Sounds basic and obvious, right? Wrong. A wheel is a combination of a rim, spokes, and a hub. The rim is the outer part of the wheel that attaches/holds the tire while the hub is the central part that connects to the suspension system. The spokes link these two parts of the wheel. You may have realized that what you incorrectly refer to as the rim is the wheel. Well, some of you at least.
Various ways of wheel fabrication are employed today and therefore there is a variety of wheels types in the market, the prime examples being forged wheels and cast wheels.
What differentiates cast wheels from forged wheels?
A cast wheel is made from casting molten aluminum into a mold. The basic process involves making a wheel mold in the shape of the desired wheel design and waiting for the wheel to dry out after pouring molten aluminum into it. The wheel is then removed from the mold, and the excess aluminum is trimmed off while minor adjustments such as drilling and surface finishing are carried out. Casting is an easy, fast and cheap way to fabricate wheels.
Forged wheels, on the other hand, start off as a solid piece of aluminum called a billet, as opposed to the molten aluminum used in cast wheels. This billet is passed through forging dies under extreme heat and pressure and result in what is called raw forgings. CNC machines are used to transform the raw forgings into the desired final wheel profiles that have a useful application.
The diverse ways in which these two wheel types are fabricated makes them have very different physical and chemical characteristics. The process of cooling off molten aluminum in cast wheels make them porous, and these can lead to severe issues if unchecked. Such issues include oxidation and cracking as well as pitting in the finishing, with the general result being the diminishing of the wheels structural soundness. In order to attain the desired level of soundness while accounting for porosity, designers are compelled to work with large tolerances hence resulting in heavier wheels. The process of casting also has the risk of incorporating impurities into the wheel’s composition and at the same time, the grain structure in cast wheels is not aligned. This translates to cast wheels being weaker and less durable.
Forged wheels are not exposed to porosity thanks to the massive pressure they undergo, and the fact that the billet is not broken up (or melted) makes for a perfectly aligned grain structure. These factors make oxidation practically impossible, and they also translate to more strength and durability. The consistency involved in forging makes it possible to use less material to achieve a certain level of structural soundness compared to casting. Forged wheels offer more flexibility to designers as they can be fabricated into the classic monoblock wheels or the two piece and three piece wheels versions. This is mainly thanks to their superior strength.
Forged wheels are therefore lighter and offer a higher level of performance, resistance to corrosion and durability relative to their counterparts and are seen as being superior. They are, however, more time consuming to fabricate and are priced higher.