OMNIBUS. A term often shortened to " 'bus," signifying a public passenger-carrying vehicle of large seating capacity. It has become synonymous in popular use with the word "motorbus." Horse drawn and steam driven omnibuses have been superseded by motor propelled omnibuses.
In several particulars an omnibus must conform with regula tions laid down by public authorities, especially in connection with dimensions and weights. The carried load, consisting of the passengers and the omnibus body, will at most be less than the regulation weight by a figure termed the chassis weight. The proportion borne by the carried load to the total weight, is largely determined by the speed, hill climbing and other performances characteristic of the omnibus and such proportion is always made as large as possible to obtain the maximum earning power. For any specified purpose, designers have in recent years been able to increase this proportion without prejudicing either the reliability of the vehicles or the lowness of their running costs.
Body design aims at making the proportion borne by the pas senger load to the carried load as great as possible, and is influ enced very greatly by public requirements in the matter of com fort and safety. This proportion tends to become less as more exacting conditions have to be met in the provision of seating, shelter, ventilation and lighting. Passengers may be taken to weigh on an average one sixteenth of a ton each, but so soon as an omnibus body is provided to carry them, the weight that must be credited to each passenger becomes much greater.
The almost universal practice in omnibus design in 1928 was to employ two axles, the rear wheels being used for driving and braking, and the front wheels being used for steering. The weight distribution, as indicated by the front and rear axle weights, must be such as to give the adhesion necessary for rapid acceleration and good braking on normal road surfaces. Skidding and inferior performance are the natural consequences of bad weight distribu tion. The limit is set to front axle weight by the consideration that the driver must be able to turn the front wheels easily in order to steer the vehicle. This limit is reached before the other, which otherwise would affect both axles alike (but which, in con sequence of the earlier limit on the front axle, affects the rear axle alone), viz. the limiting axle weight tolerated by local author ities.
Low Centre of Gravity.—In the complete omnibus, under all conditions of loading, the centre of gravity must be so low in relation to the width of the vehicle as to render exceedingly remote the possibilities of over-turning. Where conditions permit, the centre of gravity should occupy a position even lower than that which gives the desired stability. All deviations of a 'bus from uniform motion in a straight line will bring about changes in the loading of every wheel, and these changes become smaller as the centre of gravity approaches the plane of the road. With a low centre of gravity, it will be usual to work very close to the adhesion figures calculated for the stationary omnibus, since the greatest acceleration and braking effects that these figures allow, can be actually approached in practice under all conditions. Where the centre of gravity is high, braking is liable to promote skidding at corners, and on cambered surfaces ; rapid acceleration will give rise to the same tendency. Since the carried load in a modern omnibus forms the large proportion of the total weight, and since the carried load comprises very nearly everything which is above floor level, it follows that the problem of making further reductions in the height of the centre of gravity, resolves itself simply into the problem of reducing the floor height.
To obtain the lowest possible floor height, omnibus designers have developed the double reduction driving axle. In this axle, the driving shafts do not transmit the torque direct to the road wheels, but communicate their motion to them through gearing. Each wheel bears a drum coaxial with the brake drum, but of smaller diameter, on which teeth are cut internally. Pinions on the ends of the driving shafts engage with these drums at their low est points. This arrangement permits the use of full size driving wheels with an axle whose height is considerably less than that of the wheel centres. It permits, moreover, the use of a smaller housing for the right angled drive in the centre of the axle. This drive no longer effects the whole torque multiplication, but only a part of it ; whereas the whole multiplication may be nearly i o to I, the right angled drive may be called upon to give a multipli cation of only 2 to I, and may therefore be much more compact than one giving the full multiplication.