Opinions and Queries
Page 54
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Bigger Tyres for Bigger Loads
I READ with considerable interest the comments an I page 169 of your issue dated September 16, 1955, under the heading "New Gross Weight Brings Tyre Problem."
In effect, the increase in permitted gross loading of vehicles weighing over 4 tons tare and the rise in the permitted axle load from 8 tons to 9 tons do make inadequate the tyre equipments used for the former gross loads and axle weights; this applies more particularly to the two-axled vehicle, which formerly had a gross permitted weight of 12 tons and a maximum axle weight of 8 tons, now increased to 14 tons gross and 9 tons .axle weight.
However, the equipping of vehicles with tyres suitable for the new permitted loadings does not present a really grave problem, even for the two-axled type.
Let us consider them separately:—
For the two-axled type, 14 tons gross with a 9-ton rear-axle load gives 5 tons on the front axle. That means a tyre load of 45 cwt. for rear tyres and 50 cwt. for front tyres.
Obviously, the tyre equipment formerly used on such vehicles when weighing only 12 tons gross with an 8-ton axle load, viz. the 9.00-20 12 PR carrying only 40 cwt. per tyre, is quite inadequate for the new conditions of loading.
Most vehicles fitted with 9.00-20 12 PR tyres have a wheel offset of 51 in.
On such wheels may be fitted D.20 Metallic tyres, which have practically the same section as the 9.00-20 12 PR. As, however, the theoretical offset of 5 in. is found to be sometimes slightly under, we recommend the placing of a i-in, spacer plate between the wheels when D.20 are fitted as twins.
The D.20 tyre has a carrying capacity of 51 cwt. and is, therefore, entirely adequate for the front tyre load of 50 cwt. and has a margin of safety. for the rear tyre load of 45 cwt.
It is, therefore, a complete solution for two-axled vehicles weighing 14 tons gross and having a 9-ton rear axle load.
No 9.00-20 tyre is adequate.
It might be thought that the solution would be the fitting of 10.00-20 14 PR tyres, which have a loadcarrying capacity of 464 cwt. Such tyres, whilst being capable of taking the load on the rear wheels, are not adequate for that at the front.
Apart from adequacy, however, other difficulties would be encountered in fitting them on existing vehicles. The 10.00-20 14 PR is much bigger in section than the 9.00-20 12 PR and could not be fitted on existing wheels. New wheels of larger offset would be needed and, even if they were fitted, it might not be possible for the vehicle to accommodate the 10.00-20 14 PR because the combined effect of increased offset and increased tyre section would mean a reduction in lateral clearance of about 1 in. which, on many vehicles, would not be possible.
As regards rigid three-axled vehicles, single-steered with rear bogie, these, which may now have a gross weight of 20 tons, weigh about 5 tons on the front axle, i.e., a tyre load of 50 cwt. The rear tyre loading is about 41 cwt. per tyre.
There is no real problem in equipping such vehicles c14 with adequate tyres, because the D.20 Metallic is suitable for the front axle on existing wheels; either the C.20 Metallic or 9.00-20 "X," each with a carrying capacity of 43 cwt. per tyre, are suitable for the rear bogie and can be fitted to existing wheels.
Rigid four-axled vehicles laden to 24 tons gross give a load per tyre on the rear bogie of about, 42 cwt.; the front tyres are not quite so heavily laden. Once again C.20 Metallics or 9.00-20 "X" are adequate and can be fitted to existing wheels.
It will be seen, therefore, that the fitting of adequate tyres on vehicles used at the newly permitted maxima of gross weight and axle load does not mean " over-tyring " as suggested by your comment—" fitting the next size would mean over-tyring and thus add unnecessary expense and weight."
London, S.W.3. M. R. Goma, Michelin Tyre Co., Ltd.
" Truc-trains " for Britain?
THE statement made by Lord Forbes regarding the 1 conveyance of loaded lorries or trailers lay rail, which was recorded in your issue dated December 23, was most interesting. I consider that the subject is one of great importance in this country as our road traffic is reaching the overflow point.
The only restricting factor, in my view, is the British loading gauge, enforced by the many old bridges and tunnels with which the railways have to contend. The present dimensional limits are: height, 13 ft. 6 in. from track level to bridge crown, overall width, 9 ft. 6 in. In the U.S.A. bridges over the main lines are more elevated and allow a full height of 16 ft. and a width of 10 ft. 6 in.
The only rail vehicles in Great Britain which would be suitable for " truc-train " working would be the 50-ton bogie bolsters employed for carrying long girders or loads of rails.
It is doubtful whether road vehicles would be able to clear the loading gauge unless they were mounted on small wheels, such as those on the Karrier Bantam. Again, in the U.S.A., special low-loading bogie rail vehicles with end ramps have been constructed for such work, and the leading railways are building trucks capable of taking two semi-trailers of average size. These truc-train services are in operation between New York and Boston, Philadelphia, Chicago, Buffalo and leading -cities in the southern states; also between Chicago and
St. Paul and other mid-west cities..
They constitute a joint affair between the trucking companies and the railways. Where outside transport concerns use the services, the road people provide tractors and.crews at each end of the run. Deliveries are being maintained between New York and Chicago, a distance of 960 miles and the trains comprise 50 trucks carrying 100 semi-trailers. They are fully equipped with air brakes and roller-bearing axle boxes. The average speed can be as high as 70 m.p.h. and true-trains are given priority over ordinary freight. Unloading docks are provided at all depots which serve them.
In Britain we must make more use of the railways in some way such as this to relieve the strain on the .roads.
Doncaster. J. W. MORRIS.