Solid Rubber Tyres.
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Important Points which are oiten ignored by Users.
By J. M. Maelulich.
A detailed study of the commercial motor cannot fail to be of interest to " a nation of shopkeepers "; and in this broad sense every Briton is interested directly or indirectly in commercial transport. There are now many types of successful vehicles on the market, and the chief point to be considered is the cost of maintenance. This depends largely on the driving, but in the first instance on the choice of tyres—iron tyres being out of the question where speeds exceed six or seven miles per hour. City roads are, alas, too full of holes, and even the roads of our small towns are too roughly kept to allow a non-elastic wheel to carry a mechanical vehicle over them without jarring it to pieces in a very short time. Springs are very good to absorb the large up and down shocks, but they do not absorb the continuous small shocks which are the most injurious. An elastic body is required at the root of the evil ; and solar nothing so elastic and tough as rubber has been discovered which successfully absorbs the small vibrations and breaks the sudden shocks caused to the wheels when rolling up or down obstructions on the road ; good springs can do the rest. Nor is it likely that anything will supersede the rubber tyre when we realise how extraordinarily durable the hest rubber tyres are; in fact, it is no exaggeration to say that such rubber tyres are more durable than even steel tyres, and more economical even for heavy goods traffic when everything has been considered. There are, however, solids and solids, and the construction as well as the size and quality have to be considered ; mechanical fastenings such as wires or metal bands are very good so long as they last without breaking or tearing through the tyre, making it impossible to repair or re-fit; unfortunately this happens sooner or later, in most cases without warning, and the trouble being internal is not seen until the Lyre is cut off and destroyed. It should be borne in mind when selecting a tyre that it's essential to have a sufficiently large section to carry the load at the required speed without overworking or injuring the rubber, so that the surface only will wear away slowly : increased speed on a heavy car is equivalent to increased weight on the tyres. A tyre without bands or wires is necessary, and one which is not spread out over the edges of the rim and will not spread by being too small for the load and so get cut by the rim. Such pinching of the rubber between the road and the edges of the rim will soon destroy the best solid rubber tyre; but squeezing the rubber inside the walls of the flanges can do no harm, provided those interior walls have no internal projections which would cause the rubber to cut or split on them when passing between the wall and the road. Squeezing to the ordinary extent inside the rim keeps that portion of the tyre which is not pressed upon the ground on the bed of the rim ; that is, the rubber has sufficient strength in itself when squeezed to counteract the centrifugal force which is small as compared with road strains ; this squeezing inside the rim is totally, distinct from squeezing or nipping on top of the rim, and it prevents the rubber tyre springing off the bed of the rim as the wheel revolves and then being hammered down again by the sudden pressure on the road at each revolution and so becoming pulverised and destroyed in the base. It is essential to have lateral and longitudinal compression to make the rubber extra tough and more elastic and resilient. It is very important to have free access of the rubber into the mouth of the rim; that is, to have the rubber out:flue of the rim narrower than the mouth ot the rim, so that the base of the tyre inside the rim acts as a cushion, relieving the wearing surface and making it last longer. It has been thought that you cannot get both lateral and longitudinal compression; but obviously this opinion is wrong, for to prevent the base working as a cushion, a hard unyielding plate would have to be placed on the top of the rim across the mouth so that the rubber outside the rim would meet the rigid resisting surface which would obstruct the cushion below the level of the top of the rim. Wires or bands inserted in the base of the tyre, to hold it on, to a large extent prevent the rubber inside the rim acting as a cushion under any circumstances.
The higher the wheel (within reason) the better, because a larger portion of the circumference of the tyre is made to carry the load, and the revolutions or hammerings of the tyre on the road are less frequent, which gives it time to recover and prevent slipping to a large extent.
Twin tyres prevent side-slip for the simple reason that they give two edges to cause friction of the road, and should the 'bus slip slightly one edge clears away the grease before the other and gives it a firm grip on the road. In many places twins on the front wheels are advisable as well as twins on the driving wheels. These remarks apply equally, of course, to triplet tyres.
Having given some details of the class of tyre which will be most reliable and economical, let us consider the sizes most successful in general use for the various loads. In speaking of the load we must consider the total load including spares and passengers or goods ; that is, the maximum load the tyres will ever be called upon to carry. For iscwts. on the tyres of the four wheels the most suitable and economical size is a rubber tyre which in its widest part measures 2 in. full; and. for 23cwts. one which in its widest part measures 3in. full; and for zscwt. 41in. full. Above those weights twins or triplets are required on the driving wheels; for instance, for three tons twin 3iin. full ; for over three tons twin, 44in. full or triplets. It is a mistake to have a large single tyre, for it will destroy itself by being too much alive owing to the greater amount of swaying on the rubber. One point in connection with the commercial vehicle which does not seem to he fully realised is how much depends upon the way in which the vehicle is driven. The driver, to be economical, must feel the motion in his car and the tyres and not cause shocks to either ; in fact he must almost imagine the vehicle to be alive and with nerves like his own ; he must drive it with feeling, as a careful cyclist would ride or a careful driver of horses would drive, the result 'being almost a total ahsence of sudden jerks backwards and forwards or sideways in starting, stopping, twisting, and turning in the traffic; he must in fact nurse the vehicle and tires under his care with advantage to the goods it is carrying or comfort to the passengers he is driving. Otherwise, the best of vehicles will be needlessly condemned.