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CONTROLLING THE SIX-WHEELED VEHICLE.

6th December 1921
Page 16
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Page 16, 6th December 1921 — CONTROLLING THE SIX-WHEELED VEHICLE.
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The Increased Momentum. The Brakes Usually Provided. How to Apply them. Gear Changing. Turning and Reversing.

0 WING to the increasing use of six-wheeled vehicles in this country and their difference from the normal type of four-wheel lorry, the question of driving needs careful consideration.

The heaviest load carried on the body of normal lorries is in the neighbourhood. of six tons, but with a. six-wheeler loads of from ten to twelve tons are quite usual. Driving a vehicle with such heavy loads requires a good deal more care and judgment, owing to the increased momentum, and to ensure perfect safety the latest type of six-wheeler is provided with three independent brakes.

Two hand brakes are fitted: one acting on the carrier wheels and the other on the driving wheels. A foot brake operating between the gearbox and jackshaft is provided for emergency purposes. The order of application of these brakes is most important, and in order to explain this it is necessary to give a short description of the construction of the six-wheeler. The motive unit carrying the transmission is mounted on a short four-wheeled chassis. The front portion of the body rests on a turntable suitably sprung on the driving axle, with the rear portion of the body resting on springs mounted on the carrier wheels.

It will, therefore, be seen that the body and carrier are pivoted on this turntable and this pivoting a,etion governs the whole application of the brakes.

The correct method is that of applying the carrier wheel brakes first, in order to stop the load at the rear of the vehicle and so check the cause of the momentum. This action has the effect of pulling the front portion of the machine backwards, thus straightening the whole vehicle out like a piece of string in tension, owing to the pivoting action on the turntable.

When the momentum has been sufficiently checked, the driving wheel brakes can be applied safely and later the foot brake if necessary.

If this application is reversed and the foot brake

or driving wheel brakes applied before the carrier wheel brakes, it would have the effect of checking the comparatively light motive unit against the load, thus causing the carrier and load to overrun the Motive unit.

In this case the rear wheels of the vehicle might tend to skid to' the right or left of the driving wheels, owing to the turntable action.

If the brakes are applied in the correct manner, a six-wheeler has considerably less tendency to skid than the normal lorry, even with such an increase in weight.

Gear changing on a six-wheeler requires a good deal of thought and judgment, because of the heavy load carried and consequent sudden loss of road speed as soon as a gear is disengaged, Running up a hill which necessitates changing from top gear through second to bottom_ gear is no easy matter with a ten-ton load, but this is facilitated enormously if there is a good drop in ratio between each gear, so allowing the engine to speed up quickly when each separate gear is engaged. The quicker the change the better, as once momentum is lost the vehicle will very quickly come to a standstill, and starting on a steep hill with a ten ton load has its disadvantages_ A good many drivers have a habit of running clomp hill out of gear," with the throttle almost closed and the engine just turning over. While this may cool the engine and be a possible nieans of saving netrol, there seems to be a considerable danger in iree-wheeling down hills. Astonishing speeds may be obtained in this way, relative to a great extent to the load carried, and if by any chance the engine stops, as it often does when running with a closed throttle, there is little possibility 'of getting back into gear. The checking of the speed is then entirely dependent on the brakes, and if these are at all weak there would be very little chance of pulling up in an emergency. A good deal has been written concerning the difficulty in backing six-wheeled vehicles, but, as a matter of fact, once a driver has become acquainted with the correct method there is no reason why sixwheelers should not be reversed as easily as fourwheelers. In fact, it is possible to manoeuvre these vehicles into places where no four-wheeler could be taken.

If, in the first place, six-wheelers had been the standard vehicle and four-wheelers had 'come along after, the same difficulty would be experienced.

After, driving a six-wheeler continuously for two or three months, it requires quite a lot of thought to reverse a four-wheeler. In backing a six-wheeler the motive unit should be regarded as a bogey simply to control the body and carrier wheels backwards.

In order to reverse the vehicle to the left, the front wheels are turned to the right, and, once the body and carrier, swings to the left, the front wheels are straightened slightly, when the whole vehicle will then describe an arc to the left.

The longer the front wheels are kept on one lock the smaller will be the arc, until it is possible to bring the motive unit at right angles to the body aad carrier wheels. Actually, the process is very similar to backing—a-horse and cart, except that the horse can be moved at right angles to tlae---cart without moving the wheels of the-cart.

In the six-wheeler the whole vehicle, must move slightly backwards in order to give. the motive unit time to arrive at right angles,to the body. If the horse, shafts, and front wheels of the cart are looked upon as the m,otive unit the whole process becomes apparent.

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