shoe. The servo action is less, per shoe, than that acting on the leading shoe on the earlier systems, but the two shoes together give a more powerful, progressive and stable brake. I don't know of any Singers that had this type of brake - do you?

In Part 2 we will look at hydraulically operated brakes.

Editor's note: Roadsters have hydro-mechanical brakes, the front being hydraulic and the rear mechanical.