The
single-leading-shoe drum brake (SLS), a.k.a. "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 traveling 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 traveling forwards
becomes the trailing shoe, and vice-versa.
The
twin-leading-shoe brake (2LS) is a type of drum brake that has two
leading shoes, rather than the single leading shoe and a single trailing shoe of
a single-leading shoe (SLS) drum brake. A leading shoe has a self-servo effect,
so an advantage of a 2LS is that it provides the maximum retardation in its
intended direction of travel, i.e. forwards. 2LS brakes are fitted on the front
axle of automobiles, or the front wheel of a motorcycle. A 2LS brake is more
powerful than an SLS design.
SLS
brakes are still used for the on the rear wheel of many motorcycles, and for the
front wheel of smaller bikes and scooters. An SLS brake is less powerful than a
TLS, but that is not an issue for a motorcycle's rear brake. Compared to cars,
bike are higher and have a shorter wheelbase, so weight-transference under
braking is much more pronounced. Excessive braking force can cause the rear
wheel to lock, so it is normal for a bike's rear brake to be much less powerful
than the front. Since 1969, modern motorcycles tend to have disc brake(s) on the
front wheel, and sometimes a smaller disc brake on the rear wheel. Before disc
brakes became commonplace on bikes, performance machines tended to have
twin-leading-shoe drum brakes, and some racing machines even had four-leading
shoe drum brakes.
Fig 1
Let us put all the above in context.
Firstly, what is a drum brake?
A drum brake for road vehicles consists of a number of lined
shoes located within a drum that rotates with the wheel. To slow or stop the
vehicle the shoes are pressed against the inside surface of the drum to create a
friction force. Correct drum brake geometry is important in order to ensure
that:
· Brake linings wear evenly.
· Brake output torque is appropriate to the application.
Fig 12
Figure 1 shows a typical arrangement of the shoes and other
stationary components in a drum brake. The whole assembly is mounted on the back
plate. For clarity, only the inner surface of the drum is indicated, but in
practice its rotating outer surface is very close to the back plate perimeter
flange. The small clearance between them reduces the risk of dust, water and
foreign bodies entering the drum. The brake shoes shown in Figure 1 are forced
apart and into contact with the drum by the small hydraulic cylinder, or
expander, shown on the right. They pivot about the shoe abutment on the left and
are restored to the 'brakes off' position by the shoe return springs.
Leading and trailing shoes
From Figure 1 you will see that, with the drum rotating in the direction shown,
the upper shoe is ahead of its pivot point. It is said to be a leading shoe.
Similarly the lower shoe trails behind its pivot point and is called a trailing
shoe. There is an important difference in the way leading and trailing shoes act
under braking.
Figure
2 shows the forces acting when a leading shoe is applied. Notice that the
frictional drag force has a moment about the pivot point. This increases the
input load and hence increases the drag. In other words, there is a self-servo
action, which increases the braking effect.
Figure 3 shows the forces acting when a trailing shoe is
applied. In this case the moment of the frictional drag force about the pivot
point opposes the input load, thereby reducing the drag and the braking effect.