Artics on Ice
Page 52
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By P. A. C. BROCKINGTON, n.m.i.mech.E.
0 NE of the most significant aspects of the comments made by Mr. P. F. Foice, Superintendent in charge of the Traffic Division of Gloucestershire County Constabulary during my visit to h:s office in Cheltenham last week, is that the average driver of an articulated outfit welcomes the action of the police in banning artics from ice-covered trunk roads pending grit-spreading operations by the County Council. This attitude is the more significant because it is generally applicable to drivers from all parts of the country. In Gloucestershire two police notices are available for use in critical weather, one of which is posted to ban all vehicles if the road is blocked and one to ban. artics from icy roads, but when a blocked road has been cleared vehicles are allowed through according to grade, cars and light commercials first, then heavy rigids and, lastly, artics. Normally a prohibition notice is posted some distance from the critical section of the route, for example, near a café, so that drivers have the opportunity of turning back or of obtaining refreshment or accommodation.
Hilly routes have created the greatest hazards, notably the London A40 road with gradients up to 1 in 13.5 and the A48 route to Chepstow, the steepest gradient on which is 1 in 8. Incidentally, snow clearance is assisted by a radio network established by the County Council Highways Department, which includes radio control of 84 lorries and 10 fitters' vans, ancris operated in close co-operation with the police.
The banning of articulated vehicles from unsanded icy roads was approved in December, 1960, after a meeting of the Constabulary with members of the Road Haulage Association, the decision to ban discriminately being inspired by repeated road blockage by artics in a severe spell in January of that year. Up to the time that the County was hit by the first blizzard of the current winter on December 30, 1962, bans on artics had been enforced on about 10 days in a 12-month period, and on average vehicles had been individually delayed for about two hours. Despite the far more severe weather this year, compared with January, 1960, fewer roads have been blocked and the majority of avoidable hold-ups have been caused by artics which happened to be caught in rapidly worsening conditions.
In Mr. Foice's view, the most serious shortcoming of the typical artic is the poor traction provided by the single driving axle (it is pointed out that this also applies to a multi-wheeler with a trailing axle) which has frequently immobilized the veh ice during steep climbs on a slippery surface. Lack of traction is regarded as a more serious factor in the case of an artic because, combined with poor weight distribution, it promotes 826 jack-knifing tendencies. Moreover, an artic that is bogged down is far more difficult to recover than a rigid vehicle. Defaulting artics have held up many hundreds of vehicles for several hours, and discrimination against this type of outfit is cited as a measure that " is in the interests of the majority of road users to the detriment of the few ".
It is axiomatic that some artics are worse than others (on ice) and it may be true that some attics are as good as rigids.
Mr. A. C. Jones, service manager of the York Trailer Co. Ltd., Corby, Northants, considers that articulated vehicles can be as safe on icy and greasy roads as rigid types given that they are correctly designed, and this, he states, is substantiated by the experience of operators in North America. In a York leaflet "A Straight Answer to A Straight Question" various factors are listed that contribute to jack-knifing, namely inadequate couplings, faulty weight distribution and inefficient trailer brakes. It is pointed out that the S.A.E. coupling is the only type that affords a bearing surface sufficient to give lateral stability and that with certain types of coupling it is necessary, to transfer a substantial proportion of the load from the rear tractive-unit axle to the trailer axle by moving the latter forward, which increases overhang and promotes jackknifing.
Independent Control The most important braking requirement, listed in the leaflet and cited as universal practice in North America, is the provision of independent control of the trailer brakes, by which the driver can apply the brakes before those of the tractive unit Inadequate lining area and time lag in trailer-brake application are also mentioned as common faults.
A shoe width of at least 6 in. is mentioned as essential for a payload of over 12 tons, and preference is given to the moulded type of lining. A time lag of less than 1 sec. can, it is claimed, result in jack-knifing, and, in the case of airpressure brakes it is recommended that two diaphragms per axle be used in conjunction with a reasonably short and rigidly supported camshaft. Brake equalization and the elimination of tyre patter are also regarded as important measures in the operation of tandem-axle trailers. Attention is drawn to the relatively low efficiency of single-line vacuum brakes, but it is conceded that an increase in friction-material area can improve the retardation provided by this type of system. With the air-pressure brakes of York trailers the operating pressure is 80 p.s.i., whereas the rated pressure of some tractive units is 60 p.s.i., but this differential can be rectified by the use of a pressure-differential valve, which is a
standard fitting on a number of tractive units. It is considered that, if the vehicle is equipped with an efficient braking system, a stabilizer should not be necessary.
Some of these observations are mainly applicable to hard braking when the vehicle is travelling at relatively high speed, rather than light braking when it is moving at reduced speed on an icy surface, but are nevertheless pertinent.
Support for the contention that an articulated outfit equipped with an Adrolic stabilizer has handling characteristics comparable to those of a rigid vehicle comes from Mr. A. Ross, transport manager of Robert Croan and Sons Ltd., Edinburgh, and Mr. C. S. Wood, of C1BA (A.R.L.) Ltd., Sawston, Cambridgeshire. All the 22 Bedford-Scammell outfits of the Croan fleet (with capacities up to 12 tons) are now equipped with stabilizers and there has not been a single case of jack-knifing in any weather, notably in the wintry period this year.
The Croan vehicles are employed on trunking runs with fish to Melton Mowbray and for collection along the coast roads of Scotland and driving conditions have been exceptionally severe. Vehicles in the C1BA fleet comprise three tankers and two platform outfits, hauled by Leyland Comet and Bedford tractive units, and deliveries are made to all parts of the country.
Mr. Wood reports that drivers would refuse to take out a vehicle that was not fitted with a stabilizer.
A device that automatically adjusted the braking force applied to the trailer wheels to match the imposed weight would have obvious advantages, and .)f allied interest the anti-lock type of brake developed by the Lockheed company for private cars might entirely obviate jack-knifing if fitted to the trailer. In the critical event of full braking being employed on a slippery road, a heavily loaded trailer axle would provide a greater braking force than either of the two skidding axles of the tractive unit and this would be achieved without sacrificing lateral stability. Ideally, perhaps, the rear axle of the tractive unit should also have anti-lock brakes.
The importance attached to " balanced braking" of artics in Germany is shown by a clause in the official regulations which stipulates that semi-trailers of 7.5-metric-ton capacity and over be fitted with a device that automatically adjusts braking force to the load imposed on the axle or axles. The regulation will be extended to cover all articulated trailers on July 1 of this year and will be applied to drawbar trailers if results with semitrailers indicate that this is advisable. A suitable system mentioned in the regulations is of the air pressure type, the operation of which is a function of the distance between the axle and the body.