Principal among the indispensable parts intervening between engine and road wheels, and one which may be a source of great joy or correspondingly great wrath, according to whether it be well or poorly designed and fitted, is the clutch. There are six forms into which clutches may be divided, although not all of them are in general use in the automobile. These different forms are:

(1) Cone clutches.

(2) Band or drum clutches.

(3) Expanding ring clutches.

(4) Disk and friction clutches.

(5) Hydraulic or fluid clutches.

(6) Magnetic or electric clutches.

In general, only the first four of these six forms of clutches are used on automobiles and in fact the number of adoptions of the cone and disk clutches so far outnumber the others that they practically exclude the other types. The hydraulic type is used mainly on commercial vehicles and the magnetic type was also developed first on heavy trucks. Within the last few years, however, a magnetic clutch has been placed on a touring car and bids fair to become popular. Further discussion of this will be given later.

The necessity for a clutch lies in the fact that the best results are obtained in an automobile engine when run at constant speed. Inasmuch as the speed of the car cannot, from the nature of its use, be constant, it requires some form of speed variator. This is the usual gear box or transmission, but in addition, there is the necessity of disconnecting this from the motor upon starting, since the engine cannot start under a load. There is also the necessity for disconnecting the two when it is desired to change from one speed to another either by way of an increase or a decrease. So, also, when one wishes to stop the car, there must be some form of disconnection. There are then three real and weighty reasons for having a clutch.