Wheel Offset

Wheel offset is the required distance from the centerline of the wheel, to the face of the mounting flange, provided by wheel rim. It is the difference between the wheel rim center and the wheel rim mounting face.

A wheel may be inset (positive offset), zero set or outset (negative offset), depending upon the position of the rim in relation to attachment face of the disc. See below figure.

Types of wheel-offset [1]

In the inset wheel the centerline of the rim is, located inboard of the attachment face of the disc. Inset is distance the distance from the attachment face of the disc to the centerline of the rim. A zero set wheel is the one in which the rim centerline coincides with the attachment face of the disc while in the outset wheel the centerline of the rim is located outboard of the attachment face of the disc.

A wheel whose disc can be mounted on either face to provide inset or outset, thus decreasing or increasing the wheel track is called reversible wheel. Wheel constructed in two parts, which when securely fastened together combine to form a rim having two fixed flanges is called a divided wheel rim.

Offset is important because it brings the tyre centerline into close alignment with the larger inner hub bearing and reduces load on the stub axle. This allows the inside of the wheel center to be shaped to provide space for the brake assembly, usually located inside the wheel. Ventilation slots allow air to circulate around the brakes.

The rim must be accurately shaped and dimensioned and strong enough to support the tyre under the load of the vehicle and the forces generated by the motion of the vehicle. When inflated, the tyre is locked to the rim by tapering the bead seat towards the flange, or by safety ridges or humps, close to the flange. In the event of sudden deflation, or blow-out, safety ridges prevent the tyre moving down into the well. This helps maintain & control of the vehicle while the brakes are applied.

Care should be taken to match any replacement wheel with the same offset wheel as originally equipped. Failure to match offset can result in reducing the load carrying capacity of axle. This is one of the parameter for selection of wheel.

Limitations for Wheel Offset:

A wheel with too little positive offset will be closer to edge of the fender. This can cause clearance issues between the tyre and the fender. One that has too much positive offset will be closer to the suspension components and could cause the tyre to rub on them. Wheel width, offset, and its accompanying tyre size all determine how a particular wheel/tyre combination will fit on a given vehicle. Offset also affects the scrub radius of the steering and it is advisable to stay within the limits allowed by the vehicle manufacturer. Because wheel offset changes the lever-arm length between the center of the tyre and the centerline of the steering knuckle, the way bumps, road imperfections, and acceleration/braking forces turn into steering torques (bump-steer, torque-steer, etc.) and thus, will change the drivability of the vehicle depending on wheel offset. Likewise, the wheel bearings will see increased thrust loads if the wheel centerline is moved away from the bearing centerline [2].

Next Post: "Wheel Size"

References / External Links:

[1] https://www.tyroola.com.au/guides/wheel-offset-calculator/

[2] https://en.wikipedia.org/wiki/Wheel_sizing#Offset