Yamaha, in 1980, produced an otherwise conventional chassis made from square extruded aluminum tubing. In both cases, the aluminum used was equivalent to forged, as the high pressure of extrusion has the effect of refining grain structure and closing up any voids or porosity in the material, thereby greatly increasing its resistance to fatigue.
What is metal fatigue, and how does it work? If we pull out the textbooks we find fatigue failure has three stages:
- Crack Initiation: As the material is strained by cyclical applied loads, gradually the atomic bonds under greatest stress relax by breaking, cumulative damage is done to the coherence of the material, and eventually here or there an actual detectable crack forms.<br/>
- Crack Propagation: Any microcrack grows perpendicular to applied tensile (pulling) stress.<br/>
- Rupture: As the crack grows, the intensity of stress rises because part cross-section is being reduced. Eventually the part breaks.<br/>
Now the really interesting part. Whereas Stage 1 (initiation) can take a very long time in steel, it was found that in cast aluminum Stage 1 was either quite short or even completely missing—crack propagation began as soon as stress cycling was applied to a cast test specimen.
Fairly recently researchers looked into why cast aluminum parts have behaved this way. They knew that a film of aluminum oxide forms on the surface of molten aluminum, but what they didn’t realize for a long time was that bits of this film are entrained in liquid aluminum when it is poured rapidly, from above, into conventional casting molds.
Anyone who has made pudding on the stovetop knows that as pudding cools, a fairly tough skin forms on its surface (hence the expression “My bike’s too weak to pull the skin off a pudding.”).
Stir the cooled pudding and the skin disappears as it absorbs water, ceasing to be a skin.
The same does not happen in the case of aluminum oxide films carried into metal poured into a casting mold. The oxide is not soluble in aluminum, and does not transmit stress well. Therefore every bit of aluminum oxide entrained into an aluminum casting becomes a pre-existing crack once the material solidifies. The existence of such cracks is why Stage 1 in the fatigue process in cast aluminum has been either very short or nonexistent.
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To avoid the need to employ castings, Yamaha and other early adopters of aluminum chassis for roadracing used extruded tubes or beams formed from rolled sheet, joined to very expensive, machined-from-solid wrought stock steering-head and swingarm uprights (the vertical members at the rear of the frame, between which the swingarm is located). This was perfectly acceptable for factory racebikes but far too expensive a construction ever to be adopted for production motorcycles.
I was therefore greatly surprised when I flew to England in the late ’90s to see the Al Melling-designed “Norton-of-the-moment.” It had a cast chassis! How could this be?
Soon thereafter, Yamaha began to produce cast parts for chassis by its “CF” or controlled fill casting process. Miguel DuHamel’s longtime mechanic Al Ludington complained to me around the year 2000 that suddenly Yamaha’s 600 Supersport bikes were more than 30 pounds lighter than Honda’s CBR. And later again, Ducati adopted the Ritter Vacural casting process for its major castings.
Yamaha’s description of its CF process reveals that it can cast thicknesses one-half those of previous methods—1.7mm rather than the previous 3.5mm (0.070 inch versus 0.138 inch). That’s really thin! In addition, and quite unlike extruded tube or sheet fabrications, local part thickness could be made infinitely variable in proportion to applied stress.
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A major advantage of the new casting processes was that they were deliberately designed to prevent the entrainment of surface films of aluminum oxide into the flow of liquid metal entering the mold. By filling from the bottom at low metal velocity, oxide films would remain floating on the upper surface of the hot metal, eventually to be lifted out the vents at the top of the mold. Vacuum could also be applied to make less oxygen available to the hot metal.
Previously, researchers had learned something by filtering aluminum through porous ceramics before it entered molds, discovering that this somewhat improved fatigue performance. That stimulated further research.
Now that the fatigue performance of bottom-fill, low-turbulence casting systems has been well established, Stage 1 of the fatigue process has been greatly extended, making cast aluminum chassis the most economical way of producing production bike chassis. Little or no welding is now required—some chassis are one-piece and others consist of right and left halves, bolted or otherwise fastened together (as in Harley’s LiveWire and Aprilia’s 660).
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When an English engineer put me on the spot a few years ago by asking, “What is the greatest engine design innovation of the 21st century?” he took pity on my hesitating ignorance and said, “Modern casting methods, which have so greatly reduced engine weight without reducing strength.”
There is more to the new methods than just their great reduction in oxide entrainment, but that is the largest contributor to the result, which are cast parts with near-forged fatigue properties. Twenty-first century methods for casting aluminum have brought to production the advantages of aluminum chassis formerly too costly for any application but factory racing.
