Wheelchairs often have slanted wheels. This is called camber, and the purpose of camber is to provide more manoeuvrability by having decreased rolling resistance.
This means that there is less friction between the ground and the wheel; when wheels are straight, there is more wheel surface connecting with the ground and therefore more grip. While this seems like a good thing, it makes the chair harder to push, and although it is not a big difference, this difference in resistance can be significant over a long time.
Camber also gives an increased base of support which gives more stability. This is because the chair has a wider surface area; the wheels stick out further than the base of the seat.
A wheelchair user can be thought of as a triangle shape, with the head at the top and the wheels at the bottom, and the bigger the base, the more stable the whole triangle.
Slanted wheels are closer at the top to the occupant, which means that propulsion is more efficient. The bio-mechanical movement of your shoulder – the way you use your shoulders to move your arms to push the wheels – wants to be as efficient as possible to conserve energy and to protect your shoulders from damage.
Commonly, wheels are slanted at three degrees of camber, but in the case of sports wheelchairs, much greater angles are used to allow for more stability and manoeuvrability.
A downside of slanted wheels is that the overall width of your chair is greater, and this may have an impact on your ability to get through doorways. This is important to consider as it can be easily overlooked. Standard doorways in the UK tend to be 28 inches wide.
Chairs can have camber added to them using tools or specific parts, but this is dependant on the type of chair and tends to be available to those of a higher active user specification. Some chairs can be easily adjusted in camber using tools, but others cannot and specific parts may be required.
For a list of the pros and cons of camber on wheelchairs, check out this great article from the United Spinal Association.