Data? I don't see that in terms of a back wheel.

In terms of front? If you take crash data into account then maybe a 808 or Enve 9 is faster up there (I have no crash data). My understanding based on 2012 olympics is that you can still run a disk up front for road TT…..although very few did.

Maurice

Here's a Tour test that shows a Hed Jet C2 disc is faster (lower watts) than a Zipp 808 (although the test/wheels are 2011...)

http://www.tour-magazin.de/...011/page92.html#/106

Some deep wheels will test slightly faster than a disc at low yaw, but across a range of yaw angles a disc is much faster.




Disc are pretty close to one another, though some testing has shown that lenticular discs do marginally better than flat ones on bike w/o a seattube (ex. Diamond) while flat discs do better when drafting the seattube.

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I know that there have been isolated examples of deep wheels being faster than discs. On certain frames. In certain tests.
Tour magazine's test where the Canyon Speedmax tested faster with an 808 than with a super 9 is the only one I can think of.

You do wonder how bike or yaw specific these claims are. Though I must say I seem to be going ok on Swiss side deep sections compared to previous Seasons on a disc.

But, the question is, if you're at the pointy edge of the spear, how much time realistically will you be at over 15 deg. yaw in a race?

Because, in the 0-15 deg. range, the difference between a great spoked wheel and a disc sure looks trivial.

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15 deg yaw is a fairly mild crosswind, even for those at the pointy end (40kph+). To your point, you are not always going to have a pure crosswind, but why not use the fastest wheel?

I agree that deep wheels are not that much slower than a disc, I'm just not inclined to settle for "not that much slower" (one of the reasons I have one of your bentos behind my stem BTW).

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Good points, kind of hard to argue with that ...

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Pro cyclists rarely race in anything over 15 degrees and typical yaw angles are much lower, but you always see them with discs. Were they not faster why would they use them? I've only ever aero tested my disc, but I'd give a deep section a go if I thought it would be faster (it won't).

A few things:

Pro cyclists seem to race very short TTs.
Not many 56 or 112 mile TTs in pro cycling events.
At least to the best of my knowledge.

Discs have always been pretty fast.
But deep wheels that are very fast are a pretty new technical development.
Maybe there is a bit of tradition or inertia at play ... ?

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Length has nothing to do with it as if you save watts in short TT's you save in long ones too...

Agreed.
But structural discs tend to have an unforgiving ride when compared to spoked wheels (even deep ones), at least in my experience. That can be a factor when you're on your bike for many hours.


In the final analysis, not sure why pro cyclists make the choices they do, but it sure looks like the data says that advanced aero spoked wheels are just as fast as discs in the 0-15 deg yaw range.

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I agree that some deep sections are faster than some discs, but not many (confirmed by Simon Smart). Pros (at least the top ones) are all about marginal gains and will ride what gives them an advantage. (which you can see by the amount of H3 and Autobahns out there).

Comfort simply isn't an issue. I rode my last 12hr TT on an Autobahn+Podium 19mm at 140psi and had no issues. Just as comfy as a couple years before when I used a shallow spoked wheel on a 23mm.

1.) Measurement error--are we sure that modern rims/discs are truly faster, or do their error bars overlap? No one ever overlays error bars. This is still a testament to how good modern rims are!
2.) Setup distinctions. Put a 27mm tire on a 19mm rim bed disc and perhaps a modern, wider rim that better manages airflow with a wide tire does minutely better? Discs are getting wider at the rim bed as time progresses, but it still pays dividends to match wheel/tire regardless the setup.
3.) Power to spin is largely ignored in measurements. This is going to make a spoked rim look better in the wind tunnel, but slower in reality. (Advantage of sanity checking WRT to field testing is that it catches these discrepancies, since nothing hides in a field test)
4.) A few spurious measurement doesn't a conclusion make, then again, you really like to be unconventional and contrarian for the sake of your own rationale rather than by solid reasoning (see your backwards nature in terms of road bars on a TT bike...), so I'm wont to provide a counterpoint to your ramblings, lest someone less experience accidently take them seriously.
5.) Who cares about distance? If anything, slow AG'ers by this distinction would benefit MORE from the disc, seeing as there's little differentiation in drag between a good 30mm alloy rim (appropriately matched to the tire) and a disc at zero yaw, but magnitudes more at higher yaws. Yet, any situation that allows twin discs to be used (Track pursuit) is jumped upon universally. Hmmmmmmmm....

Moral of the story? Deep wheels are fast. There is a lot of overlap in drag of various wheels at low yaw, so sometimes there are some odd matches that give unexpected results. Make sure your tires match your wheels. Go worry about other things.

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Wind tunnels don't capture the wattage needed to spin the wheel. JackMott has a spreadsheet on his webpage with these figures and the extra wattage needed to turn a spoked wheel makes them clearly inferior.

Also, check out what Contador rode for wheels in the 59k TT at the Giro. HED Trispoke and Lightweight disc. If those guys are going to piss off their sponsor by riding someone else's wheels, you know they are picking the fastest combo out there.

Dowsett's hour puts that test under question. Of course we don't know exactly how much tunnel time he has spent but I have got an impression that he tested several different revisions of skinsuit so forgetting to test the fastest wheel configuration doesn't sound right.

First time i hear that.

Since I'm an owner of an 808 FC Clincher on a P5, I always worry about a disk being maybe faster. I indeed bought the 808 after the Tour test, which tested the 808 on a speedmax faster than the zipp disk (although I had an OLD speedmax at the time).

But returning to the wattage to spin the wheel, I always thought that modern windtunnel tests test with turning wheels? So why is the spinning not taken into account?

The drag of spinning wheel is measured but not the power to actually spin the wheel.

translational drag is measured, not rotational, unless you have the instrumentation set up to measure it. LWST has some data on biketechreview on that.

Indeed that makes sense. Never thought of that.

(Although it could be measured in theory in measuring the wattage
of the drums driving the wheels)

So can I believe I'm 5 minutes faster on 180 km with a disc compared to my 808 FC Clincher?
Than I'll buy one next winter (if I can afford it :-))

Or spin the wheel up to target speed in free air on the trainer, without any contact on the tyre/flywheel interface. See how many watts to deduct from a target speed, give a rough guide for the back wheel I guess.

What is telling is that all of the hour records you will see will be on double discs, and they are riding pretty low yaw.

True.

But what is the wattage needed to spin a disc? Common sense would say that it is less than the wattage to spin an advanced 20 spoke wheel, but common sense would also say that a spinning disc also generates some rotational aero drag. The question is, how much?

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http://ir.canterbury.ac.nz/.../thesis_fulltext.pdf

some reading

Good reading, thanks.
It looks like discs take considerable power to spin as well, although a bit less than spoked wheels.

Interesting that the drag increase with going with a wider tire is significant. It makes the researcher conclude that there is never a good reason to go to a wider tire.

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