Aluminum casting can be done in several different ways, but the most common (to my knowledge) for wheels are permanent-mold casting such as Gravity Casting, Pressure Casting, or Spin Casting. As with most practical metals, Aluminum is not pure when cast - it is most commonly mixed with Copper, Silicon, Magnesium, Zinc, or Tin. Each of these metals serves a purpose in the alloy - for example, silicone content adds to the fluidity of the molten alloy ... magnesium and copper increase the alloy's strength. Another issue with cast pieces (that exists with forged pieces and any other piece as well) is that they often require precise post-machining to clean up the surface of the cast left uneven by the mold - the problem with this for a cast piece is that if left undone it can cause some of the stress-risers and creases that have been shown to let the center of a Rota rip right off its outer ring :lol:
Forging is the other "big" metal-working type, and can best be described as pressing the alloy into the desired shape using either a hammer or a press. Hammer Forging is exactly what it sounds like, repeatedly beating the metal's surface into the shape, whereas Press Forging the material is slowly rolled or pressed into its final shape. Of course these things aren’t done with your everyday Home Depot hammer, but with large machines off in some factory in China, Japan, or Korea . One of the best things about forging is that it cuts down on "air bubbles" in the metal, especially when pressed, which is one of the biggest criticisms of Cast Aluminum alloy wheels. Along with making the metal less porous, forging also has the advantage of creating a more homogenous alloy - and this means a stronger alloy.
Think of alloys as a wood grain - and this is actually how they look under a microscope, which doesn’t require much magnification actually ... the "better" the manufacturing process, the more even the grain will be in the alloy - which means greater strength and therefore less likely to deform or fracture.
Now that explains the science of the Cast Wheel vs. Forged Wheel in a nutshell, but there is much more to it. Two of my favorite lines in the ongoing argument between the two sides are the following:
1) His Volks will bend and crack the same as my Rotas if we both hit something hard enough!
2) My wheels are forged, therefore they are superior to your cast wheels!
The problem with statement #1 is that it’s just flat out wrong - there are so many variables to be considered when looking at why a wheel bent or cracked. Among them are damper & spring setup, tire size & selection, wheel dimensions, car/wheel speed, car's angle when striking object that breaks wheel, etc. And unless you own the Society of Automotive Engineers or something, I doubt you are going to be able to recreate two identical scenarios to truly prove that your Rota Grids are just as strong as the next person's Volk TE37s. However, since people will forever hold on to experience and examples (be it those who have had Rotas that didn’t break, or those who have seen all the pictures of broken Rotas) ... know that the MOST likely result of two identical setups in a harsh collision is that the Forged wheel is more likely to bend, while the Cast wheel is more likely to crack - this is just due to the material's structure after its creation (remember the wood grain analogy).
The problem with statement #2 is something I sort of just touched on in the last few sentences above, as well as this: just because a wheel is marked "forged" does not mean it’s stronger or better than one marked "cast". You will notice that even some of the biggest name companies make both Forged and Cast wheels, and their Cast wheels don’t have a history of exploding or anything ridiculous. What it really comes down to is the process that the company uses (yes, there is a noticeable difference in a piece that is Spun Cast versus Pressure Cast), and what the Quality Control and Post-Processing is like for that company - some companies can get away with making cheaply-cast wheels, but they ensure that the alloy is relatively well-kept/consistent and the piece is post-machined to near perfection.
As far as what wheels to choose based on application, this discussion could go on for pages but I will try to keep it concise - Daily Driving needs are much different from Track needs, and while you can break wheels in both cases, the likelihood of doing so in one is much greater than in the other. A true wheel enthusiast with a car that is put on the track will not only go with a "high-quality" wheel, but also one built for his or her application. Like I mentioned in another thread, wheels designed for Drag Racing should look different from wheels designed for Autocrossing. Just as a brief example, examine this Bogart 2-piece Weldment Billet Aluminum Wheel:
While we use this for autocrossing, its actually a drag wheel by design - its a 4-spoke design that is essentially a flat plate.... which means it will take loading along the direction of the surface (aka: driving forward or reverse), but side-load will cause the spokes to want to bend, which is what you always want to avoid when dealing with metal - bending will almost guarantee failure ... lucky for us, the Bogarts are overbuilt (as are most things in the racing world) and haven’t broken - a much BETTER design, however, would be one like this:
Since this wheel is designed for loading on more than just one plane, you can get away with using a lighter material (carbon fiber, in this case, obviously) without sacrificing strength...
That is just a Drag vs. AutoX short comparison - as for Daily-Driving needs, most people find that almost any wheel will do the job ... and thats where it just becomes arguing opinions about stealing other company designs, looks, baller status wheels, and all of that good stuff that we all know by now ... and we're all hypocritical to some point .... i myself plan on getting ASA AR1s which are basically the BBS RS-GTs, since ASA and BBS worked together to build up ASA's collection (so yes its a knock-off essentially, but at least its not a stolen design ... rather a cooperative effort)
... i know that was a lot of text - but hopefully at least some of it is read and understood :thumbup: