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Guide to Bicycle Technology (article index)
This section explains the most significant technical aspects relating the choice of materials and their properties, as well as the construction techniques used. This will apply equally for the bicycle itself as for its individual components. Wherever possible, this will be done in such a way that even the technically uninitiated can understand it. A knowledge of these concepts will enable you to recognize quality when evaluating a bicycle or selecting components. In addition, the purpose of these explanations at this point is to avoid the need for repeated coverage of these technical details for each of the components covered in the sections of Part II. A high-quality bicycle is a successful synthesis of materials, dimensions and construction methods that gets the most out of the available potential of each. That means e.g. that when a different material is selected, the relative dimensions and the construction techniques must also be selected specifically to match the potentials and limitations of that particular material. Or conversely, when dimensions are altered to make a very small or big bike, or one for greater tire clearances or specific purposes, the material choice and joining methods may have to be adapted to these dimensions as well. Weight and Materials Selection The reason why a modern quality bike may weigh as little as 10kg (22 lbs), without being weaker than a much heavier model, lies in this successful synthesis of materials, dimensions and construction methods. More than for almost any other product today, weight saving is a primary aim of the designers of bikes and components. Everything possible is done to make each component as light as possible within the existing constraints. 4.1. An example of the sad result of in correct material choice and/or manufacturing techniques. 4.2. Strain gauge analysis used on a bicycle frame by frame tubing manufacturer Columbus to optimize tubing thickness. Anybody who wonders whether a minor weight reduction is really worth the cost and effort is encouraged to try it out for himself. Just compare the feel of a light bike with that of a heavier model when riding, even if the total difference is only a very small percentage of the total weight of bike and rider. The lighter bike, s if it also has significantly lighter wheels, is not only easier to get into motion, it also transmits less bocks and oscillations. These shocks and oscillations all result in the loss of energy that would otherwise be available for forward motion. Although some shocks can be eliminated by lowering the tire pressure, this in creases the rolling resistance, resulting in even more wasted energy. To explain this and other phenomena, the relevant physical and mechanical concepts will be briefly explained below. Mass and Weight The most significant effect of the mass (often casually referred to as weight) is when increasing or decreasing speed. Mass, measured in kg (kilogram), is the total amount of matter, quite independent of the force by which it is attracted due to the earth’s gravitational field, which is represented by the weight, measured in N ( Newton). At any one location relative to the center of the earth, mass and weight are of course so closely related that there is no harm — except in scientific accuracy of terminology — in using weight as an indicator for mass. At sea level and at most latitudes, an object with a mass of 1kg weighs 9.8N. Acceleration is the increase in speed, while deceleration is speed reduction (negative acceleration), and both are measured in terms of m/sec.2.The force required to accelerate a certain mass is a function of the mass and the square of the acceleration. Thus to accelerate a 14kg bicycle will require considerably a more force then a lighter one, while the same force will suffice to accelerate the lighter bike faster. Maintained over a period of distance or time, the force is multiplied by the distance to calculate the energy, or work required, expressed in Nm (Newton-meter). The power limited by the rider’s output potential, is computed as the quotient of energy and time, and is expressed in Nm/s. 4.3. Fatigue testing of the bicycle under cyclical loading at Raleigh. Done to make sure a bike or a component does not fail after a number of miles of use. Of course, the rider’s weight must be considered too. For a 65kg (145 lbs) rider on a 10kg bike, the difference of one kg amounts to only 1.3%. However, since in bicycling one is always working with marginal values and differences, such a relatively small difference is clearly detected. The difference becomes even more significant when the effect of rotating mass is considered, such as that of the wheels and, to a lesser extent, that of cranks, chain and pedals. This is due to the fact that such parts are not only accelerated forward, but also around. Simplifying, it can be said that the mass of the rotating parts should be doubled for purposes of acceleration calculations. Thus, if a 10kg bike has 6kg in fixed mass and 4kg rotating mass, the effect will amount to 14kg. Each kg saved on the rotating mass will reduce the mass effect by the equivalent of 2kg — that is 2.5% of the total for a 65kg rider on a 10kg bike. Weight may be minimized by several different means. As far as the choice of materials is concerned, the most obvious way is by selecting a lighter material of the same strength (if possible). The lighter material has a lower density (usually given without indicating the units, but measured in g/cm^3. Given the same dimensions, an aluminum alloy part (density 2.7) will weigh about 1/3 of an identical one made of steel (density 7.8). Of course, it still has to be strong enough, and that may require making it bigger if the lighter material is not equally strong as the heavier one, so the final re suit is rarely quite as dramatic as the difference in density would suggest. Other Material Properties As suggested above, weight can only be considered in conjunction with other material properties, primarily its strength. The bicycle and its components must be strong enough to withstand the loading without breaking or permanent deformation. The latter is referred to as plastic deformation, as opposed to the elastic deformation from which the material recovers, or springs back. The strength of a material is usually determined by its tensile strength, which is expressed in N/mm2 — the force that can be applied when pulling on the end of a rod of the material with a cross section of 1mm Of the various measures used, the tensile yield strength is the most useful, being the value at which plastic deformation sets in. Some manufacturers unfortunately still provide this and other technical details in archaic terms such as tons (meaning tons/square inch), lbs (meaning lbs per square inch) or kg/mm2. Table 10 lists conversion factors for these and other dimensions into their scientific equivalents. Even the elastic (temporary) deformation should not be so great that stability is affected negatively, even though some shock absorption in the vertical direction may be desirable. Rigidity is the quality that deter mines how well a bike or its components resist this kind of flexing. It is affected by several factors, including a material property referred to as modulus of elasticity, expressed in N/mm2. The greater the modulus of elasticity is, the more rigid a part of given dimensions under a given load. 4.4. Fatigue testing of cranks. Aluminum parts such as these must be tested to the actual maximum number of anticipated load cycles, whereas steel parts only have to meet a certain limit to allow a prediction of their useful life. The bicycle or its individual components should not suddenly break, even after many cyclical changes of loading. This kind of damage, known as fatigue failure, is dependent on the mutual coordination of material strength, loading, cross section and detail design (the latter particularly with respect to sudden changes of shape or thickness). The number of times the load changes may be very high, such as in the case of loads induced by road surface irregularities. The ability of a material to withstand this kind of load is expressed by the number of loading cycles it can withstand. For steel parts, there is a distinct limit above which a part will withstand any number of cycles, so a steel part need only be tested to this value (10 million cycles). Aluminum does not have such a distinct limit, but must be tested at least to the actual number of cycles anticipated. Hardness is a desirable quality to resist wear. This is particularly important for parts that move relative to each other, such as bearing and drivetrain parts. Unfortunately, harder parts are invariably less ductile, meaning that they may break when subjected to a sudden impacting force. For this reason, many parts are treated to produce a hard, wear-resistant outside, while retaining a ductile, impact-resistant core. De pending on the kind of material, this property is ex pressed by either Brinell or Rockwell hardness figures. These figures relate the force applied to the resulting deformation, higher values indicating greater hardness. 4.5. Testing a drivetrain for shifting ease and wear. The melting point of a material is of some relevance in the bicycle trade when the materials are to be joined by means of welding or brazing. Of course, this property is even more significant for the filler materials used in those processes. But the melting point also matters in the selection of such non-metallic materials as those for brake blocks and minor structures — witness the bike stand made of resin-reinforced plastic that collapsed under the effect of the sun behind glass shop windows. Corrosion resistance, finally, is a property that is less easily measured, yet can be significant for many parts of the bike, especially those that come in contact with each other or cannot be painted for other reasons. In general, the cheapest steels do not corrode as easily as stronger ones, since the latter contain more carbon. Aluminum, titanium and stainless steel all corrode less, although untreated aluminum may suffer badly when subjected to a salty atmosphere. Not only these material properties, but also the weight and the price of a bike and individual components are determined in a process of weighing up the relative ad vantages and disadvantages. The designers of the various bits and pieces can predict on the basis of basic engineering knowledge what the properties of certain designs will be. Unfortunately, many forget to take that trouble, so hopelessly improper designs for the selected materials are presented at regular intervals. Just the same, awareness for these finer points has in creased significantly in recent years. Choice of Materials The major categories of materials used for bicycle frames and components are steel and aluminum alloys. In addition to these common metallic materials, at least one other metal, titanium, and several other materials may be used for various parts of the bike. The latter include natural and synthetic rubber used for tires, as well as various plastic and resin-embedded fibers (usually referred to as composites). At this point, it should be clarified what the word alloy really means: it is any mixture of two or more materials of which at least one is a metal. Actually, in bicycle engineering circles, the term is usually reserved for materials comprising at least two metals. Steel alloys are mixtures of mainly steel with small percentages of one or more other metals, while aluminum alloys comprise mainly aluminum with minor percentages of one or more other metals. Thus it is patently incorrect to assume that alloy means aluminum alloy only, or that chrome-moly (meaning chrome-molybdenum steel) is not steel, as is often assumed or implied. Often manufacturers put up a smoke screen of technical sounding terms that mean nothing specific, or at least nothing special. Virtually all frames are made of the same rather limited range of steel and aluminum alloys, while most components are made of one of an equally narrow range of aluminum alloys. The fancy names some manufacturers use to describe their pro ducts rarely indicate anything that differs from the standard materials. 4.6. Fatigue testing. Here pedals are loaded cyclically in Raleigh’s materials and component testing laboratory. Amongst the material properties described in the preceding section, one should distinguish between physical and mechanical properties. Alloying can be used to improve the mechanical properties — strength, melting point hardness and ductility. The physical properties — density and modulus of elasticity — remain virtually unchanged and can not be affected by alloying. The same applies to the various thermal or mechanical treatments that (either intentionally or incidentally) affect only the physical properties of the material. Steel and its Alloys Steel is iron to which a small percentage of carbon (usually about 1%) has been added — thus, metallurgically, it is an alloy itself. Steel alloys are mixtures of this material with one or more other metals. Although the tensile strength of various steels and steel alloys can vary widely, the density and the modulus of elasticity remain essentially unaffected. Thus, given a certain dimension, the strongest alloy may resist a much higher force than a weaker one, yet it will bend, twist and torque equally far under this effect, and it will weigh just as much. If made with a smaller wall thickness, the stronger alloy will provide a tube as strong as one made with weaker steel and a bigger wall thickness, but then it will be less rigid, leading to more flex. Despite its rather high density of 7.8, steel is a very suitable material: strong, rigid, cheap, predictable with respect to fatigue, and easy to work with. Depending on the specific purpose, certain material properties can be enhanced. Thus, the surface hardness can be in creased by means of a form of heat treatment to give good wear properties for bearing parts, or the tensile strength may be increased by means of alloying, heat treatment and mechanical work, allowing the use of thin walled tubes to save weight. Tubing Types There are four different methods of making tubular shapes: welded, drawn, welded and drawn, and extruded. For steel, only the first three are appropriate, while the latter may be suitable for making aluminum and plastic tubes. Figures 4.7 and 4.8 show how the tube is formed by each of these methods. 4.7. Welded tubing is rolled into shape and then welded at the seam. 4.8. Seamless tubing is pierced and then pulled over dies until the required diameter and wall thickness are achieved. A welded tube is made by bending, or rolling, a flat narrow plate into the appropriate shape, after which the matching ends are butt-welded together. The weld seam remains visible on the inside (unless it is ground off locally, and can generally be felt through the bottom bracket as a distinct ridge. Although this method of construction is not necessarily bad, it is only used for relatively weak, cheap tubes, and the weld seam is generally considered to reduce the overall strength to 80% of that of a seamless tube with the same dimensions. Drawn tubes are made by pulling a heated steel bar over a form and then repeating this process, also rolling it in from the outside, until the desired dimensions have been achieved. The last stage is done cold — at least that’s what it’s called in metallurgical circles, meaning at temperatures not exceeding half the melting point temperature (expressed in degrees C). This total operation enhances the strength and hardness of the tube, lending itself to application with strong steels. Welded and drawn means just that: a welded steel tube is cold-drawn afterwards. This operation removes the weld seam and enhances strength and hardness, al though this material is rarely quite as strong as cold drawn tubing. All of these methods can not only be used for the manufacture of frame tubes, but also for other components with tubular cross sections, while the process of cold work for strength enhancement is equally used on non-tubular products. The tensile strength of ordinary welded carbon steel tubing, as used on simple bikes is about 400N/mm As a result, frame tubes for normal road bikes constructed of this material, when made with the conventional diameters, must have a wall thickness of about 1.4mm. For mountain bikes, tandems and other heavily loaded machines, they should be at least 0.2mm more. Some cheap bikes have frames of this material that seem quite light; they turn out to have a wall thickness of only 1.0mm, explaining the low weight, but indicating hopelessly inadequate strength, as was demonstrated clearly in a test conducted by the Dutch Consumer Association in which virtually all such bike frames eventually broke after hard use on rough roads. Increasing Material Strength The easiest and cheapest way to increase the material strength is by adding more carbon to the steel, resulting in what is referred to as ‘High Ten’ (for high tensile strength) or high carbon steels. This kind of tubing is also welded and has a tensile strength of about 450 - 480N/mm2. The same strength provided by simple carbon steel tubing with 1.4mm wall thickness can be obtained with a wall thickness of about 1.2mm when this material is used. To increase the strength further, allowing the use of even thinner wall tubing, other metallic components are added to obtain a steel alloy of greater strength. One percent manganese gives 10 - 15% more strength, reducing the required wall thickness to something like 1.0mm without loss of overall strength, even for welded tubes. 4.9. ‘Honking’ test. Done to establish the fatigue limit of aluminum handlebars when used to provide leverage during climbing. Things start to get quite a bit more expensive when the strength has to be increased even further. This is done by using other materials in the alloy — either more manganese in combination with molybdenum or chromium and molybdenum. These materials, referred to as manganese-molybdenum and chrome-molybdenum steel, respectively (or by their manufacturers’ trade names), are usually cold drawn, although some manufacturers offer welded and drawn tubes of the same quality at lower prices. Either process results in extremely strong tubes that could theoretically be as thin as 0.6mm, providing the same strength as the 1.4mm simple carbon steel tubes. By means of an additional heat treatment and drawing step, their strength can go up enough to allow even thinner walls, such as those used in Reynolds 753 and Tange Prestige tubing (Vitus 853 and Excel, which also contain vanadium, are at least as strong, but less well known). These very strongest, heat treated and cold drawn tubes are so hard that they can not be bent or shaped afterwards. It should be noted that all steels used for frame tubing, however exotic, are in a metallurgical sense referred to as low alloys, meaning they contain less than 5% total alloying metals besides steel (measured by weight). A few exceptions do exist, such as the stainless steel frames for utility bikes made by the German mass producer Kynast. By and large, stainless steels are not stronger than most of the low alloy steels used for bicycle construction, and their properties cannot be enhanced as readily by means of heat treatment or drawing operations — however attractive it is to have a material that does not corrode and can be left unpainted. Butted and Reinforced Tubes All the thin tubes, starting at about 1.0mm, are usually supplied butted, which means that only the central section is actually that thin, while the ends, or butts, have a greater wall thickness. This increase in wall thickness serves to avoid damage done by the heating and cooling process during welding or brazing of the frame, which will be discussed in a separate section. An additional benefit of differential wall thicknesses over the length of the tubes is that they can be de signed to match the local thickness to the relevant loading at any point along the tube, without sacrificing the weight and flexibility advantages of the thinner tubes in those areas where they are adequate to handle the local stresses. Fig. 4.10 shows, slightly overstated, the way single and double butted tubes are usually arranged in the frame. Note that the seat tube and the steerer tube, or fork shaft, are generally single butted, i.e. have a thick- walled section on only one end, to provide a constant wall thickness where seat post and handlebar stem are inserted. 4.10. Orientation of single and double butted tubing. 4.11. The three basic tubing types: welded, plain gauge seamless and butted. Many frame and tubing manufacturers claim their tubes to be triple or even quadruple butted, a misnomer if there ever was one, since no tube can have more than two butts (ends). What they mean is fair enough though, namely that there are three or four different wall thicknesses along the length of the tube, presumably to best handle the relevant loading locally. Other methods of reinforcing tubes include helical internal ridges, as used by Columbus, and external but ting. The helical ribs have the inherent disadvantage of forming sudden differences in wall thickness at heavily loaded areas (referred to as stress raisers), which makes them susceptible to fatigue failure. Maximizing Rigidity To increase the rigidity of the tubes, without sacrificing weight or strength, several methods have recently been introduced. Columbus has presented its Max tubes that are flattened somewhat at the ends, with the longer axis coinciding each time with the orientation that calls for the greatest bending force. Another method is to form the tube into a kind of lozenge shape or to put a distinct ridge along the largest part of its length. In the case of the very strongest cold drawn and heat treated materials, this must be done before the final drawing or heat treating operation. On mountain bikes, tubes have been made to greater outside diameters than has become the standard for road bikes. This provides a greater strength and a significantly increased rigidity, even ii the wall thickness is reduced somewhat. This same method, with notably thin walls, was recently also introduced by several manufacturers of road bikes, while many mountain bike manufacturers have been taking this method even further. Tubes made along these lines are referred to as OS, or oversize, tubes. Aluminum Alloys Aluminum is frequently used for many bicycle components because its low density of 2.7 promises considerable weight savings over steel components. How ever, pure aluminum is too weak and soft for this purpose: Its tensile strength is only 120N/mm2 which would require three times the steel tube’s wall thickness, resulting in the same weight. A number of aluminum alloys have been developed to overcome this problem. However, the modulus of elasticity of aluminum is also about 1/3 of the value for steel. 4.12. Cannondale bicycle with welded aluminum frame. Even so, aluminum can be used to advantage — providing it is used with the material’s properties in mind. This has long been done by making e.g. aluminum brake arms about 1.7 times as thick as steel versions, resulting in a weight savings without reducing the rigidity. Similar tricks can be played with frame tubes, as aluminum frame pioneers such as Gary Klein and Charles Cunningham initially showed, and has since been confirmed by numerous other frame builders and major manufacturers. For many years, alloys designated as 7000 series, con- taming about 5 - 7.5% zinc, have been used, especially for frame tubes. Their disadvantage is that the frames have to undergo heat treatment after welding. Most of these materials owe their high strength to a particular heat treatment procedure during the tube-manufacturing process (i.e. before welding), identified with a suffix such as -T6 added to the basic material designation number. More recently, 7005 and 7039, two interesting varieties of the 7000 series (all of which contain zinc) that do not require after-weld heat treatment, have been applied successfully for mass-produced bikes. Aluminum alloys of the 5000 series are not weldable, although they include some of the strongest types. Whenever these are used they are joined either by bonding with an anaerobic setting epoxy resin or by means of some high-tech process based on literally shrinking the tubes around the lugs, fusing them in an electric induction process. 4.13. Detail of bonded (‘screwed and glued’) aluminum frame. Titanium and its Alloys Titanium and its alloys have a density of about 4.6, i.e. 60% of the value for steel, leading to a significant weight reduction. However, it also has a lower modulus of elasticity, which means the way titanium parts are dimensioned is almost as important as it is for aluminum. Other disadvantages of titanium are its high price and the difficulty encountered when it is machined, formed or welded. It is used both for frames and for small components. Magnesium 4.14. Bicycle with one-piece cast magnesium alloy frame from Kirk Precision. This is the lightest of the structurally suitable metals, with a density of 1.7, or 22% of the corresponding value for steel. Its strength is very much lower, as is its modulus of elasticity. Only by means of significant alloying with other metals does the strength come in the useful range for bicycle applications, but even then the modulus of elasticity makes for inadequate rigidity. At least that’s what everybody thought until recently. This view was changed by the British entrepreneur Kirk, who introduced a cast magnesium frame. An interesting construction, it is reasonably cheap (due to mass production techniques and publicity as well as funding from parent company Norsk Hydro, the world’s major magnesium supplier), reasonably light, fabulously strong and extremely rigid. Actually, its major attraction is probably its unusual shape. As with most other bicycle frames made with exotic materials, the end result is not really any lighter than comparable steel frames. Just the same, it is a refreshing departure, escaping from the ingrained conventional thinking in the industry that characterizes too many of the other pro ducts in the industry. Plastics and Composites By and large, you can ignore ordinary plastics for the construction of most useful bicycle components. Of course, ordinary plastic is too imprecise a concept, but the statement applies accurately to all plastics tried so far for load bearing parts on bicycles. An experimental plastic-framed bike, which actually went into production in the early nineteen-eighties, was heavy, noisy and uncomfortable, without solving any of the real problems of the bicycle, such as the fact that the drive- train is exposed to the elements and corrodes easily. 4.15. Kestrel frame made of Kevlar fiber- reinforced epoxy resin. Clearly, too much experimenting with the bike ignores the real problems: there is nothing wrong with a steel frame that a layer of paint can’t solve, yet that’s all these experimenters seem to think of. Alter all, innovation in the motor industry has not concentrated on the bodywork, but on what’s inside — and during the time when all the manufacturers did was change the body from year to year, there was technical stagnation in that industry. Just the same, certain plastics have specific applications. One of these is Kevlar, or more correctly aramid, the generic name for a substance that has been under dispute between DuPont and the Dutch AKZO company, each claiming to hold the original patent on its manufacture. This artificial fiber is as strong and unstretchable as steel, yet many times lighter and more flexible, making it suitable for reinforcing structures in tires. 4.16. Yesteryear’s composite: hickory frame ‘tubes’ and brass tugs were combined on this American bicycle built around 1900. Both aramid and various other (mainly natural) fibers can be used to advantage as harnessing structures in a matrix of epoxy resin to provide light and strong so called composite materials. The other fibers used for this purpose are carbon and boron. As long as the resin selected hardens fully and the fibers are long, very sound structures can be made, suitable for frame tubes and other parts. However, they can also be used to advantage in a mold, in a method similar to what is used’ in fiberglass construction, to form composite frames that differ from the run of the mill, departing from the conventional tubular structure. To date, nobody seems to have been able to make a frame of this material that is as strong as a steel frame without making it equally heavy, which makes you wonder whether there is any justification for such expensive techniques. Dimensions and Construction These two factors are at least as important as the choice of materials, and must always be considered in conjunction with the latter. Often a part can actually be made lighter with steel than with some of the lighter materials, providing the dimensions and the construction methods are selected to make the most of the material. Thus, recently introduced thin-walled, high- strength steel handlebar stems have proven to be lighter than the aluminum versions that were considered the cat’s whiskers up to then. Similarly, it makes little sense to make handlebars and seat posts of aluminum, since given the diameter used, they could be lighter and stronger if made of thin-walled, high-strength tubular steel. Even cranks could be made lighter if hollow steel were used instead of solid aluminum. Lately, there has indeed been a rediscovery of steel, to which the introduction of (small) stainless steel chainrings for mountain bikes testifies. 4.17. Factors that affect rigidity of a simple structural member: The less it bends, the more rigid it is. 4.18. Factors that affect rigidity of a tubular member. Again, the more rigid member is the one that bends least. The answer to many of the bicycle’s particular demands seems to lie in the use of tubular members, as was discovered over a century ago when engineers first got seriously involved with bicycles in Victorian England. Under tensile (pulling) loading, it does not matter whether a member is tubular or a simple rod, as long as the same total material cross section is used, expressed in mm Thus, a 6mm (1/4”) diameter rod would be as strong as a 28mm (1 1/8”) diameter tube with 1mm wall thickness. However, frame tubes are not loaded only in tension, but alternately in compression and torque as well, and for this kind of loading the dimension perpendicular to the load must be maximized, which is elegantly achieved with round tubing. The resistance against bending due to perpendicularly applied forces is dependent on a number of factors. The resulting deflection is:
Consequently, to avoid excessive deformation with a given force and material, components must be designed so that they are as short as possible, and their thickness perpendicular to the direction of the force must be maximized. This explains why brake arms are made as chubby as they are and why the wheel clearances are minimized to make them as short as possible. On the other hand, that does not justify making frames smaller than necessary to fit the rider: it is the finished geometrical structure that counts, and on a frame, the maximum rigidity is achieved if the structure extends to the limits, with the members joined rigidly at the corners of the resulting polygon. Applied to cylindrical members, be they rods or tubes, these same factors can be expressed slightly differently. In this case, deformation is determined as follows:
Within limits, the biggest diameter is the best, even at the expense of wall thickness, thus retaining the same weight. Fig. 4.18 illustrates this. The limit is a function of the risk of collapse — the aluminum beer can effect. To prevent this in parts that are anchored at both ends, such as frame tubes, the ratio of wall thickness to diameter should not exceed 1:30. However, while cantilevered parts such as the fork must have significantly greater wall thickness, also related to the suspended length and the load, and must be thicker near the fixed point than near the end that is loaded, to avoid collapse. 4.19. Detail of a lugless (fillet brazed’) steel frame on an older Gary Fisher mountain bike. Although beautiful, the build-up of brazing metal in the fillet does add to the frame’s weight. For aluminum tubing (which has less inherent rigidity than steel ones), the principle of maximized outside diameter, rather than wall thickness is used more and more. After all, the same effect as a 35% increase in wall thickness can be achieved with only a 10% in crease in outside diameter — with a correspondingly lower weight penalty. Recently, several manufacturers have applied the same principle to steel tubing as well. All of the above is not limited to frame tubes. Tubular steel luggage racks and many other components can be designed with these principles in mind, providing in creased stability with a minimal weight penalty — even with a weight reduction in certain cases. It is worth noting in this context that the benefit of large diameter tubing only applies to bending and torque forces. Tension forces are just as well taken up by relatively narrow members of adequate diameter, while the remarks about buckling apply to compression forces. Finally, it should be noted that both tension and compression forces must be taken up in line with the force, since the supporting member would otherwise bend. Thus, e.g. the stays of luggage racks must be straight. Joining Methods In addition to the replaceable screw-threaded and wedged connections covered in Section 3, other techniques are used to permanently connect parts of the frame and other bicycle components. These methods, which will be covered below, include various welding processes, brazing (i.e. hard soldering), and resin bonding. The curiosity of casting a lug around the tubes has been used at times for cheap bikes — technically there is nothing wrong with it, but aesthetically it is an abomination. Brazing This is still the most common method of frame construction when steel tubes are used. Brazing is a process by which the tubes are heated to temperatures well below their melting point, after which a filler material with a lower melting point is allowed to melt and run between the joint, forming a permanent bond (the molecules of the filler material intermingle with those of the base metal) when cooled. For steel tubes, it is done at temperatures between 650 and 960 degrees C, depending on the particular brazing material used. Only relatively thick-walled tubes of the less exotic types of steel should be brazed at the higher temperature, because the sequence of heating could weaken a thin point at a distance of about 10—20mm from the hottest joint, referred to as the heat affected zone. Low temperature brazing requires the use of brazing rods with a high percentage of silver. This process is usually referred to as silver brazing, even though the material used is still predominantly copper and zinc. 4.20. Unusual industrial joining method: Aluminum tugs cast around the stain less steel tubes on a Bridgestone fame. Welding In the welding process, the base metal is heated to the melting point and the parts are fused together either with or without the aid of a compatible filler metal, forming one piece after cooling. Because a much smaller area has to be heated for a shorter time, al though the temperatures involved are higher, welding is actually more time- and energy-efficient than brazing. Due to the high temperatures required, the base metal must be relatively thick for successful welding. About 1.2mm is the minimum wall thickness under most circumstances. To prevent corrosion, welding is often done by the TIG (tungsten inert gas) process, by which a tungsten welding element (which itself does not melt, due to its extremely high melting point) is used and the joint is protected against oxidation by means of a blanket of inert gas. Welding can be used for steel as well as for aluminum and titanium. The TIG process is used when joining the latter two metals, since oxidation would otherwise make a reliable bond impossible. When welding the strong heat treated alloys of the 7000 series, the material properties are affected negatively by the heat that resuJts from welding. This effect must be reversed subsequently by a controlled heating process referred to as post weld heat treatment. Bonding and Other Processes If nonmetallic materials have to be joined, it is usually done by other methods, such as bonding, and even aluminum is frequently joined this way. Bonding is usually done by means of an anaerobic hardening epoxy resin, made by mixing two reacting components shortly before applying them. Depending on the sub stance used, the gaps between the parts may have to be relatively generous or very tight, but the surfaces must always be quite uniform and scrupulously clean. In some cases, the joint is screwed or clamped as well as bonded. Recently, the use of strong non-weldable alloys has led to the development of different joining methods, including a high voltage electro-magnetic shrink-fusion process pioneered in Austria. Finishing Processes The different parts of the bicycles are protected against corrosion, or just made to look prettier, by means of several different processes. Steel and aluminum frames are generally painted or lacquered. Sometimes chrome plating is used on steel, while both aluminum and titanium may be anodized. The latter is a galvanic process that transfers the outer layer into a hard, impregnable form of oxide, which in turn protects the rest of the material. Modern paint processes generally use an epoxy resin, which reacts anaerobically into a hard layer. Some frames are still lacquered, using many thin layers of a highly diluted material, usually finished off with a transparent layer. In recent years it has become popular to color even aluminum parts, which may be achieved either by adding a dye to the anodization or by means of resin coating. The latter process is used on cheap parts in order to save the rather expensive polishing that would be necessary to obtain a smooth, clean look on bare metal parts. On components that come into contact with others, a blank finish is preferable, since the coating or anodization soon wears off, making the whole thing look rather shabby. 4.21. Another mass-production joint details. Carbon fiber-reinforced epoxy lug, cast in place around the frame tubes. Increasing Surface Hardness Finishing processes are also used to increase the surface hardness for increased wear resistance. Thus, rims are often anodized to a greater depth — 0.050mm, rather than the 0.005mm depth used for simple corrosion protection. This process may also be used on aluminum chainwheels. Steel parts are often protected and made wear-resistant by means of hard chrome plating. Even more common is the use of chemical oxidization, which results in a matt black surface (although the use of a dye can produce almost any color effect). Soft metal parts, finally, may be nickel plated, since the harder chrome would easily peal off when the parts are deformed — it is also an excellent finish for frames. This process is used on spoke nipples. Stainless steel parts are usually left completely untreated. Actually, stainless steel does corrode, but this material instantaneously forms its own shiny surface film. 4.22. An interesting historic bike. This 1890 cross-frame may be seen as the prototype for the De La Haye and Breeze frames shown in Section 1. |
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