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ARTICS OUT OF CONTROL

15th April 1966, Page 59
15th April 1966
Page 59
Page 59, 15th April 1966 — ARTICS OUT OF CONTROL
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Fork-lift tractive unit from Ulster

A study of jack-knifing accidents

RECENT study of accidents in Warwickshire involving articulated vehicles has shown that loss of control caused by jackknifing occurred in the case of 18 (23 per cent.) of a sample of 79 accidents.

This information was given in a paper "The behaviour of vehicles when braking in emergenciesby Mr. R. D. Lister of the Road Research Laboratory, read at the 13th annual congress of the Federation Internationale des experts en Automobiles in London last week. The congress was arranged in conjunction with The Institute of Automobile Assessors.

In a definition of jack-knifing, Mr. Lister said that it occurred when both rear wheels of the tractive unit locked, when there was a "very fast yawing" of the unit which then swung round and hit the trailer, usually crushing the corner of the driver's cab.

Jack-knifing was particularly violent if the wheels locked when the vehicle was cornering because an unstable yawing moment was produced about the tractor's centre of gravity owing to the side force generated by the front tractor wheels.

Recovery from such a jack-knife was almost impossible because steered front wheels soon swung past the slip angle for maximum side force and releasing the brakes was neither sufficient nor quick enough to afford correction.

After noting that an artic may jack-knife when the wheels of both tractive-unit axles were locked. Mr. Lister observed that if the front wheels locked at some finite time after the rears had locked, this could also be re garded as a normal jack-knife. If, however, the front wheel locked first the unit continued on a straight line until it was about to come to rest, when on occasion it jack-knifed.

On a bend the unit usually straightened up before it jack-knifed, which was possibly caused by the side force from the rear wheels giving a stable turning moment until they locked. A difference in drag between the front wheels and any trailer-swing tendency could initiate jack-knifing, which might be in either direction. This type of jack-knifing was relatively gentle and was termed delayed jack-knifing.

Trailer swing had been shown to be far less dangerous and critical than jack-knifing, claimed Mr. Lister. It occurred when the trailer wheels locked and could be corrected by releasing the brakes. At higher speeds of, say, 30 m.p.h. or more, only gentle turning was needed to initiate swing and this could occur on a straight road.

Earlier in the paper, Mr. Lister mentioned that because of the high ratio of laden to unladen weight of some commercial vehicles, wheel locking readily occurred when the vehicle was unladen, given that the braking force was sufficient to satisfy the laden condition.

Referring to the distribution of braking force, Mr. Lister pointed out that the application of apportioning valves to commercial vehicles to regulate the braking on one or more axles according to the load carried was becoming more important with the demand for improved performance.,

Tags

People: R. D. Lister
Locations: London

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