Comparison between disc and drum brake

1.6. Comparison between disc and drum brake:

1. In case of disc brakes friction surfaces are directly exposed to the cooling air, whereas in the drum type, the friction occurs on the internal surfaces, from which heat can be dissipated only after it has passed by conduction through the drum.

2. The friction pads in case of disc brakes are flat as compared to curved friction linings in case of drum brakes. This means that in disc brakes, there is uniform wear of friction pads. Moreover, the friction pad material is not subjected to any bending, thereby increasing the range of materials from which to choose the suitable one. Generally use asbestos fibre with metal oxide fillers bonded with organic compounds as the material for friction pads.

Fig 1.12: Drum and disc effects of expansion

3. Unlike the conventional drum brake, the design of disc brake is such that there is no loss efficiency due to expansion. Referring to Fig 1.12, as the system becomes hot, expansion of a drum of internally -expanding shoe type of brake tends to move the friction surfaces apart, causing a loss of effective pedal travel. On the other hand, disc expansion merely changes the relative positions on the friction surfaces slightly without tending to increase the clearance.

4. Disc brakes weigh less than their conventional drum type counterpart-a saving of approximately 20 per cent being possible.

5. Disc brakes have comparatively better anti -fade characteristics. It is seen that using drum brakes the pedal load required to produce a vehicle deceleration of 5 meters per sec. per sec. increased from 30 N to almost 450 N. On the other hand, using disc brakes the pedal load remained constant at 250 N. after an initial rise from 200 N. In these particular fade tests the car was stopped from a speed of 80 km per hour. At still higher speeds, the superiority of disc brakes would be greater.

6. The sensitivity of a brake to changes in the friction coefficient at the rubbing surfaces can be shown by plotting brake factor against friction coefficient. Brake factor may be defined as the self-energising factor. A series of brake factor curves changes in brake factor for unit change in friction coefficient is much less in the case of disc brakes than it is for the two-leading shoe or the simple, leading and trailing shoe brake. This consistency of braking in disc brakes is due to absence of any self -servo action in these.

7. Compared to the drum type, the disc brakes are simple in design. There are very small numbers of parts to wear or not function properly.

8. It is very easy to replace the friction pads when required, compared to the drum type where the brake linings have to be either riveted or fixed with adhesives to the brake shoes.

9. Total  frictional area of pads in spot brakes is very less as compared with the conventional  drum type brakes, the approximate ratio being 1 4. This means that in disc brakes, the pressure intensity must be considerably greater than in the drum type. This implies that frequent relining would be necessary, due to increased rate of wear.

10. Compared to drum brakes, disc brakes offer better stopping performance, because the disc is more readily cooled. As a consequence discs are less prone to the "brake fade"; and disc brakes recover more quickly from immersion (wet brakes are less effective). Drum brakes are more liable to fade under heavy braking than disc brakes, because they are harder to cool.

11. Most drum brake designs have at least one leading shoe, which gives a servo-effect. By contrast, a disc brake has no self-servo effect and its braking force is always proportional to the pressure placed on the brake pad by the braking system via any brake servo, braking pedal or lever, this tends to give the driver better "feel" to avoid impending lockup.