Working out wheel fitment involves understanding several key measurements to ensure wheels properly and safely attach to a vehicle and sit correctly within the wheel wells. It's more than just one number; it's a combination of factors.
Here’s a breakdown of the key elements of wheel fitment and how to approach them:
Key Wheel Fitment Metrics
To determine if a wheel fits your vehicle, you need to consider the following specifications:
- Diameter: The diameter of the wheel rim, typically measured in inches (e.g., 17", 18").
- Width: The width of the wheel rim, measured from bead seat to bead seat, also typically in inches (e.g., 7.5", 8").
- Bolt Pattern (PCD): The arrangement of the bolt holes that attach the wheel to the vehicle's hub. It's described by the number of bolts and the diameter of the circle on which they lie (e.g., 5x114.3mm, 4x100mm). PCD stands for Pitch Circle Diameter.
- Offset (ET): The distance from the wheel's mounting face (the surface that contacts the hub) to the wheel's true centerline. It's measured in millimeters and can be positive, negative, or zero.
- Center Bore (CB): The size of the hole in the center of the wheel that fits over the vehicle's hub. This needs to match the vehicle's hub diameter or be larger (in which case hub-centric rings are needed). Measured in millimeters.
- Backspace: The distance from the wheel's mounting face to the rear edge of the wheel rim. Measured in inches or millimeters. Backspace and offset are directly related.
Understanding Offset and Backspace
Offset and backspace are critical for determining how far the wheel sits inward or outward relative to the hub and fender.
- Offset:
- Positive Offset: The mounting face is outboard of the wheel's centerline. This pulls the wheel inward towards the vehicle's suspension. Common on front-wheel-drive cars.
- Negative Offset: The mounting face is inboard of the wheel's centerline. This pushes the wheel outward towards the fender. Common on rear-wheel-drive vehicles and trucks wanting a deep-dish look.
- Zero Offset: The mounting face is exactly at the wheel's centerline.
- Backspace: The distance from the mounting face to the rear edge of the wheel.
Offset and backspace are related by the wheel's width. The formula is:
Offset = Backspace - (Wheel Width / 2) (Note: Ensure units are consistent, e.g., convert width to mm if offset is in mm).
Measuring Backspacing (Referenced Method)
You can measure backspacing using a straight edge and a ruler or tape measure, even with a tyre fitted according to the following method:
- Place the wheel face down on a flat surface (like the ground or a table) so the back lip is resting on the surface.
- Place a straight edge across the back of the wheel, spanning the diameter.
- Referenced Step 1: With your wheel facing down on the ground even with a tyre fitted, measure from the ground through the centre bore hole to your straight edge placed across the top of the wheel, then divide that number by 2. (This step seems to measure the overall wheel radius including the lip and divides it by two). Let's call this value X.
- Referenced Step 2: Then measure from the mounting face (the part that bolts to the car hub) up to your straight edge. This measurement is the backspacing. Let's call this value Y.
Value Y is the backspacing measurement. You can then use this backspacing value and the wheel's overall width to calculate the offset.
Other Measurements
- Diameter & Width: These are usually stamped on the wheel or tyre sidewall (e.g., P225/45ZR17 implies a 17" wheel diameter). Wheel width is the rim width, not the tyre section width.
- Bolt Pattern (PCD): Measure the distance between studs. For a 4-lug wheel, measure center-to-center of opposite studs. For a 5-lug wheel, measure from the back of one stud to the center of the stud two positions away.
- Center Bore: Use calipers or a measuring tape to measure the diameter of the center hole on the back of the wheel.
Why Proper Fitment Matters
Using wheels with incorrect fitment can lead to various problems:
- Safety Issues: Wheels may not sit flush against the hub, bolt pattern may not match (dangerous!), or center bore incompatibility can cause vibrations.
- Clearance Problems: The wheel or tyre may rub against brake calipers, suspension components, or fenders when turning or under compression.
- Premature Wear: Incorrect offset can put undue stress on wheel bearings and suspension parts.
- Poor Handling: Changes in track width due to incorrect offset can negatively impact steering and stability.
Checking Fitment for Your Vehicle
The best way to work out wheel fitment for your specific vehicle is to:
- Consult Your Vehicle's Manual: This provides the original wheel specifications (diameter, width, bolt pattern, offset, center bore).
- Check Online Databases: Reputable wheel retailers often have databases listing confirmed fitments for various makes and models.
- Measure Your Current Setup: If your current wheels fit well, measure them using the methods described above as a starting point.
- Consider Modifications: If your vehicle has been lowered, lifted, or has larger brakes, this will affect required wheel fitment.
By carefully considering and measuring these factors, you can determine the correct wheel fitment for your vehicle, ensuring safety, performance, and proper clearance.
