EXPERIENCES WITH• GIANT PNEUMATIC TYRES.
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Seven Million Miles Run on " Midland " Buses. The Advantages Offered _ by Pneumatics. Methods of Operation and Maintenance.
By L. G. Wyndham Shire, Chief Engineer to the Birmingham and Midland Motor Omnibus Co., Ltd.
IF four years ago, when we were beginning to experiment with the pneumatic tyre,, someone had told me that it would prove one of the most revolutionary factors in the operation of Single-deck omnibuses, I think I should have smiled ; certainly I would hardly have believed them. At that time, of course, the pneumatic tyre for heavy vehicles was more or less in its infancy, . and, consequently, manufacttirers of the tyres had very little running experience or data upon which to draw. To-day, however, the position is vastly different, for,. during, the past year or so, large fleets of heavy vehicles have been fitted with these tyres, and, speaking from my own experience, I shall be very glad indeed when: the whole of the fleet for winch I am responsible is running on pneumatics. From the foregoing one will gather that to-clay I am now enthusiastic on the pneumatic tyre for public-service vehicles. One cannot, however, be an enthusiast in regard to items which affect vehicle maintenance unless the cost side be satisfactory. I am pleased to say that the costs of the pneumatic .tyre compare very favourably indeed with the costs of the equivalent solid tyre, taking into account the maintenance advantages which the former offers.
This may be regarded as rather a broad and sweeping statement, and I feel that it is only right to give support to it by producing concrete facts.
In the first place, the majority of the vehicles with which, my experience Is connected are of 32 tons unladen weight, fitted with 36-in. by 6-in. tyres on the front wheels and 40-in. by 8-in, single tyres on the rear. The vehicles each seat 31 passengers, and at overload Times nine additional standing passengers are carried. The maximum passenger load is, roughly, 211 tons for this particular type of vehicle. When laden with 40 passengers, as mentioned above, the distribution of weight is such that the front axle is carrying 1 ton 17 cwt. and the rear axle 4 tons 8 cwt., from which we Jim] that the rear tyres individually are carrying 4,928 lb. and the frOnt tyres 2,072 lb.
In addition to the light 31-ton vehicles, we are also operating a number of vehicles of the 3-ton to 4-ton chassis type on similar wheels and tyres with similar passenger loads, the loads on each of the tyres in this instance being 2,240 lb. on the front and 5,320 lb. on the back.
Up to now we have run some seven million miles on pneumatic tyres, and as the bulk of this mileage has been spread over the past two years, we are in the position of being able very carefully to weigh up the pros and cons of the subject.
From the property owner's point of view, and from the point df view of those people who live in houses adjoining a main road where there is very heavy traffic, the pneumatic tyre is a revelation, for vehicles shod therewith do not cause the slightest tremor or vibration as they pass by. This alone is a very important point, for with the ever growing volume of motor traffic and the increase in the number of goods-carrying vehicles which travel at night, the roads are seldom free from traffic of some kind.
From the passenger's point of view the use of the pneumatic tyre, even if the springing of the vehicle be • not so good as it might be, provides what might be termed luxury riding, for the pneumatic tyre at once smooths out all the annoying body rattles and sharp jerks caused by an uneven road surface.
I am of the opinion that to obtain the best results from any given vehicle, when pneumatic tyres are fitted the old spring leaves should be regraded to allow the spring to flex more readily. A sOlid tyre requires a spring much stiffer to withstand the sudden sharp shock which is produced by the solid-tyred wheel striking an obstruction, whereas in the case of the pneumatic tyre the latter, to a certain extent, absorbs the obstruction and does not communicate any sharp blows to the axle.
Another important feature is the wonderful roadgripping quality which the large pneumatic tyre possesses, due largely, of course, to its greater area of contact. This feature allows of that very quick acceleration, without wheelspin, which is required to-day, and at the same time provides the grip on the road for rapid braking which is also so necessary. I am of the opinion that a motor omnibus shod with pneumatic tyres is a far safer vehicle and is one which may be driven Much faster with a greater degree of safety than a similar vehicle with solid tyres: If any proof is required on this matter, this was given during the recent bad spell of snowy weather. Here the pneumatic-tyrecl vehicles were the only vehicles that could keep the schedules going, and were, in a great many instances, the only vehicles that could keep running at all on the frozen and snow-bound roads. In this connection it is also interesting to note that the tendency of vehicles to skid on greasy roads is reduced very much indeed, and when a pneumatic-tyred vehicle is braked Suddenly and the wheel locks, the vehicle does not skid sideways, but the wheels skid in their own tracks, so that the driver does not lose command of the vehicle.
The Economy in Maintenance Arising from the Use of Pneumatics.
On the maintenance side, if the road wheels be carried on tapered roller bearings and if the brakes be so designed that they do not need frequent adjustment within the brake drum, then there is no reason why the wheel centre and brake gear need be removed or interfered with. The tyre itself may be removed either by means of the detachable flange or the detachable disc.
ib This is one of the points which should be taken into account as reducing vehicle maintenance costs. With solid-tyred vehicles the wheels have to be removed, and it is not always possible with big fleets to keep the vehicle standing idle while the old tyres are pressed off and new tyres pressedon ; consequently, spare wheels have to be brought into use, and in a good many instances brake shoes have either to be renewed or adjusted, all of which means time and material, which cost money.
With regard to the question of whether one should fit single or twin tyres on the rear wheels, I favour the single giant tyre as being the more satisfactory proposition. A good deal has been written on this subject, so that I do not think I need say anything further, except that the single tyre does lend itself to easier maintenance and better design of wheel and brake gear, and, incidentally, reduces by two the number of tyres and tubes which can get punctured or worn, and my experience is such that I should not wish to depart from the single tyre. I am quite aware that there is a limitation in regard to the size of single tyre which should be used, and I think an 8-in. tyre is about the largest which can be dealt with conveniently. Larger size tyres require thicker walls and greater air pressures, so that the resiliency of the tyre when partly loaded is not so good as it should be. '
So far I have devoted my consideration to the good points of the pneumatic tyre, so that I now think it only fair to set out a few of what might be termed the disadvantages.
The first of these is, obviously, the risk of deflation, caused either by a puncture, a burst or a leaky valve. We have had our fair share of troubles in this direction, but so also have we had trouble caused by solid tyres flatting and the band breaking away from the vultanitE base, which trouble was very prevalent at one time. When a tyre is punctured, and even when it deflates rapidly, it is really very difficult, owing to the great thickness of the cover and the strength of the walls, for a driver to know the instant the tyre has no air in it. If the vehicle be run for any distance under these conditions then the cover and tube are both ruined, the cords in the cover being separated and damaged and the tube cut to ribbons. Again, we find that when a driver has noticed a punctured tyre at an early stage he has generally applied his brakes hard and locked the wheel, causing the tyre to skid and to rotate on the rim, thus damaging both the tyre and the tube. .
However, I am of the opinion that the pneumatic tyre on omnibuses gives as reliable a service as that given on private cars—and perhaps even more reliable—and we know that the service given by pneumatic tyres on private cars is very wmarkable, on account of the absence of trouble.
Swarf and Metal on–Black Country Roads.
The actual number of punctures depends very largely on the type of roads over which the vehicles rue.. In one area in a manufacturing district in the Black Country the greater part of our punctures has been caused by pieces of swarf and metal picked up on the road. Here, therefore, we have a condition which is very difficult to overcome or to guard against.
The next important cause of deflation is the chafing of the inner tube against the protection flap which protects the tube from the rim. This trouble has been gone into very fully with the tyre manufacturers, and as a result of a reduction of the width of the band-and of making the edges more pliable this trouble will, we
think, disappear.
The tube which we find most satisfactory and which gives best service is that manufactured by the extruded• or drawn method, similar to a drawn-steel tube, for there is then very little likelihood of the rubber slitting when punctured, which has been found to be the case with the moulded tube, and I believe the former method is becoming general.
Dealing with Injuries to Tyres and Covers.
In the ease of the covers, it is absolutely imperative, if the full life is to be obtained, that all cuts, both small and large, should be immediately cleaned out and filled up with one of the many vulcanizing compounds, and then thoroughly vulcanized. The method we have standardized for dealing with cuts on the outer cover and where the cord plies have not been seriously damaged, is to use a low-temperature filling compound and to cook this compound by placing over the spot a small tank holding about a gallon of hot water, which is kept at boiling point by means of an electric hot-point. It takes, roughly, about 40 minutes thoroughly to vulcanize an ordinary sized cut of about 1 sq. in. Bigger cuts take proportionately longer and the operator working the tank soon gets to know what time should be allowed. The advantages of dealing with cuts by this method are, in the first case, the tyre has not to be deflated and if the vehicle can be moved readily to the tyre repair shop the work can be done with the tyre in position on the vehicle. The other advantage is that if the tank be. large enough and the cuts are close together two or more cuts can be vulcanized at once.
Repairs to tubes are carried out by means of the many small vulcanizing outfits which are specially designed so that over-cooking cannot occur, and this work is not a very costly matter. In regard_ to very bad cuts, or where the cord plies have been damaged or the tubes have been chafing very badly, We find it better policy to return these to the makers for their attention.
I think that old adage "A stitch in time" was in advance of its day, for it so aptly applies to the pneumatic tyre, for if a stitch be given in time the cost per mile can be materially affected.
In regard to the. debatable question as to whether it is desirable to have covers retreaded or not, I am of the opinion that if a tyre has given a really good return of anything between twenty and thirty thousand miles, the cords and the cover have given the service for which they were actually designed. Our experience in regard to the tyres we have had retreaded has been that in nearly every case either the walls have given way or the treads have come loose from the cords, and I am therefore opposed to retreading.
How Inflation is Effected.
The most serious item in maintenance work is keeping the tyres inflated and inflating new tyres when fitted. The pressure we use for the 6-in. tyre is round about 75 lb. per sq. in. and that for the 8-in, round about 95 lb. per sq. in. We find that to inflate a 40-in. by 8-in. tube, when fitted in its cover, to the necessary pressure of 95 lb.. per sq. in., requires a volume of, roughly, 14 cub. ft. of air at N.T.P., which is equivalent to, approximately, 2.15 cub. It. at 95 lb per sq. in. On maintenance work, as time is such an important factor, apart from the cost of labour, it is absolutely imperative that some means be provided for rapidly charging these tyres. The method we adopt for this purpose is to use 'steel air cylinders, each of which contains .83 cub. ft. at 1,800 lb. per sq. in, pressure. Each cylinder Is capable of completely filling approximately six empty 40-in, • by 8-in. tyres to a pressure of 95 lb. per sq. in. The cost of one of these cylinder charges of air is, roughly, 5s., so that, by this method, to inflate one of these tyres when empty costs, roughly, is like and favour this method, which is clean, safe, quick and also portable, inasmuch as the cylinders may be carried about on small trucks to wherever they are required in the garage. For road work, when sending out to serious troubles, we use a smaller cylinder of .33 cub. ft. capacity, charged to the same pressure and capable of filling two empty 40-in. by 8-in. tyres.
The cylinder method of providing the inflation is only possible where facilities exist for obtaining good supplies of charged cylinders or • where an apparatus is installed for charging the cylinders at one's own works. Even where compressors are installed in the works, we find it is necessary to have some such means of bringing the air pressure to_ stationary vehicles in garages.
Where one attempts to run a sub-garage by means of air cylinders only, without any compressor for recharging them, the cost of transporting the cylinders to and from the main compressing station is an expensive matter. I think at the moment there exists a big field for a really good electrically driven two, three or four-stage Compressor which would give satisfactory results. Whatever compressor be used: however, I am still of the opinion that it will be better to use the steel cylinders in conjuuction with the compressor, for the cylinders may be charged by a small compressor over a lengthy period and are available for quick discharge at a moment's notice.
Tyre Inflation by Man-power Not Practicable. The other means for filling tyres is by the ordinary hand or foot-operated pump. We have tried a good many inflators but have not yet found one which will fill an empty 40-in. by 8-in, tube within what might be termed a reasonable time. Furthermore, the human element becomes thoroughly exhausted and requires much "revving." _ I consider this question of providing quick air charges for large pneumatic tyres is one which, sooner or later,, will have to receive the attention of some of the motoring bodies. I would even go so far as to say that I would not be surprised to see in the future that one could purchase the contents of air cylinders at the roadside in the same way as one can purchase the contents of petrol tins at the present moment. I am led to believe that some such arrangement already exists in America.
I am quite aware that a number of heavf-vehicles is fitted with small air compressors driven either directly from the engine or from the gearbox. My experience in regard to these small compressors is that they are hardly worth their initial cost and the cost of running them, for the time taken to fill a large, empty tyre is very little less than that taken with a good foot -pump. To-day one must look after every 'minor item of maintenance, and I, for one, am against the maintenance of an item of this kind, preferring to carry a small air cylinder on long-distance runs.
Carrying Spare Rims and Wheels.
The carriage of spare rims or wheels on the Vehicle is also a matter which calls for a lot of criticism, for unless these spares are watched carefully, the tyres are often found to be deflated when required. Furthermore, if not carefully secured down to the carrier, they are found to be worn badly in places where they have been chafing. The spare tyre, however, is necessary, but it must receive attention if it is going to fulfil the purpose for which it is required. A good plan is to have the spare tyres removed periodically—say every three months—and to have these put into circulation, replacing them by other new tyres. In our own case we carry on each bus one 6-in, spare tyre, which is capable of being fitted either to a front or rear wheel. When the spare tyre is required to befitted on the rear wheel a special adapter ring, which has been designed for the purpose, is fitted on with the rim. This method has worked out very successfully in the past and has enabled vehicles to run many miles until a new 8-in. tyre has been picked up at one of the various garages.
A further source of trouble is that caused by drivers running the sides of the tyres against the kerbstones. Although we ourselves have not suffered to any appreciable extent, it is quite clear that in congested areas, where the vehicles have to crawl along hugging the side of the kerb, serious damage may be done, but this largely depends on the district and the driver. Possibly the manufacturers might be prepared to mould a continuous ring of hard rubber at a point on the outer walls of the cover which would act as a rubbing ring in cases such as the above.
As to which is the most satisfactory method of fitting the tyre and rim to the wheel proper, I favour the detachable rim type, the rim being secured to the wheel by means of a tapered wedge ring, which, in turn, is clamped in position by a number of cleats and bolts. Possibly my principal reason for preferring this method is that we started with it, and, as it proved satisfactory, we have now standardized it. This method allows of a cast-steel wheel being used, and, in my opinion, it provides a more rigid base for the tyre than the disc type of wheel, although this latter gives good service, as we have ourselves found.