When buying new alloy wheels you will often encounter technical terms such as Cast, Flow Formed (or Rotary Forged) and Forged. These terms refer to how the aluminium is produced and worked. The manufacturing method has a huge impact on the grain structure, the stiffness, and the weight savings (the unsprung weight) of your vehicle. In this guide the specialists at Wheels 'N Customs explain the differences in detail.

Casting is the most traditional and affordable manufacturing method. Molten aluminium is poured into a mould (Gravity Casting or Low-Pressure Casting) and then cooled.

• Grain structure: During cooling a relatively porous and random grain structure forms in the aluminium, sometimes with microscopic air bubbles.
• Weight & Stiffness: To make the wheel strong enough and ensure fracture resistance, the manufacturer must make the spokes and rim bed thicker. This results in a heavier wheel.
• Application: Ideal for everyday road use where extreme performance is not required. It offers excellent value for money.

Frequently asked question: "Are flow formed wheels stronger than cast wheels?" The answer is a firm yes. Flow Forming (also called Rotary Forging or Flow Forging) is the perfect bridge between casting and forging. The wheel centre is first cast, but the rim barrel is produced using an advanced rolling process.

• The Process: The wheel is placed on a rotating mould and, under extreme heat, the rim barrel is stretched and compressed by steel hydraulic rollers.
• Grain Structure & Fracture Resistance: Due to the pressure and heat the aluminium grain structure of the rim barrel becomes hugely compacted and aligned. The material becomes tougher, which dramatically increases fracture resistance. Instead of cracking on heavy impact, a Flow Formed wheel will more likely bend slightly.
• Unsprung Weight: Because the material is much stronger, the rim barrel can be made significantly thinner. This yields weight savings of up to 25% per wheel. This reduces rotating mass and unsprung weight, resulting in much faster acceleration, shorter braking distances and more responsive steering. Well-known brands in this segment include Concaver, Wheelforce and the JR-Wheels SL series.

Forging is the absolute top in wheel engineering. No molten aluminium is involved. You start with a solid billet of high-grade aerospace aluminium (often 6061-T6).

• The Process: The billet is pressed under thousands of tonnes of hydraulic pressure into the rough shape of a wheel, after which CNC mills machine the design to the millimetre.
• Ultimate Stiffness & Structure: Because the metal is pressed under immense pressure rather than cast, there is a perfect and uninterrupted grain and crystal structure. Porosity is virtually zero.
• Advantages: Forged wheels are the lightest, stiffest and strongest wheels on the market. They provide ultimate motorsport performance and 100% freedom in custom sizing.