Main wheel Alignment
To allow the wheelchair to roll with the least resistance, it is critical that the mainwheels point straight ahead.
If the mainwheels point slightly outward (toe-out) or slightly inward (toe-in), it significantly slows down the wheelchair.
O'Reagan and co-workers (1981) showed that, for some tires, toe-in or toe out of as little as 3 degrees in creased rolling resistance tenfold.
Since the front wheels (castors) are free to move to allow turning to take place, they are not subject to the same toe-in/tow-out problems as mainwheels.
They do however, increase rolling resistance greatly when their bearings become worn (Van de Woude, 1991).