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S ELECTING the right vehicle for a particular type of work

20th April 1951, Page 62
20th April 1951
Page 62
Page 63
Page 64
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Page 62, 20th April 1951 — S ELECTING the right vehicle for a particular type of work
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Which of the following most accurately describes the problem?

can be a difficult task for any operator or transport engineer who is not in the fortunate position of being able to employ a consultant or technician to examine the characteristics of a number of chassis, in relation to his needs, before making a purchase. The potential buyer of a chassis is offered a large variety of makes and sizes of vehicle from which to choose, and there are added complications of alternative type of engine, gear ratios, axle ratios and tyre sizes to be considered before making a final selection.

How, then, does the average operator choose a make and type of chassis, with the correct combination of technical features, for any given class of work? The initial cost of the vehicle, the price of reconditioned components and the quality of service facilities are often strong influences. The possibility of early delivery of a new model is also important to the home operator. Overseas users. particularly those in small, isolated towns such as in Australia or Africa, attach great importance to the availability of spares and service facilities.

Often the manufacturer who has taken care to provide adequate service arrangements overseas reaps the benefit by large export orders. Those who fit popular. well-established makes of proprietary power unit and other parts, also earn substantial goodwill overseas.

Although many operators may select vehicles for the reasons given, they are sometimes disappointed with the results, It would not be true to say that every user is satisfied with the vehicle he has chosen. In many cdses, the vehicle is grossly overladen, so that the performance is poor and mechanical failure and tyre troubles develop. Operators often complain to "The Commercial Motor" of vehicles that will not run satisfactorily, usually with an overload.

TO A South American operator of a British 5-tonner wrote, "1 am most dissatisfied with this vehicle, which has developed numerous premature mechanical troubles. Admittedly, it often carries a 2-ton overload and pulls a trailer, but this is normal for any chassis operating in the area." This, to any mechanical engineer, is a flagrant misuse of equipment, especially where the vehicle is operating in difficult territory.

Although this was an obvious case of overloading. it has many -parallels in the United Kingdom and abroad. If an operator cannot he relied upon to employ a machine of the right capacity for his normal work, it is obvious that the importance of specifying appropriate axle ratios and tyre sizes is lost to him.

Frequently, well-established operators and potential users of goods and passenger vehicles ask "The Commercial Motor" for guidance in selecting a vehicle for their work. The number of letters has increased recently, possibly because of the greater variety of "ready-made" chassis available.

The standard vehicle produced by manufacturers must necessarily be a compromise, because it is has to fulfil widely assorted types of duty. It is possible to find the one most suited to certain conditions by tabulating the " possibles " and eliminating those which clearly do not satisfy specific needs. Thus the choice can be narrowed to a few makes, one of which may be preferred. As a word of warning to the unwary, the cheapest vehicle to buy is not necessarily the most economical to operate, and, where a large annual mileage is covered, the extra cost of an oil engine may soon be recouped Assuming that a standard vehicle is to be purchased, the first thing to do is to rule and head a data sheet, as shovin in Fig. 1. It is necessary at this stage to ascertain the gross vehicle weight. which includes the chassis, body and payload The chassis weight of all the standard models can be obtained from the vehicle specifications contained in this issue. To this figure should be added the weight of the body and payload. The total weight calculated should not exceed the figure given in the specification tables under the heading of maximum gross vehicle weight, but for economical operation, it should be as near as possible to it.

A summary of vehicles fulfilling these requirements, together with the main chassis dimensions, should then be entered on the data sheet, leaving three or four spaces between each to allow for variations in axles • ratios and tyre sizes, which provide, in effect, additional models with different performances.

The chassis dimensions will enable the potential user to assess its manoeuvrability, the overall size of the platform, weight distribution between axles and parking space required in a garage. Obviously there will be some misfits on the list and the choice of chassis will be narrowed accordingly. In some cases a short wheelbase and narrow track will be preferred, because the operational area .entails . close manoeuvring, or because heavy concentrated loads are carried. Where the lorry is engaged on general haulage, the longwheelbase chassis will be required.

The length from the back of the cab to the centre of the rear axle will locate the position of wheel-arches inside the body, and this distance, deducted from the measurement between the back of the cab and the end of the frame, will give the rear overhang. Overseas users may wish to extend the body beyond the rear of the chassis frame, because it is only British legislation that restricts the overhang to a maximum of 50 per cent. of the wheelbase.

When buying a vehicle the operator is, in effect, purchasing performance, and therefore the next step is to

determine the capabilities of the chassis under normal road conditions and on various surfaces. The potential user requires to know that the vehicle has a good topgear performance with full load and that all gradients can be climbed with a reserve of power. In conjunction with the tables in this issue and manufacturers' specifications, more details can be added to the data sheet.

First the operator has to decide whether a petrol or oil engine is more economic for his work. Because of the higher cost of petrol in the United Kingdom and the greater annual mileages which maximum-load chassis normally cover, British manufacturers normally fit oil engines in all vehicles of 8-ton payload capacity and upwards. "The Commercial Motor" costs expert has established also that the use of oil engines in vehicles of 5-7-ton capacity is justified where the annual mileage is 15,000 or greater, and the extra first cost is recovered in fuel economy alone in the first 60,000 miles. This calculation is based on a fuel-cost differential of 3d. per gallon.

Having established the type of engine to be employed, the relevant details can be entered on the data sheet, but care is required in noting the maximum power and torque of the unit. It is usual to quote maximum output figures of engine performance on the test-bed, where the unit may be without its fan, dynamo and other auxiliaries. This might give a false impression of the capabilities of a vehicle measured when the unit is installed in the chassis. A list of power units, with their respective net ratings, has been compiled in the accompanying table. These portray the engine performance complete with fan, dynamo, pumps and normal exhaust system, but without brake compressor or exhauster motor. As the brake motor is idling for long periods and relatively little power is being absorbed, the power required to operate it is included in general mechanical losses, which are mentioned later.

The power ratings are applicable at sea level, but where vehicles are operated at high altitudes, such as over the Andes in South America, the Table Mountain in South Africa or in similar areas, it would be necessary to compensate for power loss. As a rough guide, approximately 3 per cent. of power is lost in every 1,000-ft. rise above sea level.

Another point of importance is the effect of ambient temperature on power output. Normally the output is quoted for a standard temperature of 60 degrees F., and every 10 degrees F. above this point reduces torque and power by 2 per cent. It is important that the makers are informed if a vehicle is to be operated at abnormal altitudes or temperatures, because special settings for the fuel-injection pump or carburetter are essential to economical and satisfactory operation.

Next there is the gearbox, which, in conjunction with the final-drive ratio and engine torque, will determine the power available at the wheels. The highest and B30 lowest overall gear ratios, calculated by the product of the gearbox and final-drive ratios, can then be added, to the sheet. Where an alternative final-drive unit is available, the second set of ratios should be entered on a lower line against the particular model.

In the medium range. of chassis, the two-speed axle is another factor to be considered in calculating the overall ratios. Assuming that the two-speed axle has ratios of 6.8 and 8.4.to 1, the average of 7.6 to 1 should be used as the basis for calculations. Because of the almost instantaneous change of ratio afforded by the two-speed axle, there will be many opportunities of using the higher gear on long inclines, especially when the full payload is not carried.

There are transmission losses in the gearbox and final drive, caused by gear friction and oil churning, the loss of efficiency varying according to the form of gear used. It is safe to assume 93 per cent, efficiency for the spiralbevel or hypoid final-drive unit when in direct drive in the gearbox. and 90 per cent, for a worm-driven or double-reduction unit. A further 5 per cent. efficiency may be lost when the indirect ratios of the gearbox ale in use.

The final details to be gleaned from the vehicle specification are the tyre sizes available. The roll;nd: radius of the tyre is required to calculate the tractive effort available and this dimension can be ascertained from Table II.

Measuring Tractive Effort

The operator is now in a position to calculate the tractive effort, which is the power available from the engine and transmission to move the vehicle. In America this is known as the " rim pull." It is given by the formula Tractive effort= Torque x gear ratio x transmission efficiency Effective radius of tyres

The tractive effort will be expressed in lb. if the torque be given in lb.-ins, and the tyre radius in. ins. If the torque be measured in lb.-ft., the tyre radius must also be in ft.

The tractive effort may also be expressed as a

proportion of the gross vehicle weight. It is frequently calculated in lb. per ton where

'1.13. in lb.

Tractive effort in lb. per ton= G.V W. in tons

In America, tractive effort is expressed in lb. per 1,000 lb. of gross vehicle weight.

For the purpose of calculation the most useful way is to express it as a percentage, where

r.E in lb. x 100

Percentage tractive effort

G.V.W. in lb.

This tractive effort is absorbed in overcoming three different resistances to motion: (a) Rolling resistance, (b) gradient resistance, (c) air resistance.

Air Resistance Negligible Of these, air resistance can be regarded as negligible at speeds up to the maximum legally permitted in Britain. It may become an important factor for coaches operating at high speeds overseas, particularly where overdrives or axles of very high ratio are employed, but for the purpose of this article it is ignored.

The rolling resistance is appreciable, because it may vary from 10 lb. to 200 lb. per ton weight of the vehicle, according to the nature of the road surfaces. Road surfaces can, in the main, be divided into three categories—good, second class and poor. Rolling resistances per ton load of the vehicle are on an average 25 lb., 40 lb. and 60 lb per ton respectively, according to the surface. A good average figure to work on is 45 lb. per ton, which gives the convenient percentage of 2. This affords a margin of safety for operation on all but the worst of made-up surfaces.

The gradient resistance is the pull exerted by the force of gravity when ascending a hill. This is usually expressed as a percentage of gross vehicle weight, and is equal to the gradient itself expressed as 'a percentage. In some parts of the world it is normal to talk about 10-per-cent, gradients and so on, but it is simple to convert the conventional British method to percentages; thus, 1 in 4 equals 25 per cent., 1 in 10 equals 10 per cent., 1 in 50 equals 2 per cent., and so on.

Power for Acceleration Any balance of tractive effort available after deducting the total of the rolling resistance, the gradient resistance and the air resistance (the last-named being assumed as negligible), is available for accelerating the vehicle. In assessing the performance of a vehicle, however, it is assumed that there is no balance of tractive effort available, and the maximum gradient climbable is calculated by deducting the rolling resistance from the tractive effort, as follows Maximum gradient per cent.— I.E. per cent.—rolling resistance per cent.

If the rolling resistance be taken as 2 per cent., the formula then becomes Maximum gradient per cent.--I.E. per cent.-2.

it is customary to calculate the maximum gradients Aimbable in top and bottom gears, the top-gear gradient 'xiving a useful indication of the overall performance. For peak performance in a hilly area, a top-gear gradient of 1 in 30 (3.3 per cent.) is a useful mark at which to aim with a four-wheeled machine of less than 12 tons gross, 1 in 50 (2 per cent.) for a 12-ton-gross four-wheeler, and 1 in 60 (1.6 per cent.) to 1 in 80 (1.2 per cent.) for a multi-wheeler. These calculations assume 2-per-cent rolling resistance.

It is important to ens ri.e that the vehicle is capable of ascending the steepest gradient which is likely to be encountered in any part of its duties. If it is to be used for local delivery work in a very hilly district, it may be desirable to ensure that it can climb the worst gradients in second gear, so that there is an adequate margin available for restarting in bottom gear with possibly an accidental overload.

It can normally be assumed that a vehicle with a friction clutch can be restarted by a skilled driver on the steepest gradient which it will climb. It is important, however, to remember that some vehicles with special transmissions, such as fluid couplings, cannot be restarted on such steep gradients, because the transmission does not allow engine speed to build up to the speed of maximum torque before the drive is taken up. As a guide, it is generally assumed that where the vehicle will climb a gradient of 1 in x from a standing start, the fluid transmission will reduce the gradient climbable to 1 in x I. Calculating Tractive Effort Returning to the performance of the hypothetical chassis used as the example, all details have been transposed from the specification tables to the appropriate columns. We now have to calculate the tractive effort per cent., which, employing the formula given, is: 4,854 x 6.0 x 90 per cent, in top gear 18.5 x 26800 and 4,854 x 48.3 x 85

-40.1 per cent. in bottom. gear,.

18.5 x 26,800

it being assumed that the chassis has a worm-driven or double-reduction axle of 90-per-cent, efficiency and is equipped with 36 by 8-in. tyres. When assessing the performance in the lowest ratio of the gearbox, account must be taken of a further 5 per cent, loss in transmission efficiency.

It has already been established that rolling resistance to the motion of a vehicle can vary according to .the type of road surface, but for our example the normal figure of 2 per cent, will be used. Thus, by deducting the rolling resistance from the tractive effort, there remains a force, expressed as a percentage, which is available for overcoming the gradient resistance provided by the force of gravity when ascending a hill.

Top-gear Effort A maximum-load four-wheeler operating at 26,800 lb. gross weight is often called upon to climb a I in 50 in top gear, but, according to the calculations, the imaginary vehicle can climb a 3.3-per-cent. (1-in-30J gradient in " top " and could haul a trailer. Its hill-clinibing capacity in bottom gear is again more than would be required in normal service because gradients of I in 2.6 are rarely found except in cross-country operation..

There arc optional tyre sizes and axle ratios available for most chassis of this type and, as an example, the 10.00-20 low-pressure tyre equipment and 7.040-1 axle ratio have been assumed to be the alternatives in the specification. Calculations can be made from the given formula and the performance With respective variations entered on the data sheet.

. This vehicle belongs to the " large-engine" group of heavies and employs a power unit which is also installed in maximum-load four-, six—and eight-wheelers and in heavy-duty articulated units. When operating solo, it would be eminently suitable for exceptionally hilly areas, but would be generally considered overpowered for flatcountry work unless coupled to a trailer.

Assuming that the operator will use this as a trailer model, running at a gross vehicle weight of 49,280 lb, '(22 tons), the payload capacity will be increased to approximately 32,030 lb. The engine torque will remain as before, but it would perhaps be preferable to employ the low-ratio axle, the performance for this being worked out for the data sheet.

From the results it is found that the maximum gradient climbable, when employed with a trailer, in top gear, not forgetting that 2 per cent. rolling resistance is equal to operation on second-class roads, is 1 in 85. This would provide a good performance for a trailer unit in undulating territory and the vehicle should be able to negotiate a 1-in-41 gradient in the lowest gearbox ratio.

Usually, the operator requires to know the maximum speed, a point which is especially important to overseas users. This may be difficult to establish in the case of petrol-engined chassis, because except where a velocity governor is fitted, the engine speed can be pushed up well past the engine r.p.m. at maximum b.h.p.

Where the engine is governed to a maximum limit. is in compression-ignition units and petrol engines with a velocity control, the maximum speed can be approximated from the governed r.p.m., given in the specification tables, and allowing 10 per cent. increase for over-running the governor.

The formula for speed on level ground is maximum m.p.h. = (Engine r.p.m. + 10 per cent.) x Effective radius of tyres 168 x Top-gear ratio x Axle ratio where 168 is a constant and engine r.p.m. is the speed at which maximum power is developed.

In the hypothetical vehicle it has been assumed that the maximum power is developed at 1,800 r.p.m., that high-pressure tyres are fitted and a 6.0-to-1 ratio axle installed. Thus, the estimated speed on level ground would be 1,980 x 18.5 168 x 6 — 36,35 m.p.h.

The overseas operator might prefer to fit low-pressure equipment, in which case the rolling radius is affected and the maximum speed is raised to approximately 39 m.p.h.

The foregoing is intended to help the potential buyer in the selection of a vehicle for normal road operation, but where cross-country, or other unusual, conditions are encountered, he would be well advised to refer to the vehicle manufacturer or a professional consultant before making a final choice.

The low power-to-weight ratio class of goods vehicle is eminently suitable for economical operation in the Low Countries where. the maximum gradients encountered in normal service are not severe.

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