ASK THE MXPERTS: “WILL THE BIG FOUR SWITCH TO STEEL FRAMES?”

When the MXA wrecking crew was snooping around the KTM factory, we spotted this prototype aluminum KTM frame in the corner of the R&D department. They tested it and didn’t like it.

ALUMINUM VERSUS STEEL

Dear MXA,
I thought the major reason most manufacturers switched to aluminum frames was to reduce weight, and yet the 2018 KTM 450SXF is 9 to 19 pounds lighter than its competitors, some of which don’t even have electric start. Is it time for the Japanese manufacturers to rethink steel?

First, all of the KTM’s weight savings over the Honda, Yamaha, Suzuki and Kawasaki are not just a result of the weight difference between aluminum and steel. KTM spent a lot of years, time and money casting smaller and more compact engines to save weight (and that also includes a 1-pound-lighter 2019 engine). As any racer who ever tried to knock weight off his race bike knows, no single component is going to get the job done. It requires an overall approach that worries not just about pounds and ounces on the frame, suspension and engine, but grams on virtually every component. As far as the difference between KTM’s chromoly steel frame and the aluminum frames of the “Big Four” Japanese brands, there are lots of market forces at work. Here is a quick list.

The 2019 Kawasaki frame shows all of the different methods of extruding, casting and forging aluminum pieces to build a strong frame.

(1) Brooklyn Bridge. Originally, aluminum frames were lighter than the steel frames they replaced, but over time, they began to gain weight. As aluminum tubes were replaced with castings and, eventually, castings with forgings, the aluminum frames became stronger, more rigid and offered increased durability. But, they got heavier. Engineers knew this as far back as the 1990s. In fact, in the 1990s, ATK designer Horst Leitner got tired of beefing up the aluminum swingarms that came on ATK 604s and welded up a chromoly swingarm of the same dimensions. Shockingly, he discovered that the old-fashioned steel swingarm was several pounds lighter. The days of the simple aluminum swingarm had been replaced by Brooklyn Bridge-style structures that were no longer lighter than a steel swingarm. KTM stuck with a chromoly frame because its test riders preferred the feel, but if you look above, you will see a prototype KTM aluminum frame that they built to test the feasibility of going to aluminum. They decided not to.

(2) Cost factor. Making a chromoly frame is a time-consuming and expensive venture. The chromoly tubes have to be bent, mitered, gussetted, welded, heat treated and cold set. There are many more steps required to make a classic steel frame than an aluminum frame. But, chromoly leaves lots of R&D leeway to make quick and simple test frames. Aluminum frames are much cheaper to build (once you amortize the casting costs). The aluminum pieces simply plug into each other and then can be welded together on a jig. They don’t need immediate heat treating because they achieve their strength as they travel across the oceans. With as few as 10 aluminum pieces, no bending and no mitering or gusseting, mass-producing aluminum frames is a money- and time-saver.

(3) Steel future. If we were betting men, we’d bet the house that there will be no steel-framed Japanese motocross bikes in the future.

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