Tr c Recent advances in heavy vehicle braking including air-operated
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disc brakes, ABS and EBS have transformed modern truck braking performance. But the pace of development is such that the engineers have moved ahead of the law-makers in Brussels. Gibb Grace reviews the current state-of-the-art braking options, and looks at how it the legislators are responding. After 40 years of development, the drum brake is about to be consigned to the scrap heap. It started out as a mechanical system, with the brakes actuated by rod and cable linkages, and progressed through hydraulic operation to the full air systems we know today. But, while the operating system has improved, the physics of the basic drum with its expanding shoes have not, and this remains its Achilles Heel. With trucks getting heavier, and running at higher average speeds thanks to ever more powerful engines, a more effective brake is needed, and the disc brake is ready to take on that role. Though not a new idea, it needed a considerable amount of development work to get it right Mentor's disc brake guru Paul Pentin explains that both disc and pad materials are very different from those used for drums and linings, and finding the right combination took time.
Equivalent drum brake Today the disc brake has reached a certain maturity and its simple concept works in everybody's favour, allowing it out-perform an equivalent drum brake in every area. For a start, the flat pad makes perfect contact with the flat disc, regardless of whether the disc is below freezing or glowing cherry red. Both pads and disc are rigid and do not distort under even the most severe loads, and this means the actuating system is efficient in terms of the amount of air used, and faithful in the way it translates the driver's demand at the pedal into braking at the wheels.
Unlike drum brakes, discs have no built-in multiplying effect due their inherent geometry. Whereas a twin-leading-shoe drum brake can have a brake factor of 3.8, and thus increase the input force by as much as 3.8 times, discs have a factor of around 0.75. As a result, discs suffer hardly any of the fluctuations in brake torque which are so prevalent with drums and, being much more tolerant of friction level changes, they exhibit little steering instability under braking. Finally, discs have outstanding no-fade characteristics and show hardly any performance deterioration under sustained heavy braking.
These attributes are impressive enough in themselves: when they are combined with anti-lock braking (ABS) and electronic braking control (EBS), the case for discs becomes overwhelming. This is particularly the case on drawbar and artic outfits, where the trailer/semi-trailer is also equipped with disc brakes.
Tests carried out by Mentor, using identical tractors hauling drum and disc braked semi-trailers, showed that even from 60km/h the disc/disc vehicle can out-brake the disc/drum vehicle by its own length. But equally, if not more impressive, is the superior control of the disc/disc vehicle under braking. The quicker reaction time, the smooth, simultaneous build up of braking torque at all axles, and the fact that the brakes adapt to meet the dynamic load imposed on each axle, combine to produce a significantly shorter, yet safer stop.
Smart electronic control The combination of disc brakes and smart electronic control has enabled engineers to surpass current legislative requirements with ease. The latest EU legislation requires the braking system to achieve a minimum deceleration of 5m/sec2, or 0.5g. This is not difficult to achieve with modern technology, which begs the question if the legislation should acknowledge these advances with a tougher target of, say, 7m/sec2.
There's also the problem of compatibility between old and new tractors and trailers, and the legislation would have to recognise and allow for this situation. In any case, although the manufacturers have demonstrated these braking advances, they have been on a limited scale. Mentor's Paul Pentin admits experience with high-mileage, discbraked, heavy vehicles is still limited.
"We are confident of the results we get from our test rigs and development vehicles, but need to have them confirmed by operator experience," he says.
In the real world operators tend to be more concerned with downtime and running costs than ultimate braking performance, so they're unlikely to switch to discs unless there is a "carrot" in the form of lower operating costs or a "stick" in the form of tougher braking legislation. Certainly the case for lower running costs is strong, as there are fewer parts to go wrong; and changing pads, for example, takes a fraction of the time needed to change linings. But legislation takes years rather than months to reach the statute book. In reality, heavy truck braking is an increasingly complex subject and the legislators are hard-pushed simply to keep abreast of developments such as ABS and EBS at the current braking level.
Whole-vehicle type approval Last month an amendment was published to the EU braking Directive 71/320/EEC which, if adopted, will mean that ABS will be required on all commercial vehicles above 3.5 tonnes gross by 2001. However, this ruling will not automatically become effective in the UK, as commercial vehicles are not yet covered by whole-vehicle type approval so the Government would have to introduce secondary legislation.
While Brussels has just about caught up on ABS, however, the ECE in Geneva has already been discussing amending its braking Regulation 13 to take account of EBS. The ECE usually leads the thinking on legislation and trading nations, and blocs such as the EU, after an appropriate time for discussion, generally use it as a basis for their own legislation. However, such is the need to formalise the rapid advances in EBS, that on this occasion the EU is likely to propose changes to the braking Directive based entirely on these developmelits in the ECE.
The philosophy behind such legislation is to take account of new technology, while at the same time avoiding any increased risk, setting a minimum performance standard and, if justifiable, requiring it to be fitted to all new vehicles built after a given date. On this basis Brussels can acknowledge the advances made in braking and prepare the way for its application without actually demanding an improvement in stopping distance.
As ABS, disc brakes and EBS become the norm, braking will improve significantly anyway. And legislation is always written as a minimum requirement, so any manufacturer which thinks it can sell better braking without incurring higher running costs is welcome to try. Hauliers will end up the winners. Design advantages of EBS. controlled disc brakes + Flat pads contact a flat disc resulting in even pressure and even wear; + No distortion of the disc or pads under load leads to lower air use; + Lower air use means that the brake can be applied and released faster, making ABS more responsive; + Fade is all but eliminated, so braking torque remains constant regardless of temperature; + A significantly lower "brake factor" means that any changes in friction level between disc and pads produce proportionately lower variations in braking torque; + Lower hysteresis, or "give" in the actuating system produces a more direct feel at the pedal.
Operational advantages of EBS-controlled disc brakes + Fewer moving parts to wear, so fewer parts to replace; + All brakes wear at the same rate; + Disc pads are easier and quicker to replace than shoes; + Pads do away with the need for profiling and running in; + Less maintenance is required; + Automatic compensation for wear; + Discs can remove the need for a traditional load apportioning valve.
Performance advantages of EBS-controlled disc brakes + Reduced stopping distance; + Reduced braking delay; + Simultaneous braking on all axles; + Each axle brakes according to the weight imposed on it; + Smoother, more controlled braking.