EXPERIENCES WITH SIX -WHEELERS IN PARIS.
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Some Details of the Results Obtained with Two Different Types of Six-wheeled Single-deck Bus.
IN the course of a recent paper, M. L. 13acqueyrisse, the general director of the Societe des Transports en Commnn de la Region Parisienne, which operates the motorbus and tramway services in the Paris district, gave some interesting information regarding the company's trials of and experiences with sixwheeled motor vehicles. As we have alrealy intimated in our columns, the company has experimented with two types of six-wheeler, both fitted with single-deck bus bodies, one being a Renault 25-seater and the other a modified form of the standard De Dion fourwheeled bus having accommodation for 48 persons. The experiments date back to 1921, since which time 51 of the larger vehicles have been in service on the main boulevards and crowded arteries of Paris.
The distance between front axle and forward rear axle is 14 ft. 3i ins, and between the two rear wheels 7 ft. The approximate axle weights are as follow : hindmost wheels are loose on their axle and are mounted so that they work in conjunction with the front wheels and facilitate the steering of the vehicle around sharp turns, the steering gear being connected both to the front and rear wheels.
The suspension of each of the three axles is by two semi-elliptic springs. The two front springs are, of course, independent. The middle and rear springs at each side are connected by a lever oscillating about a pin fixed to the chassis. The result is that the loads on the wheels are independent of the profile of the roadway and that the loads on the middle and rear axles are in a constant ratio determined by the ratio of the arms of the lever. The loads on
the two steering axles can be made approximately equal by a suitable choice of this ratio, which is advantageous from the point of view of ease in steering, absence of skidding and road adherence of the middle pair of wheels.
M. Bacqueyrisse claims that this arrangement has very clear advantages from the point of view of suspension, and independent movements of the middle and rear sets of springs are per mitted, although, owing to the interconnection through the balance levers, any one spring brought into action can transmit part . of its load on to the spring to which it is connected through
the balancer. Thus it is possible to consider each spring as being an elastic shock absorber for its corresponding spring on the other axle. The system works excellently for the reason that the balancers are seldom, or never, brought into action together.
Reactions on the ground, on the one hand, and on the chassis on the other, are thus reduced. Proof of these qualities was obtained by comparative tests carried out on one of the 48-passenger six-wheeled buses and on a 38-passenger four-wheeled vehicle having a wheel base of 14 ft. 3i ins, and fitted with solid tyres, twins being used on the rear wheels. The axle weights of the fourwheeled vehicle are given below :— The tests consisted in'the measurement of the dynamic action on the ground and road shocks to the vehicles.
The wheel to be tested was run at various speeds over an obstacle of determined shape placed on a specially made pedal which indented a test piece of copper or mild steel (taken from a number of test pieces of the same metal) by means of a very bard steel sphere. The law of the variation of the diameters of indentations as a function of the pressure brought to bear on the sphere was previously determined in the press, using a similar number of test pieces, which enables the coefficient of pressure due to dynamic action to be calculated for each test, viz. :— P p where P is the dynamic load and p the static load.
The following average results were obtained during the testa :—
AVERAGE CO-EFFICIENTS OF PRESSURE DUE TO DYNAMIC ACTION.
On middle On rear Advantage aide of axle of in favour
six-wheeled fourof six bus. wheeled bus. wheels.
Empty ... 1.36 1.64 17.1 p.c.
Loaded ... 0.89 1.03 13.7 p.c.
The comparative easy-riding tests were carried out with the aid of a liquid accelerograph designed by the S.T.C.R.P., having a period of its Own of 1-200th of a second, this being fixed to the right of the driving axle on the floor of each vehicle. The buses were run over an obstacle of determined shape at various speeds, the tests, which were the same for both vehicles, viz., at 15 m.p.h. and 12i m.p.h., showed that the average coefficient of improvement in the freedom from jolting for the six-wheeled vehicle was 12 per cent. when loaded and 25 per cent, when empty.
M. Bacqueyrisse reported that these six-wheeled buses had been in service for six years and had given entire satisfaction as regards fuel consumption, cost of maintenance and comfort of the passengers.
The design of the Renault six-wheeled
chassis, which is capable of carrying a useful load of 31 tons and on which a 25-passenger bus body has been fitted, differs from the 48-passenger vehicle in that there are two driving axles. Each of these is connected to the chassis on each side by a cantilever spring, the two springs on the same side being hinged at their centre on the chassis, their ends resting on the two axles. Therefore, in addition to the participation of the two suspensions in absorbing the reactions received by one of the two axles, there is a reduction in the unsusperided deadweight of each, which represents a further cause of improvement in the suspension. The bus is mounted on 1,000 ram. by 190 mm. lowpressure pneumatic tyres inflated to a pressure of 40 lb. per sq. in.
During the suspension and shockabsorbing competition organized last year by the City of Paris it was found
that this vehicle, when compared with a standard four-wheeled bus, which was similar to it from all points of view, including the use of low-pressure tyres, showed an improvement of 55 per cent. as regards the pressure on the ground due to dynamic action, and a average improvement of an per cent, as regards freedom from shock due to driving over bad roads, measured in the interior of the vehicle.
Finally, the author reported that the tests had shown that the six-wheeled principle enables the use of more flexible springs than on four-wheeled vehicles. He concluded by expressing the view that, in addition to the advantages already referred to, the six-wheeler effects a reduction not only in road deterioration but also in tyre wear, whilst there is a marked reduction in ebasis maintenance costs a well as in body repairs,