2013 Scion FR-S: A Closer Look At Volk Racing TE37SL Wheels
October 8, 2012
Twenty-four pounds. That's the weight difference between Project FR-S' stock cast wheels and the forged Rays Volk Racing TE37SL wheels we recently installed and tested. I knew they'd be light, but I never would have guessed they'd be six pounds lighter apiece...especially as the Volks are fully 1.5-inches wider than the stockers.
Forging is a big factor here, but it turns out to be more complex than that. That's what I learned when I inquired with the folks at Mackin Industries, US importer of Volk Racing and other brands manufactured by Rays of Japan, to find out how they heck they do it.
In general terms, wheels are either forged or cast. Forging involves taking a slab of metal and squeezing the bejeezus out of it. The squeezing is done by a giant press that delivers thousands of pounds of force, and the higher the force, the stronger the resulting part. Casting, by contrast, does not mechanically manipulate the part, making it a less expensive process that produces a weaker part.
Forging eliminates essentially every trace of porosity, which manifests as little voids inside the slab. Removing the voids makes for a denser, more uniform part. Additionally, if you were to place the forged part under a microscope, you'd see that its microstructure -- comprised of tiny individual crystals, zillions of them -- is finer, more uniform and forms a kind of "grain" in the direction that the material flowed during the squeezing process. The reduced porosity and refined microstructure provided by forging significantly increases the metal's strength.
Forged wheels are not all alike. If you were to squeeze a slab of metal and then machine away anything that didn't look like a wheel, you'd end up with what would technically be a forged wheel. This is the way many forged wheels are made -- a billet or forged solid metal pancake is CNC-machined all over, and out pops a wheel. However, in machining all the surfaces of the pancake, those critical grain flows in the microstructure are interrupted, and the strength of the finished part is thus compromised.
The way Rays forges their wheels is far more involved and results in a near-net shape part (see below for a visual approximation of the resulting grain flows) that requires little machining. They call their process mold form forging, and it is a type of progressive-die forging.
Here's how Rays process works. A slab of 6061 aluminum is heated and forged in order to increase its density. Then the slug is forged again to flesh out the features on the face of the wheel. Next, Rays applies a 10,000-ton press -- theirs is said to be the highest-tonnage press anywhere in Japan -- to the slab while it is spun, refining the wheel face and forming the rim portion. This working of the metal strengthens it.
Any remaining metal between the spokes is then punched out, then the rim region is cold-spun and formed. After a heat treatment is applied, the details of the wheel are finish-machined, magnafluxed, shot-peened and painted. Many of the wheel's primary load-bearing features require no machining at all, so the grain remains unmolested.
As you might imagine, Rays' mold form forging process is capital- and knowledge-intensive, and the resulting parts are expensive. According to the company, BBS is the only other wheel manufacturer on the globe with this kind of capability.
The TE37SLs on our FR-S are, appropriately enough, kind of a throwback wheel -- the TE37 was Rays' first forged wheel to cross over from the racing world into the road wheel market back in the early '90s. To date 300,000 TE37s have been sold.
It's been a successful venture across the board. Rays currently produces forged wheels for OEMs (Nissan is a longtime Rays wheels user), factory production-based race cars (Porsche 911 GT3 R) and purpose-built motorsports (Formula1, LeMans, WTCC). All of these wheels are forged using the same fundamental process outlined above.
In the US, the aftermarket for wheels is kind of a wild-west culture -- regulations here are loose, so how can you be sure a given wheel is any good? In Japan, the JWL (Japanese Light Alloy Wheel) standard defines the minimum strength standard for passenger car wheels in Japan. Rays forged wheels are engineered to pass double the 500,000-rotation JWL standard for radial stress and double the 100,000-rotation JWL standard for rotary bending fatigue.
There's more. Beyond those standards, the company employs a 90-degree impact test, whereby a one-ton weight is dropped onto a wheel from height of 5.5 inches and the wheel inspected for air leaks. Another test drops said one-ton weight onto the wheel from a shorter height and then subjects the wheel to a 800,000-rotation test, after which it is checked for cracks, deformation or nut loosening. According to the company, some of their standards far exceed those stipulated by Germany's stringent TUV requirements.
Even the world's strongest wheels are not invulverable, and that's why the way a wheel fails becomes critical. Forged wheels are more ductile, meaning they will bend rather than break. When this happens the tire has a chance of holding air and you have a chance at safely limping it back to the pits or to the side of the road.
Cheap cast wheels will not only take less abuse, but when they fail they are much more likely to fracture, which is a potentially catastrophic failure mode -- the tire is much more likely to un-bead itself from the wheel. In which case, hang on. Reputable cast wheels like Rays Gram Light range are less likely to fail in this manner, but keep in mind that no cast wheel can exhibit the elevated strength (or weight) characteristics of a forged wheel.
Yup, it was an informative meeting.
--Jason Kavanagh, Engineering Editor