MR. E. G. FITZCOMBE, A.M.I.MECH.E., A.M.I.R.T.E., DESCRIBES, IN A PAPER READ TO THE INSTITUTE OF ROAD TRANSPORT ENGINEERS,
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How to Select Your Vehicle
AT a meeting of the Institute of Road Transport Engineers, held at the Royal Society of Arts on February 26, Mr. E. G. Fitzcombe, A.M.I.Mech.E., A.M.I.R.T.E., read a paper, "Factors Influencing the Selection of Road Transport Vehicles." He pointed out that to minimize operation and maintenance costs and obtain maximum efficiency, the primary object is the correct selection of vehicles for their work. Operating personnel may he well trained and maintenance well planned, but these benefits will not offset extra costs caused by misapplied machines.
There are plenty of models, but the rules of correct selection must be applied. The technical Press affords much assistance by giving the characteristics of vehicles.
• Chassis produced by any maker must be compromises, as operating conditions vary considerably. They constitute the best arrangements of engine power, gear ratios, axles, and frame construction to suit recommended payloads and gross vehicle weights under average conditions. The majority in current production would operate relatively efficiently under these conditions and be able to to climb gradients of I in 60 to I in 40 in top gear, carrying the recommended payloads. Such compromises, however, may result in vehicles operating in circumstances for which they are unsuited, e.g., arduous conditions under which they may be overloaded and underpowered, or easy ones where extra payload could be carried.
Factors to be Considered
The following factors must be borne in mind in the selection:—(1) Resistance to motion offered by road surfaces and gradients; (2) type of load, including weight and volume; (3) depending upon 2, whether the load can be accommodated on a rigid vehicle with two, three or four axles, or whether an articulated unit be necessary (operating conditions may warrant the articulated type); (4) performance in service, i.e., tractive effort available for propulsion; (5) load space available and load distribution on axles; (6) cab design and general appearance; (7) comparative prices of suitable chassis and local availability of parts and Servicing facilities.
In assessing rolling resistances, values usually accepted are good. surfaces. (cement, asphalt, macadam, etc.) up to 30 lb /ton gross vehicle weight; poor surfaces (dry clay, unnude roads, etc.) up to 60 lb./ton, In Most instances, road conditions, type of load and chassis weight are known. The weight confines the choice, and it is necessary to estimate gross vehicle weight to include payload, body, crew and equipment.
Power units have tended to develop into two standard forms—oil engines for the heavier machines and petrol engines for the lighter. The position of the oil engine is established as a result of improved efficiency, flatter torque curve and longer periods between overhauls. The trend in engine design is mainly towards increased capacity and greater power, with improved power-to-weight ratio. Complaints against under-powering are being eliminated, and the improvement will be felt in faster acceleration, shorter trip time, and reduced maintenance costs. Fuel consumption need not be increased, for by careful selection of transmission-gear ratios, bigger engines can still be economical for lesser demands.
As regards back axles, the operator need not take loadcarrying capacity into account; it is a matter for the maker. It remains to him to choose the design and ratio. Designs available are: (a) single reduction by bevel gears or by worm and worm wheel, (b) double-reduction single-speed through conventional bevel gears and an additional set of spur gears, (c) the two-speed type, popular in America and recently introduced in Britain.. In this, usually, the spiral bevel primary reduction is followed by an epicyclic secondary which can be engaged as required, the change being effected by manual, hydraulic or vacuum operation. Ratios for two-speed axles should be such that the difference between high and low should give about half the drop in speed of one change in the gearbox, thus doubling the usual number of ratios.
A34 Several ratios are usually available for each chassis model. and it is important to select one which permits the engine to operate at its "ideal "speed under conditions of normal road speed, full load, and direct drive.
The range for minimum consumption is normally just above the speed of maximum engine torque. If the road speed at this point be near to that required by the operator under normal conditions of full load, top gear, and the axle ratio selected, then performance should be satisfactory.
As regards tractive effort, the torque developed should be such as on top gear to overcornt the various resistances, leaving sufficient margin for acceleration and for climbing gradients of at least I in 50. The two-speed axle• provides the best compromise between conditions of peak demand and running light.
Performance comprises, mainly, hill-climbing ability and maximum operating speed. In respect of the first, the tractive effort at the wheels must be determined, and then account taken of the various resistances, these being (a) transmission, (b) rolling; (c) wind. From this, the additional effort for acceleration and hill-climbing can be found.
Tractive effort in pounds is the product of engine torque in lb./ft., gear ratio and transmission efficiency, divided by tyre rolling -radius in feet. The information required is usually available from specifications, but as there is often optional equipment, each combination should be checked to determine the most suitable.
Transmission efficiency on direct drive may be taken as 90 per cent. for a bevel axle and 85 per cent. for the worm type. In the lower gears reasonable values would be 85 per cent. and 80 per cent. respectively.
Tractive effort available for propulsion is obtained by subtracting from the total tractive effort the total road resistance on the level, this being the sum of the rolling resistance for the gross weight and the wind resistance, but the last may almost be ignored.
To convert the result into terms of gradient climbable, consider a vehicle travelling uphill. The forces acting upon it will be: (a) tractive effort available for hill-climbing, (b) pull exerted by gravity. By resolving these forces in relation to the angle of the gradient, that climbable can be determined by dividing gross weight by tractive effort available. The value obtained, say 50, indicates that the vehicle would climb I in 50 under the conditions obtaining.
Performance in Top Gear
The final factor in respect of performance is to checkthe maximum speed to ascertain whether the vehicle will operate comfortably in top gear at that required.
If d be the effective diameter in feet, then the distance travelled by one revolution of the wheel will be d/ft., and if the wheel be rotating at n revs, per minute, then the vehicle velocity will be 7r dn ft per minute or 60 X dn
• miles per hour.
5280 Therefore, if n, the number of revolutions of the wheel, be replaced by the engine revs, per minute, then, dividing by the overall gear ratio, the vehicle speed in miles per hour will be:— 60 rt d x engine revs, per minute 5,280 x overall gear ratio.
Those who do not like calculations may use charts, or calculators based upon the principle of the slide rule.
Next comes the selection of a model which has sufficient load space to accommodate the body and payload, and proper weight distribution on the axles in accordance with tyre capacities or regulations. Sequence of operation is: (I) determine load space needed, (2) determine position of centre of gravity of , body and payload. (3) locate load centre on chassis to give correct distribution on axles.
' The author goes into considerable details on these matters, but space prevents their inclusion