SAE J694‑2021: A Practical Guide to Disc Wheel and Hub Interface Dimensions for Commercial Vehicles

SAE J694‑2021 is the latest revision of a long‑standing recommended practice that defines dimensions and tolerances for the interface between disc wheels and hubs or drums in truck and bus applications. Updated in April 2021, it adds guidance on two‑piece sleeve flange nuts and reinforces warnings against mis‑assembly of mounting systems. Divided into two sections, the standard aligns with ISO 4107 while also documenting North American practices.

🔍 Overview and Scope

The standard applies to disc wheel systems designed for dual wheel applications on trucks and buses; light‑truck single wheels and special applications are excluded. The document is split into two sections:

Section I: Covers mounting systems compatible with ISO 4107 and ISO 7575 nuts, used internationally.
Section II: Records mounting systems commonly used in North America.

Each mounting system is uniquely identified by its number of bolt holes, bolt circle diameter, and fastener type. The standard stresses that even if two systems share a common bolt pattern, their components must not be intermixed.

🛠️ Key Dimensions and Mounting Systems

For a given mounting system, wheels and hubs must match in critical dimensions such as center hole diameter (C), bolt hole diameter (D), disc flat clearance diameter (E), and pilot diameter (C1). Fastener type is tied to wheel material: flange nuts (FN) for ferrous wheels and sleeved flange nuts (SFN) for aluminum wheels to accommodate the larger bolt hole needed for the sleeve. Ball seat nuts (BSN) are also referenced.

The table below illustrates sample metric system mountings from Section I (all dimensions in millimeters):

Mounting System	No. of Bolt Holes (A)	Bolt Circle Diam. (B) ⊕0.40	Fastener Type	Wheel Material	Center Hole Diam. (C) +0.2,–0	Bolt Hole Diam. (D) +1,–0
II	8	275	FN	Ferrous	221.0	24
II	8	275	SFN	Aluminum	221.1	30
III	10	285.75	FN	Ferrous	220.0	26
IV	10	335	FN	Ferrous	281.0	26
XIII	6	205	FN	Ferrous	161.0	21

Full tables in the standard also include disc thickness, clearance diameters, and rim‑width specific values. Always consult the latest version of SAE J694 for complete data.

⚠️ Mis‑assembly Warning

Components of different mounting systems with common bolt patterns can be mistakenly assembled together. This can lead to wheel loosening or structural failure. Always verify the mounting system number before selecting wheels, hubs, and fasteners.

Design Insights and Frequently Asked Questions

The standard provides several critical design reminders:

Scalloped hubs – Hubs with material removed from the wheel backup diameter (scallops) can cause cyclical fatigue cracks in disc wheels. SAE J694 recommends against using scalloped hubs when the disc wheel mounts directly to the hub face. If necessary, consult the wheel manufacturer. Use with outboard brake drums is generally acceptable.
Stud standout (G) – Proper thread engagement is ensured by calculating standout based on maximum wheel thickness plus maximum nut thickness. Tables in the standard give minimum standout values for single and dual wheels.
Two‑piece sleeve flange nuts – The 2021 revision adds guidance for these fasteners, reflecting their increasing use in North America.

🔍 Harmonization with ISO

Section I directly aligns with ISO 4107 and ISO 7575, promoting global interchangeability for metric mountings. However, fastener type and disc thickness are not part of the ISO standard and are specified in SAE J694.

Frequently Asked Questions

What defines a mounting system in SAE J694?
A mounting system is uniquely defined by the number of bolt holes, the bolt circle diameter, and the fastener type. Each system is assigned a Roman numeral in the tables. Within a given system, wheels and hubs are interchangeable only when the correct fasteners and appropriate dimensions for the wheel material are used.
Why is it dangerous to intermix parts from different mounting systems?
Even if two systems share the same bolt pattern, differences in pilot diameters, center hole dimensions, fastener seat design, or standout can lead to improper clamping, wheel slippage, or fatigue failure. The standard explicitly warns against such intermixing.
When should scalloped hubs be avoided?
Scalloped hubs (with an interrupted backup face) should not be used when the disc wheel is mounted directly to the hub mounting face, as they have been linked to wheel cracks. Their use with outboard brake drums is generally acceptable, but consulting the wheel manufacturer is recommended.
How does the standard differentiate between ferrous and aluminum wheels?
Aluminum wheels typically require larger bolt holes or special fasteners like sleeved flange nuts (SFN) to accommodate the fastener sleeve. The tables provide separate dimension lines for ferrous and aluminum material for each mounting system.