How To Measure A Bolt Pattern

What is the bolt pattern on your trailer wheels?  For the most part it doesn’t matter because you’re not changing wheels that often, yet when it does matter, it really matters.  Here is some good info to measure and calculate just what that bolt pattern is.

There are actually a bunch of different hole combinations for connecting the wheels on cars, trucks and trailers.  The set of holes for the bolts attaching the wheel to the axle is the “Bolt Pattern”.  While it’s not usually a concern, if you get new wheels, it’s important so the new wheels fit.  If the pattern is wrong, then of course, the wheel is not of much use.

Another detail, if you ever wonder about load capacity of an axle, the bolt pattern can tell you something.  It’s not exact, but in combination with other info, it may tell you something.

Terms For A Bolt Pattern

When trying to identify a specific bolt pattern, there are some descriptive terms worth knowing.  Here are some phrases to remember (with the image to reference).Dimensions in a Bolt Pattern

  • Bolt Pattern — The set of holes, or more exactly, the combination of holes and locations for the bolts attaching the wheel to the axle.  This includes the number N of bolts.
  • Bolt Hole Size — The size of the holes the bolts pass through, or really, the size of the bolts.  The holes will be larger than the actual bolt diameter, but not by a lot.
  • BCD, or Bolt Circle Diameter — When the bolts are in a circle around the center of the wheel, the diameter of that circle is the BCD, or Bolt Circle Diameter.
  • Hub Diameter — Size of the hub, or really, the size of the hole in the center of the wheel.  The axle hub passes through this hole when the wheel goes on.
  • Distance Between Bolts — (actually, Distance Between Adjacent Bolts) — A measurement of the distance from the center of one bolt, to the center of the next.  That is called “center-to-center” as well.

While there are a lot more terms to describe a wheel, we covered many of them in our Wheels and Tires article.   That’s a good one to read as well for more info.  However, in this article we’re going to focus on the bolt pattern.

Standard Patterns

A very common way to refer to the various bolt patterns is “the number of bolts” on a “bolt circle diameter”.  Something like 5 on 4.5 — meaning 5 bolts on a 4.5″ diameter circle.  It’s a short-hand reference, but it does the job in communication.

Some of the more common sizes for bolt patterns include:  4 Bolts on 4″ Diameter,  5 Bolts on 4.5″ Diameter,  6 Bolts on 5.5″ Diameter,  and 8 Bolts on 6.5″ Diameter.  There are several others like 5 on 5 that do show up, but they are less common.  Hey, there’s even some 10 bolt patterns for the heavy ones.

Use this nomenclature when you’re shopping for wheels.  It helps a lot knowing the terms.  Here are some visual examples.  Note that the hub hole also get’s larger as the number of bolt holes increases.  That center hole also has some standards, but that is not something you need to know to order wheels.

Pick Your Bolt Pattern

Though it’s not specifically mentioned for most bolt patterns, the size of the bolt also matters.  Most are 1/2″-20 threads, but check just to verify.  Here’s another article with more info about trailer wheel lug nuts and lug bolts.

Identifying A Wheel Bolt Pattern

How do we measure?  When you have an even number of bolts, you can often measure directly across the wheel for opposite bolts to get the Bolt Circle Diameter (BCD).  That seems easy enough, but what if the wheel is mounted and the hub is in the way?

For odd numbers, or when the hub is in the way, it’s not that easy to directly measure.  So, calculate it.  Here’s the equation that gives an exact result based on a simple bolt to bolt measurement.  (Hole to hole works too.)

BCD=LBB/(sin(180/N))

Since measurements are not always perfect, just do the calculation and round a little to the bolt pattern that makes most sense.  For instance, a 5 bolt pattern is not likely to be 5 on 4.467″, but we know that a common pattern is 5 on 4.5″, so round a little, and you’ve got your answer.

Before worrying about equations, remember, the easiest way to get BCD for a bolt pattern with even numbers of bolts is direct measurement.  Strait across from one bolt or hole, to the opposite bolt or hole on the other side of the hub.  There are only a few standard patterns, so you don’t have to be super exact in measuring.

If you don’t want to calculate, use this table.  Here are a few of the common patterns and the theoretical perfect distance between the bolts.

Look Up Table

More Reading

For odd bolt patterns — like 5 bolts — in the size we’re talking about for trailer wheels, there is a cheat method that mostly works.  As an engineer, I like the direct methods above, but this measuring trick is worth knowing.  It’s not exact, but it gets close — well, for these specific circumstances.

We have a few more articles on the subject of Axles, Wheels, and Tires.  First, the Trailer Axles 101 article for an overview of axles in general.  Next, the article reference above for Trailer Wheels and Tires — which is about the tire specifications.  Just add the above information to the Specs for your wheels and you’ll have the full set.  Then, we also have an article about when it’s time to replace trailer tires.

Thank you for joining us for the read.  Have a Wonderful Day.

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