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Rigidity, Compliance, & Frame Material(4 posts)

Rigidity, Compliance, & Frame MaterialMikeC
Oct 24, 2001 1:21 PM
The following link takes you to a feature on Seven's site where they compare a couple of basic attributes as they apply to frame material.
Yeah, it's just one company's opinion, and it's obviously commercial, but it's kinda interesting, (particularly for newbies?).
re: Frame Material (long)Tig
Oct 24, 2001 2:20 PM
Good stuff, even if a little general and opinionated.
Here's something similar from Calfee Designs. As we know, it's the useage and design of the material that counts. Each one has it's strengths and weaknesses. Craig Calfee certainly has done his homework.

Material Facts

Steel, aluminum, titanium and carbon fiber all attempt to achieve the above criteria, but differ from each other in strength, stiffness, weight, fatigue resistance, corrosion, etc. For example using aluminum or titanium in the same tube dimensions as a traditional steel frame would reduce weight but would produce excessive flexibility. So non-ferrous metal frames typically have larger tube diameters than steel ones to gain rigidity.

Metal frames usually do not fail due to a single catastrophic load but because of small, repeated stresses (called "fatigue"). Steel and titanium have defined minimum fatigue limits - if the stresses are smaller than these limits, these smaller forces generally don't shorten the fatigue life of the frame. Aluminum has no such specific endurance limit, so each stress cycle, however small, takes the material that much closer to fatigue failure. This sounds worse than it is, however - designers realize this limitation and attempt to "over build" their frames for a lifetime of use.

Titanium's high strength, light weight, resilience, and resistance to corrosion make it a well-suited frame material. However, being a metal, many of the same mechanical properties that limit steel and aluminum also limit titanium: metals are equally strong and stiff in all directions (a property called "isotropy"). Once the cross section geometry of a metal pipe is determined to meet strength or stiffness requirements in one plane, an engineer lacks the freedom to meet varying demands for strength or stiffness in any other plane. In metal tubes, by setting diameter and wall thicknesses to meet bending standards, this automatically determines torsional and lateral bending stiffness.

Metal frames are just variations on a single theme compared to composites. Composites consist of reinforcing fibers, particles, or whiskers that are embedded in a matrix material. Advanced composites are composed of engineered fibers and polymer, metal, or ceramic matrices combined to form fabrics. Combining these woven fabrics with a thermosetting adhesive (using the hair-like fibers of carbon, glass, and boron) create amazing strength and stiffness. They make structures that are as strong and rigid as metal ones of equal size, but weigh much less. Furthermore, until the binder is hardened by a chemical reaction or heat, the resin-soaked fibers can be molded or formed into virtually any shape.

Unlike isotropic metals, composites are anisotropic - their strength and stiffness is only realized along the axis of the fibers which can be arranged in any desired pattern. Thus, to absorb the variable stresses of a given bicycle frame, composite frames can use multiple layers with different fiber angles for each. This puts strength only where it is needed while minimizing weight.

Along with traditional round tube and lug frame designs, composite frames can be molded with the use of internal bladders and foam in either one-piece ("monocoque construction") or multi-section frames. Also, they can be formed in a high pressure lamination process combining the frame tubes into one integral piece.

The Benefits of Carbon

A bike frame is a considerably complex structure with performance characteristics that include: lightness, rigidity, durability, and shock absorption. Aluminum and titanium frames have become popular because they challenge steel frames in at least two areas of performance - lightness and shock absorption. But, at the high end of the industry, composites will likely eclipse frames made from any metals in all performance areas.

The metallurgical composition of a metal tube can't be varied over the length of the tube. In contrast, composites can be infinitely varied over the length of the tube. Some of the variations include: different fiber angles, different plies, different ply thicknesses, different combinations of materials. So the properties of the end product made from composites can be tailored to specifications.

Composite tubes are typically formed around a mandrel (a hard round insert, typically steel, that is later withdrawn) by either "filament winding" (winding strands at various angles), "roll wrapping" or "braiding." Some tubes combine methods, using a top woven layer for appearance and protection of the underlying wound ones. Another method called "pultrusion" pulls fibers through a heated die that melts a thermoplastic matrix. Each manufacturer has its own special number of layers and orientations of fibers to create its desired combination of strength, weight, and stiffness. This is the beauty of carbon fiber: with metals the choices are much more limited, but with carbon fiber they are almost limitless.

Tailoring of a bicycle frame is not new; it's been done with steel frames for years through the butting process, where tubes are thickened at the joints to handle stress and thinned out in their long center spans to reduce weight. What if the size and shape of each tube are matched precisely to the predicted loads of pedaling and road shock? What if the material could be distributed precisely where it is needed. What if the rigidity of each tube, through some complicated shaping or milling process, varied from one plane of bending to another or from one end to another? The frame could be built to be rigid to lateral pedaling loads, but fine-tuned in the vertical plane for compliance to road shock. Shaping and milling a metal frame in this manner would be nearly impossible.

Composites can be molded into structural members with complex cross sections with relative ease. They also have some very impressive mechanical properties. The 6061 aluminum mostly used in bike frames is roughly one-third as heavy as steel, one-third as stiff, and, at best, is about 80 percent as strong as the 4130 cro-moly steel used in most bike frames. Titanium is roughly two-thirds the weight of steel, one-half as stiff, and about 60 percent as strong as steel. The carbon fiber composite most used by bicycle manufacturers is less than one-quarter the weight of steel, but it is about as stiff (which makes it almost four times as stiff on a weight-to-weight basis), and it is roughly four times as strong in tension. Carbon fiber also has a better fatigue life than steel, titanium, or aluminum, and the resins typically used to bond the fibers offer extremely good vibration damping.

Vibration and shock damping are two important factors that affect the cyclist. However, they are two of the least understood subjects in materials science. There are so many variables involved - including how atoms in a material absorb and dissipate vibrational energy, how the structure is built, what type of paint and plating are applied - that it is hard to predict how a structure will react to vibrational input. Composite's shock and vibration-damping are far superior to any metal. Its strength- and stiffness-to-weight ratios are far superior to any metal.

Sophisticated finite element analysis programs and laminated-plate theory help define the properties of a composite structure. An inherent difference between composites and metals is that composite products are constructed in layers, or plies, of directional material. Interfacial adhesion and the potential for delamination (separation) under shear or compressive loads must be considered when analyzing an advanced composite design. This information is essential when addressing the variable requirements of a bicycle.

Composites differ from metals in that they don't carry loads equally in all directions, but bear loads best in tension. A composite is similar to a bundle of strings soaked in a layer of glue or resin. The bundle can bear more weight, and flex less, if pulled from end to end or flexed like a diving board than if compressed or loaded transversely. The changing face of the bundle's performance occurs because the real strength of the bundle comes from the string, not from the resin. The primary function of the resin is to lock the fibers in place, transfer loads among fibers, protect the fibers from environmental forces, and give the structure impact strength. The directional nature of the fibers' load-bearing abilities changes the rules of structural design.
Whew, that was comprehensive.Leisure
Oct 25, 2001 5:11 AM
And well done. I said it on the redundant post, the real problem with carbon (and composites in general) is the lack of real genius in the bike industry to harness what the material has to offer. One thing that never gets addressed anywhere I've seen is the epoxy properties. The primary benefit of composites is always in the tensile properties of the fibers, but compression and shear become largely dependant on the epoxy matrix, the properties of which are often glossed over and probably not very well researched by many of the companies that use them. I speculate this is one of the reasons that composite products have not lived up to consumers' expectations. And while I'm at it, why are we stuck using epoxy anyway; what other materials can/could be used for the matrix that might have better thermal resistance or compressive strength?
Calfee's new Dragonfly frame has boron, weighs 2 pounds -NMTig
Oct 25, 2001 8:19 AM