Wheelchair tyre sizing can seem complex due to the use of different measurement systems, but it generally involves specifying the diameter, width, and sometimes the bead seat diameter. Let's break down the example "24x1 3/8 (37-540)":

1. Diameter (24 inches): The first number, "24," represents the diameter of the tyre in inches. This measurement is typically the outer diameter of the tyre when inflated and mounted on the wheel.

2. Width (1 3/8 inches): The second part, "1 3/8," indicates the width of the tyre. In this example, it is expressed in inches. This measurement is from one side of the tyre to the other, perpendicular to the direction of travel.

3. Bead Seat Diameter (37-540): The numbers in parentheses, "37-540," provide additional information about the bead seat diameter in millimeters. The first number (37) represents the width of the tyre when mounted on the rim, and the second number (540) denotes the overall diameter of the rim the tyre will fit onto.

In summary, the tyre size "24x1 3/8 (37-540)" provides the following information:

• Diameter: 24 inches
• Width: 1 3/8 inches
• Bead Seat Diameter (ISO size): 37-540 (37mm width when mounted on the rim, 540mm overall diameter)

Understanding these specifications is crucial when selecting wheelchair tyres to ensure compatibility with the wheelchair's rims and frame. It's common for wheelchair tyres to be labeled with both imperial and ISO (International Organization for Standardization) measurements to provide users with different measurement preferences. Always refer to the wheelchair manufacturer's specifications or user manual for guidance on appropriate tyre sizes.