An Analysis of the Driving Position in the Modern Motor Car.

BRYAN MCILWRAITH Bsc (Hons) Ost. Med. DO.

 

Reprinted and edited from British Osteopathic Journal Vol XI pp27-34 1993.

 

(It is probably easiest to print out this paper rather than read on screen)


 

 

Most people are probably aware that driving is detrimental to the general health of the spine, and many authorities believe it is an aetiological factor in many conditions. The most obvious reasons are the loss of the lumbar curve during prolonged driving and the vibration and jolting to which the spine is subjected. Whilst these factors are of great importance there are other less obvious ones at work all of which are due to the design of the modern motor car. The object of this article is to highlight the design faults to which the hapless motorist is subjected on a daily basis.

    To illustrate the nature of these faults it is necessary to understand how they arose and to do this a brief look at the history of the motor car is helpful. The earliest cars were little more than adaptations of horse drawn carriages and passengers sat in upholstered upright bench seats. By the turn of the century the shape of the motor car owed much less to the carriage trade and was evolving into a shape of its own which was dictated by its function as a mode of transport and the need to contain the mechanical parts within its framework. Thus was born the "three box" design which is with us to this day. One box for the engine, one for the passengers, and one as a luggage compartment at the rear. Nearly all cars are still built to this pattern and the modern hatchback is nothing more than a "two box" variation on the main theme.

    The earliest cars were exceedingly expensive by the standards of their day and sales were limited by this. However such luminaries as Herbert Austin were quickly on the scene to provide an obvious solution. Make the car smaller and so cheaper to produce whilst at the same time carrying four people. The Austin 7 with its tiny engine and body successfully promulgated this theme for half a century and,  with others like it made motoring available to those of more modest means. The seating position in these small cars however was still quite good. Aerodynamics had not yet become a styling consideration and so the passenger box was still just that; upright and square and so the passengers sat almost as one does in a dining chair. More important however were the design constraints imposed by the mechanical components. The front wheels were mounted on a solid axle which had to be positioned in front of the engine and so the length of the engine "box" could not readily be reduced. The whole structure was based on a separate chassis or frame on which the passenger compartment was placed and this thwarted most attempts to lower roof lines.

Car designers constantly sought to repackage the essential elements of the automobile to maximise the available space and by combining technical innovation with radical thinking some ingenious solutions were devised. In this respect the name of Alec Issigonis is probably the best known and he is normally regarded as a genius in his field. Unfortunately,for our purposes he is also the villain of the piece. Although best remembered for the creation of the Mini, he also designed the Morris Minor and it is worth looking at his work on this car as it shows how his thinking was progressing. Issigonis realised that if he could place the front wheels alongside the engine then he could increase the size of the passenger comparment and yet still have a relatively small car. To do this a new form of front suspension where the wheels were not linked by an axle was needed. This was termed "independent" and is now taken for granted as the method of choice for the front wheels of any car. It must be remembered, however, that many other designers were coming up with similar ideas at the same time and that Issigonis was not the first to utilize the technique but it was his ability to combine the different elements into one design which made his name legendary.

   Having successfully shortened the engine compartment it was also possible to make use of the latest technology to reduce the height of the passenger "box" for the purposes of streamlining the body shape as designers were beginning to realise that aerodynamics were not just a styling fad but were vital for performance and fuel economy. Unitary construction where the body panels are welded together to create an eggshell effect, meant that the chassis could be dispensed with as the body would be rigid and self supporting. The whole of the vehicle would consequently be reduced in height but there was a penalty to be paid in that unfortunately the human occupants remained the same size as before. In order to accommodate them, the designers introduced the bucket seat in which the seat squab is dropped nearer to the floor and angled down towards the rear, thus straightening the driver's legs but also placing his hips below the level of his knees. The seat back must also angled backwards since it is uncomfortable to sit with the hips flexed beyond ninety degrees.

   The full implications of the changes in seat design will be discussed later but, for the meantime, let us return to the mid 1950,s when

Issigonis began to sketch out his designs for the Mini. For this car he took, quite literally, a clean sheet of paper, which is unusual in the world of car design since most models have a great carry over of parts and design concepts from their forerunners. He drew a rectangle with a tiny wheel at each corner. He simplified the suspension until it was little more than large rubber bump stops. Then he added his "piece de resistance". This time the engine would not only be placed between the front wheels, but it was turned through ninety degrees so that it lay across the car, thus dramatically shortening the engine compartment. Furthermore he placed the gearbox internals below the engine in what would normally be the oil sump, so that his new car had neither gearbox nor transmission tunnel to intrude into the passenger compartment. The Mini measured 3.45m (10.5ft) long  and yet 70% of its length was given over to carrying people which was, after all, the main objective.

   The Mini stunned the motoring public when it was announced in 1959 and immediately dealt a death blow to a rash of micro cars which had appeared in the post war years. It was praised for its maneouvrability and handling, and the other manufacturers could not compete with it for price.This was hardly surprising since history has shown that it was under priced and probably did not make any profit at all at the time. The car was indeed technically innovative and broke much new ground but, unfortunately for the driver, there was a trade off. The front wheels which used to be at a distance to the driver because of the previously mentioned front axle, had come ever closer with the advent of independent front suspension, and now the front wheel was intruding directly into the footwell where the control pedals for the driver were supposed to be mounted. The only solution to this problem was to move the pedals towards the centre of the car so that they were no longer mounted directly in front of the driver but off to one side, and since the gearbox had been moved this could easily be done. This is known as pedal offset and although, in truth, it has always been used by car designers, with the birth of the Mini it reached new heights and became an acceptable standard for small car layout thereafter.

   Pedal offset is a result of the juggling act which is forced on the small car designer who has to accomodate passengers and components in a limited space, and so there are mitigating factors for its inclusion. However, examination of the steering wheel in many cars, both large and small, will show that it too is offset for which there is absolutely no justification at all. Up to the end of the forties most cars featured a solid steering column connected to a steering box. This layout was superceded by rack and pinion steering which was more accurate and, due to the revision in the geometry of the steering gear, it became necessary to include universal joints in the steering column. With this new layout it was possible to site the steering wheel almost anywhere on the dashboard. Unfortunately the designers did not take advantage of this and, until very recently, the greater percentage of cars produced featured offset steering wheels. This is an example of sloppy engineering practice which has simply been allowed to persist.

These are the main shortcomings in the driving position but let us examine just how the driver is affected by them, in reverse order:

  Wheel Offset: Obviously the driver may be forced to hold his arms at a slight angle to the body. If the wheel is parallel to the seat back, then this is possible since this arrangement will describe a parallelogram.

  Wheel Angle: Some cars have the wheel mounted at an angle to the dash i.e. with the left hand edge nearer the driver than the right so that the driver sits with left elbow more flexed than the right. In either case, the net result is that most drivers tend to rotate the dorsal spine to the left so that they face the steering wheel.

  Pedal Offset: When sitting in a car seat the thighs naturally tend to abduct. The left foot is then usually close to the clutch pedal. As previously described, however, the right foot will not be close to the accelerator due to the offset and so the driver will have to adopt one of several stategies to bring the foot in contact with the pedal. Whilst it may be that some drivers will sit with their pelvis at an angle to the seat back or possibly they may adduct the right thigh, observation shows that most drivers leave the right thigh abducted but externally rotate it. Due to the flexion of the knee this brings the right foot back to the midline and into the desired position.

  The Accelerator. It is necessary to select a seat position which allows the driver to fully depress the clutch but, although the accelerator may need to be fully depressed at times, it is usually held somewhere between one third or half of its range. Even though many designers place the accelerator slightly below the other two pedals, this still tends to lead to a situation


where the right thigh may be unsupported by the seat squab. If this happens, the weight of the thigh is transmitted down through the calf to the ankle, reaching the foot which is supported only on the point of the heel. The weight is generally distal to this pivot point so the foot tends to fall away from the driver pressing more heavily on the pedal. To control the vehicle speed the driver is forced to actively dorsiflex the foot and perhaps even flex the thigh. This is very tiring and most drivers quickly adopt a solution which is based on the previous one. The thigh is further abducted and externally rotated whilst decreasing the flexion until it rests on the seat squab. The foot is now at a considerable angle to the plane of movement of the accelerator but this suits the driver quite well. The next part of the strategy is to allow the foot to invert passively under the weight of the calf until the ankle locks up. This curious option successfully destroys any leverage about the point of the heel that develops due to the calf bearing down. The driver, having settled down to cruising on the motorway, can now make very slight alterations in speed by small dorsi- or plantar-flexion movements. If this position is hard to visualise, then observation of most drivers will illustrate it readily. Examination of the accelerator pedal in most cars will confirm that the right hand edge is worn heavily confirming that the position described is almost universal even in larger cars where there is no pedal offset.

  The Bucket Seat:  As previously mentioned, lowering the roofline of cars reduced the space available for the passengers and required a modification of the seating position. The author is 173cm (5ft 8in) tall and requires 132 cm (52in) to sit upright in a dining chair i.e. with hips and knees flexed at 90 degrees, but reference to the appended tables will show that this figure for overall interior height is not available in saloon cars, and is only acheived in some four wheel drives and people carriers.

To accommodate the driver in this reduced space, the seat is lowered and tipped backwards. This will have two basic effects. Firstly the knees will start to extend, straightening the leg. This will create a pull on the hamstrings which in turn pull on the ischial tuberosities and tend to roll the pelvis backwards, forcing the lumbar spine into flexion (forward bending). In this position the intravertebral discs of the lumbar spine are subjected to increased pressure and are susceptible to damage. Secondly, with the straightening of the legs, it becomes

 

 

 

 

A worst-case scenario in which which wheel  offset,

wheel angle and pedal offset are shown. The

 pelvis and the shoulder girdle are set at an

 angle to each other. The right foot has come

across to the mid line and the righ leg has been

forced into external rotation.

 

 

uncomfortable, (indeed impossible) for the driver’s body to be in the vertical position, and so the seat is tipped backwards, typically to an angle of 20 degrees from the vertical, which means that in order to look straight ahead the driver must flex the cervical or cervicodorsal spine by 20 degrees to compensate. The head must be held stable in this position and, although it may seem quite a relaxed posture, there is a large amount of jolting and vibration to counter, which means that the cervical musclature may have to work relatively hard and will inevitably exhibit increased tone and possibly shortening of the muscle groups after long periods of driving. (Remember that a rotation of the cervicodorsal area has already been described and you will begin to appreciate the complex nature of problems which may arise.)

 

   As previously mentioned the seat back may be  inclined at an angle of twenty degrees (minimum),and this allows a person of average height to sit reasonably comfortably. However, the restriction on headroom means that taller people will have to tilt the seat further back to give sufficient headroom which will result in an even greater flexion of the cervical or cervicodorsal spine.

   A worrying trend in new cars recently is the introduction of full sliding sunroofs by many manufacturers as standard on most models. Sometimes this has been done without restyling the shell which results in a loss of between 2 and 4 cm of potential headroom. All too often this is exacerbated by the fact that the sunroof has a thick rim or edge which sits directly above the drivers head. Paradoxically the sunroof is only standard in the British market where company car sales are still of great importance to the dealers.

 

Figure A shows 80's large saloon (adapted from maker's own data). Figure B shows 1957 Wolseley 15/50 saloon to same scale for comparison. The lower roof line has forced seat to be angled back and lowered. Note increased flexion of cervico-dorsal region. In smaller saloons the height of the seat squab must be reduced further which will increase angle at knees.

 

 

 

Drivers who are more than 1.83m (6ft) tall simply do not have sufficient headroom in many cars, yet despite  this the author has found that very few of the many drivers questioned seem to be aware of the problem. How then do tall drivers cope with fitting a quart into a pint bottle? Once again observation provides the answer. In addition to tilting the seat back the tall driver tends to move the hips forward on the seat squab and allow the lumbar spine to roll backwards into flexion. In other words they deliberately slide down the seat into a slouched position. Apart from the obvious detrimental effect on the lumbar spine, this position may also lead to an increased flexion of the knees leading once again to the distal part of the thigh being unsupported.

  To recap so far; the driver is likely to be sitting with the cervicodorsal spine flexed and rotated, the lumbar spine flexed and the right thigh abducted and externally rotated, and the right foot inverted. Bearing in mind that this position may be held for hours at a stretch this begins to look like a recipe for disaster and certainly nothing like the "ergonomic design" so frequently quoted in advertising material.

    The faults described so far are by no means a complete list. They are general to many cars but some models have their own particular ones. In rounding off the corners of the passenger box to achieve a low drag factor some stylists have brought the door tops very close to the side of the drivers head. Although they are in no danger of coming into contact with the door many drivers instinctively sidebend their necks away from the door towards the centre of the car. Once again you may observe this for yourself when following some vehicles.

   The design of the seats themselves in some cars is suspect. To give the illusion of a roomy interior some designers shorten the seat squab by three or four centimetres which, although it may not sound a lot, it is the difference between  support or lack of it for the thighs. Some manufacturers proudly proclaim that their seats have a lumbar support. Unfortunately, the seat comes in one size but humans do not and even those of the same size may have vastly different lumbar spines where the lordosis may be short or long, absent or pronounced which means that the chances of the support fitting more than a small percentage of drivers is remote. In the course of measuring vehicles the author found some seats where the squab was not aligned with the seat back. Some had a slight lateral shift but were still perpendicular to the seat back whilst some were set at an angle, usually to clear a large gearbox housing.

   So far only the use of the accelerator pedal has been discussed. Many back sufferers will complain that using the clutch causes pain which demonstrates that the clutch transmits a certain amount of force to the low back. Whether or not that force is capable of producing low back conditions is difficult to assess but it seems likely that prolonged city driving with multiple gear changes will transmit an asymetric force to the pelvis which may well be of aetiological significance. It was felt however that discussion of this and also the position of the clutch foot when not in use, was beyond the scope of this paper.

   The tables which follow are an attempt at measuring the driver's seat and its relationship to the steering wheel and control pedals in a variety of models of cars both large and small. Measuring a car is, however, a particularly difficult thing to do. Unlike a building where a floor or wall can be used as a reference point, a car has very few flat surfaces which can be used for this purpose and so the figures given in the tables should not be regarded as definitive. In all cases the manufacturer has been given the benefit of the doubt and the figures rounded up or down in their favour. I will now describe the method in which these measurements were taken.

   With the seat positioned for a driver of average height (the author), the total height from the floor to the roof of the vehicle in the centre of and behind the driver's seat was recorded.(H1) In the same position, the height between the seat squab where the driver's ischial tuberosities should rest and the roof was also recorded.(H2) (The shape of some seats made this measurement difficult to take.) H3 is the difference between these two figures and is not actually a measurement. It is important because it represents the (theoretically) available legroom but it must be remembered that some floor pans sweep up at the forward end which will reduce this figure. A low figure for H3 is undesirable as, the lower it is, the straighter the leg will be held, possibly leading to damage to the lumbar intervertebral discs as previously discussed. For H1-H3 the higher each figure is, the more room is available to the driver. The maximum and minimum for H2 are 92cm and 86cm respectively which gives a range of 6cm. This is not a lot of room to play with and, bearing in mind that the author found difficulty with headroom in many models, it is fairly clear that tall people may be forced to recline the seat, not for comfort, but simply to fit into the car at all. Where H1 is prefixed by * this indicates that the model was fitted with a sun roof.

  WO is the measurement in cm by which the steering wheel is offset in relation to the centre of the seat back. WA is the angle that the steering wheel is set at relative to the seat back and the driver’s shoulders. There is no excuse for either of these figures being anything other than zero although this is frequently not the case.

  To measure these wheel layout defects a set of  plastic profiles was constructed. One profile was laid across the car through the open side windows to form a perpendicular to the long axis of the car useing the door pillars of the vehicle as a reference point. This gave a datum line from which from whic the centre of the seat back could be found, and then a perpendicular struck from there to the wheel. The amount in cm by which this point deviated from the centre of the wheel was recorded but in all cases was rounded down to the nearest half cm.

   Using the datum line and another straight edge applied across the steering wheel, it was possible to observe whether or not the wheel was set at an angle relative to the seat back. As can be seen from the tables it is not an uncommon fault and is caused by the steering column coming through the bulkhead at a slight angle. To try to compensate for this, at least one manufacturer has made a wheel which is wedge shaped so that in the straight ahead position it lies parallel to the seat back but when turned through half a turn it lies at an angle of nearly ten degrees.

  Pedal offset is extememly difficult to measure in a way that gives any real meaning, but is quite obvious to the experiened eye, and here is based on a visual assessment. It should be noted that, by reducing the spacing of the three pedals, it is possible to disguise a relatively large offset of the accelerator pedal and so overall pedal width should be inspected.

 Having asessed the overall layout of the controls, it is worth examining the rest of the driver’s environment for attention to ergonomic details.  The support given by the seat back should extend up to the top of the shoulder blades. A figure of approximately 55cm is a desirable minimum and most manufacturers achieve this, although, once again, tall drivers may require a higher figure. Some seat backs tend to "roll off" towards the rear from quite low down making the effective support much less. Seat backs which measure 53cm or below, should be regarded with suspicion for all but the smallest drivers. One seat back measures less than 50cm. Apart from not providing sufficient support, such a seat could even be considered dangerous in the event of accident as it would allow hyperextension of the cervicodorsal area to occur.

 The length of the seat squab in most models falls into the 47 - 50cm range with a small number above and below. For comfortable driving, the thigh should be supported well down almost to the back of the knee and a figure of 46 or less is unlikely to do this for all but the smallest drivers. Once again some models feature excessive roll off at the front of the seat which reduces the effective support. A disturbing finding is the fact that a few seats squabs are uneven,ie. they are not directly in front of the seat back and have been shifted laterally by a small amount whilst some are even set at an angle to the seat back. This technique seems to be a device to clear a large gearbox housing but illustrates how some designers are prepared to compromise the human occupants.

   The vertical inclination of  the steering wheel should also be considered. A figure of between 20 and 30 degrees is acceptable for a saloon car where the wheel is mounted roughly at mid chest height, as this allows control of the wheel with a minimum of movement of the upper extremity. A figure of over 30 degrees is really  only suitable for a van or lorry where the wheel is mounted much lower. In a car it will force the driver to abduct the upper arm and require flexion and extension movements of the elbow and wrist and possibly pro-supination of the forearm when turning the wheel. A figure of below 20 degrees is only suitable for sports cars where the driving position is quite different to that which is at present under discussion. Suffice to say that it should not be encountered in saloon cars. Fortunately, most cars fall into this range, but a few have a figure of more than 30deg.

  The  accelerator pedal should  also be inspected. It should be set below the level of the other pedals as this is generally required for driver comfort; ocassionally  the pedal is nearer the driver than the other two. Fortunately this is rarely found as it is exceedingly uncomfortable for anything but the shortest trip as it results in the leg being completely unsupported.

  Pedal width over the three control pedals sholud be checked as previously mentioned. Pedal widths of 25cm or less represent a very tight spacing between the pedals and may cause difficulty for larger drivers. Such close spacing is frequently (though not always) caused by the intrusion of the wheel arch into the footwell as previously described. 

  Finally, but very importantly, the kerb height (KH) of the vehicle should be assesed. The lack of overall height in modern cars can cause problems for drivers in a way that is not associated with the seat but is actually caused by the low kerb height. In a very low vehicle the height of the seat squab when measured from the ground as distinct from the floorpan may be such that when the driver attempts to exit the vehicle his knee joint will be higher than his hip joint. With the lower kerb height the driver will be forced to increase the flexion of the lumbar spine as he tries to move his centre of gravity over the foot which is outside the car This will be accompanied by a measure of sidebending and rotation. It is in precisely this position that the intervertebral discs of the lumbar spine are most susceptible to injury due to the orientation of the laminae  of the annulus fibrosus. This manoeuvre will be even more hazardous after a long journey when the lumbar spine has already been passively moved into flexion and the erector spinae muscles have been likewise elongated.

   By contrast, in the higher vehicle the hip is above the level of the knee. There is already a move towards extension of the hip and the driver can continue to extend the hip and low back as he exits. Try it for yourself if you find it difficult to visualize. Compare exiting from a low saloon and a fourtrack style vehicle and observe the difference.

Utilising the data in the tables is perhaps more difficult than it may appear. For example it is perfectly possible for two cars to have the same figure for H2 but if one of them has a softer seat it will allow the driver to sink down more and increase the available headroom. The figures for the steering wheel are more definitive and can be used directly to select a model where the wheel is not set at an angle nor is it offset.

   Although the tables do show up obvious faults like wheel offset, they do not obviate the fact that drivers come in all shapes and sizes, and it was felt it would be too difficult to interpret them in a meaningful way for the average car purchaser. For this purpose the author has devised five simple tests which can be carried out in any car without the need for tools or instruments as follows;

1) The Praying Test. Sitting in the driver's seat, place the hands together, fingertips and palms touching, pointing outwards from the chest as if praying. The wrists should actually be touching the chest. In this position the hands will form a fairly accurate perpendicular to the body and it should be possible to see if they are pointing at the centre of the steering wheel. If they are not then the wheel may be offset.

2) The Fist Test. With the seat in the normal driving position ( i.e. a position where the clutch can be fully depressed without stretching and the hips  well back into the seat) make a fist with the left hand keeping the thumb to the side of the index finger. The depth of such a fist will measure approximately 50mm and it should be possible to place the fist on the crown of the head. If it is only possible to insert the flat of the hand between the roof and head then there is insufficient headroom.

3) The Look Down Test. With both hands placed evenly on the steering wheel look down at the legs. It should be possible to see equal amounts of both legs between the arms. Frequently the left leg will be visible but the right leg will be obscured by the right arm which may indicate that the shoulder girdle is rotated to the left in relation to the pelvis.

4) The Right Leg Test. This test should be performed after driving the car for a short while. Once again, look down and examine the position of the right leg. Is it elevated above the level of the left or has it fallen out towards the edge of the seat? Is the right foot roughly in line with the thigh as it should be, or has it had to come across towards the centre of the car?

5) The Kerb Height Test. Swing the right leg out of the car as though getting out, and place the right foot on the ground. Try and ensure the lower leg (shin & calf ) is in a vertical position. Now look at the surface of the right thigh. It should be sloping down towards the knee. If it is sloping upwards (ie if the knee is higher than the hip) you will have difficulty when exiting this vehicle.

  If the car can pass these four simple tests then there is a good chance that it is suitable for that particular driver. By utilising the tests, a prospective purchaser should be able to produce a short list of suitable vehicles,  from which they can then make a choice.

 

Now refer to tables in:

A SURVEY OF  WHEEL AND PEDAL LAYOUT HEADROOM AND KERBHEIGHT IN CURRENT MOTOR CARS (1998)