Saturday, April 23, 2011

"aero road frame" versus front fender

I'm working my way backwards through Bicycle Quarterly and am currently at the Fall 2007 edition. There's a very interesting article there: Jan Heine went to the wind tunnel to test his Alex Singer Randonneuring bike with various options. One of the tests was the effect of a front fender.

Bicycle Quarterly


As he expected, the front fender appeared to reduce wind resistance. He used a telescoping fender to test the effect of length. I plot the result here: "N" corresponds to no fender, "S" for short, "M" for medium length, "L" for long, "XL" for extra-long, and "MF" for a mud-flap on an extra-long fender.

plot


There appears to be an optimal length: the fender blocks the wind from entering the brake area, then going longer than that is counter-productive.

Jan dismisses the effect as relatively small. However, ignoring his "repeatability" tests to establish error bars, I'll plot the naked numbers of the difference in CdA from the fenders to the difference in CdA reported by Scott for "aero" mass-start bike frames relative to a "round-tube" frame, my guess the long-head-tubed Scott CR1. Scott's tests were with a dummy mounted on the bike. Jan's tests were with a rendered outline on a display against which is aligned an image of his body for repeatability when conducting the wind tunnel tests. I use the zero-yaw data from the Scott test since that is how the fender test was done.

plot


Curiously, the short front fender seems to offer a greater advantage than the Cervelo and Felt, and not much less than the Scott Foil. Velo Orange fenders cost around $60. The Scott Foil frame is probably around $3500.

5 comments:

TnA said...

Dan, I'm not sure if I'm following what this sentence means: "Jan's tests were with a rendered outline on a display against which is aligned an image of his body for repeatability when conducting the wind tunnel tests."

You mentioned he also did "repeatability" runs...how much variation across runs was there?

djconnel said...

"A live image of the rider was projected on the wind tunnel floor. Traced outlines mark the three rider positions." ... there is an image of a display showing a reference rider outline in black, sampled at the start of the test, superposed with a live image which the rider attempts to register with the outline.

3 runs were done w/ a bare frame yielding 8.14, 8.16, and 8.16 N. Runs done with a rider and bike were 26.06, 25.74, and 26.36 N, all at 22 mph. But it is noted that rides done close in time tend to be more repeatable than those done hours apart or, worse, on separate days (tests were done on two days). The fender tests, which focus on forward extension of the fender, were likely done within a short time interval. The smoothness of the curve appears to indicate a lack of random noise, although a systematic drift cannot be ruled out.

Unfortunately, with the Scott wind tunnel test, we have virtually no information about protocol, and for the Tour magazine test, I cannot read the German (my copy is only scanned, so I can't easily send it to Google Translate, either). There is clearly the opportunity for substantial variability in these tests, so data needs to be treated sceptically. For example why would the Foil, which was comparable, maybe slightly inferior to, the Cervelo S3 and Felt AR1 suddenly have less drag with a dummy rider? Some advantage of the Cervelo and Felt neutralized by the rider? I don't know what that would be. Or maybe the dummy placement wasn't perfectly the same? That seems more likely to me.

TnA said...

Aaah...I see, they used an outline projection to attempt to control the rider position. For some reason I was originally reading that like they used some sort of cardboard cutout or something ;-)

Also, I didn't understand that the lengths were in relation to forward extension. Any dimensions that correspond to the "S, M, L, etc."?

Interesting that apparently redirecting air away from the brake area using a fender resulted in a CdA drop of ~.010 m^2...and yet some folks have a hard time believing that just streamlining the brake itself could result in an ~.005 m^2 reduction in CdA :-)

djconnel said...

The fender extension is either "short", "medium", "long", or "extra-long" extending to the front. There's multiple effects. One is that the air may be directed away from the fork bridge & brake. Another is that the air is no longer hitting a moving tire. Under typical wind tunnel approximations, the tire is moving the same speed as the relative wind for zero yaw (I don't know what they do for non-zero yaw). Perhaps it makes a difference if air hits a moving tire versus a stationary fender.

BTW, the "repeatability" numbers I gave were spread out over two days. It seems a major source of variability is clothing. Jan wore a wool jersey, which if it has folds in different position can affect Cd, even if A is matched. A skin suit would have been better, even if the purpose of the test was "real-world" conditions for Paris-Brest-Paris.

TnA said...

Sure...but how long is "long", for example? I'm just trying to get a feel for the lengths...

Wool jersey?