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Bicycle Frame Geometry

A 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 consists of two triangles, a main triangle and a paired rear triangle. This is known as the diamond frame, and this article is about them.

In 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 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 (often at or near the same point as the top tube) to the rear dropouts.           

Steel frame and carbon fiber fork of 2000 LeMond Zurich racing bicycle mounted in a work stand.
 

 

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 via its steerer tube. In an integrated thread less headset, the bearings interface directly with the metal surface on the inside of the head tube

 

 Top tube

The top tube (popularly referred to as the crossbar) connects the top of the head tube to the top of the seat tube. In a traditional-geometry racing bicycle frame, the top tube is horizontal. In a compact-geometry frame, the top tube is sloped downward toward the seat tube. In a mountain bike frame, the top tube is almost always sloped downward toward the seat tube.

Control cables are routed along mounts on the top tube, or sometimes inside the top tube. Most commonly, this includes the cable for the rear brake, but some mountain bikes and hybrid bicycles 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 height lever.

 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, or inside the down tube. On older racing bicycles, the shift 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, usually on the top side, sometimes also on the bottom side. In addition to bottle cages, small air pumps may be fitted to these mounts as well.

Seat tube

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

The seat tube also may have braze-ons for mounting a bottle cage or front derailleur.

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. Occasionally (principally on frames made in the late 1990s) mountings for disc brakes will be attached to the chain stays. There may be a small brace that connect the chain stays in front of the rear wheel and behind the bottom bracket shell.

Chain stays can be straight or tapered tubes. Sometimes, on higher-end bikes, they are sculpted to allow clearance for the rear wheel and crank arms.

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. A style of seat stay that extends forward of the seat tube, below the rear end of the top tube and connects to the top tube in front of the seat tube, creating a small triangle, is called Hellenic after the British frame builder Fred Hellens who introduced them in 1923.

The expressions single seat stay, mono stay, or wishbone all refer to seat stays which 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 which meet the front triangle of the bicycle at two separate points, usually side-by-side. A single stay can provide stiffer mounting points for cantilever brakes.

There may be a bridge or brace that connects the stays above the rear wheel and below the connection with the seat tube. Besides additional bracing, this provides a mounting point for rear brakes, fenders, and racks. The seat stays themselves may also provide a mounting point for rear rim or disc brakes. Usually, no rear mount is provided on a fixed gear or track frame.

When the rear derailleur cable is routed partially along the top tube, it is also usually routed along the seat stay. One combination aluminum/carbon fiber racing frame design 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.

Bottom bracket shell

The bottom bracket shell is a short and wide tube, relative to the other tubes in the frame, that runs side to side and holds the bottom bracket. It is usually threaded, often left-hand threaded on the right (drive) side of the bike to prevent loosening by fretting induced precession, and right-hand threaded on the left (non-drive) side. It will be over-sized, unthreaded, and possibly split in the case of an eccentric bottom bracket. The chain stays, seat tube, and down tube all connect to the bottom bracket shell.

There are a few standard shell widths (68, 70 or 73 mm). Road bikes usually use 68 mm; Italian road bikes use 70 mm; Early model mountain bikes use 73 mm later models (1995 and newer) use 68 mm more commonly. The shell width influences the Q factor or tread of the bike. There are a few standard shell diameters (34.798 - 36 mm) with associated thread pitches (24 - 28 tpi).

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 adjusts the relative positions of the saddle, pedals and handlebars:

  • saddle height, the distance from the center of the bottom bracket to the point of reference on top of the middle 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 front of the saddle 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 handlebars in a lower and further position relative to the saddle giving a more crouched riding position; whereas a utility bicycle emphasizes comfort and has higher handlebars resulting in an upright riding position.

Frame geometry also affects handling characteristics. For more information, see the Bicycle and motorcycle geometry and the Bicycle and motorcycle dynamics articles.

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; see the discussion by Sheldon Brown. Touring frames tend to be longer, while racing frames are more compact.

 

Road and triathlon bicycles

A road racing bicycle is designed for efficient power transfer at minimum weight and drag. Broadly speaking, the road bicycle geometry is categorized as either a traditional geometry with a horizontal top tube, or a compact geometry with a sloping top tube.

Traditional geometry road frames are often associated with more comfort and greater stability, and tend to have a longer wheelbase which contribute to these two aspects. Compact geometry road frames have a lower center of gravity and tend to have a shorter wheelbase and smaller rear triangle, which give the bike quicker handling. Compact geometry also allows the top of the head tube to be above the top of the seat tube, increasing stand over clearance, and lowering the center of gravity. Opinion is divided on the riding merits of the compact frame, but several manufacturers claim that a reduced range of sizes can fit most riders, and that it is easier to build a frame without a perfectly level top tube.

Road bicycles for racing tend to have a steeper seat tube angle, measured from the horizontal plane. This positions the rider aerodynamically and arguably in a stronger stroking position. The trade-off is comfort. Touring and comfort bicycles tend to have more slack seat tube angle traditionally. This positions the rider more on his sit bones and takes weight off of the wrists, arms, neck and, for men, improves circulation to the urinary and reproductive areas. With slacker angle, designers lengthen the chain stay so that the center of gravity (that would otherwise be farther to the back over the wheel) is more ideally repositioned over the middle of the bike frame. The longer wheelbase contributes to effective shock absorption. In modern mass manufactured touring and comfort bikes, the seat tube angle is negligibly slacker, perhaps because of the need to otherwise reset welding jigs in automated processes and thus increase manufacturing costs, and thus do not provide the comfort of traditionally made or custom made frames which do have noticeably slacker seat tube angles.

Road racing bicycles are governed by UCI regulations, which state among other things that the frame must consist of two triangles. Hence the designs that lack a seat tube or top tube are not allowed in UCI-sanctioned road races.

Triathlon or time trial specific frames rotate the rider forward around the axis of the bottom bracket of the bicycle as compared to the standard road bicycle frame. The reason for this is to put the rider in an even lower, more aerodynamic position. While handling and stability is reduced, these bicycles are designed to be ridden in environments with less group riding aspects. These frames tend to have steep seat tube angles and low head tubes, and shorter wheelbase for the correct reach from the saddle to the handlebar.

Track frames have much in common with road and time trial frames, but come with rear facing fork ends that allow one to adjust the position of the rear wheel horizontally to set the proper chain tension. Also the seat tube angle is steeper than road racing bikes, making a track frame a more nervous bike to ride.

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.

Variations

There are other 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. This is also known as a step-through frame. Another style that accomplishes similar results is the mixter.

It is also possible to add couplers either during manufacturing or as a retrofit so that the frame can be disassembled into smaller pieces to facilitate packing and travel.

 Frame materials

Historically, the tubes of the frame have been made of steel. While steel is still used, newer frames can also be made from aluminum alloys, titanium, carbon fiber, and even bamboo. Occasionally, diamond frames have been formed from sections other than tubes. These include I-beams and monocoque. Materials that have been used in these frames include wood (solid or laminate), magnesium (cast I-beams), and thermoplastic. Several properties of a material help decide whether it is an appropriate in the construction of bicycle frame:

  • Density We'll start with an easy one. Density describes the weight of a given volume of a material. For example, 6061 aluminum weighs 0.098 pounds per cubic inch. 4130 steel weighs 0.283 lb/in3, and 3/2.5 titanium is 0.160 lb/in3. This is an important and easy relationship to remember: titanium is about half the density of steel, aluminum is about one-third the density of steel. Use those figures as a guideline, then start to look at other properties, like strength and stiffness. For instance, you might ask, 'Why doesn't my aluminum frame weigh one third as much as my friend's steel frame?' Read on...
  • Stiffness (or elastic modulus) can in theory affect the ride comfort and power transmission efficiency. 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. Lateral stiffness is far more difficult to achieve because of the narrow profile of a frame, and too much flexibility can affect power transmission, primarily through tire scrub on the road due to rear triangle distortion, brakes rubbing on the rims and the chain rubbing on gear mechanisms. In extreme cases gears can change themselves when the rider applies high torque out of the saddle.
  • Yield strength determines how much force is needed to permanently deform the material (for crash-worthiness).
  • Elongation determines how much deformity the material allows before cracking (for crash-worthiness).
  • Fatigue limit and Endurance limit determines the durability of the frame when subjected to cyclical stress from pedaling or ride bumps.

Tube engineering and frame geometry can overcome much of the perceived shortcomings of these particular materials.

Steel

Steel frames are often built using various types of steel alloys including  chromium-molybdenum steel. They are strong, easy to work, and relatively inexpensive, but more dense (heavier) than many other structural materials. Steel tubing in traditional standard diameters is often less rigid than oversized tubing in other materials; this flex allows for some shock absorption giving the rider a slightly less jarring ride compared to other more rigid tubing's such as oversized aluminum.

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. 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 tube 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 which 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 notched or 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 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 "chrome moly" 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 chrome moly steel for the top and bottom tubes but 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 to accelerate on the flat. Also many riders feel thin-walled lightweight steel frames have a "liveliness" or "springiness" quality to their ride.

If the tubing label has been lost, a high-quality (chrome moly 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 a lower density and lower strength compared with steel alloys, but what interests us here, is the better strength-to-weight ratio of aluminum giving it significant weight savings over steel. Early aluminum structures have shown to be more vulnerable to fatigue, such as due to vibrations, either due to ineffective alloys, or perfectible welding technique being used. This contrasts to some steel and titanium alloys, which have clear fatigue limits and are easier to weld or braze together. However, this has changed, with more skilled labor capable of producing better quality welds, automation (especially in Taiwan where they have developed an expertise for this), and the greater accessibility of the same modern aluminum alloys as used in commercial airliners' structures, assuring strength and reliability comparable to any steel frame. Aluminum's attractive strength to weight ratio as compared to steel, and certain mechanical properties, assure it a place among the favored frame-building materials (for example, a very strong rider, who does lots of hill-climbing, may prefer the stiffness of aluminum). It's disadvantages are that an aluminum frame doesn't have the same "feel" to an experienced cyclist as a steel frame, and there is a new trend, among bicycling enthusiasts and advanced riders, to going back to steel, trading the dead feel of aluminum for the live, springy responsiveness of steel (or that of titanium, for those who can afford it).

 

Shaped aluminum downtube with keyhole cross-section. It is connected to a dual chain stay made from carbon fiber. The aluminum parts were TIG-welded, and the carbon fiber parts are glued onto the aluminum sections.Shaped aluminum down tube with keyhole cross-section. It is connected to a dual chain stay made from carbon fiber. The aluminum parts were TIG-welded, and the carbon fiber parts are glued onto the aluminum sections.

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 became economical in the 1970s.

Aluminum has a different optimal wall thickness to tubing diameter than steel. It is at it's strongest at around 200:1 (diameter: wall thickness), whereas steel is a small fraction of that. However, at this ratio, the wall thickness would be comparable to that of a beverage can, far too fragile against impacts. Thus, aluminum bicycle tubing is a compromise, offering a wall thickness to diameter ratio that it not of utmost efficiency, but gives us oversized tubing of more reasonable aerodynamically acceptable proportions and good resistance to impact. This results in a frame that is significantly stiffer than steel. While many riders claim that steel frames give a smoother ride than aluminum because aluminum frames are designed to be stiffer, that claim is of questionable validity: the bicycle frame itself is extremely stiff vertically because it is made of triangles, the sides of which do not change in length under stress.On the other hand, lateral and twisting          ( torsion ) stiffness improves acceleration and handling in some circumstances.

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 are made to be thinner than the end sections—are used by some manufacturers for weight savings. Beware of marketing-motivated "innovations" which include the shaping of the cross-section of the tubes, especially that of the (large diagonal main) down tube, such as oval or teardrop shapes, to reduce wind resistance. The down tube is considered the "backbone" of the bicycle, and resists strong torsion forces from the rider while keeping everything together. Turbulence from the front wheel and head tube, plus the fact that a tube at an angle already looks like an oval into the direction of the wind, shed serious doubt on any important benefits of using such mechanically unsound shapes to cheat the wind. Indeed, an ovalized, teardrop, or triangular shaped tube is much easier to buckle or collapse under extreme torsion stress testing and will fail far before a perfectly round one shows any signs of flinching. The (almost vertical) seat tube is also often ovalized to cheat the wind. This again, is marketing motivated, as the rider pays as some of the pedaling energy is lost as it is diverted into flexing this tube from side to side. The seat tube is already in the wind-shadow of the front wheel and down tube, thus, aerodynamic benefits are very limited. Some very expensive bicycles, costing in the thousands of dollars, can be found on the market, where marketing instead of sound engineering principles lead the way. Buyer beware.

 Titanium

Titanium is perhaps the most exotic and 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) allow for many titanium frames to be constructed with "standard" tube sizes comparable 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), although vacuum brazing has been used on early frames. 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 an increasingly popular non-metallic material commonly used for bicycle frames.Although expensive, it is light-weight, corrosion-resistant and strong, and can be formed into 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.

Some carbon fiber frames use cylindrical tubes that are joined with adhesives and lugs, in a method somewhat analogous to a lugged steel frame. Another type of carbon fiber frames are manufactured in a single piece, called monocoque construction. While these composite materials provide light weight as well as high 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 with use over a long period of time.

Many 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, or would be excessively heavy in other materials. While this type of frame may in fact be heavier than others, its aerodynamic efficiency may help the cyclist to attain a higher speed and consequently outweigh other considerations in such events.

Other materials besides carbon fiber, such as metallic boron, can be added to the matrix to enhance stiffness further.

 Thermoplastic

An Itera plastic bicycle from the early 1980s.
An Itera plastic bicycle from the early 1980s.

Thermoplastics, according to a study from 2001 done by the Advanced Technology Project (ATP), is a new material that is still within testing.[1] It was originally developed by "Ford Motor Company Scientific Research Laboratory" and "General Electric Research and Development" within a joint venture. The ATP pioneered the use of cyclic thermoplastics in automotive components. Such parts are used in Ford's Aston Martin model automobile. Intellectual proprietary rights were sold to Cyclics Corporation which is using the process to produce such items as recyclable bicycle frames.

One implementation of thermoplastic bicycle frames are essentially carbon fiber frames with the fibers embedded in a thermoplastic material rather than the more common epoxy materials. GT Bicycles was one of the first major manufacturers to produce a thermoplastic frame with their STS System frames in the mid 1990s. The carbon fibers were loosely woven into a tube along with fibers of thermoplastic. This tube was placed into a mould with a bladder inside which was then inflated to force the carbon and plastic tube against the inside of the mould. The mould was then heated to melt the thermoplastic. Once the thermoplastic cooled it was removed from the mould in its final form.

 Magnesium

A handful of bicycle frames are made from magnesium which has around 64% the density of aluminum. In the 1980s, an engineer, Frank Kirk, devised a novel form of frame that was die cast in one piece and composed of I-shaped I beams rather than tubes. A company, Kirk Precision Ltd, was established in Britain to manufacture both road bike and mountain bike frames with this technology. However, despite some early commercial success, there were problems with reliability and manufacture stopped in 1992. The small number of modern magnesium frames in production are constructed conventionally using tubes.

Reportedly, a major problem with these frames is corrosion caused by the chemical reactivity of magnesium. Unless care is taken during assembly of the bicycle, there is likely to be galvanic corrosion at points where steel or aluminum components attach to the frame.

 Bamboo

Several bicycle frames have been made of bamboo tubes connected with steel or carbon fiber lugs. Aesthetic appeal has often been as much of a motivator as ride characteristics.

 Wood

Several bicycle frames have been made of wood, either solid or laminate. Although one survived 265 grueling kilometers of the Paris-Roubaix race, aesthetic appeal has often been as much of a motivator as ride characteristics. Wood is used to fashion bicycles in East Africa.

 Combinations

A recent innovation is the construction of frames out of tubes of different materials. This is intended to provide the desired stiffness, compliance, or damping in different areas better than can be accomplished with a single material. The combined materials are usually carbon fiber and a metal, either steel, aluminum, or titanium. One implementation of this approach includes a metal down tube and chain stays with carbon top tube, seat tube, and seat stays. Another is a metal main triangle and chain stays with just carbon seat stays.

 Braze-on

A variety of small features, bottle cage mounting holes, shifter bosses, cable stops, pump pegs, etc., are described as braze-on because they were originally, and sometimes still are, brazed on.

 Suspension

Many bicycles, especially mountain bikes, have suspension built into the frame.