4-bike frame joint carbon

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36 REINFORCEDplastics July/August 2003 C arbon fibre composites are nearly an ideal material for racing bike frames. They have exceptionally high strength to weight and stiffness to weight ratios. They also have better vibratory damping characteristics than metals, contributing to a smoother feel when riding. Carbon fibre composites are nearly an ideal material for racing bike frames. Traditionally, bicycle frames are con- structed of metal tubes joined at their ends by welding, or are brazed or sol- dered onto metal lugs, forming the frame. Composite materials have a lower density, higher specific strength and stiffness, and better damping qual- ities than traditional metals, and there- by provide an increase in frame strength and stiffness with a reduction in weight, as compared to earlier metal- lic frames. Design requirements There are many design requirements for road racing bike frames, but the fore- most are light weight and high lateral stiffness. Light weight is essential to minimize energy consumption up hills or during accelerations, and provide a more responsive feel to the rider’s movements. The stiffness characteristics of the frame are important because they contribute to how the bicycle rides. Resistance to lateral deflections and frame twisting under pedalling loads minimizes energy loss that would go into flexing the frame rather than into forward propulsion. Lateral stiffness also provides a stable feel, for example when descending or cornering, and it provides confidence in the frame’s response to rider actions. Lateral twisting of the frame around the steering column can cause self rein- forcing vibrations at high speeds, i.e. wobbling of the frame and steering col- umn. In its worst case this can cause a loss of control of the bicycle. In motor- cycles this vibratory mode is called a ‘tank slapping vibration’, referring to the handlebars oscillating back and forth so much they hit the gas tank. The ability of the frame to damp road vibrations and provide vertical compli- ance to absorb shock are also important. 0034-3617/03 ©2003 Elsevier Ltd. All rights reserved. Bike frame races carbon consumer goods forward Carbon fibre composite bicycle frames have developed into a high volume consumer application over the past decade, and are now dominate on high-end racing bikes. Ron Nelson of ClosedMold Composites, primary inventor of the Trek OCLV bicycle frame, explains how the integrated development of a manufacturing process and frame design lead to a successful commercial product line. Lance Armstrong has ridden Trek’s composite road bike to victory in four Tour de France events.

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Page 1: 4-Bike Frame Joint Carbon

36 REINFORCEDplastics Ju ly /August 2003

Carbon fibre composites are nearlyan ideal material for racing bikeframes. They have exceptionally

high strength to weight and stiffness toweight ratios. They also have bettervibratory damping characteristics thanmetals, contributing to a smoother feelwhen riding.

Carbon fibre compositesare nearly an ideal material for racing bikeframes.

Traditionally, bicycle frames are con-structed of metal tubes joined at theirends by welding, or are brazed or sol-dered onto metal lugs, forming theframe. Composite materials have alower density, higher specific strengthand stiffness, and better damping qual-ities than traditional metals, and there-by provide an increase in framestrength and stiffness with a reductionin weight, as compared to earlier metal-lic frames.

Design requirementsThere are many design requirements forroad racing bike frames, but the fore-most are light weight and high lateral stiffness. Light weight is essential tominimize energy consumption up hillsor during accelerations, and provide a

more responsive feel to the rider’smovements. The stiffness characteristicsof the frame are important because theycontribute to how the bicycle rides.Resistance to lateral deflections andframe twisting under pedalling loadsminimizes energy loss that would gointo flexing the frame rather than intoforward propulsion. Lateral stiffnessalso provides a stable feel, for examplewhen descending or cornering, and itprovides confidence in the frame’sresponse to rider actions.

Lateral twisting of the frame aroundthe steering column can cause self rein-forcing vibrations at high speeds, i.e.wobbling of the frame and steering col-umn. In its worst case this can cause aloss of control of the bicycle. In motor-cycles this vibratory mode is called a‘tank slapping vibration’, referring to thehandlebars oscillating back and forth somuch they hit the gas tank.

The ability of the frame to damp roadvibrations and provide vertical compli-ance to absorb shock are also important.

0034-3617/03 ©2003 Elsevier Ltd. All rights reserved.

Bike frame races carbonconsumer goods forwardCarbon fibre composite bicycle frames have developed into a high volume consumerapplication over the past decade, and are now dominate on high-end racing bikes.Ron Nelson of ClosedMold Composites, primary inventor of the Trek OCLV bicycleframe, explains how the integrated development of a manufacturing process andframe design lead to a successful commercial product line.

Lance Armstrong has ridden Trek’s composite road bike to victory in four Tour de France events.

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This contributes to a smoother and lessfatiguing ride. Carbon fibre is noted forits ability to damp road vibrations rela-tive to metal frames.

Strength requirementsFrame strength characteristics aresomething which the rider hopefullynever has to experience since the frameshould never break.Reliably producinghigh static andfatigue strengths isessential to mini-mize in service fail-ures which affectprofitability, productimage, and can pro-duce legal liabilities.

A bicycle frame experiences severaltypes of loads in its lifetime. The eventwhich produces the highest loads in abike frame occurs when the bike runsinto a fixed object and the kinetic ener-gy is transferred into the front area ofthe frame. This load case typicallyoccurs only once, if at all, during theframe lifetime.

Geometry and interface requirementsEven though it is much more, it has beensaid that the frame is just something tohang all the equipment on. The frameprovides the key interface for all theother componentry which comprise thebicycle. The geometry of tube centerlinesaffect rider position and handling dra-matically and must be chosen with care.Equipment which must be interfacedincludes the wheels, front fork, steerer

tube and bearing assembly, seat and seatpost, seat clamp, handlebars, derailleurs,brakes, cable routing features, pedals,cranks, bottom bracket, and water bottlemounts. There are clearances and opera-tional dimensional constraints for all theequipment.

The frame material selection affectsthe design of the frame considerably.Lower density materials such as compos-ites typically utilize larger tube diametersto increase structural efficiency. In fact,aluminium frames, which are generallylighter than steel, would not be anylighter if they used the same tube dia-meters as steel and the same frame stiff-ness was desired. The stiffness to weightratio of aluminium is actually lower than

steel. However, its much lower densityallows the larger more structurally effi-cient tube diameters to be used. Densermaterials such as steel are limited tosmaller diameters because the tube wallthicknesses become too thin at largerdiameters, and bifurcation buckling ofthe tube walls occurs.

Inefficiencies of metal lugsFrequently, carbon fibre tubes used inbike frames are adhesively joined tometal lugs. The disadvantage of usingmetallic lugs is their weight relative tocomposites. The weight of the metalliclugs significantly exceeds the weight ofthe composite frame tubes, therebygreatly limiting potential weight reduc-tions. Another problem with usingmetal lugs at the joints is that thedesigners must use smaller than optimal

lugs to reduce the lug weight, sincemetal is denser. The smaller diameterlugs use smaller diameter tubes toreduce the lug weight. The carbon fibretube diameter is therefore much smallerand less structurally efficient. The metallugs cannot really exploit the benefits ofthe lower density carbon fibre whichrequires larger tube diameters to be real-ized. The metal lugs would also nothave the superior damping qualities ofthe carbon composite. The materialdensity characteristics of metallic lugshave also prevented the development ofstructurally efficient large gussetedaerodynamic shapes for the lugs onaccount of the weight increase inherentwith such shapes. Smoothly gussetingtransitions between the main tube

members reduces stress concentra-tions, allows thinner walls, and is

more structurally efficient. The entireframe needs to be constructed of carbonfibre composite to really obtain the ben-efits of the composite material.

Previous carbon framesSeveral all-composite carbon fibreframes have been produced, with thefirst more than 40 years ago. The designand manufacturing were not both fullyoptimized in these previous attemptsand that is why they were not large com-mercial successes.

The first all-composite bike frame wasthe Spacelander invented by BenjaminBowden in 1960, which consisted of afuturistic monoque fibreglass framedbicycle. Another notable example is theKestrel frame invented by Brent Trimbleand produced by Cycle Composites Inc

Bike frame races carbon consumer goods forward

Then and now: the Bowden Spacelander(above), the first composite bicycle frame (picture: Menotomy Vintage Bicycles Inc athttp://oldroads.com), and the Trek 5900 (below).

Trek road bike frame and section.

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from 1987. The Kestrel was moulded inone piece in a single-step cure. Carbonframes inventor Craig Calfee made anall-composite frame which was marketedunder the LeMond brand name in 1991.The Calfee frames used an elegantly sim-ple design roughly analogous to steelframe construction. Small carbon lugsare cured directly onto closely miteredcarbon tubes. Small flat gussets are leftbetween most of the tubes.

One-piece framesIn a bicycle frame, stress loads are the greatest at joints, and therefore jointconstruction is a strong influence onframe design and construction. To avoidinherent problems of material discontinu-ity at frame joints, numerous designershave attempted to reduce or eliminate thenumber of joints in a frame.

The manufacture of high quality, reli-able one-piece, jointless frames hasproven difficult and expensive. Onelarge impediment involves the difficultyof reliably producing uniform high com-paction pressures in the composite lami-nate during cure, due in part to the failure to develop reliable internal pres-sure bladders to operate satisfactorilythroughout the frame.

Relatively inelastic bladder materialssuch as polyamide were used. Thesebladders could not stretch and conformto the interior surfaces of the frame.They were not shaped to mimic the inte-

rior surface of the frame either. The blad-der would frequently bridge over somedetail areas reducing compaction pres-sures dramatically. Sometimes foamingepoxy resin was used in these detail areasin an attempt to provide some com-paction pressure. However, this materialis basically parasitic and tends to deadenor reduce the liveliness of the frame.Lower than optimum compaction pres-sures in actual practice reduce materialstrength. This results in lower structuralperformance and an outer surface finishwhich requires a large amount of manuallabour to repair.

In essence, the complexity of manu-facturing one-piece carbon frames pro-duces poor laminate quality.

Trek’s OCLVOne example of a successful commercialcarbon fibre frame is Trek Bicycle Co’sOptimum Compaction Low Void (OCLV)frame. Built in Waterloo, Wisconsin, theframe was developed with Trek in theearly 1990s by a team at Salt Lake City-based Radius Engineering led by RonNelson, then president and co-founder ofthe company.

A testament to the bike’s superiority,and the reliability of the manufacturing

process, is that cyclist Lance Armstrongrode stock OCLV road frames in his fourTour de France wins from 1999-2002.

The original OCLV road framesweighed 1.1 kg (2.44 lb), the lightest pro-duction road bike frames in the world.These frames were used in the 1999 Tourde France. The original process used 150 g/m2 fibre areal weight carbon/epoxy prepreg. Since then 120 g/m2 and110 g/m2 material has been used toreduce the weight further.

Trek had previously manufactured a frame similar to Brent Trimble’sKestrel model, called the Trek 5000. Theframe consisted of two pieces (front tri-angle with rear stays as separate unit)bonded together, but it was only soldfor about one year before being takenoff the market.

Radius approached Trek after devel-oping an internal pressure bladder man-ufacturing technique for forming com-plex geometric shapes with highmoulding pressures using conformablebladders in an out-of-autoclave process.The company had previously producedtooling and manufacturing equipmentfor Cycle Composites Inc for the produc-tion of the Kestrel bike’s frames andforks, and believed its process could be

Bike frame races carbon consumer goods forward

Lug lay-up process.

Carbon composite head lug.

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used for the high volume production of anew, affordable all-carbon compositeframe.

The key to success was optimizing thecombination of manufacturing processand frame design. Previous all-carbonframes weren’t designed optimally, anddidn’t have a manufacturing processcapable of reliably producing high per-formance frames.

Evolution of design Prior to introducing the OCLV productline, Trek made aluminum frames whichuse cast aluminium lugs bonded todrawn aluminium tubing. It also made acarbon tubed frame using aluminiumlugs. This experience with bondingtubes and lugs together to form framesplayed a role in design of the first OCLVframes.

There were several key factors effect-ing development of the manufacturingprocess and frame design. An importantpart of any product and manufacturingdesign effort is to avoid infringing onexisting patents. Another requirement inthis case was for the process to be uniqueso it could be patented to protect theproduct.

Making the frame in smaller pieces allows amore specialized, morereliable manufacturingprocess to be used foreach component.

One key feature of the design revolvedaround the question of how much of theframe to manufacture in any one step.This ranged from moulding the entireframe in one piece to the other extremewhere all the pieces would be mouldedseparately, and then sub-sequently bond-ed together into a frame. In the end itwas decided to build the frame with a‘maximum componentization’ designconcept, and this had the advantage of

using Trek’s existing frame bonding pro-cedures.

Making the frame in smaller piecesallows a much more specialized, higherperformance, and more reliable manu-facturing process to be used for eachcomponent. By making the smallestcomponents possible, the process can beoptimized better, and much better struc-tural performance is obtained more reli-ably in smaller parts than with largerparts.

This is contrary to normal compositesmanufacture, where maximum partsintegration is the norm, but similar tothe more traditional manufacture ofmetal structures.

Manufacturing and productdevelopmentThe two most critical aspects were thefabrication of the lugs and the design ofthe joints between the components.

The fabrication of the hollow lugswhich connect the tubes in the framewas the heart of the design. The straighttubes used between the lugs were gener-ally made via the traditional tablerolled, oven cure process, which uses

hard metal interior mandrels, and exter-nally applied shrink tape, and freestanding oven cure. In general, thecurved and/or tapered tubes were alsomade with the same moulding processas for the lugs.

To connect the tubes and lugs, alightweight and manufacturable jointwas desired that would also integratewell with the frame bonding assembly. Amale plug extension to the lugs whichfits into female sockets was developed. Ashort tapered section at the base of theplugs reduces out-of-plane shear stressesand allows the diameter transition to bemade without the addition of any rein-forcing material. Semicircular radiallyspaced ribs or splines along the lug malemember or plug end closely control theuniform adhesive thickness, producingthereby a reliable high strength framejoint.

Frame designFinite element analysis (FEA) was per-formed early in the design phase to opti-mize the structural efficiency of theframe and size the laminates for strengthrequirements.

Bike frame races carbon consumer goods forward

A new moulding process could move the carbon prepreg lay-up from being done on the tool to beingdone on internally rigidized bladders.

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The full frame FEA was used mainlyto choose final tube diameters and theirlay-ups. There were numerous locationswhere the frame outer mould line geo-metry affected the carbon stress statedramatically because there were metal-lic fittings bonded inside. The stressstates in these areas had to be chosencarefully.

Numerous metal components are sec-ondarily bonded into the lugs, such asthe bottom bracket race seat, the headset race seats, the seat tube insert, rearbrake boss, rear drop outs, frontderailleur mount, and cable routing andwater bottle mount features. The struc-tural details for the more highly loadedof these details had to be addressed inthe lug design and manufacture.

Process designThe use of higher performance matchedmetal tooling was combined with betterbladders than had been used before toallow higher pressures, better surface fin-ish, better control of product dimen-sions, faster cycle time and better processcontrol.

Hard matched metal female cavitytooling was used in clamping presses.Previous moulded frames had frequent-ly relied on bolt together fibreglass shelltooling which was then placed in anoven. The fibreglass tooling was rela-tively flexible, which sometimes limitedthe bladder pressure which could beapplied.

A key aspect of the new process wasthe use of high performance bladderscapable of high pressures and flexibilityto apply pressure uniformly to the insideof the part. The conformable formed rub-ber or thin thermoplastic film bladderswhich are removed after cure are a bigimprovement over relatively stiff poly-amide bladders that had been left insidein previous frame designs. The stiffnessand lack of conformability also limitedthe pressures that could be used withprevious polyamide bladders.

A pressure of 1.38 MPa (200 psi) isapplied to the bladder as the closedmould is heated inside a clamping press.

After cure, when the lug has beenremoved from the tool, the bladder isdeflated and removed. The pressure usedis significantly higher than normal auto-clave processing of high performanceaerospace carbon fibre laminates whichis typically done at 0.86 MPa (125 psi). Itis also much higher than the pressuresused in the previous one piece carbonframes, which were about 0.35 MPa (50 si). The high pressure process pro-duces an exceptionally high fibre vol-ume, typically 67%, and low void con-tent. These characteristics produce alaminate which is much stronger thanlaminates produced with lower pressureprocesses.

Material lay-upThe parts are made using a standardaerospace grade carbon fibre in a sport-ing goods grade of epoxy resin. Thissomewhat simplified description of theprocess illustrates the main elements ofthe forming process. Each lug is basicallyformed from two halves of continuouscarbon fibre laminate. Unidirectionalprepreg is preplied into large flat sheetsin a quasi-isotropic orientation, i.e.0/±45/90.

For the primary preforms, shapesgenerally representing each half of eachlug are then die cut out of these eight or12 ply quasi-isotropic stacks. The need tominimize waste of carbon fibre materialrequires that the die cutting pattern forthese lug shapes be highly nested so theshapes are rotated at various angles tonest them together as tightly as possible.There are also several smaller preformsused in each lug, typically added forextra localized reinforcement or materialbuild-up.

Generally two primary preforms areused, one for each half of the matchedfemale mould. All of the lug mouldshave two halves, except for the bottombracket mould which also has a smallkey piece to form the area between therear chain stay protrusions.

The tools are usually heated up toroughly 50°C (125°F) to assist loadingthe preform into the moulds. The

preforms are then carefully pushed byhand into each mould half. On onemould half, referred to as the ‘net’ side,the preform will just come up the edgeof the mould cavity after it is pushedinto the mould. On the other side, thepreform is sized so that about 1 cm (3/8inch) of laminate rises above the mouldparting plane. This second side isreferred to as the ‘lap’ side, because itforms the lap which connects the twosides. The bladder is then placed intothe ‘lap’ side tool inside the preform.The laps are then folded in over thebladder. The net side tool half is thenquickly closed onto the lap side toolhalf before these lap pieces flop backout up and potentially get trapped inthe parting plane surface between thetool halves.

Future developmentsThe above process has remained relative-ly unchanged since its implementationin the early 1990s, but composite framemanufacture will change and improve inthe future.

Superior moulding technologies arelikely to include moving the carbonprepreg lay-up from being done on thetool to being done on internallyrigidized bladders. This has numerousbenefits including eliminating the lap orseam between halves of the parts. Itreduces fibre wrinkling, greatly increas-ing strengths and stiffnesses, allowsmuch more flexibility in fibre prepregplacement and orientation inside a lug,reduces material scrap rate dramatically,and decreases tool cycle time. ■

Bike frame races carbon consumer goods forward

Ron Nelson is president of ClosedMoldComposites and specializes in the develop-ment of consumer and aerospace carbonfibre products based on low cost high per-formance moulding technologies.

Ron Nelson; tel: +1-801-277-0309; fax:+1-801-277-0298; e-mail: [email protected]; website: www.closedmold.com.

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