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Bike Frames

A bike 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 the diamond frame. The diamond frame is made of two triangles: a main triangle and a paired rear triangle.

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, which interfaces 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 shell.

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 to the rear dropouts.
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Frame Tubes
The diamond frame consists of two triangles: a main triangle and a paired rear triangle. 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.
Head Tube
The head tube contains the headset (the interface with the fork). In an integrated threadless headset, the bearings interface directly with the metal surface on the inside of the head tube.

Top Tube
The top tube connects the head tube to the seat tube at the top. In a mountain bike frame, the top tube is almost always sloped. In a traditional-geometry racing bicycle frame, the top tube is horizontal. In a compact-geometry frame, the top tube is sloped.

Control cables are routed along mounts on the top tube. Most commonly, this includes the cable for the rear brake, but some mountain and hybrid bikes also route the front and rear derailleur cables along the top tube.

The space between the top tube and the rider's groin, while straddling the bike and standing on the ground, is called clearance. The total height from the ground to this point is called the standover height.

Down Tube
The down tube connects the head tube to the bottom bracket shell. On racing bicycles and some mountain and hybrid bikes, the derailleur cables run along the down tube. On older racing bicycles, the gear levers were mounted on the down tube. On newer ones, they are integrated with the brake levers on the handlebars.

Bottle cage mounts are also on the down tube. In addition to bottle cages, small air pumps may be fitted to these mounts as well.

Seat Tube
The seat tube contains the seatpost of the bike, which connects to the saddle. The saddle height is adjustable by changing how far the seatpost is inserted into the seat tube. On some bikes, this is achieved using a quick release lever. The seatpost must be inserted at least a certain length, which is marked with a minimum insertion mark.

The seat tube may also carry bottle cage mounts.

Chain Stays
The chain stays run parallel to the chain, connecting the bottom bracket shell to the rear dropouts. When the rear derailleur cable is routed partially along the down tube, it is also routed along the chain stay. The chain stays provide a mount for rear disc brakes.

Seat Stays
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. When the rear dérailleur cable is routed partially along the top tube, it is also routed along the seat stay. One combination is an aluminum/carbon fiber racing frame design, which uses carbon fiber for the seat stays and aluminum for all other tubes. This takes advantage of the better vibration absorption of carbon fiber compared to aluminum.

A single seat stay refers to seat stays that merge onto one section before joining the front triangle of the bicycle, thus meeting at a single point. A dual seat stay refers to seat stays that meet the front triangle of the bicycle at two separate points, usually side-by-side. The seat stays also provide a mounting point for the rear rim brakes.


Frame Geometry
The length of the tubes and the angles at which they are attached define a frame geometry. In comparing different frame geometries, designers often compare the seat tube angle, head tube angle, (virtual) top tube length, and seat tube length. To complete the specification of a bicycle for use, the rider usually needs to specify:

saddle height - the distance from the center of the bottom bracket to the point of reference on top of the saddle, which is 13cm from the rear of the saddle.
reach - the distance from the saddle to the handlebar.
drop - the vertical distance between the reference at the top of the saddle to the handlebar
setback - the horizontal distance between the saddle reference point and the center of the bottom bracket

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. In some instances, frame geometries can also contribute to high-speed wobble.

Frame Size
Frame size was traditionally measured from the center 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 racing 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. Touring frames tend to be longer, while racing frames are more compact.

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 only include shocks rising from the front wheel (in the fork). The development of sophisticated suspension systems in the 1990's quickly resulted in many modifications to the classic diamond frame.


Steel is stiff, strong, easy to work, and relatively inexpensive; however, it is more dense than many other structural materials.

A classic type of construction for both road bicycles and mountain bicycles uses standard cylindrical steel tubes that 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. Historically, the lower temperatures associated with brazing (silver brazing in particular) had less of a negative impact on the tubing strength than high temperature welding, allowing relatively light tubes to be used without loss of strength. Recent advances in metallurgy ("air hardening") have created tubing that is not adversely affected, or whose properties are even improved by high temperature welding temperatures, which has allowed both TIG & MIG welding to sideline lugged construction in all but a few high end bicycles. More expensive lugged frame bicycles have lugs that are filed by hand into fancy shapes - both for weight savings and as a sign of craftsmanship. Unlike MIG or TIG welded frames, a lugged frame can be more easily repaired in the field due to its simple construction. Also, since steel tubing can rust, the lugged frame allows a fast tube replacement with virtually no physical damage to the neighboring tubes.

A more economical method of bicycle frame construction uses cylindrical steel tubing connected by TIG 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. Fillet brazing is another method of joining frame tubes without lugs. It is more labor intensive and consequently is less likely to be used for production frames. As with TIG welding, frame tubes are precisely mitred and then a fillet of brass is melted onto the joint. Some custom frame builders and their customers prefer a fillet braze frame for aesthetic (smooth curved appearance) reasons.

Among steel frames, using butted tubing reduces weight and increases cost. Butting means that the wall thickness of the tubing changes from thick at the ends (for strength) to thinner in the middle (for lighter weight).

Cheaper steel bicycle frames are made of mild steel, such as what might be used to manufacture automobiles or other common items. However, higher-quality bicycle frames are made of high strength steel alloys (generally chromium-molybdenum, or "chromoly" steel alloys), which can be made into lightweight tubing with very thin wall gauges. One of the most successful older steels was Reynolds "531", a manganese-molybdenum alloy steel. Reynolds and Columbus are two of the most famous manufacturers of bicycle tubing. A few medium-quality bicycles used these steel alloys for only some of the frame tubes. An example was the Schwinn Le tour (at least certain models), which used chromoly steel for the top and bottom tubes, but then used lower-quality steel for the rest of the frame.

A high-quality steel frame is lighter than a regular steel frame. This lightness makes it easier to ride uphill and easier to accelerate on the flat. Also, many riders feel thin-walled lightweight steel frames add a "liveness" or "springiness" quality to their ride.

If the tubing label has been lost, a high-quality (chromoly or manganese) steel frame can be recognized by tapping it sharply with a flick of the fingernail. A high-quality frame will produce a bell-like ring where a regular-quality steel frame will produce a dull thunk. They can be also recognized by their weight (around 2.5 kg for frame and forks) and the type of lugs and dropouts used.

Aluminum alloys
Aluminum alloys have lower density and lower strength compared with steel alloys (both are reduced by approximately 2/3). Aluminum can, however, be used to build a frame that is lighter than steel. In contrast to some steel and titanium alloys, which have a fatigue endurance limit, aluminum has no such limit; but keep in mind that even the smallest stresses will eventually cause failure if repeated enough times. However, alloying, good mechanical design, and good construction practices help to extend the fatigue life of aluminum bicycle frames to acceptable lengths.

The most popular type of construction today uses aluminum alloy tubes that are connected together by Tungsten Inert Gas (TIG) welding. Welded aluminum bicycle frames started to appear in the marketplace only after this type of welding become economical in the 1970's. Comparing equal tube sizes, aluminum is less stiff than steel, but it is also lighter. In order to raise aluminum’s 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 the flex of a compliant steel frame feels more comfortable to many riders compared to an aluminum frame. On the other hand, stiffness improves acceleration and handling.

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 wall thickness of the middle sections is 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 oval or teardrop shapes, for optimizing stiffness and compliance in different directions as well as reducing wind resistance.

Titanium is perhaps the most expensive metal commonly used for bicycle frame tubes. It combines many desirable characteristics, including a high strength to weight ratio and excellent corrosion resistance. Reasonable stiffness (roughly half that of steel) allows for many titanium frames to be constructed with "standard" tube sizes compared to a traditional steel frame, although larger diameter tubing is becoming more common for more stiffness. As many titanium frames can be much more expensive than similar steel alloy frames, cost can put them out of reach for many cyclists. Many common titanium alloys and even specific tubes were originally developed for the aerospace industry.

Titanium frame tubes are almost always joined by Tungsten inert gas welding (TIG). 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 Fiber
Carbon fiber, a composite material, is the only non-metallic material commonly used for bicycle frame tubes. Although it's expensive, it is lightweight, corrosion resistant, has high strength ability, 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 fiber 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 fiber 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 fiber frames are manufactured in a single piece, called monocoque construction. While these composite materials provide light weight as well as higher 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, which consequently outweigh other considerations in such events.
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