What is cab to axle measurement?
Cab-to-axle (CA) is the distance from the back of the cab to the centerline of the drive axle. It helps determine tractor length, sleeper space, and overall maneuverability.
In commercial trucking, CA is a defined, fixed measurement used by manufacturers and fleets when sizing tractors, pairing them with trailers, and ensuring compliance with length limits. It varies with cab style, engine placement, and rear-axle configuration, and it influences ground clearance, drivetrain access, and turning dynamics.
What CA is and how it’s defined
Definition: Cab-to-axle (CA) is the horizontal distance from the back edge of the cab to the centerline of the rear (drive) axle. When a tractor has more than one drive axle, the value is typically measured to the centerline of the lead drive axle—the one closest to the cab.
How it is measured
To measure CA, start at a fixed reference point on the back of the cab and measure straight back to the centerline of the drive axle. For multi-axle configurations, ensure you’re measuring to the correct axle used for the specification. OEM spec sheets, service manuals, and VIN/build sheets normally document CA in inches or millimeters.
- The back of the cab is defined by the cab’s rear wall or firewall area, including any sleeper area that is part of the cab space.
- The drive axle centerline is the midpoint of the axle that provides propulsion; for tandem rear axles, the measurement typically references the lead drive axle.
- Manufacturers publish CA values in product brochures and spec sheets; fleets use them to assess trailer compatibility and length compliance.
Before you measure or compare CA, note the cab type and axle arrangement because these determine the reference point and how CA translates to overall length.
In summary, CA defines how far the cab sits from the rear axle and, by extension, how long the tractor will be and how it will handle with a trailer attached.
Why cab-to-axle matters for fleets and operators
CA influences several practical aspects of trucking operations, from length compliance to driving dynamics.
- Overall vehicle length and trailer compatibility: CA affects whether a tractor-trailer combination meets highway length regulations in various regions.
- Turning radius and maneuverability: A longer CA can impact steering geometry, especially in tight urban environments or loading docks.
- Sleeper and cab ergonomics: CA interacts with cab design and engine/layout, affecting interior space, access to maintenance points, and driver comfort.
- Engine and drivetrain clearance: CA can influence exhaust routing, intercooler placement, and transmission angles.
- Weight distribution and suspension: CA helps determine how weight shifts across the drive axle, impacting traction and load management.
Understanding CA helps fleets choose the right cab and axle configurations to balance length limits, payload capacity, and driver comfort.
For operators, CA is a key parameter when selecting tractors for specific routes, loads, and legal length requirements.
Typical ranges and how to find CA for a specific truck
CA values vary with cab style (short-hood vs. long-hood, conventional vs. sleeper) and whether the tractor uses one or two rear axles. In general terms, CA is measured in inches (or millimeters) and can range from relatively compact to substantial, depending on the design. To determine the exact CA for a model, consult the manufacturer’s official spec sheet or brochure, or inspect the vehicle’s VIN/build sheet. If you have direct access to a truck, you can also measure from the back of the cab to the centerline of the lead drive axle.
- Check official manufacturer specs (e.g., Freightliner, Peterbilt, Kenworth, Volvo) for the precise CA value for each cab/axle configuration.
- If you measure in the field, use a rigid measuring tape or laser distance meter from the back of the cab to the center of the lead drive axle.
- Be aware that multi-axle configurations may specify CA to the lead drive axle rather than the rearmost axle; confirm which axle is referenced in the spec.
Before using or comparing CA values, verify the exact axle reference used in the specification to avoid misinterpretation.
Accurate CA data helps ensure compliance with length laws, proper trailer matching, and optimal vehicle configuration for the intended operations.
Summary
Cab-to-axle is the distance from the back of the cab to the centerline of the drive axle, and it plays a critical role in determining a tractor’s overall length, sleeper space, and maneuverability. By consulting OEM specifications or measuring directly on the vehicle, fleets and drivers can choose configurations that meet legal requirements while matching route needs and driver comfort.
