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Magnesium is the lightest structural metal currently available in the world. Its approximately 34% lighter by volume than aluminum and 50% lighter than titanium. Besides light-weight construction, a few of the other advantages that magnesium offers are: excellent fatigue resistance, denting and buckling resistance, and the highest known damping capacity of any structural metal. The result is a much smoother ride than any other material out there.
Its no secret amongst the inner sanctum of bicycle fame builders and designers around the world that magnesium could make an amazingly rideable, high-performance bicycle. First, though, a few key issues needed to be overcome: how to extrude the perfect magnesium tubes, weld these tubes correctly, and fine-tune the ride qualities. Following four years of intensive research and lots of hard work on the drawing board and in the shop, we have done just that.
is a measure of the rate at which unforced vibration disappears, and is
an intrinsic property of the material. Because magnesium has the
highest known damping capacity of any structural metal - as much as 10
times greater than steel, titanium, or aluminum - the ride is truly
amazing. Vibration from the road literally disappears within the
structure of the frame and never reaches the rider. For this same
reason, magnesium is used in the high-tech world of aerospace hardware;
they have discovered that magnesium offers the best platform on which to
mount sensitive electronic equipment, as it is so "quiet".
Being able to ride longer and with less fatigue are the direct benefits, but the indirect benefits of magnesium might be even more important. Because vibration is not transferred through the frame, the fatigue life is nearly immeasurable. A magnesium bike will keep the exceptional initial ride qualities longer than anything else available on the market today.
|We are often asked whether Paketa's magnesium tubing is a marketing maneuver akin to "scandium" frames, for example, where the material is actually just another aluminum alloy with a small percentage of scandium - or maybe magnesium? The simple answer is, "No!" Paketa's proprietary tubing is a carefully chosen material comprising about 92% magnesium, alloyed with other metals to achieve optimal mechanical properties while facilitating the extrusion and welding processes.|
Tandems have much in common with half-bikes and also have their own peculiar design issues and trade offs. So long as there's sufficient room to accommodate the tube profiles required, magnesium becomes more advantageous in terms of stiffness, strength, fatigue resistance, dent resistance, ride quality, and durability compared to compared to other materials.
Magnesium has several properties that both alone and in synergism make it ideal for tandem frames: low density, high vibration damping capacity, good fatigue resistance, and low notch sensitivity. The low density allows the use of large-diameter tubes while still maintaining adequate tube wall thickness to avoid dents and buckling. The whole issue of tube diameters cannot be understated: a (round) tube's bending and torsional (twisting) stiffness is proportional to the fourth power of the tube's diameter, so increasing the diameter even a small amount results in a huge increase in stiffness as well as strength. For example, a mere 19% increase (the difference between 1" and 1-1/8", or 2" and 2-1/4") in diameter doubles the tube's stiffness.
In terms of dent resistance, the factor is a power of three; i.e., the resistance for a given material is a cubic function of the tube's wall thickness. Mg, being 1/3 lower density than Al (and 1/2 that of titanium or 1/4 that of steel), maintains better dent resistance to an aluminum tube of comparable stiffness, taking the difference in tube diameters into account, even though the modulus (the intrinsic bending stiffness) of Mg is about 1/3 lower than Al.
The ability to use large-diameter tubes with higher stiffness also makes the frame stronger. All else being equal, a stiffer tube is stronger by virtue of bending less under a given load: the stress-to-strain ratio, which is an intrinsic material property (modulus) but also a design parameter (tube sizes, shapes, etc.) can be effectively reduced with intelligent design that takes advantage of the material properties. As Deetz states, "stiffness and strength are simply design issues." He's dead right on this point.
Turning to the issue of durability, Mg has much lower notch sensitivity than Al or carbon fiber. Notch sensitivity is what causes carbon fiber to fracture rather than bend when struck by a sharp object. In the case of Al, notch sensitivity is what creates a stress riser from a dent or nick in the frame that can lead to fatigue failure over time. Consequently, both carbon fiber and Al frames are typically over-designed to provide an adequate safety margin against such threats. Aluminum handles impacts from sharp objects better than carbon fiber in that it won't be ruined instantly. Instead, the stress riser increases the likelihood of fatigue failure down the road. Dents and nicks are inevitable in the real world, and magnesium is inherently durable.
On all of these points, the larger, longer tubes of a tandem frame accentuate the positive attributes of the material. Larger tubes means higher stiffness and strength, with excellent durability, while keeping the weight down. An engineering analysis demonstrates conclusively that the open-frame design of the Paketa (which other manufacturers have now adopted) is more efficient than the direct-lateral design, which may be appropriate for other materials but not magnesium. Form follows function, and this is the right design for a magnesium tandem.
As applied to bicycle frames, the most interesting and unique property of Mg among all metals is its vibration damping capability. The engineering term is usually termed, "damping capacity." This varies according to the alloy composition, but it's typically about an order of magnitude--ten times--greater than steel, titanium, or aluminum. This is usually the first characteristic that test riders comment on after riding a magnesium bike: it's smoother and more comfortable than any other bike they've tried. Never mind the stiffness and handling, strength, durability, or light weight; ride quality trumps them all. It's one of those intangibles that is hard to grasp until you get on one yourself. In my experience, once a potential buyer tries a Paketa, the decision is an easy one.