Friday, March 25, 2011

VeloNews vs. Tour Magazine aero bike test: qualitative comparison

Earlier I reported on a Tour magazine windtunnel test of various "aero" mass-start road frames. Here was the plot from that magazine:

Tour magazine windtunnel data

Soon after, VeloNews, in its April edition, followed up with tests of its own on a set of its own "aero" bikes. You can read the article here, courtesy of Cervelo, which fared best in the VeloNews test.

It is curious to compare the two, because the conclusion which are drawn from the two tests are substantially different:

  • Tour used a dummy rider on each bike, using a hinged stem for handlebar placement, while VeloNews tested the bare bike
  • Tour omitted cables, a potential source of variability due to uncertain cutting and/or placement, while VeloNews included cables.
  • Tour omitted handlebar tape, while VeloNews used whatever tape was "stock" with the given bike.
  • Neither test used any waterbottles
  • VeloNews tested bikes with two sets of wheels: stock and Zipp 404s. Tour used a single set of Mavic Cosmic wheels.
  • Tour tested yaw angles of 0, ‒5, ‒10, and ‒20 degrees, while VeloNews tested more angles: ‒25 to +25 in 6.25 degree increments. Thus VeloNews didn't assume symmetry.

However such a test is done, there's going to be complaints that it was done wrong. It's way easier to critique experiments than actually conduct them. This is why I went away from experiments myself and into computer simulation. But I digress...

On the bottle issue: these are mass-start frames and mass-start frames are typically ridden with one or two bottles. The bottle has a big effect, because a bottle on the seattube will disrupt air ahead of the seat tube, while a downtube bottle will disrupt air which might otherwise have cleared an aerodynamically optimized down tube. In fact, the Litespeed Archon C1 was designed with the assumption a bottle would mounted on the downtube, so while it might fare less well than another frame without a bottle or cage, with the bottle in place it may do better. So the bottle-no bottle choice is significant. Best would be to test with both. But given only one test, a single bottle mounted to the downtube with a "typical" carbon cage cage would have been better (unless the frame comes with a specific cage). Two bottles, I feel, would be overkill, since it is quite plausible to race with one bottle, and often at the end of a race at "crunch time" only one bottle remains on the bike. And at the criteriums which constitute most of the races in the United States one bottle is the norm. So given the choice between three bikes, one better without a bottle (assumed overall fastest), one better with a single bottle, and the third best with two bottles (assumed slower than the best with one bottle), I take the one-bottle bike.

Tour clearly gets points for using a dummy. John Cobb showed, for example, that the incremental drag from a seat post is much different with a rider on the bike than without: without a rider, a bladed post may be best, while with a rider on the bike you want a wider post to fill on some of the gap between the rider's legs. One of the most prominent features of an "aero" bike is the seat post, and it's important a rider be on the bike to properly evaluate that. A real human would be best, but of course real humans can't be relied upon to hold a given position from one bike to another. So the dummy is the best solution. Cervelo, for example, specifically created a dummy of Dave Zabriskie, well known for his superb time trial position, for evaluating time trial frames. The principal downside of the dummy is it is, like the cables, another source of variability: the position will not be perfectly matched, and the articulated stem has drag which depends on its angle. Even a 1 mm difference in head and position could shift CdA by as much as 0.0004 m² (approximately 0.1% total wind drag), assuming incremental Cd of around 1 and a 40 cm shoulder width.

Next cables: I understand why Tour left the cables off. Cables move and their length isn't well defined. You could test a bike to high precision, swap out the cable housings, and then get a statistically significant difference. But cables are an important component of wind drag. Cylinders have a coefficient of drag of approximately 1. So, for example, 20 cm of 5 mm cable, perpendicular to the wind, have a CdA of 0.001 m². This compares to a typical CdA of 0.32 m² (taking the Tour result and adding 0.01 m² for a helmet). So this cable can be around 0.3% of total wind drag, closer to 1% of total bike drag (not counting the rider). If every bike had the same cable set-up, omitting cables would make sense, but aero frames usually have internally routed cables, and there are different strategies for cable entry. For example, the recently disclosed Specialized Verge is notable for its sloppy cables surrounding its carefully sculpted head tube. So points to VeloNews for including cables.

If you're going to use bar tape, use the same for each bike, preferably something minimal like Benotto. For example, the VeloNews article specifically notes the fat, padded handlebar tape on the Felt may have hurt its low-yaw performance. As I've discussed, low-yaw is the most common condition for the wind, so this may have a significant affect in how the Felt is evaluated. Handlebar tape is trivial to swap.

Not really addressed in these articles (at least from what I can tell; I don't read much German, although it's pretty easy to decode such words as "windkanaltest") is size selection. Different bikes follow different stack-reach trajectories. If the dummy is perfectly matched to the Cervelo, for example, it is unlikely any of the sizes of the other bikes will have the same stack & reach. So a decision would need to be made: do I pick the bike with the closest match in stack, the closest match in reach, or some combination? Or do I match something different, like effective top tube length or frame "size"? At some point I need to make a decision: which of two relatively similarly matched frames do I choose? Typically a larger frame will have more wind resistance than a smaller one, so this decision affects results.

Then there's yaw angle. Both magazines averaged over their tested yaws. Tour thus had an average yaw magnitude of 8.75 degrees. VeloNews, on the other hand, had an average of 13.9 degrees. I view Tour as more on target here. On the other hand, given the data, readers can make whatever weighting they prefer: no reason to blindly accept the magazine's numbers. Since VeloNews reported more yaw numbers, and reported yaw on both sides of the bike (not just one), points to VeloNews here.

VeloNews gets more points for testing both positive and negative yaw. Ironically, Tour was the one most in need of this, since they used a dummy, which was asymmeric by virtue of having one foot forward and the other back. But even on a naked bike with the drivetrain on one side and not the other, symmetry does not apply. For example, an aerodynamic crankset may help aerodynamics more with wind from one side than another, although none of the tested bikes used aerodynamic cranksets. A small error: when Tour plotted drag versus angle they fit splines through the data. Spline fits are characterized by a continuous derivative, but assumptions need to be made about the end points, for example a zero first derivative or zero second derivative. The fit chosen by Tour had a non-zero first derivative at zero yaw. Were the results symmetric, as assumed, this would result in an unrealistic slope discontinuity at zero yaw. So don't take the spline interpolation too seriously between 0 and 5 degrees of yaw.

Both magazines tested a control "non-aero" bike. Tour tested a Cannondale Ultimate System-6. VeloNews (as reported to WeightWeenies) tested a Masi Competizione with Fulcrum wheels (no Zipps). Both of these bikes are designed with fat tubes for optimal stiffness-to-weight, flaunting aerodynamics.

VeloNews then tested two sets of wheels: stock and Zipp 404 (except on their control bike, which was tested only with Fulcrums). Obviously using a fixed set of wheels is preferred: wheel and frame decisions are typically made independently. But VeloNews provides more data, showing the effect of wheel choice. More data is better, so points to VeloNews.

On the non-aero side, Tour did five bending tests: fork linear stiffness, fork torsional stiffness, fork vertical compliance ("comfort"), frame stiffness, and vertical frame compliance ("comfort"). Tour loves its bending tests. VeloNews did a single bending test: a torsional test emulating pedal loads, and for comfort and handling actually rode the bikes. The Tour test is quantitative, repeatable, and isn't affected by such things has handlebar tape selection (clearly a factor with the Felt, which had thick tape) or rider preference (in the VeloNews test, Ben Delaney Caley Fretz disagreed on the comfort of the Ridley). But then bike handling and comfort is not a linear combination of stiffness numbers: riding the bike is what it's all about, and on any stiffness component, it's been suggested there is an optimal value: more isn't always better. So I like the VeloNews approach here.

My interest is in the aerodynamics, however. So I'll focus on those numbers.

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