How drum brakes work

Know How Then & Now: How Drum Brakes Work

It is amazing how far automotive technology has come in the last few years. There are cars on the market today that can actually help steer the car down the road and stop on their own in order to avoid an accident. With all those advances some things have stayed almost the same. One example is the drum brake. Let’s take a a look at how drum brakes work see how far we have come…or not!

Let’s take a look at our project Model A and see what Ford considered “state of the art” back in 1930:

This is a clip from the 1930 Ford Model A brochure explaining the wonders of their braking system.

This is a clip from the 1930 Ford Model A brochure explaining the wonders of their braking system.

 

This is the actual front drum brake assembly from our project 1930 Model A Ford. A little worse for wear but they were still functional.

This is the actual front drum brake assembly from our project 1930 Model A Ford. A little worse for wear but they were still functional.

The brakes on a 1930 Ford Model A were actually fairly effective for the time. Having brakes on all four wheels was a big deal. They were also enclosed which was a major problem for exposed “band and drum” brake systems.

Now let’s take a look at typical modern drum brake assembly from a Ford truck:

This is the modern drum brake assembly that will be installed on our Model A project.

This is the modern drum brake assembly that will be installed on our Model A project.

They look fairly similar, don’t they? Both have brake shoes along the outside perimeter. Both have an adjuster mechanism. Both have an actuator that presses the shoes outward against the brake drum. Both use retaining springs to keep the brake shoes snug but not rigid inside the assembly. All of this is mounted to a round backing plate that sits behind the brake drum.

A modern brake drum mounted on the axle. All of the braking components are located behind the drum.

A modern brake drum mounted on the axle. All of the braking components are located behind the drum.

 

This is the brake drum from the 1930 Model A for comparison.

This is the brake drum from the 1930 Model A for comparison.

So how does all this work? The brake shoes and the brake drum are what actually do the stopping. The brake actuator pushes the brake shoes outward, which force the brake shoes to make contact with the inside of the brake drum. This create friction and eventually stops the rotation of the wheel. Once the brake pedal is released the springs retract the brake shoes and allow the wheel to rotate again. Drum brakes from 1930 work the same as those found in vehicles today, but there are a few differences.

Let’s start with the actuators. On the 1930 Model A the brake system was mechanically actuated. When the driver stepped on the brake pedal that force was transmitted through a series of rods and levers, all the way to the brake operating wedge.

The brake actuator assembly on the 1930 Model A. The V-shaped operating wedge is located in the middle behind the rollers.

The brake actuator assembly on the 1930 Model A. The V-shaped operating wedge is located in the middle behind the rollers.

The brake operating wedge pushes down against rollers that are mounted to the bottom of the brake shoes. As the rollers are moved apart by the wedge, the brake shoes are forced outward and into contact with the brake drum.

On a modern vehicle things work almost the same way but with a new technology thrown in: hydraulics. Today the force that is applied to the brake pedal is transmitted via pistons and hydraulic brake fluid through hollow brake lines. At the end of those brake lines are the wheel cylinders.

A modern drum brake actuator assembly. The hydraulic wheel cylinder is the barrel shaped part in the middle.

A modern drum brake actuator assembly. The hydraulic wheel cylinder is the barrel shaped part in the middle.

 

The pressurized hydraulic brake fluid enters the wheel cylinder, which is hollow. At each end of the hollow cylinder there is a a sealing cup, a piston, and a rubber boot. The pressurized hydraulic brake fluid forces the pistons at either end of the wheel cylinder to push outward. The outward movement is transmitted to the brake shoes, forcing them outward and into contact with the brake drum. Sound familiar?

Keeping the brake shoes the ideal distance from the brake drum is the job of the adjuster. The adjuster holds the brake shoes apart at the opposite end from the brake actuator. The brake adjuster allows for small adjustments of the brake shoe to brake drum distance. Ideally the brake shoes are adjusted to be as close as possible to the brake drum without actually touching it until the brake pedal is depressed.

This is the brake adjuster (top center) on the 1930 Model A. It is attached to the brake shoes via two adjusting shafts with pins.

This is the brake adjuster (top center) on the 1930 Model A. It is attached to the brake shoes via two adjusting shafts with pins.

The adjuster on the Model A uses a brake adjusting wedge (which looks like a cone-head bolt) that is threaded into the body of the adjuster from behind the backing plate. The cone touches the ends of the adjusting shafts inside the adjuster body. The further the adjusting wedge is threaded into the adjuster body, the further out the adjusting shafts will move. Moving the adjusting shafts outward decreases gap between the brake shoes and the brake drum. This adjustment must be manually performed as the brake shoe material wears down.

On a modern vehicle the idea is the same but the execution is slightly different. In some cases the adjustment is also automatic.

A modern drum brake adjuster (bottom cylinder), retaining spring (blue) and automatic adjuster lever (right side marked "L")

A modern drum brake adjuster (bottom cylinder), retaining spring (blue) and automatic adjuster lever (right side marked “L”)

The adjuster is is made up of a threaded screw that fits inside a cylinder. Turning the screw adjusts the distance between the brake shoes and the brake drum. The brass colored toothed wheel serves two functions. The first function is to allow adjustments to the brakes via a small window in the backing plate, all without removing the wheel. The second function is to work with the automatic adjuster lever . When the adjuster lever is activated (usually by applying the brakes in reverse or by using the parking brake) the adjuster lever moves the toothed wheel one notch, which moves the brake shoes apart by a fraction of an inch. Over time this action keeps the brake shoe to brake drum distance optimal.

Today most vehicles on the road use disc brakes on the front wheels with either drum or discs brakes on the rear. Drum brakes are perfectly fine for rear wheel braking duties since it is the front wheels that do the bulk of the braking while stopping. Drum brakes are fairly robust and usually inexpensive to maintain. Comparing a vintage drum brake system to a modern version is a great way to see how much things have changed, but also to see just how much they are still the same.

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Brian Medford

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