Hot Wheels: Choosing Aero Wheels
It’s no big secret that aerodynamic wheels provide a significant benefit when it comes time to hammer on the bike. But what’s up with all the options?
It’s no big secret that aerodynamic wheels provide a significant benefit when it comes time to hammer on the bike. But what’s up with all the options? There seem to be as many variations of bike wheels in the world as there are bike companies. While freedom of choice is great in theory, it doesn’t help you when you can only afford one set. Only a handful of athletes can stock their garage with a quiver of wheels for every type of training and racing. So how do you combine hard science with practical sense when determining the correct wheel set to supplement your collection? Finding the answer, as you see below, is a multi-step process.
Step 1: Know Your Options
We can squeeze most of the world’s wheel options into three categories.
Least-aero: Standard flat or box rims with various patterns and numbers of round metal spokes that are usually very durable and relatively inexpensive.
More-aero: Bladed spoke wheels with aerodynamic rims. The best of these will decrease a person’s 40-kilometer time-trial time on a flat course by roughly 1.5 percent (or 50 to 70 seconds) versus the standard rims above. In the “More-aero” category, there is a range of options. Aerodynamic rims come with different rim depths, and the spokes will range in number and vary from round to bladed. In a strict drag-related sense, the deeper the rims and the more bladed the spoke, the faster the wheel.
Most-aero: The most efficient set-up is a rear disk wheel with a composite-spoke front wheel. This set-up can knock another .5% (10-20 seconds) off a 40k time when matched up against the best aero rimmed and bladed spoke wheels. These are heavier wheels and are the least durable of the three options.
Step 2: Weighty Matters
It all sounds pretty simple so far: Slap on more aerodynamic wheels and you’ll reduce your time. On level roads, even the added weight of a disc wheel is negligible; since reducing the drag force (that is the force it takes to move the wheel through air) by 10 grams can negate an increase in actual weight of 1,000 grams. Problem is, once the road tilts up, weight must be considered, and the arguments over using a disc versus an aerodynamic spoked wheel get heated.
Once speeds drop under eight mph, as they would on a sustained and steep climb, aerodynamic drag contributes minimally to the total force you must overcome through pedaling, but at 25 mph this number jumps to 90 percent of the resisting force. The general speed where aerodynamic drag trumps weight and calls for aerodynamic wheels is 12 mph. And even in courses with sustained climbs, the weight of a good aero wheel set (1500+ grams) is not much heavier than a light-weight wheel set (1100+ grams), and the aerodynamic speed advantages can be significant.
This fact was demonstrated to us at this summer’s U.S. National Cycling Festival in Seven-Springs, Pennsylvania. The time-trial course included significant and steep rolling hills – enough to lead many racers to opt for lighter aero-rimmed, spoked wheel sets instead of heavier disk rear/composite front combinations. Out on the course though, we saw that those weight concerns were unfounded. Sure the climbs brought speeds below the standard 12 mph barrier, but since the high speed descents allowed momentum to be carried into and up the next uphill, the most aerodynamic set-up (disk/composite) resulted in faster times.
Step 3: Speed vs. Durability
But besides weight, there are a few other downsides to the “most-aero” option in racing wheels. One is durability: these wheels are made for racing, not for enduring thousands of miles of training. Additionally, these wheels have an “apparent mass” which makes them harder to get moving. Basically, the more weight found farther from the axle, the more force it takes to make that wheel turn.
Step 4: Race Directors
Another factor to consider is the variety of events you want to compete in. If you’re racing criteriums, time trials, road races, and possibly triathlons, a disc wheel may only be useful – or legal – in a few events each year. On the other hand, a relatively light weight, “more aero” spoked wheel may provide very good performance in the highest percentage of your competitions.
If you’re training and racing on just one set of wheels, a durable aero rim with bladed spokes is your best option. It gives you the aerodynamic advantage of reduced drag and the flexibility to reap the benefits of that advantage in almost any conditions.
Kirk Nordgren is a Coach for Colorado Springs, Colorado-based Carmichael Training Systems. For more information on the latest in training, fitness, and nutrition go to www.trainright.com.