Put a sto o overbraking
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Bill Brock tells how Girling's second-generation Skidcheck could provide the answer
OVER-BRAKING and skidding are major factors in commercial vehicle accident statistics.
One of two things can happen when both wheels of a single axle lock on a two-axled vehicle. Where it occurs on the front axle, a loss of steering control will result. If the rear wheels lock, any small sideways force, such as produced by a road camber, will induce the vehicle to begin to rotate.
With articulated vehicles and drawbar outfits the same principles apply, but because these vehicles bend in the middle the result is often much more severe.
Over-braking on individual axles has several causes. With commercial vehicles, the imposed load varies considerably from the unladed to the fully laden condition. The positioning of a load can also affect the braking distribution between axles. Braking effort needed at the wheel is dependent on the degree of grip that the tyre can give and how slippery the surface may be.
Load proportioning valves were designed to deal with
many of the problems wh arise from loading but can t. no account of road condition practice, however; they are of set incorrectly and poorly rmtained.
Anti-lock systems are rrt sophisticated and more exp sive but much more relief Significantly, most of the 10,1 or so systems that have bE sold within the UK have b( fitted to commercial vehic which are used to cao hazardous loads or have hi risk association.
Of the two British manufacl ers, Lucas Girling Ltd, with Skidcheck axle-based whE sensed anti-lock braking syst which it has produced for o four years now, has around t of the market. Greatest derm is still for retrofit, but so major manufacturers now c sider it as a desirable featurE include at the design stage. L land Vehicles' Roadtrain, for example, is equipped with all necessary wire looms to acc instal lation. fhis year Lucas Girling's ten ars of experience with anti;k systems bore fruit in the m of a second generation item code named GX, using same wheel-sensed priniles, to provide smoother redation and shorter stopping stances than could be iieved with locked wheels er the whole range of road rfaces.
Principal contents of the GX ck include an exciter, a simple el pressing with a number of Is arranged radially, which is ed to the hub; permanent ignet, mounted to the brake bookplate; a memory control relay valve, fitted in the service line after the foot valve; an electronic control module, a solid-state analogue computer embodying integrated circuitry; and a control module with monitor light.
Peak braking performance occurs just before wheel-lock where the peripheral speed of the tyre is 15 per cent slower than the linear speed of the vehicle.
As the wheel rotates the ribbed pattern of the exciter ring passes through the magnetic field projected by the sensor, generating an alternating voltage with a frequency proportional to the wheel speed, in the coil of the sensor.
The control module continuously monitors the change in frequency and thus the speed of the wheel. When the wheels begins to slow down at a rate calculated to promote a locked situation a signal is passed onto the memory controlled relay valve via the solenoid to dump air, so releasing the brakes until the wheel begins to spin again.
The objective of the system is to contain wheel slip under maximum braking to within a 5 per cent band each side of optimum performance. Control is shared. between the electric control module and the solenoid operated memory control rela) valve. This valve memorises thi control pressure of the first cycli to provide a reference allowinc more accurate control of thi subsequent cycles.
The result is a much smoothe operation with a reduction in ai consumption and increased per formance to give shorte stopping distances than wa! previously possible.
The system allows control o individual axles. Each respondi to the signal transmitted by it slowest wheel. With the artic ulated and drawbar outfits, it ii important that all makes of anti lock fitted should be compatibli with each other, to permi change of trailers.
Lucas Girling claims that th GX system can be used in cor junction with electric retarder The system is competitivel priced and designed to last th life of the vehicle. The cornpan has invested heavily to ensur reliability even though produc tion volume is still relativel small. At each stage of manufac ture the electronic assemblie are put through extensive circu testing, to ensure fail-safe mon toring.
Extensive testing of the fu system in this country, on roa and over a range of test sui faces, as well as in the Arctic E• temperatures down to 30°C, ha proved the system's ability t retain vehicle stability and steel i n g control under extrem operational conditions.
A smoother operation make the system applicable to psv fc the first time. The Mk 1 system' deep-lock cycle was considere unacceptable for passenge transport. Compatibility withrE tarder equipment allows contrc in the various modes — servic brake only — retarder only — a combination of the two.