Wind Tunnel Report

One of the 200 hp electric fans that drives the A2 wind tunnel

Chester (Chet) Kyle officially retired in 1984, one year before Mark Cote, Specialized’s aerodynamicist was born. The elder statesman of aerodynamics appears everywhere in the literature. Designer of the US team bikes at the 1984 Olympics, co-founder of the Human Powered Vehicle Association, and consultant to Specialized, Nike and others, Chet asks probing questions and prefers to record his data in a large, well-thumbed journal. Though he’s seen it all, he displays a childlike enthusiasm for his science every time he steps into the tunnel.

Mark Cote the younger, speaks rapidly and thinks even faster. He spits out numbers and does calculations on the fly befitting an MIT pedigree. His respect for Chet is obvious and genuine. Together they’re a formidable team.

Chester Kyle and Mark Cote talking about the data

Also attending, Chris Reikert, professional triathlete and recent addition to Specialized's SBCU making the trip out to better understand the wind tunnel story he'll soon have to deliver in the classroom and to provide a skin suited victim as needed. Aaron Vogel, Specialized's videographer and Chris D'Alusio, Head of Advanced R&D (owner of the legs below) arrived day two. Last but not least, Mike Giraud, former team mechanic and patient host, runs the A2 tunnel during testing. Over two days he logs at least a hundred trips between control room and test platform, changing bikes and bike attitudes vs the wind.

Chris D'Alusio has the legs to rip your off

Tunnel 101

The wind tunnel sits like a giant worm in a cavernous hanger on the outskirts of Moorseville, NC a few miles up the road from Roger Penske’s racing operation. The tail end of the tunnel is driven by four electric fans rated between 40 and 200 hp. Air is pulled rather than pushed through the tunnel by these impellers, and their exhaust is slowed by huge metallic diffusers that resemble the engine shrouds on a Saturn V.

In the wind tunnel as in all scientific experimentation, repeatability is key. The recirculating flow within the building helps maintain a constant air temperature. Temperature fluctuations would change the density of the air and consequently affect the repeatability of run after run.

At the front of the tunnel, the recirculated air is pulled in and accelerated by a smoothly reducing tunnel mouth. The A2 tunnel has a maximum test velocity of 85 mph, but cycling research generally happens in the neighborhood of 30 mph.

The accelerated air then passes through a very fine grid of hexagonal cells, think of them as short tubes that straighten the airflow into the tunnel. This is exactly the same principle as a lighting grid applied to a photographic strobe. It harnesses the light, reduces spread and produces a controlled, directional source. This is what the hex screen does to tunnel air.

Twenty feet into the tunnel the splitter plate is a raised platform 15” off the tunnel floor that isolates the test bike/subject in the airflow from the mechanicals below. Bikes are fixed at both axles by mounts which pass through the splitter to the measuring apparatus beneath the floor. There peizo electric cells/strain gauges turn force vectors into voltages which are passed to the control consoles and inserted into the sophisticated spread sheets which comprise the software behind the tests.

Tires rest on rollers that spin the wheels in sync with the air flow and a central disk within the splitter plate allows the mounted bikes to be angled into the wind (both drive and non-drive side are measured).

The tunnel engines ready for liftoff

It’s Cold in There

I’m sworn to secrecy about what was tested on that platform but details aside it all boils down to answering two questions. How much is the wind holding the rider/bike back (wind access drag) and when under yaw conditions, how much is it pushing the rider/bike over (wind access side). Who said you’d never use that trigonometry again.

It surprised me that all the calculations boil down to differences expressed in grams. Bike A, in this configuration and attitude is 20 grams better than Bike B. Moving through the positive and negative yaw angles of the testing protocol the results drop into the spread sheet and graphs.

Much of the regular testing in the A2 tunnel involves riders tuning their positioning. When you’re actually riding the cooling affect of the moving air is comfortable. Sitting on the bike in a skin suit, approximating a mannequin is a different matter entirely. It’s cold in there, and even shooting my eyes were watering at times.

Chris Reikert, Chester Kyle and Mark Cote doing some flow visualization

Get with the Flow

Flow visualization is where it really gets interesting. Out comes the tuft wand, a hollow metal rod through which a bit of string or wool is passed. Six inches or so streams about in the airflow around the bike attaching itself where the laminar flow sticks close to the frame surface, fluttering in turbulence. Chet, Mark and Head of Specialized Advanced R&D Chris D’Alusio all focused on these subtleties. Mark had a stethoscopic contraption that let him listen to the airflow around the frame, and hear the relative static of turbulence.

Two days of testing generated a lot of data that will be analyzed in the coming weeks. This kind of testing is an expensive proposition and the program was packed. Hypotheses were confirmed and judging by the grins the surprises were all positive. Of course, as you may have read in the last few days, the UCI has ruled Specialized's Shiv can no longer be used in UCI-sanctioned races (it remains legal for triathletes worldwide). I know Specialized believes they've done everything possible to conform to the standards and the "boxes" prescribed by the specifications. Baffling it has taken the UCI months to come to a conclusion, long after races have been won with the bike. Having said that I know Chris D'Alusio, Mark Cote and the rest of the Specialized engineering team will be hard at work on a solution.

Photographing the Tunnel

It's not as difficult as going down a coal mine, but a working wind tunnel is not an easy place to take pictures. Any of the desirable head on, three-quarter front views are unavailable during actual testing, even with a remote camera. And when it comes to light... well it's a TUNNEL. Like many venues we find ourselves shooting in, it wouldn't take a lot of upfront planning or cost to make a huge improvement for photographers, but somehow it's always an afterthought. Drop in a prefabricated paint booth shell, you know the ones that are daylight balanced so that painters can judge the color they're laying down, and you've got a beautiful environment for shooting the bikes and riders that come to be tested. You heard it here first.

7 Responses to Wind Tunnel Report

  1. kurt says:

    Velo,
    Once again I find myself a little, well…..envious.

    Did I mentioned I like numbers?

    Thank you for sharing.

  2. Franco says:

    Velo. Say more about the tuft wand and the string/wool attaching to the frame in the turbulence? Is that string/wool the ‘white line’ we see in aero wind tunnel simulations?

    Thanks for the review…great stuff.

    • Franco,

      The tuft “wand” is just one means of flow visualization. The short piece of string (wool or Chet’s favorite, cassette tape) flows with the moving air around the bike frame. When I say it attaches itself, I don’t actually mean it’s physically connected to the frame, just that it flow with “attached air”. I think I’m expressing this correctly… attached air is a non-turbulent layer of air close to the surface… it’s also referred to as laminar flow.

      I’m not sure if the “white line” you are referring to is smoke. A stream of white smoke is often used to demonstrate the shape of airflow around the tunnel subject. Apparently bubbles are another visualization option. Does that answer your questions?

      ::M

  3. Nick David says:

    Thanks for publishing this. My teammate visited the wind tunnel a few months ago to tweak his already tight TT position. I would like to visit this within the next 2 years. Gotta get faster first.

  4. kaiko says:

    secrets, secrets are no fun…just kidding!

    those legs definitely made my morning :)

  5. Jon Moss says:

    Fascinating stuff :-) A serious setup which shows just how much science goes into the bikes, rider and racing.

    Best,

    Jon