FRONT WHEEL AND DIAGONAL BRAKES.
Page 9
If you've noticed an error in this article please click here to report it so we can fix it.
IN DEALING RECENTLY with the prevention of skidding, reference was made in TEE COMMERCIAL MOTOR to the system of front wheel braking. The whole subject of brakes in their bearing upon skidding is a very interesting and by no means a simple one. The following notes do not profess to deal with it exhaustively, but are merely i tended to touch. upon certain theories of interest. The cause of skidding is, of co rse, lack of sufficient adhesion between. the wheel and the ground. The adhesion is partly dependent 4on the condition of the road and partly upon the ch4.racter of the tyre and the distribution of the load ipou the wheels. Anything which is liable to disturb what may be described RS the natural grip existini between the tyre and the road is, of course, liable 4o cause skidding. Thus, when a vehicle is simply proce ding in a straight line along a level road, skidding is not liable to occur, but, directly the course of the vehi e is changed or it ceases to move in a straight line, the possibility of skidding arises. If the road has a amber, then skidding may occur although the vehicle is proceeding in a straight line. Supposing, for example, that the application of the brakes on the back axle causes loss of adhesion between the tyres and the road, the back of the vehicle may tend to slip bodily sideways down the camber. The common practice is to apply brakes only to the back wheels. Obviously when we limit ourselves in this way we are not exerting the maximum braking power that could be applied if the brakes acted on all the wheels. Broadly speaking, there are two reasons why brakes to the front wheels are not commonly fitted. • One is that it is difficult to arrange the operating mechanism so that the brakes are perfectly -compensated. If one is applied more violently than the other the vehicle will tend to slew round. 4. It is comparatively easy to compensate rear wheel brakes, since the position of the wheels in relation to the chassis remains constant, but the problem is much more difficult when the wheels to which the brakes have to be applied are also used for steering. The difficulty is partly due to the fact that, when turning the vehicle, the front wheels are not parallel with one another. In Fig. 1 is illustrated the usual arrangement in which the two front wheels, when locked, form tangents to circles which have a common centre on the line of the back axle when produced. The difficulties in compensating front wheel brakes can, however, be overcome, and if this were the only point to be considered the system would ptobably he fairly common. The other pit is, however, that if brakes were suddenly applied to the front wheels causing them to skid or slide upon the road, the driver would be temporarily deprived of fly power of directing the vehicle. Supposing that in Fig. 1 the front wheels were so firmly held by. the brakes that they could not revolve, the momentum of the car would push the whole thing bodily forward so that, even though the wheels were in the position shown, the car would simply move in a straight line. A sideslip of the back wheels can generally be rectified by a, prompt movement of the front wheels, but the front wheel skid admits of no such cure. , Suppose the front-wheel skid is produced simply by the condition of the roads, aided perhaps. by the adhesion being unsettled by an endeavour to turn a corner, something can be done to improve matters by applying the rear wheel brakes. The underlying idea is illustrated-in Fig. 2a. When the rear wheel brakes are sharply applied there is a tendency similar to what would occur if the point of contact between the rear wheel and the ground were suddenly gripped in a vice. For an instant the whole car tends to rotate about this point where movement is checked. The momentum of the vehicle throws the weight forward, and if the point (13) is for the instant regarded as fixed, the weight tends to swfrig round along the line of the arrow (A). In other words, more weight is brought to bear for the moment upon the front axle, and this addition of weight may serve to secure sufficient adhesion to correct the tendency of the front wheels to skid. Supposing, on the other hand, the brakes were applied to the front wheels themselves, the tendency would be for the car to swing round the point (R, Fig. 2b) and to move upward along the line of the arrow (B), reducing the weight on the front wheels and still fiirther decreasing the adhesion. Moreover, it is. obvious that if the front wheels are already skidding, not much can be 'done in the way of checking the movement by applying the front wheel brakes. The effectiveness of any brake is always decreased directly it locks the wheels, and leads to a direct skid instead of merely a resisting force, such as is brought into being if the brakes are less fiercely applied. The genera/ conclusion is, then, that while front wheel brakes .may • help to pull a vehicle up sharply under ordinary conditions, they are liable to do more harm than good when the surface of the road is really slippery. . One scheme which aims at obtaining the advantages without the disadvantages of the front wheel brake is that known as diagonal braking. In this system one pair of brakes would act on the wheels (A and D, Fig. 1) and the other pair on the wheels (B and C). Thus, if either pair were applied with injudicious vigour, one of each pair of wheels would still be left with a sense of direction. It is not clear that, with this system, , a tendency towards a front wheel skid could bescorrected by any application of the brakes, but, with this exception, the system appears to be a sound one, provided that accurate compensation of each pair of brakes can be secured. #3duch depends upon whether this is really possible without introducing undue complications. The writer has approached this subject, not as an expert or with a view to instructing experts, but simply as one who has endeavoured to clear his own mind on a rather intereSting and difficult subject, and may in consequence be able to help others, who have not studied the subject, to form their own views as to' the merits of he various systems that ?nay come up foi consideration. T. L.