?L reports on Antilok
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port just published by the Road Labor atory—LR191—states that )1( anti-locking brake system made motive Products Ltd. completely jack-knifing of an artic which irred with the standard braking [he device prevented the tractive r wheels locking, and braking did not differ. much from those with the unmodified vehicle; stopance was slightly shorter on a urface and slightly longer on other
■ ntilok system consists of three :s, a skid-sensing unit (driven by a from the propeller shaft) which gnal when the wheels are about to
removes the signal when the .e rolling freely again; a servo unit eases and reapplies the brakes on signals from the skid sensor; and ator unit which determines the
which the brakes are released. hide used for the RR L tests was a 17.6 ton gross tractive unit coup
single-axle trailer. It had. airtydraulic braking and the Antilok coupled to the air servo for the
rear brakes. Tests were carried out unladen, fully laden and part laden on three types of road surface—wet rounded gravel (slippery), wet fine-textured asphalt (good resistance to skidding) and dry finetextured asphalt (high resistance to skidding). The relative co-efficients of the surfaces at 30 mph in the locked condition were 0.21, 0.47 and 0.55.
The main series of tests consisted of braking the vehicle to a standstill on a straight path from various speeds up to about 40 mph. With the Antilok system out of use, full brake application produced some "jack-knifing" on all the road surfaces used and with all load conditions tested. It can be presumed that the term "jack-knifing" is used in the report to indicate even a small amount of out-of-line movement of the rear end of the tractive unit because it is stated that "severe jack-knifing" was experienced at 35mph when part laden on the dry asphalt and at 30 mph when fully laden on the wet asphalt. No reference is made to maximum-pressure braking on the wet rounded-gravel surface so it can be presumed that the vehicle kept straight on these tests. But the report says that partial brake applications on all surfaces and in each load condition caused the rear wheels to lock before the fronts; the pedal force required to lock the fronts was nearly double the values required to lock the rears. The report concludes that "with the standard vehicle, jackknifing is liable to occur if the brakes are applied sufficiently to lock only the tractor rear wheels."
Brake stopping distances from 30 mph with the Antilok system in use were between 4 per cent and 7 per cent shorter on the low-adhesion surface as compared with the figures when the system was not in use. They were marginally longer—up to about 6 per cent—on the other two surfaces.
Since the publication of LR191 it has been reported that the Road Research Laboratory is at present carrying out largescale assessment of anti-lock brake devices in co-operation with British Road Services. About 300 BRS attics are being fitted with anti-lock brakes and accident records will be compared with 300 similar vehicles using standard braking systems. The trials began in October and will continue for at least a year.