What is the PCD of a wheel ?

PCD (Pitch circle diameter)

For bolt or screw spacing, pitch circle diameter or PCD, also called the bolt circle diameter or BDC, is the diameter of an imaginary circle passing through the centre of bolts distributed around a component such as a wheel.

The BCD may be expressed in millimetres or inches, and is usually given with the number of bolts. For example, a 1974 MG B has a 4/4.5 inch (4/114.3 mm) wheel hub, meaning it has a 4-bolt pattern with a 4.5 inch (114.3 mm) bolt circle diameter.

The most common BCD values are 100 mm (≈3.94 inches) and 4.5 inches (114.3 mm). Many old British cars use 4 x 4"

What is an alloy wheel offset ?

Offset of an alloy wheels

The offset, measured in millimetres, can be negative or positive, and is the distance from the hub-mounting surface to the rim's true centreline. A positive offset means the hub-mounting surface is closer to the outside edge of the wheel, i.e. the wheel wraps around the hub and brake hardware more deeply; a negative offset means the hub-mounting surface is closer to the inside edge of the wheel and wheel sticks outwards more than inwards.

An average deviation of 5 mm is acceptable.

  • A positive (+) offset is when the mounting surface is shifted from the centreline toward the front or outside of the wheel.
  • A negative (-) offset is when the mounting surface is toward the back or brake side of the wheel's centreline.
  • A zero offset is when the mounting surface is in the plane of the centreline.

Negative offsets are considered aggressive, since the mounting surface of the wheel will "hit the hub of your car much quicker, hence the wheel will stick out more."

What does ET mean on a wheel?

The offset is described by the term ET, which is from the German word 'Einpresstiefe' translated as 'insertion depth'. Most wheels have a positive wheel offset which means the mounting surface is further outboard than the wheel's centre.  

                            or the simple definition of ET is the distance between the mounting surface and the geometric centre of the wheel (the symmetry axle), expressed in millimetres. When the ET value decreases, the alloy wheels will protrude further out. An increased ET value, on the other hand, results in moving the wheel location deeper into the wheel arch.

What is "J" in a alloy wheel ?

"J" is the width of the wheel in inch . for example we regularly come across the questions that what does 7j mean on a wheel or 7.5j on a wheel. It is nothing the wheel width in Inch.

What is the bore on a alloy wheel ?

Centre Bore of alloys

The centre bore is the size of the hole in the centre of the wheel where the spigot fits through.  The centre bore, (also known as spigot size), is the diameter of this hole, usually measured in millimetres

For most road wheels, the weight of the car is transmitted from the spigot, (the bit the centre of the wheel that protrudes from the hub), to the centre hole in the wheel. The job of the wheel studs or wheel bolts is to hold the wheel in place over the spigot.  It is therefore very important that the centre bore of the wheel matches the spigot size of the car's hub.

Alloy wheel weight table

Weight of an alloy wheel varies by size (inch wise)   or types

These are some approximate weights by Inch wise

Inch

Weights

Inch

Weights

12”

4 kg - 4.5 kg

13”

5 kg - 6 kg

14”

6 kg - 7 kg

15”

7 kg - 9 kg

16”

8 kg - 10 kg

17”

9.5 kg - 12 kg

18”

11 kg – 14 kg

19”

11 kg – 15 kg

20”

13 kg – 18 kg

22”

17 kg – 22 kg

23”

17 kg – 27 kg

24”

22 kg – 23 kg