Drum Brakes
CHAPTER-2
DRUM BRAKES
2.1 Introduction:
Drum brakes were the first types of brakes used on motor vehicles. Nowadays, over
100 years after the first usage, drum brakes are still used on the rear wheels of most vehicles.
The drum brake is used widely as the rear brake particularly for small car and motorcycle.
The leading-trailing shoe design is used extensively as rear brake on passenger cars and light
weight pickup trucks. Most of the front-wheel-drive vehicles use rear leading-trailing shoe
brakes. Such design provided low sensitivity to lining friction changes and has stable torque
production (Limpert, 1999). Drum brake is composed of mobile and immobile elements.
Immobile elements are via backing plate attached to the supporting structure of the vehicle,
while moving parts (drum) are connected to wheel hub. Friction elements of drum brakes are
two symmetrically placed brake shoes and drum.
Front drum brakes may be of either design in practice, but the twin leading design is more
effective This design uses two actuating cylinders arranged so that both shoes use the self-
applying characteristic when the vehicle is moving forwards. The brake shoes pivot at
opposite points to each other his gives the maximum possible braking when moving
forwards, but is not so effective when the vehicle is traveling in reverse he optimum
arrangement of twin leading front brakes with leading/trailing brakes on the rear allows more
braking force at the front of the vehicle when it is moving forwards, with less at the rear. This
helps prevent the rear wheels from locking up, but still provides adequate braking at the rear
2.2 History of Drum Brakes:
Brake drum was invented by Louis Renault in 1902. He used woven asbestos lining
for the brake drum lining as no alternative dissipated heat like the asbestos lining, though
Maybach has used a less sophisticated brake drum. In the first brake drums, levers and rods
or cables operated the shoes mechanically. From the mid-1930’s, oil pressure in a small
wheel cylinder and pistons operated the brakes, though small vehicles continued with purely
mechanical systems for decades. Some designs have two wheel cylinders.
2.3 Working Principle:
The brake shoe is allowed to expand in both the direction. As the
brake lining of shoe touches the inner surface of the drum, friction is generated in between
the brake shoe and drum and stops the vehicle from moving.
2.4 Working of Drum Brakes:
1 - Baking plate, 2 - Drum, 3 – Brake shoe, 4 – Shoe lining, 5 - Rivet, 6 –
Brake adjuster, 7 - Elements for holding the shoes, 8 - Shorter return
spring, 9 - Longer return spring, 10 – Lever mechanism of the parking
brake 11 - The return spring
Assembly and Components of Drum Brake
1. As the brake pedal is pressed, it compresses the fluid in the master cylinder and allows the
piston of the wheel cylinder to expand outward.
2. The outward motion of the piston of wheel cylinder forces the brake shoe outward against
the brake drum.
3. As the brake shoe lining touches the inner surface of the drum, and due to the friction
generated in between the brake shoe and drum, the motion of the wheel reduces and vehicle
stops.
4. As the force is removed from the brake pedal, the retracting springs draws the brake shoe
inward and the contact between the friction lining and drum ended. Now again the brake is
ready to apply.
5. A self adjusting screw is present at the bottom, which is used to maintain a minimum gap
between the drum and brake shoe. When the lining of the brake shoe is wear out than the gap
between the drum and brake shoe increases, at that time the adjuster is adjusted again to
maintain the minimum gap.
2.5 Features of Drum Brakes:
Drum brakes are used in most heavy-duty trucks, some medium and light duty trucks,
and few cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels
since most of the stopping force is generated by the front brakes of the vehicle and
therefore the heat generated in the rear is significantly less. Drum brakes allow simple
incorporation of a parking brake.
Drum brakes are also occasionally fitted as the parking (and emergency) brake even
when the rear wheels use disc brakes as the main brakes. Many rear disc braking
systems use a parking brake in which the piston in the calliper is actuated by a cam or
screw. This compresses the pads against the rotor. However, this type of system
becomes much more complicated when the rear disc brakes use fixed, multi-piston
callipers. In this situation, a small drum is usually fitted within or as part of the brake
disc. This type of brake is also known as a banksia brake.
Disc brakes rely on pliability of calliper seals and slight runout to release pads,
leading to drag, fuel mileage loss, and disc scoring. Drum brake return springs give
more positive action and, adjusted correctly, often have less drag when released. It is
however possible to design special seals that retract the piston on a disc brake.
Drum brakes emit less particulate matter (PM) than disc brakes, as the wear-particles
are mostly sealed in. They are not better in this regard than frictionless brakes
though.
Certain heavier duty drum brake systems compensate for load when determining
wheel cylinder pressure; a feature rare when discs are employed (Hydro pneumatic
suspension systems as employed on Citroën vehicles adjust brake pressure depending
on load regardless of if drum or discs are used). One such vehicle is the Jeep
Comanche. The Comanche can automatically send more pressure to the rear drums
depending on the size of the load. Most other brands have used load sensing valves in
the hydraulics to the rear axle for decades.
Due to the fact that a drum brake's friction contact area is at the circumference of the
brake, a drum brake can provide more braking force than an equal diameter disc
brake. The increased friction contact area of drum brake shoes on the drum allows
drum brake shoes to last longer than disc brake pads used in a brake system of similar
dimensions and braking force. Drum brakes retain heat and are more complex than
disc brakes but are often the more economical and powerful brake type to use in rear
brake applications due to the low heat generation of rear brakes, a drum brake's self-
applying nature, larger friction surface contact area, and long-life wear characteristics.
2.6 To list advantages of drum brakes:
less expensive to produce
slightly lower frequency of maintenance due to better corrosion resistance compared
to disks.
built-in self energizing effect requires less input force (such as hydraulic pressure).
wheel cylinders are somewhat simpler to recondition compared to callipers.
minor weight savings, primarily from much smaller and lighter hydraulic cylinders vs.
callipers.
