Alloy metals provide superior strength and dramatic weight reductions over ferrous metals such as steel, and as such they represent the ideal material from which to create a high performance wheel. In fact, today it is hard to imagine a world class racing car or high performance road vehicle that doesn’t utilize the benefits of alloy wheels.
The alloy used in the finest road wheels today is a blend of aluminum and other elements. The term “mag wheel” is sometimes incorrectly used to describe alloy wheels. Magnesium is generally considered to be an unsuitable alloy for road usage due to its brittle nature and susceptibility to corrosion (Flammability doesn’t help either !)
While many people choose alloy wheels for their beauty, there are equally important performance benefits to be derived including…
Reduced Unsprung Weight: Compared to Steel Wheels: This is one of the most critical factors affecting a vehicle’s road holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes) and therefore most susceptible to road shock and cornering forces. By reducing unsprung weight, alloy wheels provide more precise steering input and improved “turning in” characteristics.
Improved Acceleration and Braking: By reducing the weight of the vehicle’s rotational mass, alloy wheels provide more responsive acceleration and braking.
Added Rigidity: The added strength of a quality alloy wheel can significantly reduce wheel/tire deflection in cornering. This is particularly critical with an automobile equipped with high performance tires where lateral forces may approach 1.0g.
Increased Brake Cooling: The metals in alloy wheels are excellent conductors of heat – improving heat dissipation from the brakes – reducing risk of brake fade under demanding conditions. Additionally, alloy wheels can be designed to allow more cooling air to flow over the brakes.
One-Piece Cast Wheels
This is the most common type of aluminum wheel. The casting of wheels is the process of getting molten aluminum inside a mold to form a wheel. There are different ways this can be accomplished and although it sounds simple, this is truly an art when done properly.
Gravity casting: Gravity casting is the most basic process of pouring molten aluminum into a mold utilizing the earth’s gravity to fill the mold. Gravity casting offers a very reasonable production cost and is a good method for casting designs that are more visually oriented or when reducing weight is not a primary concern. Since the process relies on gravity to fill the mold, the aluminum is not as densely packed in the mold as some other casting processes. Often gravity cast wheels will have a higher weight to achieve the required strength.
Low pressure casting: Low pressure casting uses positive pressure to move the molten aluminum into the mold quicker and achieve a finished product that has improved mechanical properties (more dense) over a gravity cast wheel. Low-pressure casting has a slightly higher production cost over gravity casting. Low pressure is the most common process approved for aluminum wheels sold to the O.E.M. market. Low-pressure cast wheels offer a good value for the aftermarket as well. Some companies offer wheels that are produced under a higher pressure in special casting equipment to create a wheel that is lighter and stronger than a wheel produced in low pressure. Once again in the quest for lighter weight, there is a higher cost associated with the process.
Spun-rim or rim rolling technology: This specialized process begins with a low pressure type of casting and uses a special machine that spins the initial casting, heats the outer portion of the casting and then uses steel rollers pressed against the rim area to pull the rim to its final width and shape. The combination of the heat, pressure and spinning create a rim area with the strength similar to a forged wheel without the high cost of the forging. Some of the special wheels produced for the O.E.M. high performance or limited production vehicles utilize this type of technology resulting in a light and strong wheel at a reasonable cost. BBS has used this technology for several years in their production of racing wheels for Formula One and Indy cars. The BBS RC wheel uses this same technology to produce a light and strong wheel for the aftermarket.
Forged or Semi-Solid Forged: The ultimate in one-piece wheels. Forging is the process of forcing a solid billet of aluminum between the forging dies under an extreme amount of pressure. This creates a finished product that is very dense, very strong and therefore can be very light. The costs of tooling, development, equipment, etc., make this type of wheel very exclusive and usually demand a high price in the aftermarket. Semi-solid forging (SSF) is a process that heats a billet of special alloy to an almost liquid state and then the aluminum is forced into a mold at a very high rate. The finished product offers mechanical properties very similar to a forged wheel without the high production and tooling costs of a forged wheel. When low weight and performance are on your priority list, the SSF technology offers an excellent value. Currently only SSR (Speed Star Racing) from Japan is licensed to use this process for the production of wheels.
This type of wheel utilizes two or three components assembled together to produce a finished wheel. Multi-piece wheels can use many different methods of manufacturing. Centers can be cast in various methods or forged. The rim sections for 3-piece wheels are normally spun from disks of aluminum. Generally, spun rim sections offer the ability to custom-tailor wheels for special applications that would not be available otherwise. The rim sections are bolted to the center and normally a sealant is applied in or on the assembly area to seal the wheel. This type of 3-piece construction was originally developed for racing in the early 1970s and has been used on cars ever since. The 3-piece wheels are most popular in the 17” and larger diameters.
There are now many options for 2-piece wheels in the market. The 2-piece wheel design does not offer as wide a range of application that a 3-piece wheel allows, however they are more common in the market and the prices start well below the average 3-piece wheel. Some 2-piece wheels have the center bolted into a cast or cast/spun rim section and other manufacturers press centers into spun rim sections and weld the unit together. When BBS developed a new 2-piece wheel to replace the previous 3-piece street wheel, they used the special rim-rolling technology (originally developed for racing wheels) to give the rim section the weight and strength advantages similar to a forged rim. On the high-end of the 2-piece wheel market you can find wheels using forged rims and forged centers. Since these are only sold in small volume and due to the high development and production costs associated with the forging process, they tend to be on the high end of the price scale.
Upgrading Your Wheels
Muscle cars came with what we would consider narrow wheels and tires. Many cars came with 14″x6″ or 15″x7″ wheels with 70 or 78 aspect tires. When considering upgrading your wheels and tires, it is important to remember that wheel height affects the readings on your speedometer and odometer and can adversely affect the handling and suspension. Therefore, be sure that the overall height of the wheel stays the same, especially if you are putting on larger wheels and tires. You can use the chart below to check your before and after wheel height. The optimal wheel width depends on the aspect ratio of the tire that will be mounted on it. A 75 or 70 series tire has high sidewalls which will curve in when mounting them on a narrow rim. This will result in poor handling as the sidewalls deform sideways significantly before the car even begins to turn! However, the high sidewalls allow you to use a narrower wheel as this bending allows the tire to remain flat on the ground. The optimal wheel width on 75 series tires is about 70-85% of the width of the tire. Lower profile tires such as 60 or 50 series, result in stiffer sidewalls. Although they improve handling due to less flexing, they need to be mounted on narrower wheels, otherwise the tread itself will flex and lead to less contact with the road. The optimal wheel width on 60 series tires is about 85-100% of the width of the tire. See the chart below for recommended tires on a given width wheel.