Wheel Mounting: offset, wheel discs and installation explained
What is a wheel disc and how does wheel mounting work?
The wheel disc forms the connection between the rim and the wheel hub. Through this disc, the wheel is mounted onto the tractor, loader, or trailer.
Functions:
- Connection between rim and vehicle hub
- Transfer of forces during driving, braking, and steering
- Structural support of the wheel assembly
Factors determining the design:
- Rim type (single-piece or multi-piece)
- Brake system (disc brake or drum brake)
- Axle construction and flange dimensions
- Required load capacity
- Desired track width
For more information about rim and tire construction, see the article on radial tire construction.
Technical dimensions for wheel mounting
When selecting or replacing a wheel, several fixed technical dimensions must always be considered. These determine whether the rim fits correctly on the vehicle.
Key wheel specifications:
- Centre bore diameter: opening that fits over the hub
- Pitch circle diameter (PCD): distance between the centres of the bolt holes
- Number of bolt holes: 4, 5, 6, 8 or more, depending on vehicle type
- Bolt hole diameter: matched to bolt type and load distribution
- Bolt seat type: conical, spherical (ball seat), or flat, depending on application
- Wheel disc position (ET / offset): determines the wheel position relative to the hub
These specifications are defined by the wheel manufacturer according to vehicle requirements. More information on agricultural rims can be found on our agricultural rims page.
Offset (ET value) explained: impact on track width and stability
The offset, also known as the ET value (Einpresstiefe), is the distance between the centreline of the rim and the mounting surface of the wheel disc. It determines how far the wheel sits inward or outward relative to the hub.
Types of offset:
- Positive offset: wheel sits further outward; improves stability at higher speeds
- Negative offset: wheel sits further inward; often used for wide tires or specific agricultural applications
Effects of offset on vehicle performance:
- Incorrect offset affects track width
- Can overload bearings, axle components, and braking systems
- Always verify ET value when changing rims
- Influences clearance for fenders, brakes, and steering components
Search queries such as “rim offset” and “adjust ET value” show that many users actively seek clarification. Correct offset is essential for the safe use of wider agricultural tires.
Practical mounting advice for wheels and rims
For safe installation, the wheel, rim, and tire must be technically compatible.
Always check:
- Recommended rim width
- Load capacity of the rim
- Construction type (single-piece or multi-piece)
- Fitment on the hub
- Matching pitch circle (PCD)
- Offset within vehicle specifications
If in doubt, compare the technical specifications of the vehicle and the rim before installation.
Frequently Asked Questions (FAQ)
What is the difference between ET value and offset?
There is no difference. ET (Einpresstiefe) is the technical term for offset.
How do I measure the pitch circle diameter (PCD)?
By measuring the distance between the centres of the bolt holes according to the PCD standard.
Can I adjust the offset of my rim?
Only with certain multi-piece rims. Standard rims have a fixed ET value.
Why is the centre bore important?
It ensures proper centring of the rim on the hub. An incorrect centre bore can cause vibrations.
Does the bolt hole type matter?
Yes. Conical, spherical (ball seat), or flat seats—each type corresponds to a specific wheel and bolt design.