This SAE Recommended Practice defines the boundary line for establishing dimensional compatibility between air disc brake calipers and 22.5 inch diameter disc wheels and includes valve stem consideration. The line establishes the minimum wheel with valve stem envelope to allow interchangeability. This document addresses dimensional characteristics only and makes no reference to the performance, operational dynamic deflections or heat dissipation of the system. It is up to the system integrator to ensure sufficient clearance exists between the caliper, wheel and valve stem to provide safe operating conditions. Mounting systems as noted are referenced in SAE J694.
The purpose of this document is to define a boundary line for current air disc brake calipers and 22.5 inch wheels similar to the clearance lines for drum brakes provided in SAE J1865. Clearance lines in SAE J1865 do not take into account the swept area of the valve stem, which allows for the possibility of contact between valve stem and the brake caliper during operation. Drum brake systems are not affected because there is no relative motion between the drum and valve stem.