N OW that rationing is here, goods-vehicle operators are faced with
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the problem of getting a quart into a pint pot. A way of achieving at least a partial solution of this difficulty is to increase payloads beyond the level stipulated by the manufacturer. Overloading is normally to be condemned, but in the present circumstances the practice, as a short-term measure, calls for examination.
Four-wheeled vehicles in the 8-10-ton class and Most six and eight-wheelers cannot be seriously overloaded without exceeding the legal maximum gross weight. It is the vehicle having an unladen weight of 31 tons or less which can be, and often is, abused through severe overloading whilst keeping within the law.
A typical example of severeoverloading. This 5-tonner,with a payload of about 8 tons, needed maximum engine torque and clutch slip to move away from rest on quite a moderate gradient Cash Gain
If the payload of a 5-tonner is increased by 2 tons to raise the gross • laden weight by approximately 25 per cent., the fuel consumption rises by abqut 20 per cent. As the extra 2 tons represent a 40-per-cent. increase in payload, there is a substantial cash advantage in terms of payload-ton-miles per gallon of fuel consumed.
Against this saving have to be set the effects of the increased load on all the mechanical components, which would tend to increase their rate of wear and even cause premature failure, and there is the detrimental effect on the tyres.
Braking would also be affected, and when using maximum retardation the stopping distance would almost certainly increase directly in proportion to the gross vehicle weight. Because of the extra weight, the springs might strike the bump or limit stops when travelling on uneven roads.
The 7.50-20-in. tyres with which the average 5-tonner is shod are capable of withstanding a load of 32 cwt. each when inflated to 95 p.s.i. ' With the payload increased to 7 tons, the load on each tyre of the rear axle is likely to be of the order of 2 tons, which is 25 per cent. above the rated capacity of each tyre. This introduces the risk of bursts and has the same effect as running underinflated.
Tyres which have been run 10 p.s.i. under-inflated have been found to wear up to 13 per cent. more quickly than those inflated to the correct pressure. To obviate completely this condition, or at worst alleviate it, it might be considered necessary to employ larger tyres, such as 8.25-20, always supposing thit the existing rim sizes and clearances permit this to be done. The cost of such a change would be over 130 per wheel.
Cost of Overloading Manufacturers' reports have shown that when vehicles are subject to continvous overloading, the incidence of half-shaft and differential failures is much more marked. These can involve an operator in heavy repair bills, sometimes as much as £150 or even more:
A 25-per-cent. increase in gross vehicle weight would seem to be a good average figure which might be applied for a short time without serious harm.
There is, however, a distinct possibility that helper springs would be required at the rear; not only to prevent bottoming pf the springs on indifferent roads, but also to increase the roll-resistance when cornering. The cost of such a conversion is likely to be in the region of £10.
My object has been to put the facts on overloading before operators, but not to recommend it. No manufacturer condones overloading, but fuel rationing will almost force it on many operators. They must make sure that an undesirable temporary expedient does not become a permanent bad habit.—J.S.T.