Below are a series of thoughts regarding tire width and Zipp's 808 Firecrest wheels. These posts took place over a series of days in another forum, so I thought that I might be useful to consolidate them on a single page.
I had a hard time getting a good shot of what a 21mm Vittoria tub looks like when mounted on a Zipp 808 Firecrest. This is a very fast configuration, and the 21mm tire does offer gains over a 23mm or wider; it's also the tire that Zipp recommends as the most aerodynamic configuration. I've run a series of field tests with my PM and agree that the 21mm is a bit faster than the 23. The mounted tire does look odd on such a beefy rim. Flatting as 21mm tub on the rim might result in some damage to the wheel. I'd think that flatting a 21mm clincher would result in issues (on the clincher version, of course).
According to Zipp, there isn't much of a difference between a 19mm and 21mm on the Firecrest 808; there's less of a penalty when using 23s on a Firecrest and a pre-Firecrest 808. Thought that I'd share this photo for those chasing seconds. The lighting isn't great, but you can get a good sense of what the rim's profile looks like on the right side of the image.
Click on the images below for a larger view.
This definitely is not the wheel for low yaw angles. I'll bring this combination to my next tunnel trip end of September, but head-on, it sure looks ugly. Around 10 degrees, though, things look pretty good. For low yaws, I still very much prefer a narrower system.
Still building my data set and want to correlate to results from my tunnel session scheduled for later this fall. Low yaw angles = <6 degrees or so; I'm seeing ~5-7 watts difference. [The tire looks small on the rim] It does--and the thickness of the rim's outer lip is significant, too, even for a tub.
Interesting stuff-above, I said that I'm seeing approximately 5w loss with the 808/21mm tire as compared to my Hed3/19mm tire in very low yaw conditions. I don't have an older 808 to test, but I suspect that the old 808 and Firecrest 808, both mounted with 21mm tires, will perform very, very similar. It's when one uses a 23mm tire that the deltas begin to grow between the two different versions of the 808. Again, I've measured the wheels (one of my spouse's clients just took delivery on a set of non-Firecrest 808s), but I haven't ridden/tested the non-Firecrest myself.
Yes, faster riders do experience lower yaw angles than slower riders, assuming similar conditions. Most riders won't experience very low yaw angles, except under rather limited circumstances--a still day and a course that trends in the same direction, for instance. For me (I tend to lump myself into the category of relatively fast riders), I see yaws between 7 and 12 degrees, when averaged across a season. During our peak racing season--July, August, there tends to be little wind, so yaws drop. Spring and autumn, on the other hand, tend to be very windy. But given all of this, I normally plan on 7 - 12 degrees for yaw; this assumption, then, leads me to using a Cervelo or QR CD 0.1 for most of my races (very good low yaw bikes). For still days, I'll use a very narrow front wheel, like a Planet X 101 with a 19mm tire, or a Hed 3, again with 19mm tire. For very windy days, I'll use the Hed 3 again. This season, for the 7-12 degree yaw days, I'll used a Stinger 9 (or now the 808 Firecrest). I suspect that most riders will experience yaws in the 10-15 degree ranges and should prepare accordingly.
The Hed 3 is an interesting design. It does better than most at low and high yaw angles, as the wheel never really stalls. Deep rims, on the other hand, are optimized for likely yaw conditions; they will perform better than the Hed 3 in their specific design ranges, and the deltas between the Hed 3 and a specific wheel's design parameters can be significant. All of this said, if you could have only one front wheel, and you'll be using it in all sorts of conditions, then the Hed 3 is a good, safe choice.
My general principle is that when yaw will be very low, go with a narrow configuration. Higher yaws make the absolute frontal area of a wheel less important, so one can get away with wider configurations.
All of this used to be so simple--just throw on a Shamal on the front and have at it.
[Problem mounting a tub on another brand's aero rim] I'm not seeing this on my Planet X wheel; what I do, though, is roll the tire/rim along a broomstick handle to ensure adequate contact of the tire across the entire rim's surface after I mount the tire. I've done this with 19mm and 21mm tires.
I remember these items--thank you for reposting. I especially like:
"As stated before, I do not claim to have any final answers with respect to optimal wheel or tire design/width. Rather, my purpose in sharing the results of these experiments is to simply encourage people to carefully and quantitatively consider various factors when attempting to determine the wheels/tires that are best for them and the conditions under which they expect to compete."
For me, I don't look at any piece of equipment as an independent entity; rather, I find it's better to look as equipment in terms of their aggregate overall effect on performance (this is what you're doing in the references above, too). In a lot of ways, it was much easier in the early 90s, as there were fewer available options, and what options that were available had very simple design principles. (In the early 90s, I'd run a front disc most of the time--but then, I was allowed to run a 24" front wheel, too.) Today's marketplace with conflicting claims and design principles make assessment of performance values difficult, unless one spends a lot of time and/or money.
All of this said, I believe that (as a general statement) narrow is better than wide for TTs, and wide wheels were developed in response to a market that demanded the use of wide tires. Again, interesting stuff.