Wind Tunnel Tested: Five Aero Wheelsets

Published January 30, 2013 03:47PM

We vetted five premier aero wheelsets in the wind tunnel and on the road to help you find the best.

This article was originally published in the Nov/Dec 2012 issue of Inside Triathlon magazine.

When choosing a set of race wheels, the decision often comes down to the one characteristic that is tough to determine: Which set is fastest? Weight is easy to measure, and weight influences tri cycling speed a little. Braking performance and ride quality can be experienced and also have a marginal impact on a bike split. But time-trial speed comes down to aerodynamic performance. We tested five pairs of the very best deep-section clincher aero wheels at the wind tunnel at Faster in Scottsdale, Ariz. Then we took them to the roads to give you the full picture and help you pick the best wheels for you.

Test Protocol

Even wind tunnel testing isn’t perfect. These limitations could possibly create a difference in the drag measured in the tunnel and the resistance when racing these wheels on the road.

A moving rider experiences wind differently than a stationary object. Rider speed and direction change the wind speed and angle he experiences. Faster riders experience lower yaw angles than slower riders. Below is a yaw and wind speed calculation for a cyclist riding 20 mph into a 10 mph wind angled 45 degrees from his direction of motion.

Wouldn’t it be nice if wind tunnel drag charts always created a clear, distinct ranking? That isn’t the case in many tests (this one included), so we calculated wind conditions to estimate “real life” in a triathlon to decipher which wheelset could be fastest based on our test results.

Wind speed impacts the wind angle experienced by the rider, which is a product of both rider speed and wind velocity, so we looked at weather data to estimate typical wind speeds experienced in races across the country. We found that 8.1 mph was the average wind speed based on data collected in 49 major cities from Seattle to Miami. Ideally, an athlete would measure real wind speed on each individual race course and run a calculation to find the drag he expects to face during a race, then pick accordingly from his stockpile of race wheels. This is, of course, not practical for most athletes, so we used this approximation.

Yaw angle also changes with rider speed (faster rider, shallower yaw; faster wind, wider yaw), so we calculated the fraction of time a rider would spend in various yaw angle ranges when riding at both 20 and 25 mph to estimate total drag difference among the five wheelsets that a cyclist averaging 22.5 mph would experience in a typical ride:

Grams of drag predicted
Mad Fiber Clincher: 796 grams
Enve SES 6.7: 766 grams
Hed Jet 6/9: 766 grams
Bontrager Aeolus D3 7: 757 grams
Zipp 404/808: 757 grams

What do these numbers add up to on the race course? Faster’s wind tunnel engineer Jay White uses a mathematical calculation to equate 100 grams of drag to 1 second per kilometer, regardless of rider speed. That equation creates the following time differences over the 112-mile Ironman bike leg:

Estimated time saved
Mad Fiber Clincher: 0
Enve SES 6.7: 54 seconds
Hed Jet 6/9: 54 seconds
Bontrager Aeolus D3 7: 70 seconds
Zipp 404/808: 70 seconds

Based on the aero data, faster riders might lean toward the Zipp wheels because they out-performed the others at shallow yaw angles, while slower cyclists could look at the Bontrager wheels for a little extra speed. Those were the top performers at broad yaw angles typically experienced by riders averaging lower speeds. Visit Insidetriathlon.com/dragdata to see all the test data and calculations used for this estimation.

ROAD TEST

Aero testing is great, but bike races aren’t a math equation. We road tested these five contenders to gauge their ability to translate wind tunnel performance to the real world.

Braking: Stopping power left a little to be desired. Instead of braking force slowly increasing as the lever is pulled, the Aeolus 7 wheels seemed to “slip a bit” through the pads, reported a tester, until he fully clamped down on the wheels. Full-force stopping power was good, however.

Cornering and acceleration: There is a tiny bit of sway in the wheels when diving into a corner or during a dead sprint, but this subtle flex is imperceptible under more moderate riding conditions. The wheels’ ability to swallow road roughness inspired the confidence to stay in the aerobars through swooping corners.

Road feel: Road vibration is almost non-existent. Whether riding on chopped pavement or fresh asphalt, testers reported the wheels “swallowed roughness and felt glassy smooth,” earning praise as the smoothest-riding wheelset in the test. The wheels’ ability to dampen vibration could be a major advantage over long rides.

Overall: Fantastically smooth on the road, and outstanding performance in the tunnel, effectively tying Zipp for the best results.

Roll it: For responsiveness. Slowing, starting, cornering, these wheels do it all

Braking: The Enve SES 6.7 wheels provide true stopping control, not just braking power. Under full stopping power, they feel similar to the others, but their ability to gradually ratchet up the stopping power instead of slipping and skipping sets the SES 6.7 wheels’ braking performance apart from the other composite wheels. This wheelset still doesn’t measure up to the performance of the aluminum brake track on the Hed Jets, however.

Cornering and acceleration: Testers found these wheels “noticeably stiffer than the Bontrager and Zipp wheels,” and they feel like they “jump back up to speed more aggressively.” The wheels respond to a strong out-of-the-saddle sprint and create the sensation of a definite connection with the road.

Road feel: Although the 6.7s are still far from rough, these wheels transmit more vibration than some of the others tested. The difference is subtle, but their impressive lateral stiffness translates slightly to vertical stiffness as well. The broad tire bed keeps the ride “smoother than [standard] training wheels,” said a tester. Handling in wind was outstanding. Whether contending with steady winds or blustery conditions, the Enve wheels felt “steady and predictable,” reported a tester.

Overall: The most responsive ride—going or slowing. Perfect race set or for big training rides.

Braking: Stopping performance falls in the middle of the wheels tested. Riders found modulation to be good (for a carbon wheel), but the Mad Fibers fall behind a bit when trying to burn speed off quickly to descend as fast as possible.

Cornering and acceleration: Start rocking the bike back and forth on a steep climb or short sprint, and the Mad Fiber wheelset comes to life. “The wheels never bend no matter how hard they’re pushed,” said a tester. “Super stiff” is how another described the ride feel. Every tester agreed these wheels are a thrill to ride thanks to their ability to accelerate instantaneously, and their impressively slim weight helped contribute to the responsive feeling.

Road feel: As the only wheels in this review with a narrow brake track, Mad Fiber’s clinchers have a ride smoothness hurdle to overcome. The wheels are not as buttery as the Bontrager, but we did a bonus back-to-back ride test comparing them to typical spoked clincher training wheels and found the Mad Fibers to be noticeably smoother. Testers felt a little less stable riding these wheels in the wind than the others, especially when gusts were irregular.

Overall: Struggled over shallower yaw angles in the tunnel, but excelled at wide angles. Thrilling acceleration.

Braking performance: It’s not even close: Hed’s Jet wheels brake better than the others. Really, it isn’t a fair fight. These wheels boast the only aluminum brake surface, which creates predictable stopping modulation, more power under emergency braking and dependable friction in rain. The braking performance of the carbon wheels in this review is a testament to the improvement in composite stopping performance, but they still can’t match alloy. Hed is the clear winner here.

Cornering and acceleration: Simplistic construction keeps the Jet wheels’ price well below the rest, but it also impacts the weight. They tip the scales substantially more than the other four sets, and that difference is perceptible on the road. Riding the Jets feels less responsive when sprinting up to speed, but their cornering is outstanding. Exceptional brake performance inspired testers to take “more aggressive lines through technical corners.”

Road feel: Ride testers repeatedly used the word “plush” to describe the feeling of riding the Jets over semi-rough pavement. Every other wheel in this review has spokes anchored directly to the deep-section rim, but the Jets’ spokes pass through the flimsy carbon fairing and lace in a shallow box rim. Crosswinds seem to push only against the wheels and not create any steering input by twisting the front wheel.

Overall: If they’re going to live on your bike, Hed’s Jets win out for braking and overall control combined with aero performance beyond their price tag.

Keep looking: If you want aluminum-rim braking, but really, these wheels have no ride-quality shortcomings

Braking performance: Another pair displaying impressive stopping performance for carbon rim wheels, Zipp’s Firecrest wheels boast the testers’ second-ranked brake feel among the all-carbon wheels. They gradually increase stopping power and gave one tester complete and unexpected confidence during a tight-packed group ride.

Cornering and acceleration: With the ability to spin up to speed, smoothly track through a corner and “stay stiff and solid while descending aggressively,” Zipp’s carbon clinchers won praise from testers for their all-around performance. Although they didn’t dominate any of the test conditions, these wheels excelled across the board.

Road feel: Steady crosswinds seemed to roll right off Zipp’s Firecrest rim shape. The wheels tracked perfectly through moderate winds and were still stable in any conditions faced during the test. With the exception of the Heds and Bontragers, these wheels dampened vibration better than the rest, creating a smooth yet responsive connection with the pavement.

Simply because more and more triathletes are racing on them. Now that clincher tire quality and wheel options have caught up to their sew-up cousins, the reasons to opt for tubulars have dwindled. In addition to the logistical benefit of riding clinchers (easier tire changes, cheaper replacements), rolling resistance and aerodynamic tests have shown they are not slower than tubulars, although they are heavier (and maybe a little less fun to ride). And if you want to enjoy your expensive set of race wheels more than a few times a year, any of these wheelsets is more than capable of training-day duty.

Why, in the era of the iPhone, do bike companies still test and refine their designs in the wind tunnel instead of on a computer screen? Because calculating airflow is insanely complex. Engineers do use computational fluid dynamics (CFD) software to simulate wind drag, but one of these programs would have to track the effect of every individual air molecule on every other molecule to create perfectly accurate simulations. The computing power needed to make that kind of a calculation simply doesn’t exist. So instead, most CFD programs estimate drag by breaking the subject into regions instead of looking at every individual molecule. Even these semi-rough calculations require incredible computing power, and processing each condition can often take hours. These difficulties are why bike and aerospace companies still rely heavily on old-fashioned wind tunnels, even if they incorporate CFD into the process.