Anyone who's read the Window Shopper column on a regular basis knows it's usually the first place most parts make their debut. Some manufacturers even wait until a release publishes before introducing a part. It's a debutante ball for parts, if you will.
So new entries often excite us. Well, so do glittery things and certain bodily functions, but that's another story. In the March '02 Window Shopper, we ran Super Bell's new forged aluminum axle. As you can imagine, it's tough to reinvent something as simple as a beam axle, so the release piqued our interests. After all, we're talking about a lightweight suspension part, and whenever you decrease unsprung mass (the stuff not supported by the springs, like axles, wheels, etc.), you improve ride quality and a wheel's ability to follow irregular surfaces and bumpy roads. When we started thinking of the weight savings over conventional iron beam and tube axles, we got downright excited.
Of course, we tempered our enthusiasm with a little dose of real-world skepticism. Yes, pound for pound aluminum is four times stronger than steel, but we're talking about a considerably lighter part: It tips the scales at about 10 pounds! How could something as critical as a load-bearing axle hold up to the strains and rigors of braking, steering, and the odd pothole?
For answers, we went straight to the horse's mouth. As many of you know, Pete & Jake's Hot Rod Parts in Peculiar, Missouri, acquired Super Bell Axle Company in June 1999. They didn't just acquire the product line, either; they acquired the Super Bell name and reputation for strong and true axles. To push the envelope, they first had to establish what traits a sufficiently strong axle maintained.
They started by stress testing both the cast I-beam and tube axles at Arrow Laboratories, an independent laboratory in Wichita, Kansas. What they found was both the tube and cast-iron I-beam axles could sustain up to 12,000 pounds of vertical load with little to no distortion. As a bonus, they found the tube axles they made in Missouri had a slight strength margin over the California-produced axles. With the benchmark established, they started on plans for an aluminum axle.
With CAD (computer-assisted design) plans in hand, Super Bell shopped the market for a proficient forging house. Super Bell said they chose to forge the axles since forging yields a stronger component over casting or machining the pieces form plain billet.
For example, when molten metal cools from casting, it forms a random, non-directional crystal structure similar to the grain pattern of sand. Heating metal to just below its melting point and forcing it into a shape (forging) will align the crystal structure, or grain pattern, making the grain pattern less like random sand and more like the grain in wood. Since the metal isn't allowed to melt and reform its crystal structure, we call this process cold working. Now the part is shaped so the grain flows where the strength is needed the most, much like wood. Forging also makes the metal denser than cast, thereby reducing or eliminating porosity and stress risers. That's why a cast part requires more material and bulk to equal the strength of a forged part.
As for raw billet, since it's cold worked during its manufacturing process, it has a grain structure of its own. However, the cold working is done in one direction and the grain only has one orientation-very similar to a 2x4, if you will. To further follow the wood grain model, cutting either material in any way disrupts this grain structure. Making a part from raw billet is akin to cutting a part from ordinary lumber. Forging a part, however, is like growing a tree exactly to the shape you want.