Most people have heard the term “centre of gravity” or, more correctly known as “centre of mass,” but not everyone knows what it really means. In wheelchair set-up lingo, we know that it refers to the fore/aft position of the rear wheels. But scientifically, the C of G (or C of M) has a very specific meaning that is impacted by the location of the rear wheels.
The centre of gravity is defined at “that point of a material body or system of bodies which moves as though the system's total mass existed at the point and all external forces were applied at the point.” What this means is that there is one point in any given object about which gravitational forces are balanced in all directions. If the “object” is changeable, like a human body whose arms extend and flex, the centre of gravity moves with every change.
For the purposes of wheelchair prescription, what we really care about is the centre of the gravity (wheelchair plus user) in a fore/aft direction. If we look at a side elevation view of a wheelchair with a person sitting in it, we will be able to identify the centre of gravity as the point that has half of the weight in front and half behind, half above and half below. If the centre of gravity is behind the rear axle, the chair will tip over rearward. If it is in front of the caster axle, it will tip forwards. The significance of knowing approximately where the C of G is located is that, with this knowledge, we can specify set-up to optimize the wheelchair performance. Things that improve performance include reducing the weight on the front casters and shifting it to the rear wheels (the only place it can go). This is accomplished either by moving the rear wheels forward, the casters forward, the client rearward or some combination of the three.
In general, you want the C of G back as far as possible such that rearward stability is functionally manageable. This will minimize the load on the casters and result in lower effort wheeling and turning. Optimizing the C of G is important for all wheelchair users, from active paraplegics to frail elderly clients.