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An all-in alloy wheel measurement guide when buying the correct alloy wheels for your car
If you’re looking into buying a set of alloy wheels for your car, whether to improve its appearance or performance, you’re going to need a set that fit correctly.
So, how do you get the correct size wheels from the myriad options available in today’s market? What do the different numbers mean? And do you need to know how to measure alloy wheels?
We’re here to walk you through everything you need to know when asking ‘How do you measure alloy wheels?’ and make sure you get the premium fit for your vehicle.
Why fit alloy wheels in the first place?
Apart from improving the appearance of your car and helping you to stand out from the crowd, a set of alloy wheels offer advantages over their steel counterparts.
- Alloy wheels reduce the unsprung weight of a vehicle.
Reduced unsprung weight offers advantages in more precise steering and a reduction in fuel consumption.
- Reduced risk of brake failure under challenging driving conditions.
Alloy is an excellent heat conductor, so your alloy wheel will help disperse additional heat produced by the braking system when functioning under excessive operation.
Is bigger really better?
Adding larger wheels and low-profile tyres can improve the handling of a car. A well-chosen and correctly fitted pair of larger alloys can provide a noticeable improvement in steering response, particularly when cornering.
How to measure alloy wheel size – what do all the numbers mean?
Typically, the alloy wheel size conforms to the following format:
20 x 8.5J
The first number, in this instance 20, is the wheel diameter.
The second part, 8.5J, is the wheel width.
That’s all pretty simple, isn’t it? So, why all the fuss? Well, that isn’t’ where the measurements end. We’re going to go over each of the other elements and sizing considerations that affect your choice of wheels to get the very best option for your car.
Wheel rim width
The wheel rim width is the full name for the wheel width.
How to measure alloy wheel width
This is the distance between the outside and inside lips of your wheel. Measured in inches, it’s part of the wheel size shown above as the second part of the measurement.
Each vehicle has a compatible rim width range. The range can vary up to a couple of inches, which means that without a fixed width, there isn’t a one-size-fits-all solution.
Wheel offset is measured in millimetres, which is a quirk in the system given that some measurements are in inches, and other millimetres.
How to measure alloy wheel offset
Offset is the distance from the hub-mounting surface to the wheel’s true centreline. Depending on where the hub-mount is, this number can be either positive or negative. It’s one of the important alloy wheel measurements.
The offset number follows the format: ET40. In this instance, it shows an offset of 40mm.
Low offset numbers dictate a wheel that sits further out from the vehicle. Higher offset numbers sit closer in. If the offset is too low, then the wheel will stick out too far from the car. If the offset is too high, the wheel may catch on the suspension or the bodywork of the vehicle.
Again, there is no fixed size for a particular vehicle wheel offset, but an offset range. The manufacturer’s accepted difference in the range is around 2%, but again, there is no hard and fast rule. Offset should be considered on a case-to-case basis for each individual vehicle.
Wheel pitch circle diameter
As rather a mouthful, the wheel pitch circle diameter shortens to Wheel PCD. PCD is measured in millimetres.
Your alloy wheel is held in place using a selection of bolts, and the number of bolts and their locations dictates your Wheel PCD.
How to measure alloy wheel PCD
The bolt holes form a circle. Unsurprisingly, this is called the bolt circle. Whether held in place by 4, 5 or 6 bolts, the diameter of the circle they sit on, aligned to the centre of each of these holes, is your Wheel PCD. As long as you’ve got the right tools, that’s how to measure PCD on alloy wheels.
If all wheels were held in place using 4 bolts, the pattern would be a simple diamond or square shape. However, they can be held in place with up to 8 bolts. Where there are more bolts, they give a more obvious impression of a circle, meaning that taking the measurement is easier. The measurement is from the centre of one bolt hole to the one opposite. For a wheel that utilises an odd number of bolts, a specialist tool determines the most accurate measurement.
Bolt holes and PCD typically appear in the following format:
4 x 100
4 is the number of bolt holes, and 100 is the diameter of the PCD in mm.
Wheel centre bore
Measured in millimetres, the wheel centre bore is the size of the big hole at the centre of the back of the wheel. This is the hole that slides over and sits on the mounting hub of the vehicle.
Most of the original manufacturers (OEM) create their wheels to have exactly the same size centre bore as the wheel hub. These are known as hubcentric alloy wheels.
However, aftermarket brands manufacture their wheels to fit a wide range of vehicles. This offers them the ability to mass-produce designs and styles, keeping production costs to a minimum. It also means that their wheels are easier to transfer between vehicles.
To deal with the variation of hub sizes, a spigot ring slots into the centre bore to create the correct size for each car’s hub.
These spigot ring adaptors create a tight fit to the hub, making them just as safe and secure as their hubcentric counterparts.
Other items to consider when choosing alloy wheels:
1. Upsizing wheels
A bigger set of wheels not only looks imposing but can offer increases in several areas of performance.
There are a few simple rules-of-thumb to follow when increasing the size of your wheels. They will ensure that you maintain accurate speedometer readings and the same fuel economy.
When increasing your car’s wheel rim diameter by 1 inch: increase the tyre section width by 10mm and decrease the tyre aspect ratio by 10 points.
When increasing your car’s wheel rim diameter by 2 inches: increase the tyre section width by 20mm and decrease the tyre aspect ratio by 20 points.
2. Following the correct order when tightening wheel bolts
There is a conventional sequence for tightening wheel bolts. The rule of thumb is to tighten opposite bolts, working around the circle. For wheels with more than 4 bolts, then the sequence will follow a path of each bolt’s opposite, and jumping back to the bolt next to the one previously tightened.
So, start by tightening bolt 1, and bolt 2 would be it’s nearest opposite. Bolt 3 would be back opposite bolt 2, next to bolt 1.
3. One, two or three-piece wheels
Alloy wheels can be made out of single or multiple construction methods. A one-piece wheel is formed using a single mould.
As you can imagine, 2 and 3 piece constructions are moulded separately and then bolted or welded them together. This type of construction lends itself to having different rim and spoke or ‘mag’ sections, creating a variety of appearances and performance.
Protecting and caring for your alloy wheels
Every manufacturer will include proper instructions on how best to care for your new wheels. However, if you’ve bought a set of used alloys, then a little research goes a long way into the best products or methods to care for your wheels.
- Use soft brushes and cloths, avoiding any type of abrasive scrubbing.
- Determine the wheel material and finish; always use a specialist polish and shampoo for the specific bare material or finish coating.
- Avoid household detergents and cleaners.
- Never use aluminium polish on painted, lacquered or clear-coated wheels.
- Waxing wheels can help to protect all types of surface.
- Avoid cleaning hot wheels after immediate use. Allow them to cool down before starting any cleaning process.
Are you looking for an alloy wheel refurbishment, repair or just to bring some old wheels back to life?
If you’re looking into rejuvenating an existing pair of alloys, you should read our article on the various finishes available, and how you can make those old wheels look as good as new.