We discuss how drum brake works along with their parts in simple terms. Drum brakes are found in most motorcycles in India…
A brake plays an important role in bringing a moving vehicle to halt. It is just opposite of a clutch. A brake is applied to rotating axle to stop the vehicle. In a two wheeler you find two types of brakes- Drum brake & Disc Brake.
Drum Brake: Drum brakes are widely used at the rear wheel. In India and many emerging nations we still have few motorcycles with drum brakes on both front and rear wheel. A drum brake uses brake shoes or friction pads to create braking force.
A drum brake assembly contains brake shoes or friction pads, brake pedal or lever, springs, brake cable, brake drum, brake arm, brake cam, Dust seal, Anchor pin washer, brake panel.
Brake shoes: The shape of brake shoe is like crescent moon. Brake shoes are made of two pieces of sheet steel welded together. The friction material which is called brake lining is attached to the lining table either by adhesive bonding or by riveting. There are holes and slots on this brake shoe for return spring, hold down hardware and self adjusting components.
The brake lining is a consumable material used to create friction. It plays an important role in entire drum brake assembly. The brake lining must be resistant against heat and wear and should have high friction coefficient. This coefficient must be as unaffected as possible by fluctuations in temperature and humidity to work the brake effectively.
Materials which make up the brake shoe include friction modifiers, powdered metal, binders and fillers. Friction modifiers such as graphite and cashew nut shells are used to alter friction coefficient. Powdered materials such as lead, zinc and brass etc increase a material’s resistance to heat fade. Fillers are added to friction material in small quantities to accomplish specific task, such as rubber chips to reduce brake noise.
Each brake assembly has two shoes, a primary and secondary. The primary shoe is located towards the front of the vehicle and has the lining positioned differently than secondary shoe. The much needed care to be taken while putting primary and secondary shoe. The primary shoe comes on left side while secondary shoe comes on right side.
Break Pedal or lever: It is that component of brake assembly through which input to brake is given. The front brake is operated through a lever whereas the rear brake is operated through a pedal.
Brake Drum: It is made of special type of cast iron. It is positioned very close to the brake shoe without actually touching it, and rotates with the wheel and axle. When the brake lining touches the drum it generates friction heat which sometimes goes up to 600F (Fahrenheit).
A brake drum must be highly heat conductive, sufficiently rigid, lightweight and resistant against wear. Apart from this it must be accurately balanced.
The gap between brake lining and drum inner surfaces is to be maintained accurately all the time. In some type of brakes this is done automatically while some brakes require to be adjusted periodically. If the gap is too large it will cause a delay in braking and loose the brake pedal. If the gap is too small it will increase the wear of brake lining. Sometimes it may also make the rear wheel to lock up.
Brake Cable: As seen in clutch, the brake cable is also the medium to pass the input given by brake lever or pedal. The rear brake sometime uses shaft system as the medium of passing the input.
Springs: The coil type springs used as return springs when the brake is actuated by pressing lever or handle this spring expands and when you release the lever/pedal the springs contract and bring the brake shoe to their original position.
Brake cam: Brake cam works like a camshaft. When the brake lever or pedal is pressed it will move which will expand the brake shoes. The one side of brake cam is positioned between front and rear brake shoes gap and other side has teeth which fit on brake arm teeth.
Brake arm: It is the part which pushes brake cam to expand and contract brake shoes. It has teeth inside which fit on brake cam teeth.
Dust seal: A dust seal is used to prevent any kind of contamination inside the brake assembly.
Brake panel: It is located outside the brake drum. It acts like cover of brake drum.
Anchor pin washer: The springs are mounted on anchor pin washer.
How Drum Brake Works?
Now let’s look how it works. In a normal condition the brake drum rotates with the wheel but as the rider presses lever or pedal. The cable pushes the brake arm which creates movement in brake cam.
Due to movement in brake cam the return springs mounted with brake shoes expand. The expansion in springs closes the gap between brake shoes and the drum. It creates friction which stops the drum from moving. As the drum is fitted into the wheel rim the wheel also stops. When the rider releases lever or pedal, brake cam returns to the original position which contracts the springs.
The contractions in springs lead the brake shoes to their original position. Now the wheel can move freely. This is the how the drum brake works.
In India we mostly find 130 mm drum brake at the front and 110 mm brakes at the rear. They are widely used due to their simple mechanism and lower costing.
Advantages of Drum brake:
- It is cheaper compared to disk brake.
- It is simpler to operate.
- It is fairly widely used
- It is more effective in rainy condition since the brake shoes are inside the drum.
Disadvantages of Drum brake:
- It is prone to overheating which leads to break fade and wear outs
- It requires more manual effort compared to disc brake
- It needs to be adjusted frequently
- It is less effective in excessive heat compared to disk brake
Since the front brake is more effective in stopping the vehicle compared to rear brake, we find disc brakes on front wheel and drum brakes at the rear wheel or two disc brakes at the front and one disc brake at the rear.
– Mahavir Kothari