How to Measure Trailer Wheel PCD (Stud Pattern)
PCD stands for pitch circle diameter. It's the diameter of the imaginary circle running through the centre of every stud on your hub, and it's the number that decides whether a rim will fit.
We get asked how to measure it most weeks, usually by someone with a wheel in the back of the ute. Getting the PCD measurement wrong is the most common cause of errors when buying new rims, or replacing hubs.
The quick way we do it at the counter
On 4, 6 and 8 stud patterns it's simple: measure straight across from the centre of one stud to the centre of the one directly opposite. That reading is the PCD, no conversion needed.
That doesn't work on a 5 stud, because nothing sits directly opposite anything. On a 5 stud, measure between the centres of two adjacent studs instead and look the reading up in the table below.
Either way, measure centre of stud to centre of stud. If you're working off the wheel rather than the hub, measure between the centres of the holes.
4 and 6 stud: measure across
Your measurement is the PCD. Find it here.
| Studs | Across opposite studs | Pattern | Also called |
|---|---|---|---|
| 4 | 101.6mm | 4 x 101.6mm | 4 x 4" |
| 4 | 107.95mm | 4 x 107.95mm | 4 x 4-1/4" |
| 4 | 114.3mm | 4 x 114.3mm | 4 x 4-1/2" |
| 6 | 139.7mm | 6 x 139.7mm | 6 x 5-1/2" |
5 stud: measure adjacent studs
Measure between two adjacent stud centres and find your reading here.
| Adjacent centres | PCD | Also called |
|---|---|---|
| 63.5mm | 5 x 107.95mm | 5 x 4-1/4" |
| 67.2mm | 5 x 114.3mm | 5 x 4-1/2", Ford |
| 70.9mm | 5 x 120.65mm | 5 x 4-3/4", Holden |
If your reading isn't on the table, the maths is straightforward. Divide your adjacent measurement by 0.588 and that gives you the PCD.
The one that catches people out
5 x 4-1/2" and 5 x 4-3/4" are only 6mm apart, which is 67.2mm against 70.9mm on the adjacent measurement. Ford against Holden. We stock rims in both, and it's easily the most common mix-up we see.
The reason it matters is that the wrong one of those two will still go on. The studs will poke through, the nuts will wind up, and it'll feel solid in the yard. But the wheel is only pulling up square on one or two studs, and it works loose. Get this one right.
If you want to be certain, take a second measurement. Go centre to centre between two studs with one stud in between them, skipping the adjacent one. On 5 x 4-1/2" that's 108.7mm and on 5 x 4-3/4" it's 114.7mm, so you're separating them by 6mm across a long measurement instead of 3.7mm across a short one. Much easier to be sure of.
PCD on its own doesn't confirm fitment
Right PCD, wrong wheel is a real thing. Four other things have to line up:
- Stud thread. Most New Zealand trailer stock is still imperial. Our Trojan hubs run 7/16" and 1/2" UNF depending on the rating, and the nuts aren't interchangeable.
- Nut seat. Taper seat or flat seat. A taper nut on a flat seat rim only touches on a thin ring and won't hold torque.
- Centre bore. The hole in the middle has to clear your hub. Plenty of car rims foul a trailer hub even with the right stud pattern.
- Offset. How far the mounting face sits from the wheel centreline. Get it wrong and the tyre fouls the guard or sits proud of it. Our Knott alloys and spoke rims run +30mm.
What we stock
Most of our trailer rims are 5 x 4-1/2" (Ford), which covers the bulk of New Zealand trailers, in 13" and 14" and in galvanised or alloy. We also carry 5 x 4-1/4", 5 x 4-3/4" (Holden) and 4 x 4-1/4" rims. We can get 6 x 5-1/2" rims in to order, so ask if that's what you're after.
On the hub side we hold integral hub kits, rotor and hub kits and hub/stub sets in 4 x 4", 4 x 4-1/2", 5 x 4-1/4", 5 x 4-1/2" and 5 x 4-3/4", plus a blank integral hub if you're drilling your own pattern.
Have a look through wheel assemblies, rims and tyres, hubs and axle components, and bearings and seals. Wheels are listed either as rim only or as a complete wheel and tyre assembly, so check which one you're ordering.
We're in Hamilton and ship New Zealand wide, usually same or next business day.
Not sure what you've got? Take the measurement and give us a call on 07 211 7490 with that number and your stud count. We'll sort it from there.
