Alternative Tyre Equipment.
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The sizes of tyres recommended for the respective weights are in accordance with particulars supplied by leading tyre manufacturers. It will be noticed that in all cases alternative outfits are mentioned in the table; this is because it frequently happens that the load to be carried is by no means the only factor governing the choice of suitable equipment. The various points which may affect the style of a conversion will now be discussed in order, although, as so many of the factors are interdependent and indissociable, an exact sequence of importance is unattainable.
The first consideration to be dealt with is the diameter of the tyres. This should be selected as near as possible to the diameter of the original solids, with a view to maintaining the clearance of the chassis from the ground and the clearances of tyres under wings and wheel arches, also to avoid affecting the apparent gear ratio. It can be assumed that the lowering of the chassis and the reduction of the apparent gear ratio due to the deflection under load of pneumatic tyres are no more than those resulting from the normal wear of solids. Nevertheless, when no exact correspondence of diameters can be achieved, It is advisable to haye the pneumatics of the nearest
larger size; the vehicle will then be slightly faster than before, which is quite permissible.
The next thing to be considered is interchange
ability. Obviously, from the point of view of convenience, whenever possible, the wheels should be interchangeable all round the vehicle, otherwise it will be necessary to carry two spares. Nevertheless, to have all tyres the same is not always the most satis
factory or economical arrangement. Provided that the diameters of all tyres in a set are the same, the use of a larger section for the rears and spare will not be found any great disadvantage when running the spare on the front in an emergency.
Referring again to the table, it is important to mention that the prices given for outfits composed of different diameters of tyres for front and rear include the cost of the necessary two spare wheels and tyres. In certain instances, however, especially with industrial vehicles, if the difference in diameter between the front and rear tyres is not more than 2 ins., and the wheels are interchangeable in respect of offset and number of attachment holes, it is just feasible, although it should not be encouraged, to rely on one spare, which, of course, should be the same as the tyres on the rear wheels. A ease has come to notice of a conversion which consisted of 40-in. by 8-in. singles on the rear, 38-in. by 7-in, on the front, and 1,010-mm. by 120-mm. solid, especially fitted on a detachable wheel, as the spare.
Points Which Require Attention.
"Offset" is the distance, measured axially, from the centre line of the rim or tyre to the nave of the wheel, and is usually figured to include the nave thIcltness.. When fitting single tyres all round, it is sometimes the practice to use what are known as centraldish wheels, in which the side of the nave is in alignment with the centre line of the tyre, that .is, the offset is zero, ignoring the nave thickness. This may reduce the cost of adaptation of the existing hubs.
Frequently it is desired to convert front wheels only, but it is always advisable to do so with reference to possible subsequent conversion of the rear wheels, otherwise it may be found impossible, later, to select equipment for the rear which will be convenient with regard to interchangeability with the wheels and tyres already in use on the front.
Some Difficulties to Overcome.
A frequent cause of difficulty in changing over to pneumatics is the obstruction of the brake drum, which may be too large in diameter to accommodate the rim of the wheel. If the obstruction be only a small amount this trouble may be overcome by replacing the drums with others of different design, and this is often done, the new drums being part of the conversion set.
With regard to side clearance, for the rear, a minimum nominal distance of 1 in. from the tyre to the springs, chainca.se, or other nearest feature of the chassis is a fair allowance to establish, but owing to the greater width of pneumatics, particularly twins, in' comparison with solids of the same capacity, the track may have to be increased. Operating toggles of external brakes, as fitted to some American chassis, are another source of interference encountered. Points which have to be watched are that the load-line of the hub is not moved too far from the middle of the bearing, and that the overall width does not exceed the figure laid down in the regulations 'under which the vehicle is licensed. As to the front wheels, it is often found necessary to increase the track in order to retain sufficient steering lock with the wider tyres, and the same attention must be given to the question of load line.
Some owners, When converting, take advantage of the opportunity to change from plain bush bearings to roller bearings. With the fronts, as a rule, this is a fairly simple matter, since the stub axles may readily be skimmed up to take the inner races as required.
With rears, however, unless the existing bearings are of the type in which the bronze bush runs on a separate sleeye keyed to the axle tube, so that the tube itself is not subject to wear, then the operation of turning down the tubes for roller bearings, involving, as it generally does, the dismantling of the entire rear axle, is rather expensive and is seldom undertaken.
Turning now to a consideration of points in connection with the actual running of pneumatics, it is most important that the premures recommended by the makers of the tyres should be adhered to, in order to obtain the maximum length of service and freedom from trouble. It is generally advised that with twins the outer tyres should be run -at a few pounds higher pressure than the inners, owing to the road camber and for various other reasons.
From the point of view of wear alone singles have a slight advantage over twins, due to the inevitable slight skidding which takes place on twin tyres when turning corners. It has been found, however, that twin pneumatics have rather more tractive adhesion than singles, a factor to be noted if trailers be used. These comparisons, naturally, only apply to tyres of capacities suited to their respective loads.
Equipment for Special Vehicles.
Some special cases of types of vehicle not included in the table will now be touched upon.
The total and distributed weights of articulatedframe six-wheelers have been found to vary so much that average values would in many cases be misleading. Rigid-frame six-wheelers, being a more recent development, are mostly fitted with pneumatics already and, incidentally, have been found very satisfactory in this respect, because the load on the tyres is evenly divided and does not fluctuate much. Even with three differentials, however, a certain amount of skidding occurs on corners, since the axles remain parallel.
The case of tractors is of a peculiar nature in several ways and demands special treatment. Equipment which has been found satisfactory on the Fordson is 765-mm. by 105-mm. or 825-mm. by 120-mm. (3.E.) front, and 1,085-mm. by 185-mm. (B.E.) twins rear. The
overhang of the rear track line in relatiOn to the hub bearing at first seems excessive, but when it is remembered that the wheels do not actually support more than a small part of the total load this is seen to be not so serious. In this connection, the tyre manufacturer should be specially consulted for the exact pressure to be adopted, as this will vary somewhat
from the normal. Quite possibly special tyres will soon be produced to meet tractor requirements and for the small-wheeled, low-platform class of vehicle.
Those vehicles arranged for forward driving' usually have the weight more equally distributed between the two axles, and this, of course, must be taken into account. It should be borne in mind, if tempted to Et singles all round, that very large section tyres on thefront are inclined to make the steering heavy. Eight-inch pneumatics are about the maximum advisable.
In conclusion, it cannot be too strongly urged that when making enquiries for a conversion set the fullest possible particulars relating to the type of chassis be given, together with the actual and separate axle weights of the vehicle when fully loaded. Most tyre manufacturers can supply the necessary gear for the more popular makes of chassis at short notice and are prepared to deal with any unusual propositions.