Fun with Discs

Friday was a beautiful day in the Bay area, and I quickly ran home for a late afternoon ride to try out my new Zipp 900 Disc wheel.
I went out on Foothill Blvd and did some non-scientific time tests. From my house to a known traffic light, I usually take about 10 minutes during my warmup. But that day, it took me 9 minutes. Hmmm, maybe there is something here. On the flats of Foothill Blvd, I felt that maybe I was going 1-2 MPH faster than I normally would on those roads. Or I could have been just dreaming. Certainly I didn’t try to put more effort into my cycling. But it did seem like I was going a bit faster than normal on the same roads without a disc wheel.
And then there was that awesome sound like that of a jet plane flying overhead, but it was my wheel vibrating against the wind. In fact, the wheel magnified my gear shifts as well and it sounded like big ka-chunks every time I shifted.
All this talk about disc wheels is particularly interesting, especially the research on negative drag and aerodynamics. A buddy of mine remarked that at IM WA in certain parts of the course where the wind would hit the disc wheel just right, that he felt that he was shooting forward with the wind just a bit. I went out on the net looking for some literature about it.
Among the references I found, they talked about “tacking against the wind” as like in sailing. The technique is described as basically just leaning into the wind slightly, in whichever direction it’s coming from. Apparently, when the wind is hitting it from the side, it creates lift on the wheel which shoots the wheel and bike forward, similar to how a wing works where if air is accelerated off a surface, it creates force to push the wing forward. Aerodynamics is improved by smoothing out the airflow off the back of the wheel. But disc wheels will create instability problems in high winds and you need to compensate by forcing the bike to stay on course.
The other references I found were that disc wheels, due to their mass, have higher rotational inertia since it is solid versus a spoked wheel which is not. That means it takes longer to get up to speed, but then when you get up to speed, it tends to help you maintain that speed. Thus, you can maintain a higher speed easier, especially on flatter courses.
I can’t wait to try this disc wheel out at Ironman Florida at the end of the year.
If you want to read some of the stuff I read, here are some links:
Help! what’s the aerodynamic difference of the two wheels?
Spokes or Discs?
Aerodynamics – The Wheel Story Crosswind Tacking

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  1. Twain Avatar

    I used to have a j-disc; mylar covered wheel that was very light and very fast. Awesome to ride. Later switched to tubular nimbles; also fast but I hated tubulars. Now I’m trying to decide between another disc or a zipp 808/hed jet 90 for the rear (hed alps in the front). I am leaning toward the hed jet b/c it’s half the cost of the 808, has very similar aero qualities, though is marginally heavier.
    Kraig Willet has some awesome data (you have to pay for it) here:
    – the debate between the specialized/hed trispoke vs the 808 as the most aero wheel
    And more info here
    not totally up to date, but he covers disc wheels and the 808.

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