PNEUMATIC TYRES ON BUSES.
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The Experiences of the Birmingham and Midland Motor Omnibus Co., Ltd., in Running Pneumatic-tyred Vehicles for Nearly 32,000,000 Miles.
AN extremely interesting paper, entitled "The Use and Maintenance of Pneumatic Tyres as Applied to Commercial Vehicles," was read last week by Mr. L. G. Wyndham Shire, the chief engineer of the Birmingham and Midland Motor Omnibus Co., Ltd., before the Institute of Transport, and as the data obtained concerns a large number of pneumatic-tyred vehicles operating over an area of some 9,000 sq. miles, and the vehicles comprising the fleet have travelled well over 32,000,000 miles, the paper is one which bears the stamp of authority.
The roads over which the vehicles operated are of every kind and condition, including those in the Black Country, which are often narrow and with granite setts and tramlines, and these cause more trouble than elsewhere as regards pneumatics because of the quantity of metal swarf, etc., found on them. In some cases the road surfaces are ordinary macadam, and in a few cases not even this.
A Fleet on Single Pneumatics.
The whole of the mileage has been completed -with the vehicles shod with single tyres front and rear, those at the front being of 6-in. section and those at the rear of 8 ins., the approximate vehicle weights being: Front axle, 2 tons 6 cwt. ; rear axle, ;tons, depending upon the number of passengers carried, the accommodation being for 37 seated, whilst standing passengers
are occasionally carried. The tyres, therefore, support greater loads than they are stated by the makers to be capable of carrying, the redeeming feature being that for the greater part of the day the vehicles carry pay loads of only some 20 per cent, of the total.
During the past five years some 7,800 covers have been used, and 1,200 of these have completed very high mileages, 575 running from 25,000 to 30,000, 204 from 31,000 to 35,004, 100 from 36,000 to 40,000 and smaller numbers up to as high as 57,000. In all these cases they have completed their life without giving any trouble, and they were given no special treatment; in e36 other words, they were not regarded as
• "mother's boys." The bulk of the tyres —approximately 80 per cent.—failed at mileages 'between 10,000 and 20,000, whilst 31 per cent, did not reach 10,000.
• Causes of Tyre Failure. The causes of tyre failure are as follow ;— (1) Something exceptional during manufacture, whereby the cord plies become detached from each other, causing early bursts.
(2) The tread wears off evenly without any signs of carcase trouble, the wheel alignment being correct.
(3) The tread develops loose patches due to lack of adherence to the carcase in the first instance. Failure due to this cause is frequent, and (MCC a cover develops a loose section of tread the trouble usually spreads over the whole of the tyre. It can be caused by cuts and punctures not being filled in, so that water penetrates between the rubber and the ply. An indication that a tread is loose is given by the growth of a blister adjacent to the loose portion, and if a blister be cut open it is found to contain powdered rubber.
(4) Over-vulcanization is showe by a series of longitudinal cracks where the tread joins the sides of the carcass or in the bottoms of the grooves.
(5) Lack of resistance to abrasion of the tread. This varies greatly in the covers supplied by the same maker and in covers of different makes.
Mileage to be Expected.
Better average mileage has been ob., tained with pneumatic tyres than with solids, the 6-in, front tyres approximating 25,000 and the 8-in, rear tyres 17,000, and, as these figures take into account the good and bad (and dada.' 1923 there were more bad than good)', the results'are the more surprising. For 1927 the averages are 26,800 miles and 18,200 miles respectively, and these averages are rising.
According to the makers' list, a set of two 36-in. by 6-in. tyres costs 426 fiss and two 40-in. by 8-in. tyres £45 18s., making a total of 172 4s. Taking the average mileage as 28,500 in the first instance and as 19,000 in the second, this gives the average figure per mile of .22d. and .58d. respectively, making a total of .8d., or approximately Id. per mile, which figure is, of course, reduced if the tyres be obtained below list prices.
It is rather interesting to learn that the cost of running particular vehicles on solid tyres is found by the company to be in emcees of this figure, being
approximately .87d. per mile. It is clear that pneumatic tyres are certainly not dearer, except in first cost.
With solid tyres, not only the tyres, but the wheel itself, have to be taken off when the tyres fail, and in the case of the rear wheels, where the brakes are applied on rear-wheel drums, the removal of a wheel nearly always necessitates the adjustment of brake shoes or possibly the changing of brake drums, for different wheels are usually fitted complete with the new tyres. The ability to detach and change a pneumatic tyre without removing the wheel is a very real advantage.
The purchase outright of pneumatic tyres instead of buying them on a mileage basis is recommended, as the owner then takes More care of his tyres through his staff than would be the ease if the manufacturer owned the tyres.
Pneumatic Tyres on Haulage Work.
The empty vehicle on solid tyres undoubtedly has a very rough time, as there is insufficient load to enable the tyres to obtain any real grip, and the speed of the vehicle is very limited, whilst in bad weather on snmoth-surfaced roads the tendency to skid is emphasized. On solid tyres a loaded vehicle, when travelling over a bad road at a good speed, is liable to suffer much damage, as the wheels can easily be seen to dance about the road. Pneumatic tyres offer many real advantages, such as increased speed, less damage to the mechanism, less damage to the goods, and the vehicle can be driven faster when empty owing to the greater absorption of minor shocks by the tyres themselves, whilst the liability to skid is reduced, but the tyres demand considerably more attention than formerly.
The author is however, firmly convinced that within a very short time the pneumatic tyre will be found fitted as standard to all goods-carrying vehicles up to 2-ton useful load capacity.
On the six-wheeler the better distribution of the load is such that in practically all cases pneumatic tyres are warranted, except, perhaps, where the very roughest type of haulage work is carried out.
Ile pointed out that it. is extremely difficult for a driver, even on a welleprung bus, to know when a rear tyre has punctured, and frequently a vehicle is driven for considerable distances before the trouble is. located, and ill many cases the inner tubes have been cut into minute pieces.
Single v. Twin Pneumatics.
As an officer responsible for the main-' tenance of a large fleet of buses, the author has found that the principal item which has caused delay on the road is the single pneumatic tyre, and in this connection he explained that up to the present he has strongly supported the single tyre, both for the front and rear axle, as it offers decided, advantages, such as simplification of the brake gear, more robust tyres, easier fitting and the use of an ordinary spoked cast-steel wheel, which assists in getting rid of the heat, whilst, as the tracks of the. rear wheels fall within the tracks of the front wheels, it is easy to avoid damage to the sides of the tyres.
The mileages obtained are greater with single tyres and the cost is less. whilst the riding is infinitely better with 8-in. tyres at the back than when twin 6-in. tyres are employed, and twin tyres decrease the miles per gallon by between half and three-quarters of a mile.
Twin tyres, however, have certain advantages. They are interchangeable front and rear, the smaller tyres are easier to handle and have lower inflation pressures, the centre of gravity of the vehicle is lowered and there is less liability to skid, whilst on muddy roads or sand the space between the two tyres tends to fill up and forms a wedse which helps to carry the vehicle and check side-slip. By carrying only one spare tyre, in an emergency the vehicle can be run on one of the twin tyres on each of the rear wheels, so that only the fourth puncture would hold up a vehicle; this is a very important point.
Punctures in Twin Tyres.
Careful experiments have been carried out with 36-in. by 6-in, twin tyres. The two inner tyres on the rear wheels of a bus were deflated, the load then being taken by the outer tyres. This vehicle ran for five weeks in this manner and completed 3,500 miles without the running staff, the road staff or the drivers reporting that the rear tyres were deflated, and the first intimation of trouble was due to the puncturing of one of the outer tyres. Three of these tyres, including the two under load, are still at work, although under normal conditions, and each has completed over 40,000 miles.
The simplest way of testing to find if a tyre be deflated has proved to be to tap the tyre with a 2-1b. hammer.
Experiments have proved that twin 36-in, by 6-in. tyres could be operated for a time with one of each of the twiu tyres deflated. To meet modern requirements the vehicles had to be brought lower to the ground, and it was decided to use a smaller diameter tyre on the rear wheels. At first attempts were made to obtain 32-in. by 8-in, single tyres, but at that time they were not made, and further experiments were made with twin 82-in. by 6-in. tyres, and similar deflation tests enabled a vehicle to accomplish over 1,000 miles on a fast read -with heavy traffic, with no trouble except chafing of the valves of the deflated tyres by the rims, which also occurred in the previous instance.
Low-pressure Tyres.
Experiments have been carried out with low-pressure tyres of 8.25-in. section at the front and 10.54n. section at the rear, the pressure being approximately 50 lb. per sq. in. So far as wear is concerned, they appear to behave in it similar manner to high-pressure tyres, but as pressure failures are more important the tyres had to be watched very carefully. In the author's opinion, the difference in riding comfort is so small as to be hardly noticeable.
The author is firmly convinced that the front axle with a short spring with thin leaves provides infinitely better ing than the longer spring, hut teas nobly long springs should be used at e rear, where a floating movement is a advantage. Rebound snubbers should b an asset.
alms and Their Effect on Tyres.
The author stated that with pneu
atie tyres wheelspin due to tractive e ort very seldom occurs, and the g tautest wear and tear are due to the r ar braking, the skidding wheel, when pulling up, being nearly always that at the off side as a result of torque action on the cross-shaft. Four-wheel brakes, therefore, offer the user every advantage, and particularly in greasy weather. The hydraulic type of brake is strongly favoured, as this gives perfect equalization of the braking effect on each wheel.
Wear on Front Tyres.
To avoid excessive wear on the front tyres there should preferably be no toein at the front, but in any case this should not exceed /I in. The inclination of the front wheels to the vertical to give camber or splay should be between one degree and two degrees, and the front axle should be so designed that the distance between two lines, one drawn centrally through the top and bottom sections of the tyre and the other drawn through the main axis of the steering knuckle joint, should be between the limit of 1 in. and 2 ins, at the ground level. This is particularly important with front-wheel brakes.
General Notes.
For 1927 the average vehicle mileage run between tire failure and tyre failure which had involved lost mileage amounted to 110,000. The average time taken to ehange' a tyre proved to be between 20 and 25 minutes, but by the introduction of twin tyres it is hoped that there will be no holding up of thevehicle on the road. Drivers are trained to observe the exact peripheral length of the tyre which should be in contact with the road, and diagrams are supplied to each garage to assist in this, this applying only to single tyres. A tyre gauge is used every week or after 1,000 miles' running, the average fall in the air pressure in this period being between 5 lb. and 10 lb. per sq. in. Wornout covers are sold for 2s. N. each. Air for tyre filling is supplied from cylinders in which 100 cubic ft. of free air is compressed up to 1,800 lb. per sq. in. These cylinders are 4 ft. 6 ins, in length and 7 ins, in diameter, and will completely fill approximately six 40-in. by 8-in. tyres to a pressure of 100 lb. per sq. in.
The Discussion.
Following the reading of the paper there was an interesting discussion, which was opened by Mr. E. S. Shrap nell-Smith, who pointed out that the encouraging fact with buses in which he was interested was that when pneumatic tyres were employed in lieu of solids they gave a much greater mileage than as expected. In his experience pneumatic tyres were dearer, but well worth the difference, and why London buses should run on solid tyres was beyond his comprehension, and, due to the over-cautiousness of the authorities of Scotland Yard, Londoners have to ride in comparative discomfort.
The next contributor was Sir George Beharrell, managing director of the Dunlop Rubber Co., Ltd. kle referred to the paper as being most valuable and hoped that it would be widely read. It was necessary to study the conditions of tyre usage, and there should be the closest co-operation between users and tyre manufacturers.
War Department Experience.
Captain C. H. Kuhne, C.B.E., D.S.O., referred to the subsidizing of 30-40-ewt. vehicles on pneumatic tyres, which, when started about 1923, was severely criticised, as it was then diffi eult to get users to take an interest in pneuntatic-tyred vehicles. It was very interesting to find such great improvement in a few years. So far as the War Department was concerned, the wedging aetion on twin tyres was found to be better for running over soft surfaces, but there is a tendency to pick up etone, which damage the tyres. Twin tyres enable a vehicle to run home on the second tyre of each pair in the case of damage. A new oval-section tyre of wide flat pattern was giving good results, and it might enable single tyres to be used instead of twins on W.D. vehicles. Experiments had been conducted with so-called puncture-proof tubes, hut not with any great success, and now a -tyre filling war being tried out in one tyre of each pair, so that the vehicle is less vulnerable.
An Expert's Views.
Mr. W. H. Paull, a technical expert of the Dunlop Rubber Co., Ltd., said he was not surprised that tyres should stand up occasionally to overloading, but the risk of puncture and failure is greatly increased in such circumstances and tyre life is lost. He referred to twin tyres which were fitted close together. This causes all the flexing or bulging to be thrown on to the outer walls, thus shortening the life. There is no difficulty, he stated, in determining how tyres fail if the cause be within the tyre ; it is external causes which are so difficult to trace and the clues to which must be given by users. Thus it is important to keep correct tyre histories. The Dunlop Co. could guarantee that tyres run under test conditions and drawn from stock vary very little in mileage results obtained, and it would be very serious for the manufacturer if there were great deviations. The whole question is one of how the tyres are treated, and he gave several instances of identical tyres operated by different companies, and with one company the average mileage proved to be over 48,000 and with another 22,000; in two other cases the mileages were 14,600 and 28,800 respectively.
Tyres for Earth Roads.
Mr. Philip Johnson made a plea for special tyres for earth roads. He pointed out that most of the world has no roads other than these and that the greatest step forward in transport will be the regular operation of vehicles over such roads. Tyres for these must have a very low pressure not exceeding 20 lb., for continual trUffie.
Mr. R. Howley, C.B.E., of the Birmingham and Midland Motor Ontnibus Co., Ltd., referred to the main difficulty of operating buses on pneumatics as being that of puncture, but, fortunately, thie fault was much. less fre
quent. Twin tyres prevented stoppages, but were more expensive to work, did not give the same average mileage. are higher in capital expenditure, and
cause more petrol to be used. He pointed out that locking rims are not yet satisfactory, and through accidents occurring when pumping up tyres guards are now employed.
Mr. R. E. Morley, of Bryant and May, Ltd., stated that he was making tests of the relative value of pneumatic and solid tyres on commercial vehicles, and that in the case of 30-cwt. machines there was an instant and appreciable increase in mileage per gallon, a decrease in oil consumption, and a reduction of 50 per cent, in repairs.