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Won't having disk shaped wheels cause more wind resistance with crosswinds? What are the benefits of having these sort of wheels?

2006_Cervelo_P3_Carbon

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It's a time trial, fairly short and generally on a course that is reasonably protected from the wind. If side wind were a problem that day they'd likely swap out the wheels for open ones, but if wind isn't a problem the wheels allow higher top speed (less air resistance, though at the expense of more weight). You don't see disk fronts not only because of the rules but because it makes the bike very hard to handle. On longer rides not only is wind more of a problem, but with lower average speeds and more climbing weight is a bigger concern. –  Daniel R Hicks Jul 29 '13 at 20:54
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3 Answers

Disc wheels have lower drag than spoke wheels. See Wheelbuilder aerodynamic data and Aerodynamics of High Performance Race Bicycle Wheels. (One link shows simulated data, the other measured).

Most of these tests run up to yaw angles of 30 degrees. The argument is typically that at higher yaw angles (closer to side-winds) the rider would choose a spoked wheel instead of a disc.

Disc wheels are typically preferred for courses and days that do not see a lot of cross-winds not because of drag, but because they present a large area for the wind to push on, and can lead to unpredictable performance in high-crosswinds. This is one reason you don't often see disc front wheels in a non-velodrome environment.

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There are also a few National governing bodies that disallow front disc wheels. UCI permits it, but NGB's often don't. –  JohnP Jul 29 '13 at 18:49
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+1. Also note that you don't need discs to experience the "blown across the road" effect. My "default" wheels have 6cm rims - so still relatively small compared to discs - but in windy weather I will swap out the front wheel for a smaller rim, just so I can ride in a straight line! –  PeteH Jul 29 '13 at 18:51
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I found a link here: http://www.wing-light.de/CFD/wheels.htm but you can also get similar graphs from manufacturer. It shows different amount of drag in different wind angle.

Crosswinds don't necessarily give high drag, obviously it should give lower drag than headwinds. Even when it's directly blowing from the sides, depending on the speed of the bike and the wind, the resultant angle is not 90 degree.

Now, here's the interesting part, if you check the graph, at around 20 degree angle the disc actually has negative drag! I reckon to get that kind of angle, you need a higher degree angle (more like crosswinds) when you're moving fast.

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I'm not sure I understand. Is that negative drag at 20 deg from a headwind? Are we talking true or apparent wind here? I can't see how you could get negative drag without a tailwind. Worth a new question? –  JamesBradbury Jul 31 '13 at 8:11
    
@JamesBradbury - The new question would be "how does an airplane fly?" or "how can a sailboat sail upwind?" –  Daniel R Hicks Jul 31 '13 at 11:13
    
@DanielRHicks, that implies the wheel is not symmetrical when viewed from the top. Is that the case? Can't see that working very well, except in a wind tunnel. –  JamesBradbury Jul 31 '13 at 13:33
    
@JamesBradbury - It doesn't necessarily imply that, just that an airfoil effect can be achieved. Air does mysterious things at times. –  Daniel R Hicks Jul 31 '13 at 15:20
    
@JamesBradbury yes that's 20 deg from headwind. In wind tunnel, because the wheel is stationary, that's 20 deg of true wind. On the road it'd be apparent wind angle. If this raises a new question, I think it would go to physics just to get a deeper understanding of wind tunnel test results. As DanielRHicks mentioned, they call it sail effect. Another interesting point, if you think it's not worth it because you're not a pro, the paper also shows that slower riders get bigger time gains! –  imel96 Jul 31 '13 at 22:46
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The benefit of a disc wheel is that you can ride faster than with a spoked wheel, on flat and rolling roads and at steady speeds. Once hills (gravity and lower speeds) or changes of pace, come into the mix the case for spoked wheels becomes stronger.

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