Moreover, what standards or measures exist for the alignment of flattened spokes to point them into wind? How can a cyclist ensure that their spokes haven’t twisted out of alignment? Maybe we should all buy spoke alignment gauges? □. Regarding spokes, I suspect that flattened spokes offer no aero benefits, and may induce extra drag, even under optimal conditions. I there seems to be little mention of parabolic versus straight lines or constant curves ( which is to say round). Bicycle designers don’t seem to consider this topic – but the presentation angle assuredly impacts separation of airflow. Therefore the common diagrams of airflow around a tire/rim (which ignore these different angles of presentation, not to mention the yaw effects noted in the article above) are meaningless. That deep-V sometimes presents itself as a leading edge, sometimes as a trailing edge, and much of it’s time its somewhere in between. Like an American football kicked for a field goal, when viewed from the side. Note also that the “airfoil” of a bicycle wheel spins through 360 degrees for each wheel rotation. A bicycle tire/rim is a small low-speed object, but I’ve yet to hear anyone take this into account regarding aero wheel design. The RC modelers who build sailplanes (where lift to drag and drag to speed are very important), know better than to use scale airfoils until their models get quite large (say 1:4 or 1:3 scale). I say this, in part, because airflow around small objects (including airfoils) at low speeds, behaves differently from large objects at higher speeds. Further that most of the variations in performance, as measured, are just down to dumb luck of the various design teams. I would suggest that there doesn’t really exist a body of validated design theory for bicycle wheels. Second is the issue of testing versus validated design theory. Its a great way to buy something showy and spendy, but if you want to go faster, just train and watch your diet (oh, and pick your parents well). My thoughts are that entirely too much time, money and effort has been wasted on this subject and it’s related wind tunnel testing.įirst is the fact that wheel drag isn’t the biggest component of overall drag, nor is it the easiest to address. You’ve clearly advanced the public’s understanding of aerodynamics especially related to wheels. This is a rule of thumb and requires considered application. When the difference becomes 190W vs 175W, that is effectively 2.5SF and would be noticeable. There is some error in there as well as geometrical differences based on the rider and their bike. That is to say that for an appreciable difference, there should be a difference of 2.5 significant figures.Įg a rider is unlikely to notice a difference between a 190W wheel and a 180W wheel (2SF). Guidance in this regard is to adopt a 2.5SF rule. WarningĪ number of cyclists get infatuated by very small differences between the wheels. The spoke area which is of high turbulence is much smaller on a deeper wheel – hence the primary drag reduction. In general, a deeper rim will always have less drag. A bigger differential will produce a bigger drop in drag but it diminishes as the wheel tends towards a disc. Small differences in watts should generally be ignored as these will be affected by arbitrary things such as shoe overlap and the size of bike frame. You can see some of the background for this data here. These graphs are now updated on a regular basis and are plotted for interactivity. Ideally, the tire and the rim should be of the same width. It should be noted that bicycle wheels are particularly sensitive to tire size and having a wheel and tire combination that bulges will cause a significant increase in drag. This dataset was originally derived from wind tunnel test data but has latterly moved over to Dinitriev number with a back calculation to give “wattage”. Popular brands such as DT Swiss, SwissSide, Shimano, Mavic and Bontrager have all been tested. To date, this is the largest independent dataset available. Over several years, bike wheels have been tested for aerodynamic performance.
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