posted Oct 25, 2010, 5:00 AM by Donald Vescio
updated Sep 12, 2011, 8:11 AM
Recently, a couple of riders asked me as to whether a Hed 3 or a Hed Jet 90 would be faster for a front wheel. Both are very fast wheels; either would do well. In my response to him, I tried to highlight some of the aerodynamic considerations that should inform wheel choice. What followed as an interesting conversation that touched upon several aerodynamic considerations that should factor into front wheel selection, namely, frontal surface area and yaw.
If you match your Hed 3 with a 19-20mm tire, you'll find that it's drag consistently drops as yaw increases. The only time that a deep rim, such as a Stinger of 808, for instance, will out perform a Hed 3 is during a limited range of yaws, say in the 7-12 degree range. Now, if you're really quick, you'll experience yaw angles less than this range, so the Hed 3 should be a better choice; similarly, if you experience yaw angles greater (you tend to race in gales, or you normally find yourself in the back 1/3 of results), then the Hed 3 is a great choice, as it won't stall like a deep V wheel once yaw trends beyond the deep V's performance window. In other words, there are some wheels that will perform better under a narrow range of yaws, but the Hed 3 is a good performer across almost any yaw angle one might encounter.
The Jets are very strong wheels; their fairing is light,but it appears to be durable. The Hed 3, on the other hand, is bombproof--I've a rear Hed 3 that I've used as a training wheel for 15 years, with no problems.
The other issue to factor in is that today's Jet's are based on a wide rim design, which means that you have to use a wide tire. This may drop rolling resistance from an equivalent narrow tire, but it increases frontal area, which isn't necessarily a good thing.
A further discussion ensured as to whether the new generation of toroidal shaped wheels with wide rims (Zipp, Hed) are intrinsically faster than a more traditional deep-V construction. The answer to this question in part depends on the tire-rim interface: Wide rims require wide tires, or they'll lose the benefits of their aerodynamic design, while a narrow rim wheel, like the Hed 3, requires the use of a narrow tire.
The point is not whether a 19mm tire on a toroidal rim would be faster or slower than a 19mm on a narrow V-rim--the issue really is two-fold: one needs to match tire and rim width; a narrow overall profile will be much faster than a wider profile, at least from a frontal area perspective. The issue that I have with the toroidal rims currently available is that they are optimized for a wide tire, in the 23mm range. Putting a good 19mm tire on the newest Zipp, for instance, would not be ideal; same for the newer Heds, too. In general, wheels with a narrow frontal profile should test faster than wheels with a wider frontal profile, especially at low yaw angles.
I don't believe that most published testing results actually factor in how well the tire matches with the rim/wheel interface. A good 19mm tire matched to a Hed 3, for instance, will test much faster than a 23mm on the same wheel; the same is true for the Planet X 101/Blackwell. I've found that the new Easton 90mm with a 20mm tire to be extremely effective, more so than what I found with some deeper wheels--but then, I don't normally encounter much in the way of high yaw angles. Now, whether a narrow toroidal rime that optimized for a narrow tire will perform better than a wide toroidal rim mounted with a wide tire is an entirely different issue--in this instance, assuming that rim shapes are optimized for both wheels, the narrower profile version will be faster at low yaw angles, and should be at least as fast at higher yaw angles, as the wider profile design.