You probably thought that with this installment - part six of our six- part series on bicycle metallurgy - we'd be done with the subject. You were wrong, and you should know me better than that by now. I've added a seventh part, because there is too much remaining to discuss. And because I'm having too much fun doing it....
This time we'll start to talk about exotics: those materials that we didn't include in our coverage of aluminum, steel, titanium and carbon-fiber composites. The final episode of our series will have more about these exotics, plus a wrap-up and maybe even a quiz: I'll present you with some materials that have fantastic "numbers," and you can try to determine what they are, and why they would stink as a bicycle material.
The previous installment of this series covered the subject of carbon-fiber composites (it's not a metal, but we explained that last time). What wasn't mentioned or covered - on purpose, I didn't want to confuse the issue - was the distinction between thermoset and thermoplastic composites. The carbon-fiber composite bikes that you're used to seeing are of the thermoset variety. A newcomer on the scene is thermoplastic composite. The difference between how the two are fabricated is analogous to the difference between bread and chocolate.
The process of making a structure out of thermoset composite goes something like this (breadmaking procedure in parentheses). You mix up some ingredients (flour, water, yeast); put them in a mold (bread pan); apply heat (oven); and a chemical reaction occurs (yeast reacting).Voilà, Wonderbread. There's no turning back from this point. You can't re-melt the bread and start over.
Thermoplastic composite construction involves mixing ingredients, then heating them up until they go through a phase change (that is, melt). To get your frame, make sure that the material freezes into the shape you want, but if it doesn't, you can actually re-melt and re-mold it. (The number of times you can do this without property degradation depends on the material you're using.) And apparently - unlike the outgassing characteristic of the themoset process - you won't get much smell when this melting process happens. The material is also supposedly capable of being recycled, both by remolding or grinding and putting into a new mixture. So, because of its ability to be reformed and recycled, and the claim that is doesn't emit nasty smells, some people are calling it a green material.
The only incarnation of this technology in our industry that I am aware of is the Yeti bike, made in conjunction with Kaiser Aerospace and Penske.A very important attribute of the thermoplastic materials is that the impact resistance is much better than with thermoset materials. Epoxy is a relatively brittle material, and brittle is not your friend in a bicycle. A negative attribute of thermoplastic is that it is hard to bond anything to it. Given the past history of composite structures, and the quantities of little bits that you need to bond to bicycle frames, this can be a formidable hurdle.
Imagine a metal with half the density of aluminum, strength better than 6061, and elongation around 10 to 11 percent. I'm describing a magnesium alloy here, currently being tested by Easton. While magnesium is not normally known for its ductility, Easton says the material looks promising with those 10 to 11 percent elongation numbers. Although the modulus is low, in the range of 6MSI, that really shouldn't be an insurmountable problem. Aluminum has a relatively low modulus, but it doesn't mean an aluminum frame can't be built stiff. The same will hold true for magnesium, in fact a lower modulus would be welcome in the eyes of many.
One issue that needs to be addressed with this metal is the extreme problem with corrosion. Leave a magnesium part out in the rain and it will disappear faster than just about anything except unpainted steel. This problem can be overcome with proper surface treatment, like painting or anodization.
One of the intangible benefits of magnesium is that if you need to start a fire for some reason, just scrape some flakes off your dropouts, and light them up. They'll easily burn. For the mini- Hindenburg effect, just add water. The oxygen and hydrogen in the water disassociate, and party down with help from the magnesium. By the way, titanium does the same thing, but it's a little harder to get it started.
I'm sure you've heard of aluminum MMC's. Specialized has marketed its M2 line of bikes featuring Duralcan MMC tubing for several years now. The Duralcan material is an alloy of aluminum (for bike-industry purposes manufacturers use either 6061 or 7005 base material) combined with a ceramic material; in the case of the M2 it's aluminum oxide (Al2O3). Duralcan has a patented process by which it adds the Al2O3 while the aluminum is molten and in a vacuum.
The benefits of the process are apparently numerous, but for we tight-wad bikies, the big advantage is that it's a cheap way to produce this material. If aluminum oxide sounds familiar to you, it might be because you've sanded something with Al2O3 sandpaper in the past. If so, you've used essentially the same stuff that goes in these tubes: 600 grit aluminum oxide. That's right, sandpaper. Different percentages of Al2O3 yield different results. The M2 bikes have about 10 percent Al2O3 (by weight) in their mix. Which means they're 90-percent aluminum. Changing the volume fraction of the ceramic allows you to adjust the mechanical properties. Add more Al2O3 and stiffness goes up, but elongation and fracture toughness suffer. With a 10-percent mix, the material has about 8-percent higher yield strength, and 20-percent greater stiffness. The trade-off is that the elongation will be reduced, but to a claimed value of approximately 10 percent, which is acceptable.
Aluminum bikes are stiff enough, you say. True, but as you also know, this is a function of individual design. Suppose you are designing the rear end of a bike, and you want a certain level of stiffness, and you also want plenty of clearance for mud, heels, tires and chainrings. Smaller diameter tubes make it easier to accomplish this, and if you want the stiffness with smaller tubes, you benefit from having a higher modulus. With higher modulus material, you can also reduce the size of your main frame tubes so they don't resemble giant sausages.
The tangible benefit in being able to change the modulus of the different tubes for different applications is that where stiffness is more important, you can have that. Where you're more concerned about the ductility issue, as around the head tube junctions, you can enhance that property. These relatively small but important advances are great examples of what will continue to drive the evolution of bicycle frames.
Heat treatment is performed virtually the same as with a 6061 alloy. If you don't want to or can't heat treat, 7005 is also used for MMCs. Although it hasn't seen much commercial use yet, I'm sure by the time the 1995 models hit the tradeshow floors, you'll see them. The strength numbers don't really change over a standard 7005 alloy, but you can get those increases in modulus mentioned previously.
I've refrained from getting into the huge subject of MMCs that aren't weldable, but exhibit excellent mechanical properties. These are viable for bicycle use, but the designer is required to use lugs, or some other method of tube joining.
Overall, when you look at the aluminum oxide MMCs, there isn't any earth-shattering news. There are some minor enhancements to the mechanical properties, and some drawbacks. What matters most, as I've pointed out numerous times in this series, is intelligent design of the entire structure. Had enough for one column, yet? For the final installment, I'll take a look at three other aluminum MMCs, do an overview of beryllium and AerMet 100, throw in a mystery metal, and provide that long-awaited wrap-up.