This is in response to Dan’s review of the Storck in the latest Inside Tri (or maybe it was Triathlete). Doesn’t matter, we all read both. Anyway, the Storck has round tubes, in similar fashion to Scott carbon frames and all Serottas. The question that Dan raised is about why a TT bike would have round tubes, and should we just throw away all the wind tunnel data that says airfoil tubing is better? I mean, look at Lance. His TT bike has aero tubes, so doesn’t that mean it is faster? The question that gets raised, and which Storck’s designer, whose name is also forget (remind me not to post away from my magazine archives) is that the data for all these aero TT bikes is based on 0 degree wind angle. Which made me question “WHY is that?” I have wind angle data for 0, 5, 10, 15, and 30 degrees for every Hed wheel. If 0 degrees of wind angle, which is head on, was all that mattered, there wouldn’t be any need for a disc wheel. (Compare Hed Disc 0.404 @ 0 deg to Hed Alps 0.406 @ O deg). But at 30 deg - Disc 0.233 vs. 0.415 for the Alps.
But log on to Trek, Cervelo, Cannondale, or ANY OTHER manufacturer and show me the wind angle data for the frames? Somehow, it appears that it doesn’t matter to them. But, it does. I will pick on Cervelo here for a couple of reasons. Vroomen and White are engineers who should know better, that is my main reason. They test all wind angles, but don’t show it to you. They just show an “average.” And second of all, they only go up to 15 degrees of yaw, instead of 30 like Hed. There is a reason for this. It is related to the “true NACA airfoil” section of Cervelo tubes. Regular airfoils, like the nice ones that keep the planes in the air when we fly to far off races, are meant to work in a very limited range of angle of attacks. For the purposes of bicycles, yaw angle is the same as angle of attack, since the airfoil tubes are turned sideways. When you get up past 12 - 15 degrees, a NACA airfoil stalls. This air no longer flows neatly around it. So while your Hed disc shines at 30 degress of wind angle, your fancy Cervelo airfoil tubes are stalling out, creating drag, and giving you zero aero benefit. To simplify what a stall means for the purposes of this board, a stall means the air is pilling up and bouncing around the tube, not flowing neatly around it.
So in the high crosswinds of the Kona Coast, where you might think an aero frame would be at its best, it is really at its worst. So why then, do people ride aero frames? Why would Lance do it if it wasn’t faster? It is not that it isn’t ever faster. In a no-wind environment, it will save time. And the faster you ride, the lower the effective yaw angle becomes for a given crosswind. At 30 mph - Lance speed - in a 5 mph crosswind, the effective yaw angle is 10 degrees (arctan(5/30)*180/PI). So you are still within the operating parameters, but if you are doing 20mph instead of 30, it becomes 15 degrees, which is the beginning of the stall range of most NACA airfoil sections. And all of that data on NACA aifoils is for clean, laminar flow. Last time I checked, there is no NACA airfoil designed to operate behind a bicycle wheel and bicycle fork. For Lance, who rides unbelievably fast, it may make sense for a lot of situations. But to buy a bike because of it, doesn’t make a lot of sense. Aero frames certainly work in the zero-crosswind environment of a velodrome, but on the Queen K? Perhaps not… Peter Reid won the race last year on a nice round tube Specialized. Round tubes can be turned for better comfort and vertical compliance than an aero frame can. And with the realistic gains from aero tubing being less than what many people believe, don’t shy away from a nice round tubed bike like the Storck instead of an “aero” tubed bike like a Cervelo.
I am sure that I will get all kinds of angry retorts quoting John Cobb’s tests and stuff like that. But much as I like John Cobb and respect him as a fellow engineer, he is not without error. I would like to point of the Oval Concepts Jetstream fork. How many stages were won on that thing? My guess is that the fundamentals of aerodynamics that went into designing the fork were sound, just like the fundamentals of designing an aero tubed bike initially seem sound. John is a brilliant guy. However, the reality is that it doesn’t measure up. Just like in reality, an aero frame isn’t really as aero as you might think.
Aerodynamics on a bicycles is all about air management of the whole system. That is why the new Zipp 808 works. They realized that designing an aero wheel was no good if you didn’t take into account what happens when you stick a tire on it. A simple thing, but they were the first to do it. I wish they would give more data on drag profiles, especially with different tires, but they address the real system of a bicycle, rather than trying to apply aerospace technology and claim it works on a bike. Hed does this as well on the H3. The “spokes” on the H3 are not airfoil shaped (they are symmetric - the leading edge is no thicker than the trailing edge) like they are on the Corima 4spoke or the XLab wheels, but the H3 is faster. I wonder if they tried an airfoil spoke, but then shied away because it stalled out in high crosswind. Either way, nobody complains that the H3 should have airfoil shaped spokes.
An interesting argument, and proof that technology is often being pushed because of image rather than results is shown on the Velomax website. Velomax answers the questions “Why doesn’t Velomax use bladed spokes” here (velomax.com - This website is for sale! - velomax velo max Resources and Information.). The answer they give is:
Simply stated, a profiled spoke in a static or head-on air stream can be very effective. However, a profiled spoke in a side wind is a propeller.
A round spoke always presents the same small profile to the air stream, whether the angle of attack is head on or from the side. When you consider the wind direction, speed of the bicycle, and rotational speed of the wheel, it becomes apparent that there is an alphabet soup of aerodynamic considerations happening throughout the course of the ride. For this reason, we believe that swaged (butted) spokes present the best real-world aerodynamic profile.
However, if you look at their deep-V wheels, the Tempests, velomax.com - This website is for sale! - velomax velo max Resources and Information..
It says “Spokes - Elliptical aero profile stainless…” What is going on? Didn’t they just tell me that aero spokes weren’t worth it? My guess, people weren’t willing to buy an aero wheel without “aero” spokes, so tthey were forced to succumb to market demands, even though they know the technology doesn’t support it. Fortunately, they don’t give a little blurb saying the spokes help, but I was still upset to see them on there.
Hed gives us data on all their wheels for a variety of real world scenarios (0-30deg). Shouldn’t the frame makers do the same? 30 degrees of wind angle is a realistic number, shouldn’t the guys at Cervelo admit that the bike is less aero in that range?
Another minor complaint against Cervelo is that the airfoil section that is shown is always cut perpendicular to the leading edge of the tube. However, the downtube of the bike is never put at a 90deg angle to the wind, so the cross-section should be cut parrallel to the wind approach angle, which it is not, which means that section is no longer the same airfoil that they showed you. Kudos to Vroomen & White for not just making an airfoil-looking tube like a lot of guys do. But, guys, if you are going to make a claim, make it 100% true. How much harder would it be to make downtubes that had a proper airfoil section when cut at an angle of approximately 45 degrees (or whatever the downtube angle is)?
To say something nice about Cervelo, I will say the P3’s rear wheel cutout is an example of rear aero, since it is clearly shielding the leading edge of the wheel. And the rear entry dropouts allow you to maximize that benefit. So clearly, they are thinking men. Which is why I wish they would own up with all the data, instead of just the stuff that makes them look good.
Anyway, hope this generates some good discussion, and sheds some light on why some of the German uberengineers continue to build round tubed tri bikes instead of electing to build “aero” tubed ones.