In side winds, deep air foils and disk wheels can create a "sail effect" and save energy
a) True b) False
A dominant share of time
(more than 94% a typical outdoor race) the wind strikes a rider at
enough angle that the airflow along a the rider and bike is disturbed and
not laminar. The stall angle (the angle that the air breaks from the
airfoil) of disks and most deeper sections is only 6 degrees off front,
an angle achieved by normal rider speeds with only a minor breeze.
This disturbed non-laminar airflow breaks up into turbulence that causes drag
and slows the rider. Rather than deep air foils which only present
low frontal area when head-on, an ideal airfoil presents a low frontal
area to many wind angles. This is smaller structure which has low frontal area
and low side area. Note that smaller structures require stronger
In our experiments, we measured dozens of variables from dozens of racers and race venues. we recorded the wind angle and speed, and rider speed, location and acceleration. We also measured the benefits of drafting and versus not drafting, the effects of climbing and riding on flat terrain. We created models of exposure time versus these various conditions and tested the performance of air foil solutions for reducing drag versus these conditions with very high resolution test instruments (high precision, +/- 1.5 grams, and high accuracy +/- 2 grams).
The perception that disks and deep section airfoils create lift is most likely borne out of low resolution (+/- 50 grams) wind tunnel instruments that do not account for all the drag forces encountering the rider. Airfoils like disks and deep rims are perpendicular to the direction of movement and therefore can not generate lift, since this lift would also be perpendicular to their surfaces and not in the direction of the movement of the bicycle. Sailboats achieve lift by swinging their sails out of the center line of the boat. The only forces disks create in side winds are destabilizing, sideway counter-steering forces. Lenticular disks improve the turbulence angle to only 9 degrees at best and share the same issue of being unable to create non-perpendicular lift. Lenticular disks also increase head-on drag by increasing their frontal area. Many of the deeper airfoils on bicycle frames share similar issues. Their low head-on frontal areas are helpful, but their large side areas are distinctly not helpful.
Nimble specifically invested in developing uniquely stronger materials and material designs to enable us to have minimal frontal area and also reduce the side area. Our designs achieve 19mm frontal widths, with rim depths of only 39 to 28 mm, and side open areas of 68 to 82%.
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