Wheel & Rim guide

When selecting wheels for your vehicle it is important that you understand the factors affecting your selection. This guide will give you a better understanding. We have used a single wheel size as an example throughout this guide.

Example size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg).


Wheel hub

It begins with your wheel hub. The mounting face of your wheel will be attached to the face of your wheel hub.

Hubs fall into two basic categories, spigot centered or bolt/stud centered. Your hub will likely have a spigot (a raised section in the middle of the hub) to which spigot rings are fitted to assist with the centralisation of the wheel. Not all hubs require a spigot. Bolt or stud centric wheel hubs use special tapered bolt heads or nuts to centralise the wheel on the hub..


Wheel & hub face

The wheel mounting face fits directly onto the face of the wheel hub. If required a spacer or adaptor may be positioned between the wheel and the hub, which will result in the outside edge of the wheel moving away from the hub. An adaptor is used when wanting to change PCD or bolt/stud configuration from what currently exists. Spacers are also used when when there is limited clearance or you wish to change the appearance.

Wheel and Hub face


Wheel diameter

Example wheel size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg).

The diameter of a wheel is the height of the wheel, measured in inches, between the tyre bead mounting faces of the wheel. The example wheel has a diameter of 18 inches, to which you would fit an 18 inch tyre. The overall diameter of the wheel will be greater than 18 inches due to the extension of the wheel rim flange which is designed to keep your tyre on the wheel.

Wheel Diameter


Wheel width

Example wheel size 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg)

The width of the wheel is the distance between the two inner mounting faces of the wheel rim flanges where the tyre beads are mounted. The example wheel has a width is 9.0 inches.

Wheel width


Wheel bolt holes

Example wheel size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg)

In our example wheel there are 6 bolt holes in the wheel. You need to match the number of bolt holes in a wheel with your hub. Your hub may have bolt holes in which case bolts will be required to secure your wheels. If you hub has bolt studs, then you will need wheel nuts to secure your wheels. Tightening of your wheel nuts or bolts should be my means of a torque wrench to ensure they are uniformly and securely mounted.

Wheel bolt holes


Wheel Pitch Circle Diameter (PCD)

Example wheel size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg)

If you draw a circle through the center of the wheels bolt holes, the diameter of this circle is your PCD. The example wheel has a PCD of 139mm. The PCD of a wheel may be illustrated as 6/139 or 6139 or as 6x139 in our example.

In some instances a wheel may have a multi PCD. An example of this would be a wheel with a PCD of 4x100/108 which would permit fitment of the same wheel to either a 100 mm PCD hub or a 108 mm PCD hub. This is normally achieved by having two sets of bolt holes on the wheel or elongated bolt holes.

Wheel PCD


Wheel offset (ET)

Example wheel size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg)

Offset is the distance between the center line of the wheel to the mounting face of the wheel. Negative offset, ET-x, means the mounting face is set closer to the inner flange of the wheel. Positve offset, ETx, means the mounting face is closer to the outside edge of the wheel. Neutral offset, ET0, means the mounting face is on the wheel center line.

The outside edge of a wheel will be positioned, relative to the wheel hub, further away with a negative offset, closer to the hub with a zero offset and closest with a positive offset.

Our example wheel has an offset of ET6, which a positive offset of 6mm, seen in the left hand illustration.



Wheel Center Bore (CB)

Example wheel size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg).

The diameter of the center bore of the example wheel is 108mm. In instances where the wheel bore is greater than the original wheels, a spigot ring is inserted to ensure the wheel is correctly located.

Wheel Center bore


Wheel Backspace

Example wheel size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg).

Backspace is a measure which is seldom used but is useful in determining suitability of wheel selection.

Backspace is the distance between the outside edge of the wheel to the mounting face, measured from inner flange of the wheel. This measurement helps in determining the clearance between the mounting face of the wheel and any potential obstruction on the vehicle.

Our example wheel has a backspace of 5.25 inches (BS5.25). The overall width of a wheel is normally around 1 inch wider than the stated size. As this is a 9.0 inch wheel, the overall width of the wheel would be 10.0 inches. A 5.25 inch backspace on a 10 inch wheel will place the wheel mounting plate towards the outside of the center line, which equates to a positive offset.

Wheel backspace


There is a relationship between backspace and offset. Our quick reference table helps you to identify the offset of a wheel with a known backspace value.

Backspace - inches

3.25

3.5

3.75

4.0

4.25

4.5

4.75

5.0

5.25

5.5

5.75

6.0

Wheel width - inches

Offset - mm

5.5

0

6

12

19

25

32

38

44

52

57

63

69

6.0

-6

0

6

12

19

25

32

38

44

52

57

63

6.5

-12

-6

0

6

12

19

25

32

38

44

52

57

7.0

-19

-12

-6

0

6

12

19

25

32

38

44

52

7.5

-25

-19

-12

-6

0

6

12

19

25

32

38

44

8.0

-32

-25

-19

-12

-6

0

6

12

19

25

32

38

8.5

-38

-32

-25

-19

-12

-6

0

6

12

19

25

32

9.0

-44

-38

-32

-25

-19

-12

-6

0

6

12

19

25

9.5

-52

-44

-38

-32

-25

-19

-12

-6

0

6

12

19

10.0

-57

-52

-44

-38

-32

-25

-19

-12

-6

0

6

12

10.5

-63

-57

-52

-44

-38

-32

-25

-19

-12

-6

0

6

11.0

-69

-63

-57

-52

-44

-38

-32

-25

-19

-12

-6

0

11.5

-75

-69

-63

-57

-52

-44

-38

-32

-25

-19

-12

-6

12.0

-81

-75

-69

-63

-57

-52

-44

-38

-32

-25

-19

-12

12.5

-87

-81

-75

-69

-63

-57

-52

-44

-38

-32

-25

-19

13.0

-93

-87

-81

-75

-69

-63

-57

-52

-44

-38

-32

-25


Wheel Load Capacity

Example wheel size: 18x9.0, 6x139 PCD, ET6, CB108, BS5.25 (1133kg).

The load that our example wheel is designed to safely carry is a maximum of 1133 kg.



Images used in this guide are for illustration purposes only.