Substantial reduction of motorbus depreciation by materially increasing the useful life of the vehicle is an important problem now facing the automotive engineer. The author contrasts present motorbus life with that of street-cars; he finds a probable life of 4 or 5 years only for the former and 20 to 25 years for the average type of trolley-car. This, in the case of the motorbus, he says, is too short a period of usefulness and directly affects operating costs, since the increased cost of motorbus maintenance offsets its lower initial-cost.
Demand for maximum comfort, safety and speed from the public and for economical operation from the operators has renewed interest in the six-wheel motorbus and given its design an impetus, although present four-wheel motorbuses of 25 to 30-passenger capacity have, and will continue to have, a very definite field and will not become obsolete due to replacement by other types. However, the exact placement of the line of demarcation between the four and the six-wheel types is a problem. Specification of maximum vehicle-weight per inch of tire width will regulate motor vehicles most logically for highway conservation, in the author's opinion, other means being stated to be manifestly unfair, unscientific and detrimental to economic progress.
Design and location of the chassis units to permit adequate ground-clearance and low-hung floors is a perplexing matter. Assumptions of seating capacity, body dimensions and power needed come first; then, the determination of the wheelbase. For the six-wheel type, the last is either the distance between the front axle and the forward rear-axle or the distance between the front axle and a point midway between the two rear-axles; but the author leaves this question open, as well as that of the proper ratio between the wheelbase and the distance between the two rear-axles, except that these axles should be as close to each other as possible to reduce axle side-slip to the minimum. The spring-suspension should consist of the simplest spring combination that will provide maximum flexibility. Numerous spring-combinations are illustrated.
Two rear-axles permit smaller axle-units, provide increased ground-clearance, decrease over-all height and reduce individual unsprung-weight, according to the author. When the wheels of one axle roll over a bump, the rise of the sprung mass is only one-half that occasioned in the conventional design, but the time required is the same; therefore, the velocity of drop of the sprung mass is halved. Since the impact force is F = ½ MV2, the body reaction and stresses induced in this type are but one-quarter of those felt in the single-rear-axle type and this fundamental advantage of the dual rear-axle produces the increased good riding-quality attained.
Other questions on which opinion is divided are whether to drive with only one of the two rear-axles, using the other as a load-carrying element, to drive through all four rear wheels to transmit power through the gears of the first axle to the second or to divide the power equally between each of the axles by using a power differential. The maximum universal-joint angle permissible with any spring and drive arrangement is mentioned. One that will permit the axles to maintain a fixed relation one to the other eliminates the angularity of the short drive-shaft between each axle.
Skidding is practically eliminated when braking all four rear wheels of a six-wheel motorbus, even on wet, greasy, smooth pavement; therefore, it is questionable if brakes on the front wheels also are needed. As the size and weight increase, servo mechanism becomes necessary; the question then is whether to cause it to operate a drive-shaft brake in conjunction with front-wheel brakes or to cause it to apply the wheel brakes only, reserving the drive-shaft unit for emergencies.
Steering must remain easy, even though the wheel-base increases. This may necessitate center-point steering, possibly having the axis of the king-pin in the plane of the wheel. The bore-and-stroke ratio of the engine must be chosen to comply with the accepted characteristics of a motorbus engine but must also combine the maximum crankcase-front-axle clearance with a low over-all-height. The author favors the four-cylinder rather than the six-cylinder engine for this service.
Frame construction presents the problem of striking a compromise between relative rigidity and excessive weight. A flexible frame demands a flexible body.
The choice lies with the flexible type of motorbus chassis and body or the super-rigid assembly. Other problems relate to ventilation, heating, freedom from road dust, prevention of exhaust gases and engine compartment vapors from entering the body and the elimination of noise. Safety appliances that will apply the brakes automatically in case of accidental loss of control by the driver are suggested.
Greater manufacturing cost, increase of total weight and higher transmission-losses constitute the price that must be paid for vehicles employing more than four wheels, but these may be offset by the increase in good riding-quality and by the greater capacity, tire-mileage and fuel-economy obtained. Future development also depends largely upon the progress made in road design and expansion of the areas traversed by suitable highways.