As regards final drive, some degree of standardisation is possible so far as the axle arms and bushes are concerned. The bearings of the front wheels are standardised so as to make the wheels interchangeable, and the diameters of both the front and rear wheels are fixed. Makers and users are left with a fairly free hand as regards the type of body to be fitted to a subsidy machine, but in general this must be a lorry body with detachable sides and ends at least two feet high, and carrying a frame to take a complete overhead cover. Use is also found for a certain number of box vans.
Many provisions are made in the subsidy scheme with a special view to making the vehicles suitable for working in convoy. In the first place, it is stipulated that all engines shall be fitted with governors which shall automatically control their speed to 1,000 revolutions per minute. This prevents drivers from racing their engines and over-speeding their cars when running light, and also determines a definite top speed, which should be almost exactly equal for all subsidy machines, as it were. Coupled with this, are the provisions as regards change-speed gear. The engine governor having limited the speed of the 3-ton type to 16 miles per hour, and the speed of the 30-cwt. type to 20 miles per hour, the next step is to secure that, when road conditions require the use of lower gears, the whole of the vehicles in the convoy will need to change gear practically at the same point, and when on lower gears will run practically at the same speeds. With this object in view, it is stipulated that the ratio between top and bottom gear shall be about five to one, giving bottom speeds of about three and four miles per hour respectively for the two classes. This provision, taken in conjunction with the stipulated engine dimensions, should secure that every vehicle is capable of tackling a gradient of one in six on normal road surface whether fully loaded or empty. It will be seen that the point kept in view has been the desirability of arranging everything so that drivers of a large number of machines working in convoy--that is to say, running along the same road at short intervals behind one another--shall not have any difficulty in keeping their proper distance, and so in preventing any risk of collisions. For similar reasons the fitting of a ground sprag is made compulsory. The object of the sprag is to prevent a vehicle from running backward if, for example, the driver misses his gear on a steep gradient. The fitting is not necessary on ordinary commercial machines, since the brakes will almost certainly hold the car before it has run more than a few yards, but in the case of one vehicle in a long convoy, those few yards may mean a collision with the following car, and a hopeless muddle causing serious delay. In view of the possibility of some one car failing, every chassis has to be fitted with towing hooks at the ends of the side members of the frame fore and aft, so that each machine can, if required, either be towed or tow another, and any “lame duck” will not obstruct the whole convoy, but can be dragged along to some point where it can be towed out of the way, and the rest can be allowed to proceed.