Mumbai University > MECH > Sem 8 > Automobile Engineering

Marks: 14 M, 5 M

Year: May 2014, May 2013, Dec 2012, May 2012, Dec 2015

There are three types of drum brake system:

• Twin Leading Shoe
• Leading/Trailing Shoe
• Duo-Servo

A. Twin Leading Shoe

• The twin leading type is the least common type in modern automotive use.
• This system was used popularly in front wheels because it is very effective at braking in forward direction.
• Twin leading shoe drum brake system use two single piston wheel cylinders –one near the top of the backing plate and one near the bottom.
• Each wheel cylinder activates one of the brake shoes.
• It is called twin leading shoe brake because both the shoes are arranged in a leading shoe configuration in the forward direction.
• When applied in the reverse direction, the breaking force is very less and hence it is usually accompanied by one of the other types of brakes on rear wheels to be used as parking brakes.
• This type is well suited for motorcycles they are driven mostly in the forward direction and rarely in reverse.

B. Trailing and Leading shoe

• Leading / trailing drum brake is a basic type of drum brake design.
• The term "leading/trailing" means that only one shoe is "leading", moving into the rotation of the drum and thus exhibiting a self-servo (or self-applying) effect.
• The leading shoe is "dragged" into the friction surface of the drum and thus achieving greater braking force.
• The other shoe is "trailing", moving against the direction of rotation, is thrown away from the friction surface of the drum and is far less effective.
• An advantage of an SLS brake is that is equally effective whether the vehicle is travelling forwards or in reverse.
• When the vehicle is moving in reverse, the role of the leading and trailing shoes is switched. What would be the leading shoe when the vehicle is travelling forwards becomes the trailing shoe, and vice-versa.
• This type of brakes is generally found only on the rear wheels of cars, motorcycles and front wheel of smaller bikes and scooters.

C. Duo-Servo

• Duo-servo drum brake systems get their name from using servo action in both forward and reverse direction.
• The system uses a single wheel cylinder with two pistons which activates the brake shoes.
• The bottom of each brake shoe is not anchored to the backing plate but is connected by an adjustable floating link. This configuration allows the bottom of the brake shoes to move in the direction of the drum.
• The anchor pin at the top of the backing plate prevents each shoe from rotating past that point. The shoes can move away from the anchor pin but they are stopped when they rotate toward it.
• When the brakes are applied, the shoe contacts the drum; the friction causes it to rotate with the drum.
• This puts a small force through the bottom connecting link and applies (servo action) the bottom of the rear shoe, pushing it into contact with the rotating drum.
• The rear shoe is pushed harder into the drum as the anchor pin prevents from rotating.
• Thus the rear shoe does most of the braking work.
• It is used in commercial vehicles that require especially large braking force.