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Bicycle FrameA bicycle frame is the main component of a bicycle, onto which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and is made of two triangles, a main triangle and a paired rear triangle. This is known as the diamond frame. The main triangle consists of the head tube, top tube, down tube and seat tube. The rear triangle consists of the seat tube, and paired chain stays and seat stays. The head tube contains the headset, the interface with the fork. The top tube connects the head tube to the seat tube at the top, and the down tube connects the head tube to the bottom bracket. The rear triangle connects to the rear dropouts, where the rear wheel is attached. It consists of the seat tube and paired chain stays and seat stays. The chain stays run parallel to the chain, connecting the bottom bracket to the rear dropouts. The seat stays connect the top of the seat tube (often at or near the same point as the top tube) to the rear dropouts. Unless otherwise specified, the remainder of this article focuses primarily on the diamond frame. Frame geometry There are variations on the basic diamond frame design. Historically, women's bicycle frames had a top tube that connected in the middle of the seat tube instead of the top, resulting in a lower standover height. This was to allow the rider to dismount while wearing a skirt or dress. Although some women's bicycles continue to use this frame style, this is also known as a step-through frame. The length of the tubes, and the angles at which they are attached define a frame geometry. The geometry of the frame depends on the intended use. For instance, a road bicycle will place the rider in a lower, more crouched position; whereas a utility bicycle emphasizes comfort and has an upright seating position. Geometry also affects handling characteristics. Frame geometries in which the wheelbase is shorter are quicker in cornering but harder to balance. Mountain bicycles For ride comfort and better handling, shock absorbers are often used; there are a number of variants, including full suspension models, which provide shock absorption for the front and rear wheels; and front suspension only models (hardtails) which deal only with shocks arising from the front wheel. The development of sophisticated suspension systems in the 1990s quickly resulted in many modifications to the classic diamond frame. Recent mountain bicycles with rear suspension systems have a pivoting rear triangle to actuate the rear shock absorber. There is much manufacturer variation in the frame design of full-suspension mountain bicycles, and different designs for different riding purposes. Frame size Frame size was traditionally measured from the centre of the bottom bracket to the top of the seat tube. Typical "medium" sizes are 21 or 23 inches (approximately 53 or 58 cm) for a European men's road bicycle or 18.5 inches (about 46 cm) for a men's mountain bicycle. The wider range of frame geometries that are now made have given rise to different ways of measuring frame size; see the discussion by Sheldon Brown. Touring frames tend to be longer, while racing frames are more compact. Frame materials Historically, the tubes of the frame have made of steel. Steel is still used, newer frames can also be made from aluminum alloys, titanium and carbon fiber. Several properties of a material help decide whether it is an appropriate in the construction of bicycle frame: - Density (or specific gravity) is a measure of how light or heavy the material per unit volume.
- Stiffness (or elastic modulus) can in theory affect the ride comfort and power transmission efficiency (but in practice, because even a very flexible frame is much more stiff than the tires and saddle, ride comfort is in the end more a factor of saddle choice, frame geometry, tire choice, and bicycle fit).
- Yield strength determines how much force is needed to deform the material. (for crash-worthiness).
- Elongation determines how much deformity the material allows before cracking (for crash-worthiness).
- Fatigue limit determines the durability of the frame when subjected to cyclical stress from pedaling.
Tube engineering and frame geometry can overcome much of the perceived shortcomings of these particular materials. Steel Steel is flexible, strong, and relatively inexpensive, but more dense than exotic materials. A classic type of construction for both road bicycles and mountain bicycles uses standard cylindrical steel tubes which are connected with lugs. Lugs are fittings made of thicker pieces of steel. The tubes are fitted into the lugs, which encircle the end of the tube, and are then brazed to the lug. More expensive bicycles have lugs which are filed into fancy shapes - both for weight savings and as a sign of craftsmanship. While lugs are not the only method of joining bicycle frame tubes, they facilitate easy field repair of a damaged frame. To clarify, when cycling in third world countries, a lugged frame can generally be repaired due to its familiar and simple construction. Also, since steel tubing can rust, the lugged frame allows a fast tube replacement with virtually no damage to the neighboring tubes. A more economical method of bicycle frame construction uses cylindrical steel tubing connected by welding or brazed welding, which does not require lugs to hold the tubes together. Instead, frame tubes are precisely aligned into a jig and fixed in place until the welding is complete. Most common steel production bicycle frames are welded, as are many high-end custom frames that are individually built for a customer. The braze welding process of joining frame tubes is more labor intensive, and consequently is less likely to be used for production frames. Some custom frame builders and their customers prefer a braze welded frame for aesthetic reasons. Among steel frames, better performance and higher cost are obtained by using butted tubing. Butting means that the inner diameter of the tubing changes from small at the ends (thicker for strength) to larger in the middle (thinner for lighter weight). The tubing would be made of special steel alloys (generally chromium-molybdenum, or "chromoly" steel alloys) chosen for their combinations of strength and lightness. Reynolds and Columbus are two of the most famous manufacturers of bicycle butted tubing. Aluminum alloys Aluminum alloys are very lightweight, but inflexible. If too much flexion is demanded of aluminum, it will fail, (i.e. the tube will crack). Also, in contrast to steel and titanium, aluminum has a notable fatigue limit; even small repeated stresses will eventually cause failure. Alloying help to extend the fatigue life. The most popular type of construction today uses aluminum alloy tubes that are connected together by Tungsten Inert Gas (TIG) welding. Aluminum bicycle frames started to appear in the marketplace only after this type of welding become economical until the 1970s. Comparing equal tube sizes, aluminum is less stiff than steel, but it is also lighter. In order to raise aluminums stiffness, the tubing diameter is increased beyond that of steel and thus known as oversized tubing. The greater diameter generally results in a frame that is significantly stiffer than steel. This is not always a benefit, since a compliant steel frame absorbs more road irregularities whereas an aluminum frame passes more of this abuse to the rider. Aluminum frames are generally recognized as having a lower weight than steel, although this is not always the case. An inexpensive aluminum frame may be heavier than an expensive steel frame. Butted aluminum tubes -- where the middle sections are made to be thinner than the end sections -- are used by some manufacturers for weight savings. Other innovations include the shaping of the cross-section of the tubes, such as in a keyhole shape, for optimizing stiffness and compliance in different directions. Titanium Titanium is perhaps the most exotic and expensive metal commonly used for bicycle frame tubes. It combines many desirable characteristics, including high strength, light weight and corrosion resistance. This combination of properties allow for most titanium frames to be constructed with "standard" tube sizes comparable to a traditional steel frame, although some larger diameter tubing is also used at times for more stiffness. At 15 times more expensive than steel, these frames are out of reach for most cyclists. Many common titanium alloys and even specific tubes were originally developed for the aerospace industry. Titanium frame tubes are typically joined by Tungsten inert gas welding (TIG) or Metal inert gas welding (MIG). It is more difficult to machine than steel or aluminum, which sometimes limits its uses and also raises the effort (and cost) associated with this type of construction. Carbon fibre Carbon fibre, a composite material, is the only non-metallic material commonly used for bicycle frame tubes. Although expensive, it has light weight, corrosion resistance and high strength, and can be formed in almost any shape desired. The result is a frame that can be fine-tuned for specific strength where it is needed (to withstand pedaling forces), while allowing flexibility in other frame sections (for comfort). Custom carbon fibre bicycle frames may even be designed with individual tubes that are strong in one direction (such as laterally), while compliant in another direction (such as vertically). The ability to design an individual composite tube with properties that vary by orientation cannot be accomplished with any metal frame construction commonly in production. Simple carbon fibre frames are assembled using cylindrical tubes that are joined with adhesives and lugs, in a method somewhat analogous to a lugged steel frame. More exotic carbon fibre frames are manufactured in a single piece, called monocoque construction. While these composite materials provide light weight as well as strength, they have much lower impact resistance and consequently are prone to damage if crashed or mishandled. It has also been suggested that these materials are vulnerable to fatigue failure, a process which occurs over a long period of time. Many specialty racing bicycles built for individual time trial races and triathlons employ composite construction because the frame can be shaped with an aerodynamic profile not possible with cylindrical tubes. While this type of frame may in fact be heavier than others, its aerodynamic efficiency may allow an individual cyclist to attain maximum speed and consequently outweigh other considerations in such events. External links Frame
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