Aerodynamics thoughts

This week, the Al33 and a number of other rims will be tested in the A2 wind tunnel in North Carolina. We aren't paying for it, it isn't our test, but we agitated for it to happen, facilitated all of the arrangements, and designed the test. Without those inputs from us, the test wouldn't have happened. Call our stake sweat equity. We won't be there, but the US distributor will be, and we will be there in virtual.

I'm excited. I love testing, I love the wind tunnel, and I've learned a lot every time we've done it.  

The test will use a 2017 model Zipp Firecrest 303 as a baseline. Tested wheels will be a HED Belgium+, a Kinlin XR31T, the Al33, and a Flo30. All wheels are 20h rims built with CX Rays and standardized hubs, except for the 303 which is of course an 18h wheel which uses Zipp's own hub and CX Sprint spokes. 

The test will be done as a wheel only test. This has plusses and minuses, but it's proven to be an accurate way to test wheels and it allows wheels to be tested in a time efficient manner, which means we can include more wheels.

Test tire will be a Continental GP4000sII in 23c size. There will also be a 25c GP4000 there, how much testing gets done with it depends on time available.

All the quantitative data on each rim/wheel will be presented - depth, weight, inner and outer width, and retail price. 

We'll be doing the standard 20* sweep in 2.5* increments, on one side. Since these are all symmetrical front wheels, doing both sides would take time that would reduce the number of wheels we could test. We will also include steering axis force data as provided by A2. 

Not entirely certain how the data will be presented. My inclination is to show the standard graph like you've all seen 100 times, and then overlay some of the more defensible angle of attack distributions over top of those. It makes the most sense to then use those distributions to create a one number score for each wheel. 

I'd encourage anyone who wants to get the most from this information to become as informed as possible about the benefits and limitations of aerodynamics testing. Tour Magazin is an amazing resource, and you can go to the App Store, download their app, and buy issues for about $3 each. Issue 8 from 2016 is particularly good. Become familiar with the other methods like Chung and Alphamantis. 

A few bullet point thoughts:

1. Any quantitative test will have some strengths and weaknesses, but no wheel can make aerodynamics claims without credible quanification that allows at least some comparison to relevant standards. It shocks me how many brands still try to skirt past with a "trust us, we're fast!" line of bull. If the whole sales proposition for any wheel is that it's fast, yet it shows no data, I think you know what I'd say to that. This principle is why we insisted that the Al33 absolutely needed to be tested. 

2. Depth and speed are not interlocked. We first showed this four years ago when doing the original Rail 52 test, where the 52 proved faster than even the 85mm wheel than we'd been using, and was faster than the deeper Zipp 404 at angles from 0 through 5 degrees. 

3. We're still using the GP4000 in 23c size because that's been the standard, and it's still a VERY widely used tire in situations where aerodynamics are important. Our previous tests showed a reliable pattern that wider tires had a linear and predictable negative effect on outright aerodynamics. 

4. The Zipp 303 gets used "as is" because it's a wheel system, and its value as a baseline is in using it as it's been used in other tests. That allows you to make worthwhile comparisons to the greater universe of what's out there.

5. Have reasonable expectations. In the Tour Magazin test I referenced earlier (seriously, download it), the difference between best and worst was 13 watts. That's 40ish seconds in a 40k TT at 30mph between a Mavic Ksyrium and a 404 and DT Swiss 65, which were the fastest wheels in the test. That's about .4mph, worst to first. Anyone telling you you're going to go 2 or even 1mph faster by just switching to more aero wheels is selling you a load of crap. 

Okay, that's it for now. Looking forward to Thursday. 


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I read a blog post a few months ago about how rounder cross sections seem to be more stable in crosswinds than rims with sharp edges. Do you have any thoughts on this? I don't suppose you could look into it if you have any time left after you complete your planned tests.


Great point about how much speed you can gain with aero wheels. Having said that, I'm curious how much faster you can go at peak speeds where aer matters more. Sayon a descent going 35mph on Ksyriums, would I be going 36mph on Zipp 404 or would that be closer to 38 or 40mph? It may not be relevant for time trials and racing, but for sheer riding exhilaration, I'd be curious to know.

Chris K

Hi Ben – Thank you for using "locate" – that's a perfect word to describe the benefit of these comparative tests.It's going to seem like we show more drag on Flo wheels than Flo does. The very simple explanation for this is that they report with "tare" removed, while we leave it in. The tare value is what you get when you run the fixture through the angle sweep with no wheels in it. Then you subtract that value from each wheel's run when you report. There's debate over whether or not it's the right way to go about things. I think it's not, but so long as people don't fall into the trap of comparing raw CDA, grams of drag, or watts figures from one test to the other (and it seems you're well aware of this) then for the most part no harm done either way.Dave


Great to see this test coming together! I'm glad you're including the Flo30 as those guys have done a lot of testing of their own wheels but without too many outside references so this should also help to locate the rest of their deeper wheels as well.

Ben Gordon

Hi Zed,Good question. Unfortunately, it's one without a clear answer. The older narrow v-shaped rims were considered to handle badly in crosswinds, but accurate measurements of it in the wind tunnel only started in the summer of 2014. At that point, the narrow v-shapes were already long past their peak of popularity. So we haven't tested anything against one since the handling component of the tunnel tests came out. Each rim is different, and any seller making claims needs to be able to back them up with good evidence. Our Rail 52 and 34 performed brilliantly against the Zipp 404 and Enve 3.4 when we tested those in August of 2014. We thought the Rails would do well thanks to some shaping that we used, but we didn't know just how well they would be relative to the other wheels we tested. Both Rail depths and the Enve and Zipp shapes would be called rounded, but they scored differently on that scale – the Rail 52 was better than the Enve 3.4, even though it was 17mm taller. Now Reynolds has a wide rim with the sharpest v point anyone's seen in a long time, and in the Tour Magazin test it proved to be remarkably good at crosswind handling. This is the same with anything aerodynamics related – any claim needs to be backed up with evidence or it's just empty talk.Just testing one random v-shaped rim versus those that we're testing on Thursday wouldn't tell anyone too much, and we're going to struggle to get everything tested in the time we have. Sorry.Dave

Dave Kirkpatrick

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