For a long time I've been alarmed by the trend in carbon bikes to go to fatter and fatter tubes in a never-ending quest for higher stiffness-to-weight ratio. Indeed the trend predates carbon fiber: it goes back far further, to Cannondale, and before that, Klein with their fat-tube aluminum bikes. To save a relatively small amount of mass, giving up a large amount of wind resistance.
But then as the quest for lighter carbon frames started approaching a limit, there was an effort to gain performance elsewhere. So wind resistance got renewed interest. Starting perhaps with the Kestrel Talon, there were a series of "aero road" frames which, at the cost of around 200 grams more or less, performed much better in the wind tunnel.
These frames had a hard time catching on with professional riders, however. Road cycling is performed mostly in packs, with fatigue and avoiding crashing both major factors, so optimizing the bike is a lot more than optimizing speed at a given power: it's about positioning yourself within a pack, of reacting to attacks, of confidence at speed. For whatever reason the aero frames didn't provide these things, and the majority of riders remained with the fatter-tube bikes designed for stiffness and handling.
This led to a compromise: bikes like the Scott Foil, Cervelo R5 (and RCa), and the Trek Madone incorporated aerodynamic features like truncated foils into their frame designs in order to split the difference aerodynamically between the "aero" bikes and the bikes designed without any consideration to aerodynamics. These, I thought, had hit the sweet spot.
But they were basically working to undo the damage which had been done going to fat tubes in the first place. What of the lowly steel bikes with their relatively narrow one-inch tubes? How would these do in the win tunnel? Tony Rominger set an impressive hour record in the 1990's on a steel tube frame, going more than 53 kph. This is fast by any standard. Of course, Rominger used EPO, which helped, but nevertheless it was clear his steel round tubes didn't handicap him too badly. Nobody has much of a financial interest in showing whether standard steel tubes do compared to, for example, a Specialized Tarmac or Trek Emonda.
Finally Specialized, ironically, delivered. Specialized built their own wind tunnel in Morgan Hill, California, which allows them to investigate a lot of questions which become harder to justify if you're using a commercial facility with a high cost/hour. But for some reason they compared an old steel-tube Specialized Allez bike with a state-of-the-art Specialized Venge, an "aero road" bike designed to be on the leading edge of UCI-mass-start-legal frames.
Here's the video:
So the result is the Venge was 50 seconds faster per 40 km. What do I do with this number? First, I need to make some comparisons.
The test rider in the video has a relatively relaxed position compared to most pro racers. Tour magazine did a wind tunnel test with a dummy rider and got a CdA of 0.32 m2. Analysis of rider speed-power data has shown a good match for climbs of a CdA = 0.35 m2. I'll assume this rider is more like the pro racer climbing position than the more aggressive Tour magazine dummy.
I'll then assume that the 40 km time is at an assumed speed of 40 kph. This is an excellent time on a road bike with drop bars.
Then I'll assume that the time comparison is under the assumption power is proportional to speed cubed. This is a standard approximation which neglects the effect of rolling resistance, but it's commonly used.
I start with the fractional time savings: 50 seconds out of 1 hour is 1.39% time saved.
I assume power saved is three times speed saved, so the power difference, and therefore the difference in CdA, is 4.17%.
Given the baseline CdA value of 0.35 m2, I then get an absolute difference in CdA of 0.0146 m2.
There's a common standard, especially in the US, that tests on wind tunnels are done at 30 mph (Specialized echews imperial units and uses 50 kph = 31.1 mph, but plots with 30 mph are common). Assuming 30 mph and an air density of 1.2 kg/m3, this difference in CdA corresponds to a difference of 21.1 watts, or a difference in force of 1.57 Newtons, which is 160.4 gram-equivalents of force, or 0.353 pounds less retarding force on the Venge versus the steel bike.
This isn't at all surprising, as was described in the video. Round tubes aren't close to optimal for wind flow, while the shaped tubes on the Venge, along with the hidden cables, are. Additionally the downtube shifters on the steel frame add some wind resistance. The other components are probably a wash.
But my interest isn't so much in the pure aero road frames, but rather in the middle-ground frames like the Madone and the Cervelo R5, and even more so in the bikes like the Emonda and the Tarmac. Is is possible my humble steel Ritchey Breakaway is actually faster on the flats than these super-expensive carbon bikes?
For comparison of results, it's best to compare with other tests which included a rider on the bike, since some parts like the seat post or handlebars may affect wind resistance more on a naked bike than with a rider. Both Cervelo and Tour magazine use dummies: Cervelo uses its dummy of Dave Zabriskie in his CSC days, while Tour used a clothing mannequin. I don't have good data from Cervelo, but I'll show some 2012 data from Tour.
Recall I concluded the CdA difference between the steel bike was 0.0146 m2. The horizontal lines on the Tour magazine plot are 0.0100 m2. A difference of 0.014 is thus comparable to the spread in values from the Tour test for moderate yaw values from 0 to 10 degrees. Curiously the Cervelo and the Venge both have issues at zero yaw, but move to the front of the field at 5 degree yaw. This was also seen by Tour in their 2011 test of the Cervelo S3. In any case, the Venge is 0.011 m2 better than the "reference bike" with the same wheels at 5 deg, and 0.029 m2 better at 10 deg, 0.027 m2 at 15 deg. Whatever the reference bike, the Specialized test of the steel bike seems to be doing a bit better, depending on what sort of yaw angle averaging Specialized used in the 50 sec/40 km number quoted in the video.
The next plot is from Bicycling magazine showing bike-only data, no rider.
The bikes here are the Cervelo S5, the Specialized Venge, the Felt AR1, the Blue AC1, the Scott Foil, and the Specialized Tarmac, all with the same wheels. The difference between the Venge and the Tarmac is approximately 250 grams equivalent force at 30 mph. Recall I calculated 160 grams equivalent foce between the Venge and the steel frame. This suggests, the steel frame is around 90 gram equivalent force less than the Tarmac.
This isn't a fair comparison: different tests and one with and the other without a rider. But it makes sense: narrower tubes = less wind resistance. But I'd love to see a straight-up head-to-head time trial comparison.