• Home
• Used vehicles
• Product news
• Dealer news
• Compliance
• Buying advice
• Archive

Eap, E ffriU Lxi TEO

by John Dickson-Simpson

ERGONOMICS, or human-factor engineering, might seem one of those gimmicky terms of modern times to some designers, but when you think about it, all it does is put a label on the welcome tendency to design the machine to suit the man instead of expecting the man to adapt himself to the machine. This gets progressively more important as machines become more complicated and as the environment —which in the case of road transport means shorter driving hours and road congestion—becomes more demanding.

On the road-transport front, ergonomics has already shown its value in improving safety standards through promoting more efficient driving. There is still quite a long way to go.

The effect of vehicle design on a driver's human efficiency can be far-reaching. The relevant ergonomic factors are mostly concerned with positioning the controls in the right place and making sure that the effort needed to operate them strikes a balance between limiting fatigue and preserving sensitivity. There are other significant influences on a driver's performance, however. Noise and Vibration are examples and these are two areas where commercial vehicles are generally deficient.

Fatigue is also fundamentally influenced by a driver's posture while he is sitting on his seat. Getting the right posture is not such an easy matter, and first impressions often do not correspond with reality. There is a wealth of difference between short-term impressions and long-term effects. For example, a seat with soft upholstery can give the impression of being extremely comfortable, but as far as long-term fatigue effects are concerned fairly hard upholstery is less tiring. Seat comfort is more a function of correct dimensions, angles, positioning and upholstery-filling frequency than of expense in cushioning trim or even range of adjustment.

For minimum long-term fatigue the seating positions should be such that the centre of gravity of the body's trunk is over the two bones at the bottom of the spine. In other words, it is healthier to sit up straight if forced to sit for a long period. The back of a seat should therefore be quite close to vertical and not be angled backwards more than about 10 degrees.

So far as the cushion is concerned the main depth of upholstery should be at the base where the major part of the driver's weight is imposed, yet many seats have the maximum depth of cushion at the front edge. Also, although it is useful to support the length of the thighs there should be none of the upward rolling of the front edge of the cushion which seems to capture the imagination of so many seat designers. A roll on the front edge of a cushion restricts the circulation of blood behind the knees and gives rise to long-term aches in the thighs.

So it can be misleading to base design on concepts which are essentially only opinions. Having said that, there are some areas of driving environment which still need to provide room for individual preferences. To make sure that short as well as tall drivers can be comfortable a substantial range of seat adjustment is essential.

It is interesting that proper use of ergonomic data rarely adds 'to the cost of the product. There are exceptions, of course; noise insulation is a case in point. But it costs no more to put a seat or an accelerator pedal in the right than in the wrong position. It costs no more to position mirrors so that they are nearer the normal line of vision than to mount them far back so that the driver has to turn his head to see in them. It costs no more to trim an interior in a colour liable to blend with the colour of the road surface than to use contrasting colours which cause eye fatigue.

There are still plenty of examples of basically poor ergonomics in some respects on both cars and commercial vehicles.

Incorrect relationship of seat, floor, pedals and steering wheel is quite common. By incorrect is meant that a significant number of people's joint angles are forced to take an attitude outside the zone of long-term comfort.

The ankle angle should lie within the 85 to 95deg zone and the knee angle should be between 100 and 120cleg. Other zones of joint-angle tolerance are 86 to 100deg for the hip joint, 80 to 90deg for the elbow, 10 to 20deg for the back inclination, 15 to 35deg for the bottom of the neck and 10 to 20deg at the top of the neck.

Steering wheel The steering wheel needs to be high enough to provide adequate knee clearance between the pedals and the steering wheel rim and yet low enough to avoid cramped angles in the arm joints. It also needs to be angled sufficiently to avoid wide varitions in the arm-joint angles when the hands are shifted from one position to another on the wheel rim.

If the driving position is such that the joint angles are not ideal then there will be aches and pains after long periods of driving. Perhaps particularly sensitive these days is the ankle-joint angle imposed by the accelerator position. If the accelerator angle is too acute it will cause a pain in the knee. This, together with dull aches in the undersides of the thighs caused by badly shaped seat cushions, is now one of the most common causes of discomfort on long motorway journeys where the accelerator tends to be kept in substantially the same position for long periods.

Also, a common mistake in the design of seats for commercial vehicles is that their dimensions are not generous enough; there is no room for slight shifts in position. Moreover, it is often the case that there is not enough natural ventilation in the upholstery material; a seat surface should always have flutes in it to provide air spaces, and a material such as leather or a woven cloth is always more comfortable for long-term sitting because it does not aggravate sweating. This becomes even more important when designing vehicles for service in hot countries.

Cab size When all the acceptable proportioning has been done on a driving position needed for long-term comfort, it will be found that at least 5ft interior depth is needed in the cab. Yet this dimension is more commonly found for overall cab-depth than for internal depth on present-day trucks. Add to this the locker space which really ought to be provided in cabs and it becomes evident that in the future truck cabs should really have a bumper-to-back depth of 70in. rather than 60in. Concessions such as this are surely to be expected if drivers are to maintain their standards of safety in a traffic environment which is getting more and more arduous.

It is often found with commercial-vehicle cabs that having set the driving position comfortably and arranged a, deep windscreen it is not so easy to position minor controls within easy reach. The instruments also tend to be rather remote from the driver ifthey are placed on the dash. There is therefore a growing preference for grouping minor controls and the instruments in a housing on the steering column. Switches and air valves can then be instantly to hand. Minor deficiencies often still remaining are that too many switches are grouped close together so that it is not easy for the driver to distinguish their functions, and the figuring of the instruments is too fussy and detailed.

Instrumentation Instruments are often expected to do too much. In the times elapsing between a driver

glancing at his instruments, something can seriously go wrong. The modern thinking is that instruments should denote only the extent to which something has gone wrong and should not be relied upon for informing the driver in the first place that something is not working properly. In other words, instruments should perform quite a secondary role. A single warning lamp or buzzer can do the primary indication. Then, only after the primary warning does the driver need to scan his instruments. This primary warning light should be a distracting one. Secondary warning lights should not be so distracting.

Internal reflections in a windscreen are often more numerous than they ought to be. They could be reduced by better concealment of instrument lighting, more use of dark matt surrounding surfaces and having the windscreen raked at least 5deg from the vertical.

The extent of a driver's vision has a major influence on safety and there are certain inherent inhibiting factors in a commercial vehicle on this score. Rearward vision in a direct line to the rear is often obscured by body or load, so that a view in the mirrors will not disclose a closely following vehicle. Reversing while looking backwards often entails opening the cab door and leaning out. It is a help if cabs are wide, if the door window is large enough for a driver to lean out, if the driving position is fairly close to the side of the cab and if the mirrors are outrigged far enough to be beyond the sides of the vehicle.

Vision to the nearside is nearly always very poor because of the height of the bottom line of the windows. Deep windscreens or deep front corner side windows relieve the problem to some extent but still leave blind a large section of the nearside. Some drivers fit an additional small mirror to the nearside, angled so that it gives a view of the area in the immediate vicinity of the cab on that side. This seems to be preferred to a kerb-sighting window in the nearside. Drivers often dislike kerb-sighting windows because of the distraction from being conscious out of the corner of their eyes of the high-speed view of the roadside.

Some degree of wrapround at the sides of a windscreen is usually beneficial in a truck cab. It moves the pillars farther back from the• normal straight-ahead field of vision, and this is particularly useful on commercial vehicles because of the thickness of the pillars (most commonly between 3+in. and 5in.). Too much wrapround, however, begins to bring shortcomings in other respects: cab strength can suffer and insufficient space is left for the doors.

Centre pillars

Centre pillars in windscreens hardly ever interfere with vision in practice. because of the eyes' parallax effect. The only practical advantage of a single-piece windscreen is the overlapping wiper area it permits. When the nearside wiper is bottom-pivoted and the offside wiper top-pivoted the windscreen is cleared in just the places through which a lorry driver, in his high position, wants to see. But the superior wet-weather vision through a single-piece windscreen can be overwhelmed by the need to keep windscreen replacement costs low, especially where there is frequent risk of breakage—as is indeed the case in many overseas territories.

On the face of it large mirrors are an advantage but the advantage can be more imagined than real. Exceptionally large mirrors are not needed and indeed they can obstruct forward vision. Convex mirrors can be quite compact, and no other type. of mirror can give the field of view which is needed for a commercial vehicle. However, a flat mirror on the driver's side is useful for judging distances accurately.

Mirrors are also often mounted too far back. When they are outriggeoforward they are much closer to the driver's peripheral field of vision so that they can be seen at a glance instead of by turning the head. In any case, when they are mounted forward, they are pushed into a fresh air stream which is not contaminated with road spray liable to deposit dirt and water on the mirrors. With the mirrors outrigged forward in this way there is a chance of their being viewed through the corners of the windscreen and perhaps even through the area swept by the wipers.

Now that commercial vehicles are generally operating at faster speeds than they did a few years ago, aerodynamic problems which were not evident before are beginning to arise. On a short, basically rectangular shape such as a truck cab the problems are extremely difficult to beat. Even where front edges have generous curves there remains a problem of air swirl around the back of the cab which soon deposits dirt on the rear windows. Much more research needs to be done on this problem, which is linked with that of spray flung from the front-wheel arches. A side benefit of a better understanding of the aerodynamics round a cab could be more efficient ventilating air-flow through the interior. At the moment there is no problem of getting sufficient ventilating air into a cab but there is much to be desired when it comes to getting air out of it.

Ventilation Ventilation is particularly vexing in 'hot countries. A vigorous through-draught of air is needed at both leg and neck levels. At the same time there must be adequate fly-screening, and vents ideally should be closable or baffled so that heavy rain is not driven into the cab. Also, insulation against the sun is extremely important. At the least, thick insulation or double-skinning needs to extend over the whole roof and down the back of the cab to waist level. While the rear-window vision needs to be adequate for safe reversing, the windows should be shallow to avoid the sun's rays causing discomfort through the back of the cab.

The European tendency to fit extremely deep windows in cabs is an embarrassment in hot countries. The need for good vision is just as strong in hot as in cool countries, however, and this is why there is a lot of attraction in Triplex Sundym glass which dramatically cuts infra-red radiation without spoiling clearness of vision materially.

The noise in the cab of a goods vehicle is

requiring accelerated attention because of rapidly changing operational environment, quite apart from the ordinary commercial pressures of competition which are generating refinements to develop sales appeal. Besides the public concern abour external noise froin heavy commercial vehicles (and too often the public criticism is emotional rather than factual) there is a fast-growing sociological aspect of the internal noise with which the drivers of the vehicles have to contend.

No reliable evidence has been submitted so far to prove that the noise to which truckdrivers are exposed does noticeably impair their health or their daily performance. But this is speaking generally. There are, after all, some heavy vehicles which suffer from a loud cyclical reverberation, often stemming from the transmission, which can produce a lulling effect over sustained periods. Also, low-frequency drumming sometimes induces a headache. It is no exaggeration to say that one or two designs have had such a high noise level that the driver has been partially deaf after a day's driving.

So there are cases of noise being severe enough actually to cause physical discomfort. But even the bulk of commercial vehicles which are not noisy to such an outrageous extent now require quietening to a marked degree.

Detrimental effects of noise are influenced as much by the time to which a human being is exposed to noise as by the intensity of the noise itself. A high noise level need not be worrying if the exposure time be short. If the noise level is halved, the exposure time can be doubled. For example, if a noise of 96 decibels were tolerable for four hours, then 93dB would be tolerable for eight hours. What needs to be remembered is that as transport productivity improves, exposure times to the noise in a cab are getting longer even though the total hours of work are being reduced. This is particularly true of long-distance vehicles. Also, running at higher speeds involves working with higher noise levels.

Experts differ to some extent when it comes to defining what sort of noise level can be considered comfortable. It is not a bad rule-of-thumb guide, however, to say that the comfort limit is the noise level at which normal conversation cannot be heard. That level can be taken as 75dB, according to one of Britain's leading experts on automotive noise, Mr C. A. G. Mills, of the Motor Industry Research Association. Such a low noise level can indeed be achieved if sufficient care is taken, but at the moment a reasonable compromise between what is desirable and what is commercially practicable is probably 850. Not many heavy commercial vehicles can hit that figure as manufactured in standard trim.

The two areas of attack on noise which require the most attention are to the engine itself and to the sound-absorption of the cab. It has been amply demonstrated that sound-deadening the cab can reduce the noise level by 10dB without involving inordinately expensive treatment.

Suppressing the noise which enters the cab is just as much a matter of care in construction as of the choice and design of materials and of sound-insulating techniques. There is absolutely no point in applying expensive sound-deadening if there are small gaps or holes in the cab floor. Sealing around pedals and levers and engine covers is therefore of paramount importance. It takes only a millimetre gap to double the noise transmission.

Cab structure

The basic cab structure should be stiff and have a good resistance to noise transmission. Here, pressed steel has an advantage. Too much reliance tends to be put on damping compounds daubed on panels; they reduce resonance but do nothing to cut radiated noise. To get worthwhile reductions in noise, soundabsorbent materials are needed to prevent a build-up of noise through reflection of sound waves. To get the basic intensity of the noise down, a backing of material, such as steel, with a high mass per unit area is needed.

The first line of defence against noise is the cab floor and any associated engine cover. A large measure of success will follow the application of a noise-absorbent layer to the underside of a steel floor (with no gaps, remember). The flanking floor as well as the area immediately above or around the engine should be treated. Further advance can be made in suppression of noise through the floor and engine cover if damping and more mass is applied on top, inside the cab. This is why a moulded-rubber covering, if possible in a single piece, is so effective, especially if backed by a thickness of felt. From then on it is a matter of applying damping compound to all panels and of fixing sound-absorbent material to the underside of the roof and the rear bulkhead, which are the major panel areas.

How successful are British commercialvehicle producers in meeting all these ergonomic requirements? So far as the practical application of ergonomics is concerned, the British automobile industry is now almost certainly leading the way, speaking generally, even if the lead in the study of ergonomics as a pure science still lies in the USA.

On the whole, the big manufacturers are more successful at meeting ergonomic requirements than the small ones. There is no real reason why this should be so, however. Most of the knowledge is available from outside institutions and is just as open to small as to big manufacturers.