Alloy Wheels Explained
Understanding alloy wheel terminology and specifications is crucial for choosing the right wheels for your vehicle. This guide will help you navigate the technical aspects of wheel selection.
Key Wheel Terminology
PCD (Pitch Circle Diameter)
The PCD describes the bolt pattern of a wheel. It consists of two numbers that represent the number of bolt holes and the diameter of the imaginary circle they form.
Example: "4x100" means the wheel has 4 bolt holes arranged in a circle with a diameter of 100mm.
Wheel Offset (ET)
Offset measures the distance from the wheel's center line to its mounting surface. It's a crucial factor in determining how the wheel sits in relation to the vehicle's suspension and body.
Positive offset: Mounting surface is toward the front face of the wheel (commonly found on front-wheel drive vehicles).
Negative offset: Mounting surface is toward the back of the wheel, creating a more "concave" appearance (often found on rear-wheel drive vehicles).
Wheel Dimensions
Wheel dimensions are typically expressed as diameter x width, with width measured between the inner flanges of the rim where the tire beads seat.
Example: "15 x 6.5J" indicates a wheel with a 15-inch diameter and 6.5-inch width. The "J" refers to a specific flange profile.
Centre Bore
The centre bore is the diameter of the centre hole in the back of the wheel that fits onto the vehicle's hub. A precise match is essential for proper centering and to prevent vibration.
Spigot rings: If a wheel's centre bore is larger than the vehicle's hub, spigot rings can be used to ensure proper fitment and centering.
Important Considerations
Correct offset is crucial for proper steering, suspension performance, and to prevent rubbing against suspension components or fenders.
Wheel spacers can be used to adjust offset if needed, pushing wheels further outward from the hub.
Tire compatibility must be considered when changing wheel dimensions. Tire aspect ratio may need to change to maintain proper overall diameter.
Load rating must be appropriate for your vehicle's weight and intended use.
Potential Issues
Vibrating steering can occur if wheels are not properly balanced or have incorrect centre bore fitment.
Rubbing or scraping against suspension components or wheel arches can occur with incorrect offset or width.
Uneven tire wear may result from improper wheel specifications or alignment issues after wheel installation.
Brake caliper clearance issues can arise, particularly with larger diameter wheels or those with less dish.
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