Watch Your Tyre Costs
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Tyre Selection is a Matter That is Seldom Given Adequate Attention, Although There is a Model for Every Purpose, Careful Choice Will Cut Costs NOWADAYS there is a tyre for every purpose, and if you select the correct tyre for your particular work, you will secure the cheapest mileage. This facility is the outcome of the modern tendency towards specialization., Manufacturers have realized that the " allround" tyre is a thing of the past— and not surprisingly, when one stops to consider the variety of conditions under which tyres are worked.
Take as an example a coal merchant's lorry. Its work consists of loading up at a wharf or station and pottering about the town, stopping at every other house to drop a few hundredweight, then off again to the next call. Roads are smooth, speeds are low and the vehicle is starting and stopping all day long.
• Overloaded •
Now, the contrast—the longdistance haulage contractor. He probably overloads to the extent of 50 per cent. (that is a mild estimate for some people I know) and covers rough and smooth roads at high speeds, with hard braking and few stops—not only all day long, but frequently all night as well.
Then there is the haulier whose men cheerfully drive over tips, unmade roads, brick ends, bottles, etc., and who do not consider their vehicles to be fully loaded until the wings are resting on the tyres.
If you want a fourth example, there is the farm lorry, which travels regularly over rough, flinty country roads at those times when it is not sunk axle deep in mud.
• Specialization •
Is it reasonable to expect that one type of tyre can serve all these pur
poses? Certainly not. So, rather than attempt to produce the Utopian universal tyre, manufacturers have evolved a tyre for every purpose.
Thus we have tyres specially built for the town vehicle; for frequent stopping and starting on good, smooth roads, and designed to give maximum mileage under not-tooexacting conditions. Then there are " long-distance " tyres, intended to withstand great frictional heat for protracted periods. Others are designed to withstand the roughest of conditions, whilst, lastly, there is a tyre designed purely for grip--a pattern that will lift a lorry out of the deepest and softest mud. This article does not name any particular make of tyre. It is, therefore, impossible to recommend the correct grade in each make for the various uses detailed above, but each of the leading manufacturers has investigated this question of specialization and offers a range which includes the tyre that you require.
Consult your dealer or manufacturer and he will quickly put you on the right track if you give particulars of your work. It is amazing how few people are aware of the wide selection of tyres that is available and of the saving effected by fitting the correct type. It is an investment that will be well rewarded.
The next question is tyre size, which is closely related to the matter
of load. You kriow, of course, that the carrying capacity of a tyre increases in proportion to the volume of air that it will hold; the air, not the tyre, carries the-load.
It is not a question of pressure, but rather one of normal air volume. Thus, a 7.50 by 20 tyre inflated to 55 lb. per sq. in. will carry more weight than a 32 by 6 truck type inflated to 75 lb., simply because 55 lb. in the larger tyre represents a greater volume of air than 75 lb. in . the 32 by 6 equipment.
The question of size is more or less determined by vehicle makers, but, unfortunately, they have a habit of turning out their products Made.quately equipped, from the tyre standpoint, for the work that they have to de. Some makers guarantee that their lorries will withstand a high percentage of overload, but they omit to say that, although the vehicles may do so, the tyres will not.
Of course, if you choose (and pay), you can have an oversize fitted before the lorry leaves the works, but even then you do not always obtain the correct size. Quite recently I had a case where an over-enthusiastic agent had sold a 3-ton lorry to a user after telling him that the 32 by 6 tyres were capable of dealing with a pay-load of 6i tons.
• Interchangeable •
Fortunately, the tyre equipment on most medium-weight commercial vehicles allows of a fair amount of interchangeability. A vehicle fitted with 6.00 by 20 tyres, for instance, has a carrying capacity of 12i cwt. per -tyre (subject to certain conditions mentioned later). If you must overload, you can fit a 32 by E truck type to the same rim (a 3.75 by 20) and increase carrying capacity to 17i cwt. per tyre.
Where 32 by 6 heavy-duty tyres are replacing truck-type equipment, a change of rim is advisable. The 4.33-in, rim is necessary in this case and the vehicle thus equipped has a capacity of 221 cwt. per tyre. Mind you, you can put the larger tyre on to the smaller rim, and there are thousands of lorries running about equipped in this way, but it is bad ' practice to compress the beads of the tyre to such an extent—over in.
• Safe Limits •
The following figures give the maximum carrying capacities of the most popular sizes for vehicles up to 4 tons, the correct pressures when fully loaded, and the correct widths of rim (actual measurements, not nominal) to which they should be fitted This table gives some guidance in choosing a tyre size according to the load, but it must be remembered that the back axle does most of the work and, even when twin tyres are fitted, the individual rear tyres carry a greater weight than do those on the front. This rule is generally applicable, although it does not apply to such an extent on lightly loaded or forward-control vehicles.
To determine the correct size of .tyre for a job, it is necessary to weigh each axle separately when the lorry is fully loaded. The weight of the back axle (including pay-load) should not exceed four times the capacity of the tyre if twins be fitted, or twice the capacity in the case of singles. The same rule applies to the front axle:
If the limits of the tyre be exceeded when the vehicle is under its usual load, an oversize should be fitted. Otherwise the percentage of loss of mileage will be far greater than the percentage of overload.
Gear ratios are always slightly affected when tyres of a bigger radius
are fitted, but most vehicles will accommodate the next larger size without any material loss of power or other ill effect. Where it is desired to go a step farther and fit an even larger size, it is advisable to consult your supplier before doing so.
Always remember that, before fitting an oversize, the lateral and circumferential clearances of the present equipment must be checked to see that the larger tyre can be accommodated. The nearest fouling 'points are usually mudguards, body sides or springs. Check also the offset of the wheels, so that adequate clearance between twins may be assured. The foregoing notes are intended to help the user to select the right type of tyre for his particular work. This is half the battle in tyre economy. Fleet owners think long and carefully before they lay out money for vehicles. What make? What model? What capacity? What type of body?
These are the questions that they ask themselves in trying to decide the best type for their work. But the tyre is given hardly a moment's thought. Little do they think that in each leading make there is a range of specialized types, each of which is supreme in its own particular sphere.