Wheel sizing

thumb|right|200px|Measuring an outside rim diameter to approximate wheel size

The wheel size for a motor vehicle or similar wheel has a number of parameters.

Units

The millimetre is most commonly used to specify dimensions in modern production, but marketing of wheel sizes towards customers is still sometimes done with traditional systems. For example, wheels for road bicycles are often referred to as 700C, when they actually measure 622 mm. Wheel diameters and widths for cars are stated in inches, while car tire bead diameters are stated in inches and widths are in millimetres.

Wheel size

thumb|right|200px|Wheels with Asanti rims on a police [[Hummer H2 car]]

The wheel size is the size designation of a wheel given by its diameter, width, and offset.

The diameter of the wheel is the diameter of the cylindrical surface on which the tire bead rides. The width is the inside distance between the bead seat faces. The offset is the distance from the wheel's true centerline (half the width) to the wheel's mounting surface. Offset is covered in more detail below. A typical wheel size will be listed beginning with the diameter, then the width, and lastly the offset (+ or - for positive or negative). Although wheel sizes are marketed with measurements in inches, the Michelin TRX introduced in 1975 was marketed in millimeters.

For example, 17 × 8.5 × +35 designates a diameter of 17 inches, width of 8.5 inches, and +35 mm positive offset (432 × 216 × +35 in fully metric numbers).

Replacing the wheels on a car with larger ones can involve using tires with a lower profile. This is done to keep the overall diameter of the tire the same as stock to ensure the same clearances are achieved. Larger wheels are typically desired for their appearance but could also offer more space for brake components. This can come at cost of performance though as larger wheels can weigh more.

Alternatively, smaller wheels are chosen to fit a specific style of vehicle. An example of this is the Lowrider Culture in which smaller wheels are largely desired.

Wheels can be widened to allow for a wider tire to be used and to poke the wheel out to the fender of the vehicle. Running a wider tire allows for more of the vehicle's power to be put to the ground because there is a larger surface area making contact with the road. This will improve a vehicle's performance when it comes to acceleration, handling, and braking.

Bolt pattern

thumb|right|The tire wheel of Subaru BRZ S (DBA-ZC6) with optional parts

The bolt pattern determines the number and position of the bolt holes on the wheel to line up with your vehicle's studs on the mounting hubs. The bolt holes are spaced evenly about the bolt hole circle. Wheel studs are the bolts that are on your mounting hub and are used along with lug nuts to attach the wheel to the car. The bolt hole circle is the circle that the center of each bolt aligns with. The second number in a bolt pattern is the diameter of this circle. The bolt circle has the same center point as the mounting hub to ensure that the wheel will be concentric with the mounting hub. The bolt circle's measurement is called the bolt circle diameter (BCD), also called the pitch circle diameter (PCD).

The bolt circle diameter is typically expressed in mm and accompanies the number of bolts in your vehicle's bolt pattern. One example of a common bolt pattern is 5x100 mm. This means there are 5 bolts evenly spaced about a 100 mm bolt circle.

The picture to the right is an example of a 5×100 mm bolt pattern on a Subaru BRZ. The wheel has 5 lug nuts and utilizes a 100 mm bolt circle diameter.

5hole_wheel.png|5-hole bolt pattern

4hole_wheel.png|4-hole bolt pattern

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Some of the most common BCD values are 100 mm (≈3.94 inches), 112 mm (≈4.41 inches), and 114.3 mm (4.5 inches). Always check your owner's manual or call your local car dealership to confirm the bolt pattern on your vehicle. Over the years, over 30 different bolt patterns have been used by car manufacturers, with most of the different bolt patterns being incompatible with each other.

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!List of different bolt patterns On vehicles without wheel studs, wheels must be fitted with the correct type of lug bolts.

Lug nuts (or bolts) will have either flat, tapered (conical), or ball (radius) seats. The type of seat a wheel requires will determine the appropriate lug nuts required to securely attach the wheel to the vehicle. A flat seat type has a flat end that puts pressure on the wheel and compress it against the mounting hub. Similarly, tapered and ball seat types have a conical or semicircular end, respectively. A place to find the lug nut type is to check OEM (Original Equipment Manufacturer) specifications if you have stock wheels or contact the wheel manufacturer if you have aftermarket wheels.

Centerbore

The centerbore of a wheel is the hole in the center of the wheel that centers it over the mounting hub of the car. Some factory wheels have a centerbore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct centerbore for the car they will be mounted on are known as hubcentric. Hubcentric wheels reduce the job of the lug nuts to center the wheel on the hub. Wheels that are not hubcentric are known as lugcentric, as the job of centering is done by the lug nuts assuming they are properly torqued down. Another, more common, term is hub piloted or stud piloted wheels and hubs. The stud piloted (lug centeric) is an older design while the hub piloted design is more commonly in use today and can provide for a more accurate connection.

Centerbore on aftermarket wheels must be equal to or greater than that of the hub, otherwise the wheel cannot be mounted on the car. Many aftermarket wheels come with "hubcentric rings" that lock or slide into the back of the wheel to adapt a wheel with a larger centerbore to a smaller hub. These adapters are usually made of plastic but also in aluminum. Plastic rings only provide initial centering, but are not strong enough to help support the wheel in case of high-speed pot hole hit. Steel ring is strongest, and aluminum is medium

Brake caliper clearance

The caliper clearance, also called the "X-factor", is the amount of clearance built into the wheel to clear the vehicle’s caliper assembly.

Tire sizes

thumb|right|Example of tire sizing on an [[all-terrain vehicle.]]

Modern road tires have several measurements associated with their size as specified by tire codes like 225/70R14. The first number in the code (e.g., "225") represents the nominal tire width in millimeters. This is followed by the aspect ratio (e.g.,"70"), which is the height of the sidewall expressed as a percentage of the nominal tire width. "R" stands for radial and relates to the tire construction. The final number in the code (e.g.,"14") is the mating wheel diameter measured in inches. The overall circumference of the tire will increase by increasing any of the tire's specifications. For example, increasing the width of the tire will also increase its circumference, because the sidewall height is a proportional dimension. Increasing the aspect ratio will increase the height of the tire and hence the circumference.

Off-roading tires may use a different measurement scheme: , followed by wheel size (all in inches) – for example 31×10.50R15 (787 mm × 267 mm R380 in metric designation). The size of the wheel, however, is denoted as . This means that the width of the wheel is and the diameter is .

Load capacity

Load capacity is the amount of mass a wheel will carry. This number will vary depending on the number of lugs, the PCD, the material used and the type of axle the wheel is used on. A wheel used on a free rolling trailer axle will carry more weight than that same wheel used on the drive or steering axle of a vehicle. All wheels will have the load capacity stamped on the back of the wheel. The Gross Vehicle Weight Rating (GVWR) is the maximum operating mass of a vehicle as specified by the manufacturer. In the United States this information is required to be on the vehicle's door placard. The load capacity of the total number of wheels on the vehicle combined must meet or exceed the vehicle's gross vehicle weight rating.

Staggered wheel fitment

thumb|right|Wide rear wheel on a 1968 [[Dodge Charger, an example of a staggered wheel fitment.]]

Staggered wheel fitment usually appears on rear-wheel drive vehicles (and in smaller numbers some all wheel drive cars), when the rear wheels are wider than the front wheels. Such a wheel setup may be found on the Ford Mustang, Infiniti G35, certain models of Mercedes and BMW, etc. A good example of such wheel combination is having in front and in the rear. Technically, wider wheels in the rear allow better grip with the road surface which is a performance benefit for better acceleration.

;Advantages

Better grip with the road for improved acceleration;
Better cornering ability;

;Disadvantages

The rear wheels cannot be rotated to the front and vice versa;
The front and rear wheels will have different tire sizes;
In case of improper installation the large rear wheel may rub suspension or wheel arches.

Another setup option of staggered wheel fitment is called double staggered, having smaller diameter narrow width wheels in the front with larger diameter and wider width wheel in the back. For example, a vehicle may feature wheels in front and in the rear. Such setups are found in the Chevrolet Corvettes, the first and second generation of the Acura NSX, and some others.