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TT Bike GeometryTT bike geometry is one answer to the conflict between efficient power delivery and aerodynamics. The technique is to adopt a more upright seat-tube and essentially rotate the rider forwards around the crank centre line.Biomechanical efficiencySome people believe that a typical Time Trial position uses very different muscle groups to conventional cycling. This may be true of the arm and shoulder muscles but if you compare the key anatomical angles of a road cyclist in an aggressive 'on the drops' position with those of a TT posture they are very similar. Both of these positions rotate the pelvis forward to stretch the Glutes and Hamstrings into a more effective region. This will be at the expense of the hip flexors which are shortened but the overall benefit is positive. Both of these positions are constrained by how far forwards the torso can be inclined as eventually the minimum hip angle (48.7° and 48.6° shown here) reduces to a point where flexibility limits are reached and blood flow is constricted. Similarly the hip angle at full leg extension could be a limiting factor as at this position (94.3°/94.5°), the Hamstrings will be fully stretched. On a positive note, biomechanical efficiency and aerodynamics are mutually compatible as both call for the rider to lean forwards, but only up to the point where the reduction in power generating capability is not offset by an improvement to drag. |
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AerodynamicsMost people are aware that aerodynamic drag of a body is a function of its drag coefficient (Cd) and frontal cross sectional area(A). A racing cyclist in a crouch position will have a Cd of around 0.9, which does not sound very good but is at least better than if they were just standing up (1.20) or a flat plate (1.17). To improve our aerodynamics (CdA) we need to improve both Cd and A, and can have a reasonable attempt at dropping cross sectional area. Without access to a Windtunnel we have to hazard a guess at improving Cd.Reducing Cross Sectional Area.Lower the front end of the bike.Make sure your elbows are inboard of the hips. Bring your knees in towards the frame. Hunch your shoulders and get your head down. Wear tight fitting clothing. |
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Improving Cd.Many people can envisage how air molecules will impinge onto the front surface of any moving body, increasing pressure and so creating drag. What can be more important though is flow separation causing a reduction in pressure behind the object. Aerodynamicists will attempt to prevent this by maintaining laminar flow around the object for as long as possible, avoiding sudden changes in shape. This gives the classic aerofoil section (such as an airship) that has a Cd of 0.02. As a rough guide any 'streamlined' bike parts need to have a width to length ratio of approx 1:4 to be effective (1:2 is only marginally better than a round tube). This is not particularly easy on a bike but we can see plenty of examples such as aero helmets, frame shapes, aerobars, spokes and deep rim or disc wheels.The use of aero parts is only limited by budget, as is a trip to a windtunnel. Simple guidelines for improving Cd are as follows. By adopting a flat back and horizontal forearms we will approach the 1:4 width to length ratio for these body parts. Reduce unnecessary turbulence by routing cables internally. Use the forearms to guide the airflow over the hips. Reduce skin friction by using tight fitting clothing. |
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ComfortThe suitability of a TT or conventional racing position can largely be answered by a simple question.By how much and when do you migrate towards the tip of the saddle whilst racing? If the answer is over 25mm and almost straight away then your position isn't optimum and you may be faster and more comfortable with some changes. Measuring what happens when we shuffle forwards by only approx 10mm we can see the following. The minimum hip angle opens up. The hip angle at maximum leg extension opens up. The shoulder angle reduces. Not shown here but also measured was a reduction in the maximum knee angle as the saddle height effectively lowered. So by coming forwards you may be finding more power by reducing any tightness in hips or hamstrings. Alternatively you may be reducing discomfort in the shoulders, or simple sliding forwards on the saddle because it is tilted too far downwards. Ideally, the bike position should allow you to stay planted in the saddle, conserving energy with relaxed arms and shoulders for most of the race. |
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Clip on Aerobars.Aerobars clipped onto the bars of conventional racing bikes are very popular. The mistake many people make is to set them too low, and needlessly carry the weight around as they cannot get comfortable on them. There are two approaches to fitting clip on aerobars, differentiated by accompanying saddle movement.1) Saddle position does not move. 2) Saddle Position moves forward. |
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