Bicycle frame
A bicycle frame is the supporting structure of a bicycle . It carries the weight of the rider and passes it on to the wheels. Driving, braking and steering forces act on the frame, as well as the bumps due to unevenness in the path. All the components required for steering , drive and other functions of the bike are attached to it.
construction
A bicycle frame usually consists of the following components:
- the seat tube or saddle tube receives the seat post and carries the bicycle saddle ;
- the head tube takes the bicycle fork through the headset , which guides the front wheel (see also: steering head );
- Top tube and down tube connect the saddle and head tube; the top tube is lowered on ladies bikes or is often missing entirely;
- the rear end traditionally consists of chain stays , seat or saddle stays and dropouts and guides the rear wheel ;
- the bottom bracket shell takes the bottom bracket on, usually it is the lower end of the seat tube and combines this with traditional frame constructions with down tube and the lower struts of the rear triangle ( "chainstays");
- smaller fastening parts for luggage racks , chain guards , mudguards , dynamo , Bowden cables , drinking bottles, air pumps , etc.
Frame shapes and geometry
There is a wide variety of frame shapes. The frame determines the external appearance of the bike and is therefore also subject to fashion.
Traditional frame shapes
- The diamond frame offered for a long time the best compromise between stability and weight. It is the classic frame for men's bicycles and is stepped over with the leg when sitting on, or the leg is swung over the rear wheel and saddle to the opposite side.
- The gooseneck frame allows easy passage between the handlebars and saddle and is the classic frame for women's bikes. It has two curved and parallel down tubes, the top tube is missing.
Truss frame
In a truss frame, each frame field forms a triangle. Such frames are in principle more rigid than frames that contain squares and are less springy.
- Traditional frames always formed a square, as the top and bottom tubes were attached to the opposite ends of the head tube. This allowed the frame to give way a little if the front wheel was hit, but it was sensitive to collisions from the front. To increase stability, the top and bottom tubes of mountain bike frames were often brought closer together and ovalized.
- The modern Alex Moulton bike has a sturdy truss frame . It is designed as a unisex frame and can be climbed relatively easily between handlebars and saddle. Its rear end is flexible and flexible.
- The rigid framework of the historic Dursley-Pedersen bike is of similar complexity.
Modern "ladies frame"
Mixed forms between diamond and gooseneck frames can be viewed as modern women's frames. They are easier to step over than the diamond frame.
- With the trapezoidal frame , the top tube is lowered. This then leads straight or slightly curved to the center of the seat tube or a little lower. If no specially reinforced seat tube is used, it can kink backwards under heavy loads . Anglaise , Mixte and Berceau frames do not have this static problem. Thanks to the two additional struts, they are almost as stable against impacts as a diamond frame.
- The Anglaise frame is similar to the trapezoidal frame . It has two additional struts which, as an extension of the lowered top tube, lead to the dropouts on the rear wheel.
- With the Mixte frame , the two struts of the Anglaise frame continue to the front of the head tube and replace the top tube. This construction is against lateral distortion less rigid than ordinary frame with top tube, as two thin struts less torsionally rigid , are as a single top tube of larger diameter.
- The Berceau frame is similar to the Mixte frame. The double top tubes are not straight, but curved for easier entry.
Low access frame
- The wafer frame is now widely used. It only has a large diameter down tube, which is curved like a wave, and often has a small tube above the bottom bracket shell for reinforcement. The step-through height is very low.
- When deep via hole ( Easy-boarding frame or low step ) is decreased the trapdoor to about 15 to 20 cm above the roadway. The sole down tube with a large diameter is U-shaped in the lower part. The chainring of the pedal drive must keep its minimum distance from the road. So that it doesn't get in the way when climbing through, the wheelbase is increased (step through in front of the chainring).
Other frame shapes
- The sloping frame has a top tube that slopes down towards the saddle (generally similar to a trapezoidal frame). It was developed for MTBs and trekking bikes before it was used in road racing bikes and later in bikes for general use. Improved materials (titanium, aluminum alloys, CFRP , colloquially called carbon ) made it possible to maintain the rigidity of diamond frames. The advantage mentioned is a lower risk of injury when stopping and descending due to the lower top tube at the rear. The mass is not less compared to the diamond frame, since the savings due to shorter rear struts are canceled out by the increased rigidity of the seat post due to its greater free length.
- With the cross frame , the seat tube, which is only extended down to accommodate the bottom bracket, and a tube that goes from the head tube to the rear wheel and then splits to the rear fork cross each other. This design is one of the oldest constructions and is not very rigid.
Monocoque frame constructions made of composite materials (especially CFRP) are often constructed similarly. Their rigidity results from the fact that the intersection is designed to be large.
Modern frame shapes known as cross frames are often standard frames that have been extended to form framework frames. They usually contain an additional pipe that crosses with a differently routed previous one. - With the Y-frame , only a forked tube leads from the steering tube to the seat tube. These frames are offered either unsprung for youth bikes or with a sprung rear end for mountain bikes.
- Recumbents have a wide variety of special frame shapes
- Do-it-yourself frames : bicycle frames made for individual items and small series. Examples: tall bike , side by side .
Frame size and height
The frame size or frame height is usually the distance between the center of the bottom bracket and the (upper) end of the seat tube, in Italy and France it is more likely to be measured to the center of the saddle sleeve (for example with de Rosa ). In German, French and Italian manufacturers are centimeters usual. Internationally, a classification according to customs prevails. The gradation is usually 2 inches, which corresponds to around 5 cm.
With most modern mountain bikes and other bikes with sloping top tubes and large tube cross-sections, the measured frame height is often not a valid criterion for the actual size of the frame. Since the seat tube protrudes above the top tube and the now common seat posts with a large diameter can bridge a large distance between the seat and the seat tube, the actual height of the seat tube is arbitrary. Some manufacturers therefore specify the so-called classic frame height , measured from the center of the bottom bracket to the point of intersection with a virtual horizontal top tube. If this information is missing, the decisive factor is the distance between the seat tube and the head tube, which should be measured horizontally from the top of the head tube to the interface with the seat tube.
Since the available stems allow a variety of handlebar positions, especially with mountain bikes, in general, the exact adaptation of the frame to the body size is no longer as important as it used to be.
When choosing the right frame size , the step length (step height, leg length) is the decisive factor. It is measured on the inside of the leg from the sole of the foot to the perineum .
- Trekking, city, touring and racing bikes are generally offered with frame sizes between 47 and 68 cm. As a rule of thumb, the frame size should be 0.66 times the stride length.
- Cross bikes have frame sizes between 41 and 61 cm, the frame size should be 0.61 times the stride length.
- Mountain bikes have smaller frames, frame sizes from 35 to 58 cm are common, the frame size should be 0.57 times the stride length.
- In the case of children's bikes, it is not the frame size but the wheel size that is given in inches.
Bicycles with predominantly sporting use in hilly terrain are often ridden standing up for up to a third of the total travel time. Since most of the force is used when standing, the handle position that supports the development of force when the bike is standing should first be determined by selecting the stem and handlebar for this purpose. If the distance between the handlebars and the bottom bracket is too short, it impairs the control of the bike and makes it easier for the front wheel to climb on steep passages. If the distance is too great, too much of the body's weight rests on the arms. In addition to the fatigue of the arms, this also makes it more difficult to apply the measured force to the rear wheel. When the handlebar position has been determined, the saddle should be moved horizontally to adapt it to the length of the frame.
Adjustment of the saddle
Regardless of the frame height, the correct adjustment of the seat height is important for energy-saving driving without pain in the knees and buttocks. In general, the seat is adjusted so high that the extended leg just touches the pedal with the heel when the crank is in the lowest position. Shoes with flat soles should be worn and the hips should not tilt to the side.
In the case of bicycles that are not ridden standing too often, the horizontal position of the saddle is determined after the correct seat height has been set by first moving a crank to the horizontal position. If the ball of the foot is now placed on the pedal in the usual way, a plumb line leading down from the driver's knee should point to the axis of the pedals.
Requirements for bicycle frames
Rigidity and strength
With the assumption of a static basic loading a loaded through the weight of the rider bicycle frame at a standstill is to bend claimed structure with the support points rear wheel axle and the fork head.
With static force application, the down tube and chain stays are subjected to tension, the top tube and the rear stays are subjected to pressure. The inclination of the head tube increases the pulling force in the down tube, and there is also a bending moment in the top tube. The inclination of the seat tube creates a pressure component in the down tube that reduces the tensile force, and an additional bending moment in the top tube.
Additional stresses when driving:
- Torsional forces from the pedaling movement
- Bending moments due to braking
- Bending moments and torsional moments due to pedaling movements, inclined travel and steering forces
The weight of the rider is transmitted through the frame from the saddle to the ground via the two wheels (on the front wheel via an intermediate fork and steerer tube). When the road surface is uneven, impact forces add to the weight, which means that the frame is subjected to alternating bending loads.
When force is exerted on the pedals, reaction forces arise on the handlebars and, if the driver does not get out of the saddle, lateral forces on the saddle as well. The increased foot force on the pedal generates a tensile force applied by the hand on the other side of the handlebar. This asymmetry causes an alternating twisting ( torsion ) of the frame around the longitudinal axis of the bicycle.
A frame is the more rigid , the less it is elastically deformed under the forces of use . This is achieved both through an advantageous frame shape and through the use of material with a high modulus of elasticity and / or the choice of large pipe diameters (results in a large area moment of inertia ).
A frame must have a minimum strength so that it does not deform plastically or even break during use . Bicycle frames are at risk of fatigue failure due to alternating stress. Such fractures can occur suddenly, which means that they are not announced by plastic deformation. Countermeasures are the choice of materials with high fatigue strength and careful design and manufacture of the connection points between the pipes. The connection points are particularly at risk because on the one hand the stress is greatest there, and on the other hand they are produced by welding or soldering. The resulting heating can reduce the initial strength of the material.
Dimensions
A bicycle is all the more pleasant to use, the better the muscle energy used to ride it, the higher its efficiency . It should therefore have a low dead weight, because the driver not only has to move but also the bike. The frame, which should be light but stiff, accounts for a large part of the weight of the bike. These conditions are met in particular by a framework designed as a latticework. In addition, all moving and rotating masses (running wheels including tires, crankset, chain, sprocket) should be kept as low as possible. This considerably reduces the effort required to accelerate. That is why structurally efficient materials such as fiber materials are always used in racing.
Visual appearance
The bicycle has changed from being a mere commodity to a mass-produced item, which is why its visual appearance is increasingly subject to fashion. In addition to a beautiful shape, the frame is also required to be easy to care for, which makes higher demands on corrosion protection , for example . The surface treatment (usually painting ) required for steel traditionally used up to now is not omitted even with the currently favored aluminum.
Materials for bicycle frames
The classic material for bicycle frames is steel . However, its use is declining. The previously common connection of pipes by inserting and soldering in sockets is being replaced more and more by "butt" welding . A special feature are butted frame tubes, the ends of which are thickened towards the inside. The accumulation of material thus achieved at the connection points corresponds to the usually increased stress there, without the weight being unnecessarily increased by constantly thicker walls. Well-known manufacturers of such frame tubes are Mannesmann , Reynolds (England), Columbus (Italy) and Tange (Taiwan).
Today aluminum alloys have established themselves as the standard material. The low density and the insensitivity of the metal to rust perforation are mentioned as sales arguments. The lower density is offset by the lower modulus of elasticity , which is why the pipes must have a larger material cross-section (large diameter). Aluminum bicycle frames are therefore not in principle lighter than frames made of steel. The principle of "butted" tubes is also used here.
Carbon fiber reinforced plastic (carbon) is used more and more often in racing bikes and mountain bikes. The ratio of stiffness or stability to weight is no better with carbon frames than with steel or aluminum. The only advantage of carbon is that it can be designed more freely.
Titanium is not widely used as a frame material. Titanium frames weigh about as much as aluminum alloy frames. The main advantage is longevity, as titanium practically does not corrode. Due to the complex production and processing of titanium, frames made of this material are very expensive. Titanium frames are lighter than all other frames for bicycles with the same rigidity. However, titanium is also the most expensive material.
The joining of the frame tubes depends on the material. Steel is mainly soldered or welded (with or without sleeves). Aluminum and titanium are almost exclusively welded and rarely glued . Carbon frames are glued from fiber mats or consist of a continuous shell, which is then called a monocoque . Bamboo or plastic pipes are more rarely glued into sleeves made of aluminum or plastic. Composite structures, such as fiber-reinforced metal frames, are also rarely found.
steel
Of the many different steel alloys , chromium- molybdenum alloys are usually used in frame construction , such as 25CrMo4 (in the USA 4130) and very rarely 34CrMo4 (US 4135), which has a slightly higher strength. Stainless steels are also rarely found. CrMo steel is a tough quenched and tempered steel, i.e. a steel in which sudden breakage rarely occurs.
Steel is easy to work with. It can be soldered and welded without any problems, even gluing is possible. Thanks to many years of experience with the material, exotic tube shapes such as butted, spiral reinforced and others can be produced.
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aluminum
Aluminum alloys (as of 2013) are often used in frame construction. The alloys commonly used have the designations 6061-T6, an alloy primarily with magnesium and silicon, and 7005, an alloy primarily with zinc and magnesium. Aluminum has only about a third of the specific stiffness (modulus of elasticity) of steel, but only a third of its density. In order to obtain a profile made of aluminum with the same bending and torsional rigidity as that of steel, a larger geometrical moment of inertia is required, which causes the typically large tube diameter for aluminum frames. The same strength is achieved through an enlarged material cross-section.
Aluminum and aluminum alloys do not have a distinct limit for fatigue strength like steel. With these materials, fatigue fractures are comparatively difficult to confirm statistically (see also Wöhler test ).
Another disadvantage of aluminum is that the welding process is more complex and more prone to failure than steel, which already generates internal stresses. In the meantime, so-called welding stress relief annealing is used as a post-treatment.
In principle, aluminum frames are not lighter than those made of steel, even if this is often suggested in advertising. Although the density of aluminum is significantly lower than that of steel, the frames must be dimensioned larger in order to have approximately the same strength. Today's steel frames are often lighter than comparable aluminum frames, since the cheap area is almost exclusively covered by mass-produced aluminum and steel frames are almost only found in the higher-quality area.
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Scandium alloy aluminum
A scandium alloy is an aluminum alloy that contains a small amount of scandium in bicycle frames . This addition can increase the tensile strength of the material by up to 20% compared to a conventional aluminum alloy, but at the same time also makes it comparatively brittle (increased risk of fatigue fracture).
titanium
Some titanium alloys are high-strength, some are even just above the steels used in frame construction. Due to its almost half as high density as steel, titanium has a better strength-to-weight ratio, but steel is about three times as stiff as titanium. The butting of titanium tubes is possible, but time-consuming and not common for bicycles. The final reinforcement of pipes is very often omitted for reasons of cost. A major disadvantage is the poor weldability of titanium, as it forms an oxide layer similar to aluminum. This increases the hardness and brittleness in the weld seam, which brings with it the risk of fatigue fracture. Thus, the frame has to be welded in noble gas or vacuum cabins, which is extremely expensive.
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Carbon fiber reinforced plastics (CFRP)
Fiber- reinforced plastics have been used successfully for a long time, for example in the manufacture of sports equipment. The theoretical stiffness and strength values are very high, but only apply in one direction, namely the fiber direction, while the strength and stiffness values for metals are the same in all directions ( isotropy ). In addition, the fibers must be supported among each other. This is done using plastics, the so-called matrix. In the case of bicycle frames, this usually consists of epoxy resin . If a force does not act in the direction of the fiber, only the matrix holds against it, and epoxy resin, like other plastics, is not very strong. The fibers must therefore be laid in several directions, which reduces the weight advantage. In addition, the force distribution in a frame must be known so that reinforcing fibers can be laid in the correct amount and direction in all directions of force. This process is associated with a lot of work. Overall, the frame is lighter, but also significantly more expensive, and hardly or not stiffer or almost as heavy with the same stiffness as steel or aluminum frames.
The various reinforcing fibers are used in three basic forms:
- as a strand or bundle of parallel fibers (rovings),
- as fabrics and braids in their most varied forms and
- as non-directional mats or random fibers.
In addition to parallel fibers, there are also yarns and twines that are created by twisting single or multiple spun threads. The twist creates a tough, tightly bound thread that can be easily processed into a textile (for example, woven).
In addition to carbon fibers (approx. 7-10 µm in diameter), glass fibers (approx. 5-15 µm in diameter) and aramid fibers (trade name, for example, Kevlar , approx. 12 µm in diameter) are used. Carbon fibers are very brittle, which is why aramid fibers are added, which have a greater elongation at break. Fiber-reinforced plastics that only contain aramid fibers are not used in frame construction because of their high price / strength ratio.
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Metal Matrix Composite (MMC)
This material actually belongs to aluminum, but also to the fiber-reinforced materials, because fibers or particles are inserted into an aluminum matrix for reinforcement. In 1998 two companies presented products: Specialized with a particle- reinforced frame (aluminum oxide particles), Univega with a boron fiber-reinforced frame. The strength of the aluminum is hardly improved, but according to the manufacturers the stiffness by up to 30%. These frames have now disappeared from the market.
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bamboo
Bikes with frame tubes made of bamboo (mostly Bambuseae ) came into fashion in the fin de siècle . The production of a durable connection between the bamboo tubes is relatively complex. While the manufacturing processes for steel tubes and frames became more efficient, the trend ebbed a few years after the turn of the century and large manufacturers such as Grundner & Lemisch also gave up production. today bicycle frames are again being made from bamboo in small numbers. Bamboo has a specific stiffness (modulus of elasticity) that corresponds to about one tenth of steel , but also only about one tenth of its density (depending on the grass used). In terms of static material strength, bamboo is somewhat behind aluminum and steel. In order to obtain a frame made of bamboo with the same flexural and torsional rigidity as that of steel, a tube diameter is necessary that is comparable to an aluminum frame. The material cross-section varies due to the different thicknesses of the bamboo tubes.
So far, no fatigue fractures are known in bamboo, but it is an organic material that can go moldy. Bamboo does not break, but splinters if the load is too high. Bamboo is flammable.
The connections of bamboo frames are usually made with several layers of fiber and epoxy resin .
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Industrial manufacturing in Asia
European and North American bicycle manufacturers have had most of their bicycle frames manufactured in the People's Republic of China and Taiwan since around the year 2000 in order to keep production costs down. Only a few manufacturers still manufacture the frames in their own factories in Europe or North America. These are then correspondingly expensive, such as. B. De Rosa wheels , but not necessarily higher quality, because the manufacturing quality hardly differs worldwide today. For their part, the Taiwanese manufacturers are already largely outsourcing production to low-wage countries such as Laos or Vietnam, since production in Taiwan for cheap mass-produced goods has already become too expensive.
Labelling
Bicycle frames are usually provided by the manufacturer with a unique frame number that identifies the bicycle, e.g. B. after a theft.
literature
- Fritz Winkler, Siegfried Rauch: Bicycle technology repair, construction, production. 10th edition. BVA Bielefelder Verlagsanstalt, Bielefeld 1999, ISBN 3-87073-131-1 .
- Michael Gressmann, Franz Beck, Rüdiger Bellersheim: specialist knowledge of bicycle technology. 1st edition. Verlag Europa Lehrmittel, Haan-Gruiten 2006, ISBN 3-8085-2291-7 .
Web links
- Adjust bike. December 14, 2013, accessed February 14, 2014 .
- Frame maker "Le Canard" - bicycle frames from Bochum at Balance. bochumschau.de, accessed on February 14, 2014 .
- Bikefitting - calculate frame geometry
- Ebikeers portal - determining the correct frame size , accessed on January 18, 2020
swell
- ↑ Smolik Velotech: head tube
- ↑ Smolik Velotech: tax rate
- ↑ Production in Saarbrücken: cross frame. utopia-velo.de, accessed on February 14, 2014 .
- ↑ Christian Smolik: Smolik Velotech: frame height , 2002, accessed October 26, 2015
- ↑ Find the right frame size . RoadBIKE magazine February 2017, p. 49
- ^ Gunda Bischoff: Measuring the stride length. March 1, 2008, accessed February 14, 2014 .
- ↑ Tomek: Adjust the cockpit and saddle correctly. February 29, 2008, accessed February 14, 2014 .
- ↑ Tobias Meyer: Frequently asked questions about bamboo bicycles. faserwerk.net, 2013, accessed February 14, 2014 .
- ↑ Beautifully formed by our carbon craftsmen, our bamboo frames perform at the highest level of the sport. calfeedesign.com, accessed February 14, 2014 .