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How Rims Work and Why They Matter

rims

What would a car be without wheels? Wheels are a defining characteristic of all cars and allow a vehicle to actually roll – clearly, you wouldn’t get far without them. Many people focus on wheels for their aesthetic appearance, but they play a vital function in how your car drives. With so many sizes and styles available, you may wonder what the difference is, how wheels work, and if the differences matter.

Components of a wheel

Outboard face: This is the main face of the wheel; what makes up the design you see on your vehicle. Besides style, the outboard face also provides structure for the wheel. This section of the wheel is also commonly known as the ‘spider’.

Center bore: An empty hole in the center of the wheel. The center bore is often covered by a cap with the manufacturer’s logo so you can’t see it when the wheel is on the car.

Plate: The central section around the center bore that contains the bolt holes for the lugs. It bolts to the wheel hub, while the spokes connect the rim to the plate.

Spokes: The structures that connect from the plate to the edge of the rim. There are a huge variety of spoke patterns, with different spoke counts, diameters, lengths, and shapes available. Spokes play a large part in determining the weight and strength of a wheel.

Rim: Outer diameter of the wheel. When discussing the size of a wheel, it is usually the rim diameter that is being measured.

Bolt circle: The circle that is created by the centers of the bolt holes, where the lugs affix the wheel to the hub.

Valve stem bore: Hole that allows air to be added to the tire. The valve stem itself be made of rubber or metal.

What wheels are made of

The material chosen to make a wheel not only affects the look of the wheel, but its performance through its strength, weight, and durability.

Steel: Simple and inexpensive, steel wheels are usually basic in design and fitted to low-cost cars. They are often covered by a hubcap to add some style. Steel wheels are often heavier than more costly equivalents.

Aluminum alloy: All types of cars from every automaker are fitted with aluminum wheels, from sedans, to trucks, to luxury cars. Aluminum strikes a solid balance between strength, weight, durability, and cost.

Carbon fiber: While very rare, an increasing number of vehicles are available with carbon fiber wheels. Wheels fashioned from this exotic material are extremely light and strong, so they make an ideal choice for track-oriented sports cars. However, carbon fiber wheels are extremely expensive, and while strong, are brittle – a single impact with a curb or piece of road debris can render them useless.

Why wheels matter

From a technical perspective, wheels provide a means of affixing tires to the vehicle in order for the engine to put its power to the ground. The engine creates power, which then moves through the drivetrain down to the wheels. Wheels bolt to the wheel hub and rotate around the axles. Friction is necessary to turn the wheels and propel a car forward, and is provided by the tires touching the ground. The wheel transfers power from the axle/hub to the tires, providing the leverage needed to move the car. The amount of leverage created depends on how large the wheel is in relation to the power that the engine generates.

Besides looks, changing your car’s wheels can have a significant impact on its alignment, handling, braking, ride quality, and acceleration, as well as the calibration of the speedometer. There are several things you need to consider before installing new aftermarket wheels:

  • Make sure it fits in the wheelwell

  • Consider the correct backspacing, which is the width of the wheel and the location of the mounting plate

  • Ensure it matches your vehicle’s wheel bolt pattern

  • Determine if there is an adequate selection of tires that fit the new wheels

  • Make sure the wheels don’t rub anything on the vehicle, especially while turning or under suspension compression.

It’s smart to work with an experienced professional or a specialty wheel shop when choosing new wheels to ensure that you select a design compatible with your vehicle.

Important wheel measurements

Picking new wheels isn’t as simple as deciding which design you like the most. There are important measurements to take into consideration before making a purchase. This process, known as wheel fitment, helps ensure that the wheels will be compatible with your car. You don’t want to end up with wheels that don’t fit or cause driveability problems.

First is diameter, or how “big” or “small” a wheel is. Big wheels are a stylish upgrade, but bigger is not always better. Wheel diameter has a direct impact on a car’s acceleration, braking, handling, transmission gearing, and speedometer reading. Large wheels may not fit easily into the wheelwell, and you may experience the wheel rubbing against the car while turning or under suspension compression. Bigger wheels also often necessitate using a lower-profile tire, which don’t provide as smooth a ride over road imperfections, and can be more susceptible to punctures. Fitting over the brakes is another consideration with diameter, but usually only when opting for a smaller-than-stock wheel.

Wheel width is another critical measurement. A wheel’s front spacing (distance from the centerline to the outer edge) and back spacing (distance from the centerline to the inner edge) make up its width. Choosing a wider rim, and subsequently a wider tire, may provide more grip and traction while turning, accelerating, or braking. However, a wider rim also has a higher probability of rubbing on the inside of the wheelwell and will likely decrease your fuel economy.

Measuring the center bore will help ensure a snug, vibration-free fit. The bore is spaced to precisely match the hub, ensuring that the wheel is positioned exactly on center as the lugs are tightened down. The center bore is a weight-bearing part of the wheel, so it’s important to match its sizing exactly to the wheel hub. Too small and it won’t fit, too big and it won’t work to support the weight of the vehicle. Some wheel manufacturers suggest using spacers to resolve this issue if the center bore is too large, however these are not recommended as a permanent solution.

Bolt pattern and bolt circle diameter are other critical measurements. Bolt pattern refers to the number of lugs that secure the wheel to the hub. Most cars have four or five lugs, but some heavy-duty vehicles have six or even eight lugs. Bolt center diameter refers to the diameter of the circle created by the center of the bolt holes. Not matching the bolt pattern or bolt center diameter means that the wheels simply won’t go on the vehicle.

Construction of aluminum wheels

Most car wheels are made of aluminum alloy, but there are a variety of methods to produce them. Forged wheels begin as a solid piece of aluminum, which is then formed under massive pressure and heat into the final shape. Forging produces extremely strong and often lightweight wheels.

Gravity casting is the simplest process and involves pouring molten metal into a mold. More metal must be used to provide the necessary strength, creating a comparatively heavier rim.

Low pressure casting forces molten aluminum into a mold to pack it tightly and increase its strength. Counter pressure casting uses the opposite concept, with a vacuum that sucks the liquid metal into the mold. The process is different, but the results are similar.

Flow formed wheels are popular in motorsport applications. They start as a basic metal casting, which is then shaped under pressure while spinning. This stretches and compresses the material, increasing its tensile strength, shock resistance, and load capacity.

Billet wheels are precision machined from a solid piece of metal stock. A machine slowly carves away material until only the desired shape is left. Because billet wheels retain the molecular structure of the stock, they are extremely strong, however usually not as lightweight as forged or flow formed alternatives.

Regardless of the method, most wheels have some sort of paint, powdercoat, or anodization to provide protection from the elements and to add color. Some wheels have machined faces to expose the bright metal underneath along the rim, spokes, or bolt center diameter, which adds an extra dash of style.

Problems with wheels

They may be solid pieces of metal that can support thousands of pounds of weight, but wheels are not impervious to problems. Besides the many fitment considerations, durability and strength are crucial to staying safe behind the wheel. Lightweight wheels may spin up faster and turn more readily, but are more susceptible to denting, cracking, and other damage. Heavy wheels might be stronger and more durable, but can reduce fuel economy. The point is that a truck that carries heavy loads, a 4x4 that is taken off-road, a nimble sports car, and a luxury sedan will all have different wheels that suits their performance best.

Wheels may be ubiquitous across automobiles, but they are not as simple as they seem. With the ranges of sizes and styles available, there’s a set of wheels to fit the needs of any driver.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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