At 1:03 PM -0700 9/22/07, Kurt Sperry wrote:
>I mostly agree, but I'd add that the assumption that fat tires are
>aerodynamically "draggier" isn't a safe one to make. Aero can be
>maddeningly counterintuitive, even identical shapes are subject to scaling
>(Reynold's Number) issues. Remember lenticular front wheels? They sure
>didn't look aero. It wouldn't surprise me a bit if fat tires (the rim
>profile's interaction with the tire's will also come into play) were found
>to be more aero. The wheel is interacting with the rest of the bike and the
>rider aerodynamically in turbulent/chaotic ways that can't be visualized in
>terms of the usual streamlines in a free flow. But you can't really tell
>without empirical data to analyse. And good, valid, repeatable aero data of
>mixed flows is famously difficult to acquire.
I have been unable to find any firm data on whether a narrow tire is more aero or not. You'd think somebody would have tested this at some point... If they have, the results are kept secret!
The only data I have found is from Continental, reported is on cyclingnews.com. It seems that they used a rider with a power meter, and sent them out once with a narrow tire and once with a wider one. While they showed a small increase (5%) in power output with the wider tire at a speed of 50 km/h (31 mph), there are so many variables that you cannot say anything with certainty. You'd need a very rigorous statistical analysis to show that your data is due to tire width, and not due to changes in wind direction or something else. Here is the link
http://www.cyclingnews.com/
As it is reported, I consider the data meaningless. At the very least, I'd like to see two runs with the same tire, and see whether the power output was the same... And of course, at lower speeds, wind resistance is less important and rolling resistance becomes a greater factor in the overall picture.
Since the data was lacking, we included this test in our wind tunnel testing - see the report in the latest issue of Bicycle Quarterly (in the mail right now, if you haven't got your copy).
We tested a Vittoria Open CX 24.5 mm tire vs. a Grand Bois Cypres 31 mm front tire. Both have similar tread patterns. We tested this both with a front fender and without. While the measurements for the narrower tire were 0.9-1% less resistance than for the wider one, the results were not statistically significant - that is, the difference was smaller than the noise of these wind tunnel tests. (The noise was not because the wind tunnel is bad, but because the pedaling rider changes position slightly, etc.)
So basically, if narrower tires are more aero, the effect is minor - compare that to the effect of lowering your stem by 20 mm (5%). This also makes me wonder about the 5% power difference in the Continental data - we don't even get 5% difference in wind resistance, much less in power output!
Wider tires have lower rolling resistance - which we confirmed in our real-road tire resistance tests - and it appears that wider tires will be faster at most speeds cyclists use. How wide? We haven't figured out at which point the returns from wider tires diminish and finally decrease (I doubt a 4" wide tire would be faster than a 2" wide one). To test this, we'd need tires with the same casing, but different widths. The only ones we have tested were Michelin Pro2 Race in 20, 23 and 25 mm. There was a clear trend, with the 20 slower than the 23, which was slower than the 25.
Classic content: the bike we used in the wind tunnel was my 1973 Alex Singer.
Jan Heine
Editor
Bicycle Quarterly
140 Lakeside Ave #C
Seattle WA 98122
http://www.bikequarterly.com