Random Thoughts on Wheel Depth

posted Jul 4, 2011, 4:22 PM by Donald Vescio   [ updated Sep 12, 2011, 8:12 AM ]
Recently, I was posting a response to a very good question: "Will I notice a performance difference between a 50mm and a 60mm front wheel?"

There is lots of anxiety about front wheels, mostly because this is one area--after optimizing one's body position--where real aero gains can be made.  

What follows are some random thoughts that I used in framing my conversation:

Hi---you're going to get lots of conflicting advice on this one ;)   There are a lot of variables to consider, apart from rim depth.  The shape/profile of the rim is important, your average speed, and the general prevailing direction of the wind all impact a wheel's performance.  Now, in the most general of terms, I don't think that you'll find a huge performance difference between 50 and 60mm, assuming that the rim shapes roughly are the same.  I've found, again assume similar rim shapes, that 20mm is the step that one needs to take in order to see performance difference. 

I wouldn't worry about body weight--I think that too many people make too big of a deal as to how body weight impacts handling.  I go about 69kg, my wife 47kg; neither of us ever has had any problems running very deep wheels in front, even in exceptionally windy conditions.  (The *shallowest* front wheel that I'll use is 90mm; my wife always uses a  front trispoke, and she's on 650s).  Last week, I ran a Planet X 101, and the wind was pretty gusty).

But for performance--if you're nervous about going with a deep front, then look at a good quality 50-60 wheel and carefully select your tires for rolling resistance and (even more importantly) how well the tire's diameter matches the diameter of the rim.  Ideally you want the the edge of the tire to blend in with the edge of the rim.  Putting a big, wide tire on a narrow rim (say, a 23mm tire on a Planet X or trispoke) will cause a big aerodynamic hit.  In other words, you can have an extremely fast 50-60mm front *if* you carefully optimize your tire selection.

The new generation Zipps and Heds are designed for people who want to run wide tires; these are fast wheels, too, but they do have design compromises.  They are really good at managing the airflow of a wide tire  onto an aero rim, but they still are *wide."  My only exception to my personally 90mm+ rule of thumb is when I'm racing on a dead still day--then, I'll bring out my Shamal, fitted with an 18mm tire.

Too much info--the short answer is, don't worry about 10mm, chances are that you won't cause a discontinuity in time/space if you go deeper, and just take care with the tires that you select.  Good luck--

(Coda: I suspect that one's speed has a greater impact on a front wheel's stability than a rider's body weight--the faster one is, the lower the apparent yaw angle will be, and the less pressure from the side on the wheel.  Of course, frame geometry and body position would trump forward speed.  Just thinking out loud how one who might have 25 kilos on me would have a harder time handling a wheel than someone my weight, or my wife's.  Not casting any criticism or aspersions, mind you!)