I'm always very skeptical about aerodynamics on bicycles. The frontal area of the rider is so much larger than that of the bicycle itself, that the whole aero fad of the early to mid 80's seems silly in retrospect. Not sure it does much good to reduce drag of the tire and rim, when that air is going to crash into the rider's legs or torso anyway. In my opinion, the only place where aerodynamics is really significant is in positioning the rider so as to minimize drag on his body. This means aero bars and stems, now banned from road racing, are worth more than all other aero components combined. So I would think the aerodynamic difference between a 19 mm and 32 mm tire probably isn't worth discussing, although as Jan says a 4" wide tire might be a problem.
I actually like the Shimano DA AX and 600 AX components, but I don't believe for a minute they deliver the benefits originally advertised for them. They just look cool.
Regards,
Jerry Moos Big Sping, TX
Ken Freeman <freesound@comcast.net> wrote: I would think that because tire air flow soon becomes rim air flow, that tire aero drag cannot be separated from rim drag. The impingng air flow is separated by the tire into a left and right passing-flow pair, and the two flows are joined again as they pass the rim, in the leading part of the wheel. Obviously the reverse should occur at the trailing part of the wheel. The only relevant measure, if this coupling is tight, is the drag caused by the tire-rim system. Practically, I could see putting one set of wheels on a bike and recording coast-down from a high speed on a level course, then repeating in the other direction to attempt to equalize windage effects. Then change wheelset and repeat. Alternatively do the test in a wind tunnel. If you measure a difference with the same bike, same rider, same protocol, and same weather (crosswinds might matter), you probably don't need to consider the rest of the bike.
On their website Zipp had at least some discussion of their wheel system aerodynamic measurements.
But unless there is a big difference in the drag coefficients of two tire-rim combinations, increasing the cross-sectional area, driven by the tire actual width if the wheels are the same diameter, should result in more aerodynamic drag. This assumes the the hub and spoking desighs of the two wheels are essentially the same.
Aerodynamic drag is well-established science. It should be necessary to make measurements to calibrate a model or to validate specific designs, but not to arrive at correct general predictions.
Or is there data that would call these general rules into question?
Ken Freeman Ann Arbor, MI USA
-----Original Message----- From: classicrendezvous-bounces@bikelist.org [mailto:classicrendezvous-bounces@bikelist.org] On Behalf Of Jan Heine Sent: Sunday, September 23, 2007 1:00 PM To: haxixe@gmail.com; hersefan@comcast.net Cc: classicrendezvous@bikelist.org Subject: Re: [CR]Wheels make you faster, not frames.
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