If you check the link to the overall Crr spreadsheet in the upper right of this blog, you'll see that there's a new top entry. It's the tire pictured above, the Specialized Turbo Cotton 24C which was introduced today. It sits firmly at the top of my list of production tires I've personally tested on the rollers, with a predicted on-road Crr of .0029, as compared to the Crr of .0031 for a trio of tires that used to be in a virtual tie (the Vittoria EVO Open Corsa Triathlon 22C, the Vittoria EVO Open Corsa Slick 23C, and the Schwalbe IronMan 22C Tubular).
The fact that it's so well rolling is not surprising considering it's construction. Like those other 3 tires, it's based on a 320tpi cotton casing. The main difference from those other tires is, of course, the tread that's glued onto it, which in this case is made from Specialized's proprietary Gripton material that they introduced last year with the S-Works Turbo tire. I've found this compound to truly live up to it's name, as can probably be guessed by the amount of wear on the labeling in the picture above.
OK...great...so it rolls really well (~4W less for a pair @40kph as compared to the "benchmark" Continental GP4000S) and rides/handles great, but is it aero? Or, at least "aero enough"? As I've written before, Crr can make up for a great deal of aerodynamic "sins" http://bikeblather.blogspot.com/2013/04/why-tire-crr-matters.html . Is the Turbo Cotton at least aero enough to take advantage of its ultra-low Crr to make it a good choice when aero is of a concern? Apparently...yes.
In interactions with the folks at Specialized, I was given a spreadsheet that shows the drag data for a test run taken with Zipp 404FC clinchers in a Venge road bike. The tires tested were the S-Works Turbo, the Turbo Cotton, and the Continental GP4000S 23C, which has been shown to have excellent aerodynamics on a variety of wheels. The plot of CdA vs. yaw angle is shown below:
As you can see, it shows that the Turbo Cotton is basically tied with the GP4000S at low yaw angles (up to 5deg), but then the GP4000S results in up to .010 m^2 lower CdA at 15deg of yaw. It's also interesting that the S-Works Turbo appears to be more aero than the Turbo Cotton. This could perhaps be because of the "lip" at the interface of the glued-on tread and the casing on the Turbo Cotton.
Another way of looking at this is by combining the Crr results with the aero results. Here's what the combined power would be for 40kph (85kg load). As can be seen, the low Crr certainly helps the Turbo Cotton beat the GP4000S all the way out to 10deg, while at 15deg of yaw the Conti actually beats it by ~3-5W. Above 15deg, the margin narrows again.
Well...it's pretty fairly obvious what I would do with this right? Time to analyze the combined aero+rolling drag for a weighted average of yaw angle, like was done in the blog post I linked to above. In short, here's how that turned out (using the Crr from my own roller measurements):
What does that mean? Well, using the Mavic generated wind yaw angle weighting (as in the previous analyses), it shows that the excellent Crr of the Turbo Cotton tire overcomes the lesser aerodynamic performance at higher yaw angles (as compared to the GP4000S), at expected apparent wind speeds up to >50 kph. And it stays faster than the S-Works Turbo all the way up to apparent wind speeds of 60kph. Fairly impressive.
I've ridden a pair of these tires on my road bike for the past few weeks...and I have to admit that I've found my new favorite "all around" tire for racing. It's a no-brainer selection for road races and crits due to its naturally (because of the construction) good ride quality and cornering grip (because of the tread compound). I've yet to TT on these tires...but it's pretty tempting. They seem to work fairly well for Tony Martin for that purpose. Then again, I have a feeling Tony Martin's average apparent wind yaw angle is pretty darned low :-)