I recently went to the San Diego Low Speed Wind Tunnel (lswt.com) with several other engineers from Trek’s Advanced Concept Group (ACG). Since this was an internal test, we were looking at some wild ideas, not necessarily related to marketing or any pro tour team or triathlete, but to better understand aerodynamics for future Trek aero bike and accessory designs. Although we’re always “thinking aero”, we prepared for months specifically for this trip, brainstorming aero concepts and details, modeling the most interesting ones in CAD and running CFD analyses; preparing physical prototypes, whether CNC foam mock ups, hand-made clay parts, SLA models, reconfigured assemblies from existing parts, or buying a few competitor brands’ bikes. One of the specific reasons we went was to get a real-world comparison of a pedaling rider on our TTX and other brands’ bikes. Bikes were built up as they would be for riding, including drivetrain, cables, etc.

Wind tunnel trips are always exciting, and this one was no exception. We can’t share publicly all of what we learned, but we’ve chosen to share data comparing a few existing bikes, namely the current Trek Equinox TTX, a 2006 Trek Madone (with the shaped tubes), a 2007 Scott Plasma, and a 2005 Cervelo P3C, all in size medium. A chart comparing them is on Chris Lieto’s blog and I've reproduced below (click on image for larger version):

We were pleasantly surprised by the response this got in various forums such as Kraig Willet’s BikeTechReview.com's Performance Forum and Dan Empfield’s Slowtwitch.com's Triathlon Forum, among others.

These discussions brought up many excellent comments, and it’s refreshing to see the level of in-depth understanding of many of the forum members. I’ve jotted down a few answers here in order to clarify some of the confusion.

1: Are the data fudged? No. Although speculation from uninformed parties can make it seem possible and tempting to do so, all the data are as described below. It might also be worth noting that the testing was carried out by Trek’s Advanced Concept Group as a pure internal R&D effort (thus a strong desire to learn the true differences), and only afterwards we made the decision to publish some of the data.

2: Who was the rider? Not Levi, not Basso, not Lieto (though Trek has been in the tunnel with each of these riders on other occasions), but ACG’s own Doug Cusack. That explains why the CdA back-calculated by sharp readers doesn’t seem to fit those riders. Doug has been involved with aero bikes for a long, long time, going back in time through every one of Lance’s TT bikes and Lance’s F1 advisory team, all the way to his own small frame building shop before he worked at Trek.

3: How consistent is a live person? Doug’s drag numbers repeated to within 20 grams in separate runs on the same bike configuration taken hours apart during the same session. That’s impressively low variation for a live human. Standard controls such as outlining the profile on the TV screen and identical bike set up, including seat height and set back, reach and drop, etc. (both tasks were part of my responsibilities for this trip) are probably obvious. In addition to that, Doug has been to various wind tunnels more than anyone else at Trek, and knowing how variable riders often are, has developed detailed techniques for holding the same position all day.

4: Are all the runs shown from the same session? Yes. While sometimes weeks in the wind tunnel can be worthwhile, confidence is highest when comparing only runs from the same session.

5: Was he pedaling? Yes, at a realistic 90 rpm. The motion of the legs may or may not be the most important detail to include, but it is of course more realistic than a stationary dummy.

6: Is this result (Equinox TTX faster than Cervelo P3C) consistent? Yes. Across multiple separate tips to various wind tunnels (with different riders), this has been the case.

7: What bottles were on the bikes? None. Although we also tested various experimental hydration systems during this session, that data is proprietary and is not shown on the published chart. Of course you might have noticed Chris Lieto’s Bontrager Speed Bottle in front of his stem…

8: Can I get a Bontrager Speed Bottle in front of my stem? Not yet. Trek ACG’s Mark Andrews designed and CNC’ed a custom stem face plate to accept a dremelled Speed Bottle cage just for Chris. And one for me (Thanks Mark!). :-) 

9: What wheels? Bontrager flat disc rear, Bontrager Aeolus ACC front, same on all bikes in the chart. The drag differences are not due to different wheels.

10: What handlebars? Bontrager Race X Lite carbon base bars and Bontrager clip on aero bars, same on all bikes in the chart. The drag differences are not due to different bars.

11: What about Specialized, Walser, Orbea, Wilier, etc.? Long ago we tested a Walser and found the then-current Trek OCLV Team Time Trial frameset to be faster. The current Trek Equinox TTX is faster still. The others weren’t available at the time of our test.

12: What about this article on BikeRadar.com that seems to say the data is from Levi? Whoops, sorry, that’s incorrect – possibly an understandable mixing of references to the many tunnel trips Trek has made.

13: Is the data for the Equinox TTX or just the new TTX SSL? Both. The Equinox TTX and Equinox TTX SSL both come from the same molds in our Waterloo carbon molding facility, so the shapes (and aero properties) are identical. The SSL has weight savings through intensive FEA analysis, lay-up schedule refinement, physical testing and carbon fiber material changes, hence the top-of-the-line OCLV “Red” designation.

14: Cervelo finishes second again! Yes, it’s true: Cervelo’s Gerard Vroomen has an excellent comment along the lines that they’re happy to finish second in other companies’ tunnel tests. We went to the tunnel for research purposes and hoped the Trek would be faster, but honestly wanted to know even if the Cervelo (or other bike) was faster.

15: Does this mean the Trek Equinox TTX is the best bike for everyone? No, of course not. For best performance, a rider must fit on the bike comfortably and powerfully. But if the Equinox TTX permits such a position for you, it is definitely that little bit faster than the others.

16: What are the differences between various Trek OCLV TT bikes? In sequential order:

     a: Trek Team Time Trial. Originally painted white for Lance, then raced by the USPS team for years. This bike got the wider spaced fork blades as a running change. Eventually sold as the Equinox 11, etc. Slack seat tube angle, three sizes.

     b: Lance’s black TTX (not “Equinox” TTX). This went into very limited production in 2006 only (just one size, Lance’s). Same slack seat tube angle as before, same wide fork.

     c: The Equinox TTX. This is the data in the graph. Started production in 2006. This is what most people think of as “the TTX”. Most were painted red, yellow, blue or silver. Changes include steeper seat tube, down tube closer to front tire, 1 1/8” steerer, fork crown surfaces matches up with frame, etc. Lighter “Reverse Bond” fork as a running change, but no difference in fork dimensions or shape (or aero properties).

     d: Trek Equinox TTX SSL. Introduced this summer, lighter frame, but same shape (and thus same aero properties) as the Trek Equinox TTX.

Sorry for such a long list, but it’s always interesting to hear what people are interested in and it’s clear to me that many people are interested in the details of the testing that produced the chart. As always, questions and comments are welcome! Thanks for reading.

-Damon Rinard

P.S. Trek's White Paper on this tunnel test is downloadable in PDF format from Slowtwitch.com here:

http://www.slowtwitch.com/images/trekwhitepaper1.pdf