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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Mike,Thanks. And I bet those Fulcrums are pretty good, but the rub is of course that they can't say how good because they'd rather sell carbon. That dynamic is one of the biggest obstacles to alloy wheels getting the consideration they deserve.Dave


If I were only allowed to read bike wheel aerodynamics info from one source it would be yours- thank you for helping me overcome my bike wheel aero research OCD tendencies. My latest build is a set of KINLIN xr31 t rimmed wheels. Since I was trying to use hubs I already had, I used an old MAVIC 28 hole front, which has front hub flanges spaced about 15mm closer together than on most hubs. When you look at the wheel from the front, it has noticeably less frontal area, which I surmise might offset the higher than I needed spoke count in terms of aero drag. As far as mid-depth AL clinchers and aerodynamics are concerned, I think a wheel/rim completely under the radar is the FULCRUM Racing Quattro LG which is 35mm deep on the front, 24mm wide, and has 16 bladed spokes. I've yet to see any test results of that wheel from Campy/Fulcrum or elsewhere, but I suspect it might be pretty awesome.


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