Casting is relatively simple: first you create a mold, then you pour molten metal into it. This only works for simple shapes though. Casting thin sections, for example, is difficult, as the liquid metal cools quickly as it enters these and solidifies before they’re filled.
Forcing the metal in under pressure fills the mold more quickly, giving the metal less time to cool, but this causes bubbles to form.
This is a problem for two reasons. One is that it weakens the metal, so you need more to retain enough strength, and it weighs more. Second, it means you can’t machine the metal very much. Go too deep and you’re into the porous inner, which means you get a much weaker structure and, on a wheel, it might not be airtight either.
Forged wheels are made very differently. You stat with a disc of aluminum alloy which is shaped in a process called flow forming. The metal is hot but not molten, and as it’s rotated on a spindle, hard steel rollers are pressed against it, changing its shape in a manner which looks similar to a potter shaping a lump of clay. With the basic shape of the wheel created, it is then machined extensively.
This means more complex shapes can be created and as the metal is not porous, it can be machined to have thinner wall thicknesses, right down to 2.5mm.
From the outside a cast wheel might looks very similar to a forged one, but it will have simpler shapes, and what you can’t see is that the metal it’s made from is thicker in many places than in the forged one.
The forged one can be just as strong using significantly less metal – the difference can be over 1.7 kilogram per wheel.