Fig 1. Definitions for the cant, rake, and wheel bank angles for a wheelchair castor wheel.

Left and middle: A demonstration model of a wheelchair with a castor wheel with a cant angle. The castor wheel at the bottom of the wheelchair is visually slanted, allowing the caregiver to push from the side without applying a continuous moment to keep the wheelchair straight. An example of outdoor use is shown in S3 Video. The people depicted in the photos and videos gave written consent to publish as outlined in the PLOS consent form. Top right: The naming convention from ISO:7176:26 [8] is used to describe the castor wheel. A castor wheel is a wheel linked to a vehicle with a rigid connector, called the castor fork. The castor fork is connected to the vehicle with the castor stem and can rotate freely along the swivel axis, denoted here as $\widehat{\mathit{w}}$. The rigid angle between the vehicle and the swivel axis $\widehat{\mathit{w}}$ in the frontal plane defines cant angle φ_x. Bottom right: The rake angle φ_y is the angle between the swivel and vertical axes in the sagittal plane. The wheel bank angle σ is defined as the angle between the swivel and wheel spin axes.