Technical Tuesday: Wheel Sizing Explained

Forward

Today is the day! You’ve finally bought a brand new set of wheels and tires for your ride and you can’t wait another second to get them bolted on and see how they look. As you lower car back down to the ground with the new wheels bolted on, you step back to admire the wheels. Your excitement turns to horror as you realize your car now looks like it skipped leg day, with over 6 inches of gap between the top of the tire and the fender, this is definitely not the stance you were going for. You realize it’s time to stop guessing at wheel and tire sizing, it’s time to nerd out and learn exactly what all these numbers mean and how to choose the right ones if you’re going to master your car’s wheel fitment. It’s time to read Firing Order’s Wheel and Tire Fitment Guide.

Image Source: Underground Photography for Expectation and j-body.org for reality. 

Overview

Welcome to this week’s edition of Technical Tuesday. Today we’re jumping into the world of fitment! We’re going to look at how to pick the perfect wheel size, with the ideal offset, and how to make sure the bolt pattern will match up to your car. After reading this article, you will have all the tools necessary to make your fitment dreams a reality. These are the topics we'll cover in this article.  

  • Bolt Pattern
  • Center Bore
  • Inner Diameter
  • Outer Diameter
  • Offset
  • Width

Without further adieu, let’s get nerdy.

Bolt Pattern

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Choosing the correct bolt pattern is incredibly important. If this is wrong, the wheels simply will not bolt up to your car. Bolt patterns are typically listed using the following format, “<Number of Lugs> x <Lug Spacing>”. So, for a bolt pattern that has 5 lugs, and a lug spacing of 114.3 millimeters, it would be listed as 5x114.3 in millimeters or 5x4.5 in inches.

The quick and easy way to measure your bolt pattern is to simply use a set of calipers or a measuring tape on the following lugs depending on your lug count. Please note, 5 lug wheels are special, and this method will get you close for 5 lug setups, but not exact. Make sure you verify your results with a trusted third party as well to be sure. 

For a quick and easy way to look up your car’s bolt pattern, check out TireSize.com’s comprehensive database of bolt patterns here.

If you would like further information on how this is calculated, check out TireRack’s comprehensive guide to bolt pattern measurement here.

Center Bore

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There are two categories of wheels you need to be aware of when choosing a Wheel with the correct center bore, hubcentric and lugcentric. Lugcentric wheels rely on the lugs to center and support the wheel, whereas hubcentric wheels fit perfectly on the hub to center the wheel, this also takes a lot of the pressure off the lugs and onto the hub. 

When looking at a set of lugcentric wheels, you simply need to make sure that the center bore of the wheel is larger than the hub on your car so that the wheel will fit. 

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When looking at hubcentric wheels, the wheel’s center bore needs to be exact. If the wheel’s centerbore is two small, it will not bolt up to the car as the wheel will not fit over the hub. If the hub size is too large, it will not center your wheel correctly and may cause vibrations. If the wheel’s center bore is too large, hubcentric rings are available to fill the gap and ensure a proper fit. 

Inner Diameter

The inner diameter of a wheel needs to be large enough to clear your brakes and any steering and suspension components that are covered by your wheel. You can refer to your owner’s manual to find out what size wheels are recommended for your vehicle. Generally speaking, as long as you do not go to a smaller wheel, they should clear. To be sure, it is a good idea to measure how much clearance you need to clear your brakes and ensure that your aftermarket wheels will not rub as some aftermarket wheels may have a smaller inner diameter due to the design of the wheel.  
Outer Diameter
The outer diameter of a wheel is measured by the tire bead surface of the wheel, which means a 17 inch wheel may measure slightly larger when measuring the visible front face of a wheel. The major item to note here is that as you go to a larger wheel, to keep the same overall diameter of the wheel and tire setup, the tire’s profile will need to be made smaller to compensate. More on that topic in the Tire Sizing section of this article. 

Wheel Width

The next measurement to take into account is the width of the wheel. Choosing a wider wheel will allow you to run wider tires, which increases the contact patch of the tire on the road to allow for better grip. Because of this, many enthusiasts often go to a wider wheel when looking at aftermarket wheels. Before you run out and buy a set of 12 inch wide wheels, you will also need to measure for any potential clearance issues. Suspension components, inner fender wells, and fenders can all get in the way of your wide wheel dreams, so make sure to double check your available space first. 

Wheel Offset

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Offset is incredibly important when choosing your new set of wheels. The wrong offset, if extreme enough in either direction, could cause the wheel’s spokes to come in contact with the brake caliper, or in the other extreme, cause the wheel to stick out past your fenders causing interference with the fender during suspension travel. The offset of a wheel is measured by the distance of the wheel’s mounting surface from the wheel’s centerline. The centerline is simply the center of the wheel’s width. So, if you have a 10 inch wide wheel, the centerline is set at 5 inches from either edge. Positive Offset is used to move the mounting surface of the wheel closer to the face of the wheel, which insets the wheel further into the wheel well. For the opposite effect of protruding the wheel further out, Negative Offset is used. 

As an arbitrary example, let’s say you want your new set of wheels to sit one inch in from your fender to avoid hitting the fender, and you want your new wheel to be as wide as possible. You’ve even done your homework and you know to avoid any potential clearance issues you can run a 10 inch wide wheel with an 18 inch outer diameter.  

To ensure that the wheel sits perfectly in the space you have measured for it, you need to define the offset. You know that you want to have the face of the wheel 1 inch inside the fender, so the first thing you will need to find out is how far the mounting surface for the wheel is from the fender. After some measuring you find that the mounting surface is 4 inches inset from the edge of the fender. As it is a 10 inch wheel, you know that the centerline is at the 5 inch mark, so a wheel with zero offset will cause the wheel to stick out past the fender by a full inch. For the desired fitment of one inch inset from the fender, the face of the wheel will need to be inset by two inches from the centerline. To inset the wheel further, Positive Offset will need to be added.  After minor frustration and mumbling something about the metric system under your breath, you convert 2 inches to 50.8mm, which rounds up to 51mm. You then know that for a 10 inch wheel, you will need a set with +51mm offset to fit as intended. 

More Resources

For a great visual representation of all these dimensions, head over to willtheyfit.com

 

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TechnicalTuesdayDylan Wiebe