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'Equalize Braking for Artic Stability'
Page 44
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THE ultimate answer—already pracec1. able—to making articulated outfits stable was to equalize the braking effort on all axles proportional to the weight that they carr:ed during braking. This assertion was made yesterday in a paper given to the Institute of Road Transport Engineers in London by John NI. Diekson-Simpson, product planning engineer of the Leyland Motor Corporation.
Mr. Dickson-Simpson opened his paper, entitled "Modern Trends in Articulation ", by recording that artics were getting more numerous, bigger, heavier and safer, and although they were more expensive to operate they gave a great deal more scope on the traffic side of road transport. Listing attractions of articulation which included manceuvrability, longer deck length, flexibility in loading and unloading, and the possibilities of less subsistence allowance payments and of inter-company operation, he pointed out that artics were nevertheless a nuisance to engineers; technical objections included greater capital outlay, higher running costs, poor traction, uncertain stability and difficulties in interchangeability.
He pointed out that artics had in some cases been used as the only way to get valuable deck length and, even under the new regulations, the artic still had this advantage over rigids; within 42 ft. '74 in. overall it was possible to get a 34-ft. deck. In practice, around 28 ft. would be more popular. The author remarked that although the needs of international haulage were claimed to be one of the reasons for the C and U amendments, full interchangeability with European outfits was, in effect, killed by stipulating a 38-ft. axle spacing for 32 tons gross on four axles.
Six-wheeler the Answer?
A six-wheeled tractor might be the answer to interchangeability at 32 tons gross, but weight was important and so was load distribution. There was a need for a lightweight six-wheeled tractor of less conventional form, and if the sixwheeler were given single drive through the rear axle (carrying twinned wheels) with a light, single-tyred axle ahead of it, the unladen weight need go up no more than three-quarters of a ton compared with a four-wheeler; if the wheels of the " slave " axle of the bogie steered, so much the better. A six-wheeled tractor of this single-single-twin layout gave increased payload capacity, and better overall compactness for the complete outfit without spoiling the interchangeability of trailers between different types of tractor.
Mr. Dickson-Simpson said that with the
higher (30/32 tons) gross weights it was a .big help if the trailer king-pin overhang was more than the SM MTrecommended 2 ft. 6 in.; some operators already ran with up to 4 ft. overhang, and tractor manufacturers should realize this.
al 0 On what to specify when buying, he remarked that while every chance of keeping down weight and cost should be taken, initial cost was not as important in long-term profitability as was weight, through its effect on payload. This, he said, was going to become a more real issue as enforcement became more strict.
There was, he maintained, a lot to be said for the economy of the big, slowrevving British diesel despite its effect on weight but there was a growing demand for lighter and cheaper tractors obtaining their power by revving faster. The high-revs-and-low-torque approach permitted lighter and cheaper transmissions but one would expect worse fuel consumption—though neither this nor shorter engine life necessarily applied in practice.
This high-rev philosophy seemed bound to find quite full application among maximum-weight artics for the long-distance runs. If higher governed engine speeds were accepted the next logical step was to move from in-line sixes to vee eights to keep inertia loads and piston speeds down and thus attempt to preserve the durability standards set by the slower engines.
How to get Stability
Turning to stabilify, Mr. DicksonSimpson said that hardly anyone had believed Mr. F. D. Hales of MIRA when, in 1963. he showed mathematically how jack-knifing might be eliminated if enough braking were put through the front wheels of the tractor. But, on the whole, the principle had now been swallowed after much testing of the theory (and the writing off of two cabs at MIRA). However, said the author, it was no use pretending that the mechanics of the artic stability problem were simply a question of boosting the front brakes. Application of the principle was complicated, but the following facts had, he claimed, been established:—
(I) an immediate substantial improvement in stability resulted from boosting the front-wheel braking proportional to the dynamic fully laden weight borne by the front wheels during braking; (2) the ultimate answer was to equalize braking effort on all axles proportional to the weight that they carried during braking (which could be done through a chassis-mounted modulating valve linked to each axle); (3) so long as tractor brakes were balanced, the trailer's braking characteristics did not matter for stability (although they affected braking efficiency.
(4) the best order of wheel locking for resistance to jack-knifing was 1, 2, 3 (counting from the front), and thus delay in trailer braking did not contribute to instability;
(5) from a stability point of view thee was no advantage in having a fifth-wheel coupling more than 12 in. ahead of the tractor rear axle—except above about
50 m.p.h., when 18 in. was desirable; (6) stability improved in direct proportion to tractor wheelbase increases, but about 10 ft. was a good compromise; (7) when roads were slippery, drivers should use the footbrake for retardation rather than engine compression via the gearbox.
Limitations of the Code The latest knowledge on braking, said Mr. Dickson-Simpson, made the Ministry of Transport's proposed code of practice on braking fall short of the ideal. Also, an opportunity for promoting stronger headboards seemed to have been lost; it was a negative outlook to limit braking because of headboard weakness and rather should headboards be strengthened, The author said that the new genera. tion of heavyweight artics was bringing its share of problems, not least on interchangeability; he provided tables ol tractor and trailer combinations showing where coupling limitations lay.
He concluded by stressing that the technical objections to attics were being overcome and nothing should now stand in their way, because of the contribution! which this type of vehicle could make to transport productivity. In particular the Government should remove the doubts about the legality of tractor. trailer interchange between companie! and should encourage piggy-back opera tion if it expected the railways also te reap benefit from the artics versatility
EEC TRANSPORT PROGRESS
From a Special Correspondent
SO long as agreement is reached on 2 forked rates system (considerable doubts still exist about the likelihood o this) the Community quotas for inter national road transport will be intro duced by the EEC on January 1 nex year. The 1966 quota will total 88( authorizations, which permit holders n move vehicles freely on transport opera tions between member states; the import ance of prior agreement on rates .i exemplified by the French attitude, whicl is that France will not allow free circu lation of goods traffic from any membe country lacking a published rates system
The decision on quotas was reachee at the Council of Ministers meeting Ins week, when it was also agreed in prin ciple to introduce the "harmonization timetable for transport. Among othe measures, this recommends the abolitim by January 1, 1966, of double taxatim on international vehicles; duty free entr of fuel in vehicles' tanks to be operativ by April 1, 1967; and the basis of calcu lation of road vehicle taxes to b standardized by January 1, 1968.
There was again failure to agree o the figure of 38 tonnes g.t.w. for roa trains and on 13 tonnes for single axle so the Commission suggested bilaterc agreements between countries who adot different standards, rather than waitin for a. Community solution.