Understanding Wheel Sizing

Posted on

Aftermarket wheels are a popular upgrade for many car enthusiasts, but not many people understand all of the details that go into selecting the right type of wheel for their car. While this may be acceptable if you are just looking to replace a broken wheel or get anything that works onto your car, having a good understanding of wheel sizing is crucial if you are modifying your car for performance. When selecting performance wheels, such as Cosmis racing wheels, proper wheel sizing is crucial both to get the right fitment for your car and to choose a wheel that meets your particular performance needs.

A Note on Terminology

One piece of wheel and tire terminology that often confuses newcomers is the "tire bead." You will sometimes also read about the "bead seat" on the wheel. To put it simply, the tire bead is simply the part of the tire that meets the wheel. You may not be surprised to learn, then, that the bead seat is where the tire bead sits on the wheel. You can generally consider the bead seat on a wheel to be the actual rim of the wheel.

How Are Wheels Sized?

Let's start with the basics: wheel sizing. The most common number that you will see when discussing a wheel is the diameter. This is the total size of the wheel if you were to draw a straight line through the center from one side to the other. In a way, you can think of this as the height of the wheel. If you read that a car comes with 17" wheels from the factory, for example, then that car is equipped with wheels that have a 17" diameter. It goes without saying, but larger wheels are generally significantly heavier.

In addition to diameter, wheels are commonly sized by width and offset. As you might imagine, wheel width is the size (given in inches) of the wheel from front to back, measured along the bead seat. Wheel width is extremely important for handling characteristics, so this is more than a simple cosmetic decision. Wheel offset (given in millimeters) indicates where the wheel's mounting surface sits relative to the centerline of the wheel. Offset is exceptionally important to prevent wheels from rubbing up against fenders or other parts of the car.

It is important to note that you cannot mix and match diameters on your vehicle in any way. All of your vehicle's wheels must have matching diameters. Having mismatched wheel sizes is not only dangerous, but it is also likely to do serious mechanical damage to your vehicle in very short order.

How Should You Select New Wheels?

Choosing new wheels isn't always easy, and it will depend on what your goals are with the purchase. If you are simply looking for wheels that look great, then you will primarily want to pay attention to wheels that fit. This means researching the proper offset for the size of wheel that you are interested in and ultimately picking out something that looks best to you.

If you're after performance, on the other hand, then you have a more difficult task ahead of you. Larger wheels will generally provide increased responsiveness and performance but at the cost of potentially reduced tire life. Additionally, those larger wheels are going to mean lower-profile tires, which will ultimately increase noise, vibration, and harshness (commonly referred to as NVH). Perhaps more importantly for the performance-minded, larger wheels can potentially be heavier. Heavier wheels carry with them a significant performance penalty, so choosing durable, lightweight wheels is the way to go for performance cars.