For all the harping on about un-sprung weight and rotational inertia being so critical to performance, I think people often ignore the more practical reality of wheels and wheel selection. While proper wheel sizing and selection is probably the second most critical decision you'll make on your car (right behind spending serious money on a good set of properly sized tires), none of our project cars have particularly fantastic $2000-apiece wheels. There's a good reason. We drive our cars hard. And while we, like everyone else, take pride in how good our cars look, we've just accepted the idea that wheels, especially on our cars, are disposable items that will take a beating, just like tires and bodywork.
Even though 90 percent of the cars you see in tuning magazines all have forged or billet, multi-piece, multi-dollar wheels, it's important to keep a few things in mind:
Cast wheels aren't bad.
Not everyone can afford mega-buck wheels.
Even if you could, you'll still end up bending or scratching one just before running it into a random pot-hole.
Bang for the buck, a good cast wheel is every bit as good as a big-dollar forged wheel. They're replaceable, affordable enough to have a second set for race tires, and sometimes not much heavier than the forged version. Do the math and you're more likely to reduce rotational inertia by downsizing a wheel than paying extra for a forged 18-incher. And are you really going to feel a three-pound difference in each wheel as you avoid speed bumps in the parking structure?
From a mechanical perspective, wheels are a trade-off between weight, strength, brittleness and the ability to take continual cyclical loading from the stress they have to endure. In the case of performance wheels, we want something that's super-light and super-strong, but at the same time won't crack when it hits something, the way glass or composites do. Such things don't exist, since one comes as a trade-off for another.
For strength, titanium or steel would be the way to go, but there's a reason no one does that. Titanium, although ludicrously light, is too brittle and can't handle the repeated minute deformations a wheel has to put up with. Steel is too heavy and, for certain alloys, is too brittle. That's why we have aluminum wheels. Balancing strength, weight and cost, aluminum is probably the best material out there. And if you're wondering about forged magnesium-sure, it's great, but it's actually softer and weaker than your average non-forged aluminum alloy. The only reason why it's so coveted in racing is because it weighs half as much. What people don't mention is what happens when you blow out a tire and drag a magnesium wheel down the street.
Wheel manufacturers know all about this and that's why companies like RAYS and Enkei, who make performance wheels as well as large-volume OEM wheels, continually strive to find better ways to build cast aluminum wheels. It cuts their bottom line in terms of time and cost, and allows them to deliver a product that's just as good for the street consumer, but at a fraction of the price.
On a recent tour of AME's manufacturing facility in Shizuoka, Japan, I finally saw its proprietary rolling forging or MAT (Most Advanced Technology). While it's recognized that machining and forging a hunk of billet aluminum into a wheel is still the best way to make a strong wheel, technologies like rolling forging can make up much of the difference. By using a low-pressure cast aluminum wheel that's later rolled to extrude the wheel hoop, the manufacturer is able to supply strong, lightweight-and affordable-wheels, wheels that share similar crystalline grain structures with forged items for brands like AME, Enkei, AVS and many others. Combine these new manufacturing technologies with modern design techniques like FEA (Finite Element Analysis) and you have non-forged wheels that are more than worthy of SCC track abuse.
The reality is professional race teams use forged wheels because the added cost is worth the insurance for a lightweight wheel that, if bent, will still hold tire pressure and get them to the finish line, instead of cracking like a cast wheel might. But if you hit something on the street hard enough to crack a cast wheel and de-bead the tire, chances are you've got bigger problems than just your wheels.