aero wheel wisdom36
by Mark V on Mar 01, 2007 at 11:43 AM
I’d like to share just a few thoughts about wheels…because too few of you are asking me. People get the most nonsensical ideas about high performance wheels, and I can only surmise that this results from hearsay, undigested marketing propaganda, and a weak understanding of science. So here follows some general guidelines for choosing high performace aero wheels…
1) The low-profile “aero” rim- if the rim is anything less than 30mm deep, it ain’t all that “aero”. With a tire on the 25mm deep rim, the cross section is at best 48mm deep and 20-23mm wide. That’s just too blunt a shape to smooth out the rupture in the air made by the tire’s leading edge. Also, low profile rims usually require more spokes, which also churn up the air. Why then do so many manufacturers make short, “teardrop”-shaped rims? Because the triangular cross-section is an efficient structure for a spoke-tensioned hoop…and they look cool. Are these rims any more aerodynamic than a box-section like a Mavic Open Pro? Probably…but probably not by a meaningful margin.
2) Front vs rear wheels- Surely you’ve noticed in time trials that most Pro-Tour riders use a carbon disc wheel on the back and a really deep section (or maybe carbon tri-sopke) aero wheel up front. You’d be right to think that a disc is the more aerodynamic wheel but wrong to think a disc was choosen as the rear wheel because the rear position is aerodynamically more important. Not true. Time trialists on the road rarely use a disc up front because a disc’s large side area makes it too sensitive to crosswinds. A disc in the rear position won’t affect steering like it would in the front. But here’s a fact that most people don’t understand: the front is the more important position. The front wheel is your leading edge into the undisturbed airstream; the rear wheel lives in the “dirty” air churned up by the front wheel, fork, frame, and your very unaerodynamic, spinning legs. The most aerodynamic wheel in the world can only do so much to smooth that airflow. What does this mean? Well, unless Zipp is sponsoring you, you likely have a limited budget for time trial wheels. You want bang-for-the-buck? Just buy a front with a deep-profile (like 50mm or more). But you ask, “What do I do if I’m a lightweight rider and there are strong crosswinds?” Well, Skippy, I guess you’re gonna have to suck it up. If it’s survival conditions on the course, just run a standard wheel up front. If you can ask me if you should buy a moderate-profile wheelset for high winds and a deep-profile set for good conditions, then I guess Zipp is sponsoring you…
3) “climbing wheels”- If you want to know if a set of chi-chi “climbing” wheels is right for you, ask yourself these questions: Is the joy of a smooth riding bike more important than a competitive edge? If you do compete, do you spend the majority of the race going uphill at less than 17mph? Does your bike spend a significant amount of time on a gram scale? If you answered “yes” to all of these questions, then go out and buy those climbing wheels now! The facts are that a real aerodynamic advantage will beat a weight advantage at any speed above a crawl but a deep-profile aero wheel is going to ride harsher. Climbing wheels are also for the vanity of people who need the lightest bike their money can buy, and personally I am so over gram-counters. Yesterday at work, some tosser interrogated me about the weights of the inner tubes we had in stock. You can go ahead with your delusions that saving 15 grams on an inner tube would have you flying like Pantani on the cols, but buy yourself a gram scale and leave me out of it. (Besides, Pantani would have never gotten rid of a few grams.)
Is light weight a good thing? Yes. Is it better than aerodynamics? Usually not. Do I have to choose between weight and aerodynamics? With the latest carbon deep-profile wheels on the market, no…you can have both…you just won’t have money left to buy food or pay rent.
Everything I just said has to do with performance with the merest mention of cost effectiveness and ease of use, but not aesthetics, service-ability, durability or any of those others issues. You want to tell me how your best friend heard from some pro that such and such brand wheel is like…the fastest? Oh yeah, anecdotal evidence has so much value. You want to know exactly which wheel is the fastest? Hire some scientists, build some test fixutres, and buy some windtunnnel time…that’s an expensive question, and one that becomes very specific to conditions of the race course. But stray not far from my wisdom and you will do no wrong.
…looks like I picked the wrong week to stop sniffing tubular glue.