Page 290 - Handbook of Electronic Assistive Technology

Chapter 9 • Powered Mobility 279

FIGURE 9-2 Rear wheel drive�
might be the case with a mid-wheel drive chair. The exception to this is where antitipping
wheels are fitted close to the ground, in which case the problem may still arise.
Some manufacturers offer a castor lock function. This is a simple addition where a pin
attached to the frame drops into a receiving hole located on the top of the castor fork (or
similar arrangement) to lock the castor in a forward position; this prevents the castor swiv-
elling, which can be helpful when:
• Driving up a ramp.
• Reversing out of a vehicle with a dropped/lowered floor: the dropped section does not
cover the full width of the vehicle, and can be narrow, meaning that the castor wheels
are prone to catch or jam against the sides of the lowered section when the castor
stems attempt to turn.
• Driving into and out of a width-restricted through-floor lift.
• Driving down a narrow alley/passage way.

Note that a castor lock does not negate directional change entirely; left/right inputs
from the joystick will still have some influence, although limited.
A failure to place a sufficient proportion of the weight of the person and the seating sys-
tem over the drive wheels, (i.e., placing the weight away from the castor wheels) can lead
to a number of problems, some of which are more serious than others:

• High rate of wear on the castor wheel tyres, their forks and stem bearings.
• Difficulty making sharp turns at slow speed caused by the resistance of the castor
stem to spin due to the load under which it is attempting to operate.
• Loss of traction through the drive wheels:
• Can cause wheel spinning, especially when making tight turns on a smooth/shiny
surface.
• When traversing a cambered pavement, particularly one sloping to a drop kerb
and particularly in the wet/damp. Loss of traction to the drive wheel at the top
of the slope means that a significantly reduced braking effect is applied, which
can result in the wheelchair turning down the slope and possibly into the road,
regardless of any corrective inputs applied through the joystick. This has clear
safety implications, and while it does not mean that rear wheel drive chairs are
inherently unsafe, it does place responsibility on the manufacturer and prescriber
to ensure that weight distribution across the front and back axles is appropriate.