Join Date: Nov 2010
Location: Northeast Louisiana
Alright, I attached a very crude drawing, that I made as I sit here helping my 8 year old do his homework, You can probably find a lot better on the internet, but hopefully you'll understand the point.
I tell my guys to think of bolt hole patterns as round clock faces and 60 minutes. You can divide the number of bolt holes into the sixty minutes, and that's actually where your studs will be in a wheel/hub
A bolt hole diameter is measured across the diameter of a circle that would go thru the middle of each stud. You can see this circle in my attached drawing.
A four hole wheel would have the studs every 15 minutes apart.... at the 12 (or 0 ) mark, the 15 minute mark, the 30 minute mark, and the 45 minute mark. 60 divided by 4 = 15
A five hole wheel would have the studs every 12 minutes apart..... at the 12 (or 0 ), the 12 minute, the 24 minute, the 36 minute, and the 48 minute marks. 60 divided by 5 = 12
A six hole wheel would have the studs every 10 minutes apart....... at the 12 (or 0 ), the 10 minute mark, the 20 minute mark, the 30 minute mark, the 40 minute mark, and the 50 minute mark. 60 divided by 6 = 10
A 8 hole wheel would have the studs every 7.5 minutes apart.....at the 12 (or 0 ) mark, the 7.5 minute mark, the 15 minute mark, the 22.5 minute mark, the 30 minute mark, the 37.5 minute mark, the 45 minute mark, and the 52.5 minute mark. 60 divided by 8 = 7.5
As you read, all the even number hole wheels have two studs directly across from each....ie the 12 (or 0 ) mark and the 30 minute mark. You measure the bolt hole diameter this way, by measuring from the center of one hole (or stud) then directly across to the center of the hole on the opposite side. This gives you the measurement.
HOWEVER, on odd number hole wheels, like a 5 bolt (or that stupid 7 bolt Ford truck wheel), you can't measure across it, because you don't have 2 holes/studs directly across from each other. If you measure from the 12 (or 0 ) mark, there is not another hole at the 30 mark, but instead it is either at the 24 or 36 minute mark.
The way you measure odd bolt wheels, in order to get a correct diameter, is measure from the outside of one hole...to the center of one of the holes closely opposite the first one. An example would be to measure from the outside of the 0 minute mark to the center of either the 24 or 36 minute mark.
When you do it this way, the half hole distance you gained by measuring from the outside of the hole, is the difference that you are missing by there not being a hole directly 180 degrees across to get the diameter like you did in even numbers.
I'm sure our math engineers here can give the geometric formula for it, but I'm trying to keep it a little simpler.
Ok, attached is my very crude drawing showing a 5, 6, and 8 bolt wheel....and how to measure across to get the bolt pattern diameter.
In a 15 inch wheel, your common ones are going to be 4.5, 4.75, 5, and 5.5 inch diameters.
Once you have the bolt hole diameter correct (let's just say it was the 4 1/2 inch one), the way you order/buy wheels is by saying you need a 15 inch, 5 hole, then the bolt hole diameter (ie 4.5)
or you can say you need a 15 inch 5 on 4 1/2 trailer wheel.
2011 Flagstaff 831 RLBSS
I used to be addicted to time travel, but that's all in the past now.