John Bullar
Website http://www.individualfurniture.com
BETTER CHAIRS: A designermaker's view, by John Bullar
First published in Furniture & Cabinetmaking, October 2004.
What other inflexible artefact makes such intimate contact with the body as a chair? If you tried on a pair of shoes, you might tell straight away if they were too small, but not much else about their comfort. Shuffling around the shoe-shop carpet in them for a few minutes, generally gives a clearer picture, but even then you can be caught out and find they are crippling a mile or two down the road. Is it any surprise then that chairs, which are often bought on appearance or if tried at all, only for seconds, can turn out to be disastrous?
FUNCTIONAL CHAIRS
Now I am not saying that good chair design - whatever form it takes - will solve all the problems associated with too much sitting, but it must be important. So how can we predict which chair designs will function well? Galen Cranz, in her book 'The Chair', addresses many aspects, including ergonomics, in remarkable detail. Professor Cranz explains that seat height should allow the heels to rest on the floor where they will carry 40% of the sitters weight. This avoids pressure on the back of the thigh muscles, which can prevent blood flow. A downward slope on the front edge of the seat also helps in this regard. However, if the seat is too low the knees will be higher than the hips, causing stress on the hips and discs in the lower back.
Seat tops are usually either padded, or else shaped from wood as in the case of Windsor chairs. Cranz says the sitter's weight should be distributed through bones, not flesh and this means that any padding should be thin.
ONE SIZE FITS ALL
Of course, people vary in proportions as well as in height but a good starting point for finding the ideal seat height is to accept that the distance from underside of the knee joint to the heel is one quarter of the persons height.
Statistical graphs of height distribution across the population show two peaks, one at 5ft 6 inches (1676 mm) and one at 5ft 10 inches (1778 mm), corresponding to average women and average men. There are wide natural variations around these peaks, as well as a tendency for young adults to be about 3 inches taller than their grandparents are. Taking the average person's height as 5ft 8 inches (1727mm) and dividing by four gives 17 inches (432mm), which is normally taken as the optimum height for a standard seat.
The majority of the adult population fall within the height range, 5 ft (1524mm) to 6ft 4 inches (1930 mm ), so at worst they have to cope with chair seats that are 2inches (50mm) too high or too low for them. Of course, this provides little comfort to those individuals who are not in the majority of the population.
CHAIRS FOR BEARS
One option is for people to have individual chairs made to measure, like the three bears that Goldilocks visited. This is a service that the individual furniture designer maker may be able offer as an extra advantage over the mass market. Of course, special small seats have always been used in primary schools, as those of us who have squeezed into them to watch school concerts, will be very aware!
Increasing the seat height for a tall person is clearly an option but reducing it for an adult who is short, or has short legs, would accentuate their lack of height when sitting with others. Normally, sitting down is a great leveller in this regard and a seat that makes short people look shorter or have an awkward reach for the table is not likely to be well received.
Another strategy may be to build in a low front rail, which braces the chair legs and may also be used as a footrest. This design feature appeared on chairs in medieval times and the extent to which these footrests were used can be seen by the amount of wear on the rail, with a high chair usually showing more wear than a low one.
BACKRESTS
The main distinction between a chair and a stool is that the chair has a backrest allowing the sitter to lean back without risk of falling. For centuries people have been making chairs with backs that are essentially straight and tilted back a few degrees behind the vertical plane. More recently, lumbar support for the lower back has become a common chair feature. To achieve this, the splat or spindles are curved to match the profile of a person sitting upright in a posture that is health for the vertebrae. This feature is somewhat controversial because people who are accustomed to slumping in chairs will find it 'digs in' to their backs and effectively forces them to adopt a healthy posture to avoid it.
It is possible to achieve a similar degree of lumbar support without a pronounced curve in the chair-back by providing an opening between the back and the seat to allow for the buttocks and coccyx.
ARMRESTS
Armrests are generally assumed to be there for resting your arms on - reasonably enough. However, in practise people who are dining will be holding knives and forks and people in meetings or offices will have their hands on pens or keyboards. There is often a table or desk to rest your arms on and if not, they will rest quite comfortably on your thighs. Overall, there does not seem to be a big requirement for an extra place to rest the arms.
My belief is that the main function of armrests is to aid the process of sitting down and standing up - something that the elderly and those with joint problems can otherwise find difficult. Maybe that is why many family dining set have two 'carvers' - dining chairs with arms, for the elder members. The arms also function as handles for moving the chair closer to the table during the process of sitting or moving it back before standing.
From the chairmaker's point of view, the armrests can also have the function of bracing between the front and back legs and restraining the upper stiles making the chair stronger. The normal height for armrests is about 8 inches (200mm) above the seat.
Many 'easy chairs' seem to defy all the rules. They have large amounts of padding with no real back support and are far too low for the feet to take any weight. Used together with a footrest they can function closer to beds than seats. Armrests are pretty near essential for these, otherwise getting out of them can be a struggle, even for the able bodied.
STRENGTH
Apart from having a poor shape, another way in which a chair could cause injuries would be by collapsing under the sitter and depositing them on the floor - something for the chair-maker to avoid at all costs! If the chairs are designed to cope with all shapes and sizes they must, of course, be built to cope with all weights.
A popular stunt performed by some sitters is to rock their chair back on its rear legs. This results in the sitters weight being cantilevered off the rear seat joints. Fortunately the trick is mostly done by young children who are light in weight, but there are some adults who will attempt it.
Ideally the rear joints would be strong enough to cope with rocking but a heavy adult would put enormous strain on a joint, inevitably loosening it if they do not immediately break it. Traditionally these would be mortise and tenon joints, pegged to stop them working loose.
Another defence against compulsive rockers to design a chair such that the rear feet are well behind the seat, making it near impossible to rock. This also improves stability and avoids accidental tipping back.
PROTOTYPING
Taste in the style of chairs comes and goes as the wheel of fashion turns, but judgements about what design is comfortable or not, remains constant - discomfort will never come into vogue. Whether you are making for yourself or for someone else, you need to get it right. Chair making takes a good deal of effort and can consume surprisingly large amounts of wood, especially if the design includes curves cut from solid. Chairs that are individually made for a client, will tend to be highly priced compared with mass-produced chairs, which are often astonishingly cheap. If you are producing a set of them it is even more important that the design you are working to functions as a comfortable seating platform. One of the best ways to confirm this is to build a prototype. Often these are made from softwood, plywood, mdf etc and may have screwed joints allowing them to be disassembled and modified as necessary. If you make it obvious that the prototype is an adjustable tester, people will not make the mistake of thinking that you just make rather crude chairs. 
TESTING
Having gone to the effort of building the prototype, it is worth giving it a thorough and critical testing, ideally by a range of people sitting in it for at least half an hour each. Then be prepared to act on the results and re-think the design where necessary.
(c) John Bullar, August 2004