Wheel Size Explained – Pros and Cons
In Part I of Where the Rubber Meets the Road, we discussed the various types of wheels from the beginning of the history of the automobile to the present day. In this blog we will look at wheels and tires sizing including width, height, offset and backspacing.
Just like the type of wheel, wheel sizing can make or break the look of the finished product. Often, we hear critics look at a vehicle and talk of the stance as perfect or in some cases less than perfect. While vehicle stance has a lot to do with suspension, it is not possible to obtain the right stance and look without selecting the perfect wheel size. You can spend a lot of time on the details of your restoration or build and when all is done, the look of the car can be made or not by your selection of wheels.
We must also remember that sometimes it is not just about look, it is also about function. Wheels will affect how the car handles. As always when choosing a wheel, think of what you are trying to accomplish and what your project will be used for. Will it be strictly a show car and spend much of its life on a trailer? Do you intend to use it as a daily driver? Will your project end up as a weekend track car? The wheels that you choose will make all the difference in the look of your vehicle, how it performs on a track and how comfortable the ride and driving experience will be.
When referring to Wheel Height only, we are typically referring to the diameter of the wheel. This is the measurement from the bottom of the wheel to the top. This is a different measurement than what is known as Section Height or sidewall height, which refers to the height of the sidewall of the tire from the top of rim to the top of the tire.
When picking the ideal set of wheels and tires for your classic car, you must consider both of the above measurements. There is only so much room in the wheel well of a car, so it is necessary to find the perfect balance between wheel diameter and section height. Since there is not an infinite amount of room in the wheel well of your classic car, you must find the balance between wheel diameter with section height for your desired look and the use of your vehicle. In essence the greater the diameter of the wheel, the lower your section height will be.
The larger the wheel diameter, the lower the section height or profile of the tire. There are both benefits and disadvantages to a lower profile tire. Tires with a shorter sidewall offer distinct handling and ride characteristics. Thin-profile tires give the driver a more precise feel of the road since there is less rubber between the rims and the driving surface; this greatly increases the ability of the driver to use tactile handling techniques in high-performance situations. Thin-profile tires also reduce the effects of lateral deflection, also called tire-fold, this benefit handling and safety. However, with greater road feel and less rubber between the rims and the driving surface it becomes an arguably less comfortable ride. Depending on the use of your project, comfort may not be the most important characteristic you want to achieve.
Tires that have a thicker sidewall, also called broad-profile tires, offer some safety and comfort advantages. Broad profile tires have a wide range of inflation adjust-ability, allowing versatility for those on rough roads or off-roading. The thicker sidewall also protects those rims you chose (following the first blog in this series) from pothole damage and from curb dings when parking. The broader sidewall allows for greater absorption of potholes and other road imperfections, thereby increasing comfort.
Wheel Width
The width of the wheels ultimately determines the width of the tires that may be safely mounted on those wheels. As one would expect, wheels and tires come in various widths, and while some drivers prefer the aesthetics of different widths, it is important to consider the pros and cons of different tire widths before making a purchase. The width of the tire or wheel in this section ultimately relates to how much rubber comes in contact with the ground. Tire thickness is essentially a zero-sum game, meaning that the advantages gained from a wider tire come at the cost of the advantages gained from running a thinner tire.
Thick Section Width Advantages
Wheels that support tires with a thick section width distribute the vehicle’s weight over a broader area, increasing grip, especially under heavy acceleration. The broader contact area of these tires allows for efficient conversion of torque to the asphalt, as well as decreasing stopping distance. Therefore many racing cars are fitted with broad section-width tires or racing slicks. The broader section width also allows the tire to distribute heat more evenly across the rubber surface, increasing the tire’s lifespan.
Thin Section Width Advantages
Tires with a thin section width offer more turning ability and reduce the risk of tire-rub during the suspension travel or during hard turning maneuvers. Thinner tires, while offering less contact area with the ground, also decrease the chances of a potentially dangerous hydroplane situation, making wet weather driving a safer endeavor. In terms of comfort, thin section-width tires offer more driving comfort since they reduce the amount of road surface imperfections transferred to the driver and passengers. Road noise is also reduced when using thin section-width tires.
Wheel Offset and Backspacing
Wheel Offset is the distance from the hub mounting surface to the center line of the wheel. The wheel offset is measured in millimeters and results in a positive, negative, or zero offset. Positive offset is when the hub mounting surface is toward the front or wheel side of the wheel. It is common to find a positive offset in newer and front wheel drive vehicles. Negative offset is when the hub mounting surface is toward the back or brake side of the wheel’s centerline. A negative offset wheel usually has a “deep lip”. The hub mounting surface is even with the centerline of the wheel.
- With a higher offset, your wheels mount farther inward, providing greater clearance between the outside edge of the tire and fender, but less clearance between the inside edge of the tire and your vehicle’s suspension.
- With a lower offset, your wheels mount farther outward, providing a wider vehicle stance. For example, a lifted truck with oversized tires often requires 25mm-50mm (approx. 1-2”) less offset than stock, so the wheel and tire are farther toward the outside. This provides more clearance from the suspension. Some lifted trucks even require a zero or negative offset rim in order to clear large tires.
Wheel backspace is the distance from the inside edge of a wheel to the mounting face. A wheel’s backspace is equal to ½ the wheel’s width + offset (in inches) + ½” (see image for a reference to demonstrate what wheel backspacing is).
- More backspace indicates that the wheel protrudes farther into the wheel well and closer to the suspension parts — which increases the risk of rubbing.
- Less backspace indicates that the wheel protrudes less into the wheel well and therefore reduces the risk of rubbing.
It is always a good idea to check with a professional to ensure that the wheels you are purchasing will give your vehicle not just the look, but the clearance you will need for your vehicle. It is never good idea to sacrifice safety for looks.
Jamie Pappas
