Fit Files: Surprising Results From Wind Tunnel Testing
In our latest installment of Fit Files, we get a chance to see one of our own fitting clients in the wind tunnel and review his results. Sam is a longtime client and a 50-year old triathlete standing 6’4″ and 198 lbs. I recently worked with him on the fitting and sale of his new Cervelo P5 and last month he made a trip to the A2 Wind Tunnel in North Carolina. We’ll summarize his results below.
Baseline Fit Werx Position
Sam’s baseline position is how we set him up at the shop. We knew he was going to visit A2 and helped him plan what he was going to test in the wind tunnel. This position allows him to be comfortable and is sustainable for his short course triathlon race distance focus while generating his best power. There are some details that keep him comfortable yet quite aerodynamic for a moderate front end position. Aerobars are angled up slightly to get the right relationship between his hands and forearms for comfort, speed, and easy visibility ahead. We’ve found his gradual back bend when combined with a well designed helmet and tri suit will keep airflow attached past his shoulders down his back. His elbows have plenty of space between them so he can breathe easily and maintain the position while narrowing up his shoulders very well for a rider his size. His hydration system sits nicely between his forearms and he’ll be moving his computer closer to his hands for easy visibility while riding.
Sam’s baseline position resulted in a CdA of 0.272 at zero yaw running his old Rudy Project Wing aero helmet without visor and Castelli tri suit. It honestly surprised both of us how fast it tested given his size. We’d worked with some programs like Best Bike Split to estimate this CdA on his old bike and calculated it to be well over 0.30, so seeing such a big drop in CdA was an exciting start to the session. A rider Sam’s size and speed will save about 19 watts for a 0.01 CdA reduction, for those out there that are move familiar with watts than CdA numbers. A2 also tests every position at 7.5deg of yaw as we’re never riding only at zero yaw, so we kept his yaw data in mind as well.
First changes include helmet visor and body position
Now the A2 staff would put Sam through a number of changes as they seek further aerodynamic gains. We won’t get into all 20 tests completed in his 2 hour session, we’ll focus on the highlights. Sam tested the same Rudy Wing helmet with the visor which reduced his CdA to 0.269. He can now weigh the aero penalty of a few watts against the benefit of more cooling on warmer race days which for bigger riders can easily offset those few watts. Sam tried a number of changes such as tucking his head lower, shrugging his shoulders, and holding his hands higher on the extensions. I spoke to him at length about these changes and while some of them reduced CdA a little they all had shortcomings. The lowering head and squeezing shoulders in eventually bothered his neck, and holding hands higher on extensions was also less comfortable and even caused him to inadvertently shift gears without wanting to. He decided not to implement any of these changes as the small gains weren’t worth the trade-offs.
Aero Helmets
Helmets came next. While A2 has a number of helmets in stock, Sam focused on a few key models that have proven fast in most recent testing. The Giro Aerohead helmet was first and increased CdA slightly to 0.270 compared to the Rudy helmet with visor. It wasn’t too long ago the Giro helmet seemed to beat everything, yet his existing Rudy Wing was faster. He then tested what would be the winner of the day – the new Rudy Project Wingdream helmet. This helmet can be seen on some pro tour teams and has a noticeably wide visor. His CdA dropped to 0.263 with the Wingdream helmet. The best aero helmet tends to be individual, based largely on rider shape and position, but this was clearly the winner for Sam. Here’s a pic with the Rudy Wingdream below:
Aerobar Extension Angle
Adjusting aerobar extension angle was next and this topic must be the top trend brought up during our triathlon bike fittings. Interestingly, angling the extensions up for Sam was slower as CdA climbed to 0.272. Keep in mind his angle is already at 10 degrees so he learned this is enough. Flatter extension angle slowed him down too. This was one of the more surprising results from the tunnel and more evidence to the jury that you don’t know what’s best for you until you test it. Often the extension angle itself isn’t what impacts your drag, but what it causes your shoulders, back, and head to do. The size of the rider, distance from the aerobars to your hips, and nearly every detail of the aerobar setup can impact whether this change will help you.
Final Aerobar Adjustments
Two consecutive changes from here led to the position Sam is sticking with. First the elbow pads moved forward which reduced CdA to 0.261. A later change raised the aerobars 1 cm higher for a CdA of 0.262 but there was a noticeable improvement in comfort resulting in the most comfortable position of the day. We can’t stress how important comfort is in a triathlon position as it can impact everything from holding that position, to power production, to running off the bike, and also efficient fueling and digestion for long course triathletes. Comfort, Power, Aero – choose all 3!
Water Bottle Placement
When I mentioned aerobar angle being the hottest mentioned trend, I seem to have forgotten about water bottle placement. Fortunately the sport no longer allows the contest of who can fit more water bottles down the front of their tri suits, which can act like a fairing breaking up the air in front of your chest and hips to reduce drag. Yet you can still often get a similar effect with water bottle placement on the aerobars. Sam uses the proven horizontal bottle between the forearms setup, but also tested raising the bottle higher as shown in the photo below. Notice how the bottle and computer are now closer to his head using a mount underneath the bottle to hold it higher. This was an interesting test as the CdA at zero yaw was lowest of the day at 0.260, but interestingly it was significantly slower at 7.5deg yaw adding 5x the amount of drag that it saved head on (it added 0.005 to the yaw CdA). These results make sense if you visualize the extra surface area the wind will see coming from the side with the bottle lifted up. We decided this gain wasn’t worth the penalty at yaw as practically speaking it would likely be slower at a range of wind angles on race day.
Trisuit Change
Sam then tried the Ryzon tri suit which he wanted to test against his existing Castelli tri suit and CdA bumped up to .0267 at zero yaw. This is another surprising results of the day as everything about this suit felt fast and it’s proven to test fast for other triathletes. The suit felt super tight without anything moving in the wind, but the numbers proved otherwise. Trisuits and skinsuits continue to be an item where results are very individual. You only know when you test on yourself. Some suits work better tighter to remove any wrinkles or material flapping in wind. On the other hand, a suit that’s too tight can cause the carefully designed surface texture of a suit open up and perform less optimally. I’ve noticed tighter skinsuits make it harder for my shoulders to roll in toward my neck too, so too tight a suit may also make your shoulders wider. Also remember you need to swim in this trisuit. For now Sam will stick with his existing Castelli suit.
Final Thoughts and how do you get watts from CdA?
Sam’s was successful in reducing his CdA from 0.272 down to 0.262 and even felt more comfortable in that final position. He also ruled out a number of changes and trends that he no longer needs to think about. This CdA change from his A2 trip alone will save him save him 19 more watts. We know Sam is going to ride just as hard, so this means at the same power he’ll go about 80 seconds faster in his next Olympic distance tri bike leg from this wind tunnel trip alone. These savings also are in addition to those he’s already reaping from the new position and bike, so we’re excited to see how next season goes.
While cyclists and triathletes are better understanding how CdA represents your aerodynamic drag coefficient, it can still be confusing how this number can be related to watts saved. Let’s face it, watts savings are something we can all understand as power meters and smart trainers are bringing watts to every rider. However, watts saved is dependent on many other things including the speed you’re riding at, your CdA, your weight, power output, and other small but important details. There are programs now available such as Best Bike Split that can help you ballpark your CdA based on a triathlon bike split time or cycling time trial. Once you know your CdA, then you can change it and see how it impacts your time. You can also change power and see how that impacts your time. I find this is much easier than using complicated Excel spreadsheets as we’d done for many years. It sure is faster too.
Good luck this season Sam and thanks for sharing your data!
