More aero wheel wisdom: the nature of the question

By day, I work at a high-end bicycle shop that happens to be two blocks from the bustling Pike Place Market, Seattle’s cruise ship terminals, and the neighborhood meth-dealers. Needless to say, we get a WIDE range of people coming into the shop. Aside from people I have to forcibly remove after they surrender my merchandise, a lot of people regardless of cycling experience ask me questions like, “Which wheels are the fastest?”

Well, different companies have different answers to that question…strangely enough their answer is usually their own wheel. So are they bluffing, because they can’t all have the fastest, right? The truth is that it depends a lot on how you ask the question.

Firstly, what do you mean by “fastest”? Since a bike wheel doesn’t move by itself, we can narrow down the search to “a wheel that allows a rider to achieve the fastest speed.” Which rider? And by “speed”, do we mean terminal velocity or an average speed? If it’s an average speed, over how long of a distance or duration?

So far we have some pretty basic but not inconsequential variables. For the sake of argument, let’s assume it’s a 180cm, 70kg elite rider on a reasonably flat 25km time trial course. Now we are getting close to what most manufacturers consider to be the important question, but we still haven’t taken wind into the equation. At this point these companies might be thinking wind tunnels.

Now you have to decide where the wind will hit the rider. If the wind is always still or head-on (0 degrees), that simplifies the question quite a bit. Many companies stop asking the question there. However, realistically the wind will be more of a factor. If the wind is at 90 degrees to the rider (directly perpendicular to the rider’s travel), then there will be an apparent wind angle somewhere between 0 and 90 degrees. That exact angle depends on the angle of the wind to the rider’s travel path, the velocity of the wind, and the rider’s velocity. So in designing the question, you’ll have to make some assumptions about the value of these variables.

Next you have to construct a testing rig for the wind tunnel. You’ll need to make sure the wheel is spinning at least. You’ll probably want to turn the wheel relative to the air flow so as to test at various wind angles (yaw). You aim to measure force on the wheel opposite the travel vector; this is expressed as drag in units of force (or gram equivalents). The effort for the rider to overcome this drag for a given unit of time is usually expressed as Watts. So less grams of drag means the rider saves Watts.

Now, are you measuring how much power is required to rotate the wheel or did you make an informed assumption as to how much that contributes? Is the wheel being held in the wind tunnel by a test rig or by a actual fork, maybe with a frame? Does it matter? Could one wheel be fast by itself but actually yield a higher drag number in a complete bike? Under what circumstances does either condition exist? What about the position of the wheel, front or rear? Is the airflow velocity even at the top and bottom of the test rig, and is that an accurate model for real world conditions? The questions go on and on.

All in all, it’s pretty complicated. The companies that really know their stuff are throwing down a lot of money to find an answer. It’s not all smoke and mirrors…aero wheels represent a real, bolt-on speed advantage for riding speeds over 17-20mph. Which wheel is the fastest is more difficult. But surely if an aero wheel manufacturer isn’t putting a lot into the question, they can’t expect to have the answer.

When people seriously ask me what aero wheel to buy, I take into consideration many factors such as the rider’s intended use, the manufacturer’s reputation and service, and the product’s price, service history, and ease of maintenance. Like performance tests, buyers should ask their question very carefully before choosing. Without realizing it, most people have more than just performance as a variable in their decision.



7 Comments

Another interesting aspect to this is the fork. Apparently Oval Concepts has an airfoil-shaped fork that sucks air from the wheel area, producing less air resistance to the spokes. Just saw today on VeloNews that Ridley has licensed the fork for their new Dean TT bike.

“the questions go on and on”...You got that right.  so much so that we’re giving serious consideration to building our own wind tunnel. 

We’ll give you some shirts to wear when in that wind tunnel!

Umm ... whoever has the fastest legs?

Legs is right, but also when you experience “lift” with a set of aero wheels or the flywheel affect, you’ll know what a rush that is and the speed you can carry. For [Roleurs](http://bikehugger.com/2007/11/jet_60_c2s_a_roleur_wheelset.htm), like me, nothing more satisfying than getting on top of the gear, and feeling that bike just roll fast.

@Brett

I was thinking about the Oval fork when I wrote this article.  The fork works by reducing air pressure on the rear-top quarter of the front wheel, but I’ve heard it suggested that the fork design benefits non-aero wheels more since a good aero wheel drag is less to begin with.  In other words, the fork will benefit both types of wheel, but it has more that it can improve upon with a conventional wheel.

So my question with Ridley/Oval’s claims is whether they are measure with conventional wheels or a really good aero wheel.

A similar thing occurs when companies claim a huge reduction in rolling resistance for tubeless road tires.  Generally, rolling resistance is measured with a large steel drum testing rig.  It works well for testing because it eliminates a large number of uncontrolled variables that you couldn’t account for in something like a hill roll-out. 

However, anyone who has used rollers for indoor training knows that a cylinder greatly magnifies rolling resistance compared to a flat surface. 

Before I jump on the tubeless bandwagon, I want to know how they got their numbers.

I’m as skeptical of a fork slot as “dimples” on wheels and tires. Or even that recycling exhaust to power a turbo makes an engine more efficient like Saab claims. Oh wait, that’s how turbos work.

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