Yamaha’s Spin-Forged Wheels

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

While the spin-forged aluminum wheels on Yamaha’s 2021 MT-09 achieve a modest weight saving of 320 grams per pair, their real value lies elsewhere: in increasing the strength and ductility, or ability to bend without breaking, of the part of the wheel closest to road impacts—the rim.

Morris Mags and the Birth of Cast Wheels

Cast wheels have been with us for more than 50 years. The distinguished late British rider/engineer Peter Williams adopted cast light-alloy wheels for his Arter Matchless roadracer for the 1970 season. It wasn’t long before conversations between US Kawasaki race-team manager Bob Hansen and race car mag-wheel pioneer Elliott Morris resulted in the US-made “Morris mags,” in 1973. MV rider Phil Read introduced Morris mags to his team and the revolution was on.

Related: Wheels of Fortune

Elliot Morris first built wheels for race cars then motorcycles. “Nature seems to like odd numbers, so that’s what I picked,” Morris said when asked why the now seven spoke pattern was chosen. (Jeff Allen/)

Cast aluminum wheels first appeared on some Yamaha production models in 1978, and they have now largely displaced laced, wire-spoked wheels on roadbikes save for a few special applications or whenever stylists wish to invoke classic themes.

Note the word “cast” above. For the strongest parts with the greatest resistance to fatigue or impact, casting is not the top choice; forging is the preferred production method. This is because the metal’s grain orientation can be controlled in forging, and gas porosity is crushed out of existence by the high pressure of the forging process.

In casting, liquid metal fills a sand mold or multipiece heated metal die whose internal cavity is the shape of the desired part. In closed-die forging a hot metal blank or preform is squeezed into the shape of the die cavity in a powerful press.

Related: Why Magnesium Is Attractive For Use On Racing Machines

In general, a cast part must contain more metal than a forging, and so is heavier to achieve equal durability.

Cast magnesium wheels were dominant in roadracing until the coming of stronger, more ductile forged mags. The raw magnesium forgings I saw at Marchesini are strange bloated-looking objects that look as though they’re made of risen bread dough. Transforming that into the beautiful finished product that we see on a MotoGP bike requires long and expensive machine time on automated turning centers (a fancy term for a lathe). This is uneconomic for production motorcycles.

So far as materials, aluminum is preferred over magnesium for production wheels because it is less vulnerable to corrosion.

Half Cast, Half Forged?

Is there some process intermediate between forging and casting, which can result in durable wheels, lighter than cast but less expensive than machining from massive forgings? The ideal would be a “net shape” or “chipless” process of some kind.

Yamaha, since its earliest days as a maker of pianos, has been a pioneer in producing metal parts using advanced casting methods. Best known to today’s motorcyclists was its deployment at the turn of this century of low-turbulence bottom-fill casting to achieve “near-forged properties.” This innovation made today’s high-production aluminum motorcycle chassis possible. A top mechanic for a competing 600 Supersport team once told me, “The Yamahas are about 32 pounds lighter than the best we can do right now.”

Today, similar processes are now widely used. But at the time it was clear that Yamaha was willing to invest serious money in advanced production technologies.

Yamaha’s Spin-Forged Wheels

Yamaha’s new wheels begin as aluminum castings, but with the rim in an unfinished state. The wheel is clamped in a fixture and heated by a gas flame while rotating. The forming tool is a roller that is brought to bear against the rim material, forcing it against a shaped mandrel beneath it. The metal is now “flow-forged” by the pressure of the moving roller, combined with the controlled temperature produced by the heating flame. Just as in a hot-forged part, the rim material is made to “flow” ahead of the roller, widening the rim and gradually conforming to the shaped mandrel.

Yamaha evaluated numerous alloys to find one that would work well with this process. The result is a tough, strong rim that will bend in severe impact rather than crack or have chunks broken out of it, possibly causing loss of tire-inflation air. Where a cast wheel rim would have to be 3.5mm thick to provide necessary strength, the “flow-forged” rim can be spin-forged as thin as 2.0mm.

Related: Behind The Design Of Motorcycle Wheels

Yamaha’s latest MT-09 hypernaked features spin-forged wheels. (Yamaha/)

Remember that as a vehicle accelerates, the material in wheels must be accelerated in two separate ways: 1) in a straight line, and 2) around its own axis. The second most greatly affects the fastest-moving part of the wheel—the rim. The main reason modern motorcycles are so responsive to steering inputs is not some special “sweet numbers” steering geometry. It is the lightness of modern wheels, tires, and brake discs.

To recap, Yamaha’s spin-forging process does reduce weight somewhat. But its major achievement is combining ductility and strength in the wheel rim that can enhance durability and rider safety.

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