Elementary Aspects of Road Bicycle Design, Engineering
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VIEW C 3 6 25 9,62 549,4 VIEW B-B SECTION - 376,4 B 16,08 5,50 8 149,7 72,82° 17,20 C B 562,4 522,9 A E E 10 5 27 150 SECTION A-A R8,20 74 46,3 R 10 9,46 32,50° 77,31° 411 574,1 A 141 147 153 brand 50teeth 53teeth Shimano 141mm 147mm 8,70 SRAM 141mm 147mm 25 Campagnolo 142mm 148mm R1,5 12 3,5° R7,6 6 5 R R 8,4 8,15 2 SECTION E-E SCALE 1 : 1 Elementary aspects of road bicycle design, engineering and fabrication © Motion Devices SA, 2015 © Motion Devices SA Page 1 Over the course of the last one hundred years, the design of the Contents diamond frame road racing bicycle has evolved surprisingly little. A quick look at frame building 2 Nonetheless, many riders complain about not feeling comfortable What has custom meant? 3 on their bicycles but do not always understand why that is the case. When is a monocoque a monocoque? 4 Few complain that their bicycles do not handle as they would wish, A quick background on carbon fiber 5 silently accepting that a new bicycle does not feel as intuitive as the Why a custom lay-up? 6 bicycles of their youth. They usually ascribe the difference to their It turns out that geometry is not as easy as it looks 7 rose colored backward looking glasses. The rule of thirds 8 Stack and reach explained 8 In fact, while correct bicycle design and engineering requires a The importance of being consistent 8 thorough understanding in order to produce a very high performing Top tube length 9 bicycle, it is not impossible to achieve. In the following pages, we Stem length 10 will attempt to outline our thoughts on the subject. Handlebar reach 10 Handlebar drop 10 Seat tube length 11 Motion Devices SA Seat tube angle 11 P.O. Box 235 Bottom bracket drop 11 109 Route d’Arlon Wheelbase 12 L-2012 Luxembourg Chainstay length 12 www.rolobikes.com Seatstays 12 [email protected] Center of gravity 13 Front-end geometry 13 Head angle and wheel flop 13 Fork offset or fork rake 14 Geometric trail 14 Crosswind 16 © Motion Devices SA Page 1 he most elementary and fundamental aspect of the bicycle is frame A quick look at frame building geometry. It determines fit, handling and comfort, the enjoyment T s metal frames evolved and the industry grew, a number of derived from riding being directly dependent on those three basic small fabricators emerged who offered a custom service. parameters. A They purchased steel and later aluminum tubing from Reynolds or In terms of importance, we order these three basic design parameters Columbus and then welded the tubes together to fit their customers. as follows: The ‘ready-to-wear’ segment generally did not fully grasp frame geometry leading to bikes with poor fit, handling and 1. Fit comfort characteristics, so the small fabricators developed 2. Handling a niche in which they offered better fit for customers who 3. Ride could afford the service. Basic comfort and handling need to be addressed through The industry metal of choice changed over time from steel, frame geometry rather than through special component to aluminum and titanium. Although aluminum and titanium choices at a later stage. Once the basic geometry has require more skill to manipulate and weld, the techniques been established to drive both the fit and the handling, were effectively the same. And since metals are isotropic and only then, can we tackle the ride aspects of the bike’s materials, the engineering skill required was within reach design, namely the structural engineering challenges. for virtually all of these fabricators. The Motion Devices process hierarchy of design When carbon became the material of choice in fabricating parameters then flows in the order to the right. frames in the late 1990s, new challenges arose. Because carbon is an anisotropic material, the engineering skill The parameters that affect a bike’s fit are more or less required suddenly moved out of reach for many fabricators. well-understood, albeit not always that well implemented. But there was a potential solution: they would continue On the other hand, the parameters that affect handling to use essentially the same manufacturing techniques as characteristics remain poorly understood. The interaction wheels tires they had before and would rely on carbon tubing which of these different parameters is complex to the point saddle they would ‘weld’ using carbon in a wet lay-up process. where still today, some fail to understand why the bicycle cockpit remains stable and upright when moving. drivetrain While metal tubing could have varying wall thicknesses, features Although most of today’s bicycle nomenclature stems from meaning the walls could be thinner at the middle of the the time when bicycles were almost exclusively made tube and thicker at the ends, it required complex and very with tube to tube metal construction, our bikes use a full monocoque expensive engineering, tooling, and methodology, such as hydro composite construction. The fundamental elements and geometry of forming. But it was more difficult to achieve. The manufacturers now the bicycle frame however remain the same. had to choose between trial and error or engineering design, which for the most part required finite element analysis (FEA). FEA suddenly © Motion Devices SA Page 2 required much more computing power than was within the reach stiffness, and by extension weight, of the frame. The customer will of all but a handful of manufacturers. To move forward, the smaller feel that he is getting his own bespoke frame to fit just him, different fabricators who lacked the resources for the advanced engineering from any other on the market. And, in a certain sense, that is true. and capital investment for tooling had to find another solution. Essentially, to this day they have to rely on the tube manufacturers While nothing can replace the feeling of having a product made just for the mechanical properties of the tubes they source. Of course, for you, there are a few important points about this idea of custom they are also limited to a choice of whatever the tube manufacturer geometry. Let’s start by looking at human physiology. sells. Usually that means a choice between two or three tube set options. Human physiology Leonardo da Vinci’s Vitruvian Man illustrates human proportions The alternative approach was to manufacture monocoques. The rather clearly. While taller people obviously have longer limbs fabricators who had the engineering expertise and access to the than shorter people, the proportions remain essentially the same computing power could now design any shape of frame imaginable, regardless of how tall someone may be. Put another way, have you choose the materials, and dial in the mechanical properties desired ever noticed that a chair’s seat is almost always the same height, at any given point on the frame. The flexibility means that any weight and you are uncomfortable if it is either too low or too high? Desks, or stiffness is achievable within the range of physical property limits dining tables, and kitchen counters too? We notice differences as of the carbon itself. But each frame size requires its own mold, its small as 1cm, although we may not always realize it. own templates for the carbon and its own engineering specification for the prototypes as well as production, which only intensifies the Most of us are physically proportioned in a tight statistical band need to get it right the first time. The initial investment was and but for some, custom geometry is the only solution, if for no other remains very significant because the molds are costly to develop reason than stock frames are not made in all sizes. Take Shaquille and make, sourcing the carbon is not always straightforward, and O’Neal as an example. He is 216cm (7’1”) tall and weighs roughly the manufacturing process involves considerable manual labor and 150kgs (330lbs). There is no manufacturer today who makes a stringent quality control, particularly for low volume production. standard frame large enough and strong enough to accommodate These issues help explain why virtually all of the ‘monocoque’ him - his only option is a custom tube to tube bike. fabricators moved their production to low cost environments. Some of us also have somewhat above mean asymmetries between What has custom meant? our right and left sides such as between our left and right limbs, or our right and left foot. A bike frame however is a symmetrical n the past, and even still today, a custom frame means custom structure so these types of variations from the mean usually cannot geometry using tube to tube construction. A customer calls one of I be properly addressed even through custom frame geometry. the custom fabricators and begins a process of designing the frame. Together they discuss head tube and seat tube angle, and adjust the But for most of us, if we start out with a correctly designed basic length of the top tube. After what is usually a long discussion, they geometry, we can easily fit very comfortably on a bike and make agree to the geometry and the customer can then choose between small adjustments through various components. Let’s take a quick two, or at most three, sets of tubing which will determine the overall look at what we mean. © Motion Devices SA Page 3 Seatposts can range from no setback to up to 25mm of setback.