bike physics on wheels

dazman said:

Hi,

I saw bikephysics.comlistOpen ↗ was linked on another wheel question. I was very surprised about the low aero rating for zipp 404 (hub rating not so surprising). Do you guys believe this - There are so many pro triathletes using these for aero advantages that I have my doubts....should my next wheels by shimano wh540s?

Hi, I looked at the link but I don't see the Zipp listed. I have 303 Zipp wheels. Are you saying they aren't very aero? I know what you mean about seeing them everywhere and also thought they were good.

I'm also surprised by these results, which have a rear Ksyrium ES (the archetypal wheel with no attention paid to aerodynamics!) performing better than a rear Zipp 404!!!! I'm finding this pretty hard to swallow, particularly as the ES did very badly in the roues artisanales testing. John Swanson, any comments?

Hi, everyone. Sorry I didn't pickup on this thread a bit earlier. I'd agree that the results for the rear wheels aren't intuitive and require a bit of analysis. I'll just preface this by saying I wish I hadn't screwed up when I measured the *front* Zipp 404 wheel.

Through my testing, I've noticedsome general trends about the aerodynamics of a wheel. The aerodynamics are affected by the number, shape and size of the spokes, as well as the shape of the rim. It's no surprise that if you have more spokes and larger spokes, the aerodynamics will be kind of awful. Round is also much worse than bladed or aero shape.

When it comes to rear wheels, you must also consider the amount of dish. This places the drive side spokes in behind the rim and creates drag that is different from front wheels.

And lastly, we need to consider that I'm only looking at the rotational drag, not the "translational" drag of the rim moving forwards through the air.

So keeping all those things in mind, let's see if it makes sense that the Zipp 404 and Ksyrium ES wheels have roughly the same rotational drag. The ES has big spokes, but there are only 20 compared to Zipp's 24. The Zipp spokes are also round, if I remember correctly. And since dish also matters, I can see why the results might be close.

The last thing to consider is that the deep Zipp rim will drag along a larger boundary layer than a shorter rim like the Ksyrium. Dragging along more viscous fluid (air) does affect the overall aerodynamics, though I don't know how much.

So overall, I can see why the Zipp 404 and Ksyrium ES may be close in my testing, but not in a wind tunnel, where the translational aerodynamics matters a lot. In fact, that's one limitation of my testing is that it'sprobably only valid to compare results of wheels with similar rim depth. I hope this helps...

John Swanson
www.bikephysics.com

To add to what Señor Swanson has said, a rotating bicycle wheel is one of the hardest things to accurately model aerodynamically. It's not a trivial solution. A lot of things can be assumed, but when it comes down to it, it takes a wind tunnel to get reliable data for conclusions. Theoretical results will vary depending on what turbulence model is chosen and how the wheel is modeled. The spoke root at the rim is particularly problematic: at that location, you can have a quartering air flow, turbulence as a result of boundary layer detachment upstream on the rim, air being dragged partially around the rim by the motion of the spokes (quasi-rotational flow). Then what happens, theoretically, downstream of the spokes is a toss-up.

Even if you've got a nice, straightforward theoretical model, there's generally always surprises in store at a wind tunnel. As much as aircraft of modeled and tested in CFD, modifications are often made at or after the windtunnel in order to improve or change unexpected pressure distributions.

Hi John,

Thanks for the explanation - so maybe a footnote with such info on your site would be useful? Would you also consider catergorising the diffeerent wheels (e.g traditional, semi/mid/deep/disk aero) in the future or adding a coefficient that assumes deeper rims will end up with a better result in the real world? I can understand that actually getting your hands on all these wheels for testing is not a trivial affair also!

....Back to buying some Zipp 303s as my new wheelset rather than shimano traditional wheels....

ScienceIsCool said:

So overall, I can see why the Zipp 404 and Ksyrium ES may be close in my testing, but not in a wind tunnel, where the translational aerodynamics matters a lot. In fact, that's one limitation of my testing is that it'sprobably only valid to compare results of wheels with similar rim depth. I hope this helps...

John Swanson
www.bikephysics.com