The centre of gravity is lower in the "N.S." omnibus than in other types. Under the worst conditions, when the upper deck is laden and the lower deck is unladen, the ground may tilt to the extent of 28° beneath the car before over-turning can occur. The best figure obtained with the more usual construction, and without a top cover, was 25°. With the advent of cross seating, omnibus axle weights became so great as to approach the limit tolerated by road authorities. Rear axle weights, reaching that limit, could no longer be increased, and the need arose, as bigger omnibus weights were required, for throwing the weight forward. To this end the forward drive omnibus was introduced in London, and subsequently in all parts of the world.
The modern 'bus body is 'continued forward to a point only an inch or two behind the rear cylinder block. The driver sits forward alongside the engine, and is actually in a much more favourable position to steer his vehicle round blind corners than formerly. The drawbacks to the forward drive omnibus, are, firstly, the heaviness of steering consequent upon the increased front axle loading, and secondly, the difficulty of access to the engine on the off-side. Neither of these drawbacks has, however, proved serious. There is a tendency among vehicle builders of six wheelers to return to the conventional driving position of many years ago, but the day cannot long be postponed when forward drive will be forced upon designers of six wheelers, as it has been forced upon designers of four wheelers.
Transmission Problem.—Common to all road vehicles em ploying the internal combustion engine, is the problem of trans mitting the power developed by a relatively inflexible prime mover to the road wheels. In the omnibus, above all other vehicles, this problem is one of great difficulty to which no entirely satisfactory solution has yet been found. The high power weight ratio of an ordinary car makes possible the employment of a lightly con structed clutch and gearbox, since the car will normally run in top or direct gear. Only motoring enthusiasts care for driving on the gears and obtaining thereby a splendid performance out of a small capacity engine. Nevertheless, the omnibus driver is called upon to do this, and in consequence, the transmission must be of great robustness and be simple to manipulate. In addition to the need for gear changing, ever present on undulating roads, with the relatively low powered omnibus there is the constant stopping to let down and take up passengers which also calls for gear manip ulation. With the very heavy omnibus used in services necessitat ing a great many stops, it has become impossible to achieve, with out high maintenance costs, the propulsion of the load through the agency of the clutch and gearbox. It is probably only a mat ter of a year or two before other forms of transmission—electric transmission in particular—will live down their many drawbacks, and show on the whole a saving in costs.
For the right angled drive in an omnibus rear axle, bevel, and other forms of gearing have yet to be proved superior to worm gearing. Within recent years, the old fashioned motion communi cating screw and wheel has been developed to meet modern auto mobile and industrial requirements, and in its original form it can scarcely claim a relationship with the highly efficient rever sible worm gearing of the present day. The difficulties inherent
in the many forms of bevel drive are to make them compact and also silent.
Suspension.—The suspension of an omnibus is effected through longitudinally disposed semi-elliptic leaf springs mounted on the axles, supplemented by rubber buffers, steel volute springs, or other energy absorbing devices over the axles. The latter augment the stiffness of the suspension to meet exceptional load or road conditions.
Rear springs are often allowed to take the driving and braking torque reactions of the rear axle. Where this is not permissible, a torque arm is provided, and the rear springs are shackled at both ends. Tractive effects are of ten communicated to the vehicle through the rear springs also, though, where a torque arm is pro vided, the springs must, in view of their being shackled at both ends be relieved of this duty. Where it is provided, the torque arm usually swings from a well braced frame cross-member that has sufficient strength to withstand the pushing and tugging in a hor izontal plane to which it will be subjected by the driving and braking exertions of the rear axle. Some vehicles are made with a torque tube surrounding the propeller shaft.
No effort is made by designers to produce completely rigid chas sis frames for omnibuses. All things carried on the frame are mounted so as to flex with it or be able to take up a new position without strain. Such rigid structures as engine, gearbox and radi ator, are mounted on frame attachments or bearers with rubber or other resilient pads interposing and permitting small relative movements. Where severe conditions must be met, the engine and gearbox are mounted on three such bearers only. Three points of support, however, situated, remain always in a plane, and there can be no distortion from twisting with three point suspension. The drive between units, disposed none too rigidly in relation to one another, is effected by means of shafts bearing flexible couplings or universal joints at their extremities.
The enormous mileages accomplished by the omnibus as corn pared with the light car, prohibit the adoption by omnibus en gineers of light car greasing methods. There is no part of an omnibus which can be packed with grease and left to itself from one year's end to another. Grease nipples and filler caps are pro vided everywhere in accessible positions, and positive action grease feeding guns which can be relied upon to urge lubricant where lubrication is necessary, are used under service conditions at frequent intervals.
Tyre-makers, anticipating the call for higher omnibus speeds which had been heard for some considerable time, developed the pneumatic tyre that will carry heavy loads. With higher run ning speed, better braking must be provided on all vehicles. Rear wheel adhesion becomes insufficient and all four wheels must be fitted with drums and brake shoes. Four wheel braking is, in any event, often forced upon designers by the difficulty of accommo dating two sets of brake drums on the very small wheel centres that accompany these deep section tyres.