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What I learned about aerodynamics from the Specialized Win Tunnel

Specialized Win Tunnel tour
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Specialized Win Tunnel tour

While in Palo Alto for the Specialized Turbo Vado e-bike launch, I made a side trip to their Morgan Hill, CA, headquarters. The whole thing is impressive, and throughout this week you’ll see more of it, but I’m starting with the Win Tunnel.

It’s housed in the rear-most building, which is the original Morgan Hill headquarters. Mike Sinyard moved the company from San Jose and grew it inside these walls until it was bursting. Once they’d moved everyone to a new, larger building just in front of this one (you can see the red trim of it in the window’s reflection), this one was gutted to make room for the Win Tunnel. Not only did I get to see what it’s like to stand inside while it maxes out at 70mph (video on our Facebook page), I learned why virtually every wind tunnel test produces results at 40km per hour or more…

Specialized Win Tunnel tour

I don’t know about you, but I rarely (read: never) sustain 24.85 mph over more than a few hundred feet. Unless it’s all down hill. But that’s what 40km/h translates to, and that’s the number we see in virtually every bike or wheel manufacturer’s aero claims. Why? Turns out, there’s a good reason. But first, a little tour.

Enter the building, which requires a key card, and you’ll see the front end of the tunnel. This is where the air is sucked in…

Specialized Win Tunnel tour

…through a 20″ deep honeycomb wall. The divisions are paper thin and easy to bend, so it’s best not to touch them. This system gets the air moving into the tunnel in a straight path with minimal turbulence. Normally, a wind tunnel like this would cost many millions more than even a Specialized-sized company could afford. But, based on conversations over drinks at the Vado launch, I learned that it came about thanks to fortuitous hires of people skilled in such things and beneficial friendships with folks outside their company that just seemed eager to impart knowledge.

Specialized Win Tunnel tour

Entry to the wind tunnel comes through the command center.

Specialized Win Tunnel tour

From the inside, where the bike sits, the honeycomb wall just looks like a big screen. The fans are in the back and pull air through the tunnel so they’re not sending messy, turbulent air across the bike and rider. The sensor gauge sits atop a platform with its own channels for air to move through, which they say helps reduce any boundary layer air situations that could skew results.

Specialized Win Tunnel tour

The strain gauges that attach to the axle are essentially like weight scales turned on their side. As the wind pushes the rider back, it measures how much “weight” is created, which is calculated into grams of drag. From there, they can figure out all sorts of things. The platform rotates, letting them simulate yaw (cross winds), and at its base well underneath the platform, it’s about twice the diameter you can see here.

Specialized Win Tunnel tour

Projected on the floor in front of the rider is wind speed, angle and the amount of drag. The latter is shown as a graph over time, and the blue hills are from them pressing down on the sensor.

Specialized Win Tunnel tour

At the back are six giant fans. Here, too, is where they were able to cut costs. They used readily available commercial HVAC fans that are designed to run continuously, day in and day out. That means they’re low maintenance, and repairs and replacements are easy. Compare that to custom, one-off high speed fans that need custom, one-off replacement parts and cost many multiples more and you see how they used off the shelf parts to their advantage. The carbon fiber surrounds were laid up in house and custom made. The first piece didn’t have a smooth enough finish, so they keep that in the office for star athletes to sign when visiting the tunnel.

Specialized Purist water bottles are made in the USA

The back and side of the building are used for their Purist water bottle finishing. They’re blow molded elsewhere in California, then have their glass-like Purist interior coating applied in house because they didn’t want to share that technology. They also print them in-house with non-toxic inks, assemble the caps (some of those parts are made elsewhere), and pack ’em up in these boxes.

Now, about those aerodynamics numbers…

How bicycle aerodynamics tests translate to the real world

The formula above basically translates to this:

Power = 1/2 x Air Density x Velocity³ x Coefficient of Drag x Area. The Cd•A part of that doesn’t change unless you change your position or frontal area, so that’s considered a constant when figuring out the power required to overcome that drag. Velocity and Air density obviously change, and since Velocity is cubed, the faster you go, the bigger impact it has on the power number. Which is why it takes exponentially more power to continue to increase your speed. Air Density, represented by Rho (ρ), is calculated with barometric pressure, relative humidity and temperature (more on that here). Change that and you change how easy or hard it is to plow through it.

Note there’s no mention of “grams of drag”. Because that number is variable, and it doesn’t matter when calculating how much power you need. Instead, the formula above shows how much power you need to move at a certain speed with a fixed Coefficient of Drag Area. Some of the exact terms might be minced thanks to my rapid note taking, but the principles are what’s important: Reduce your frontal area or optimize your shape, and you need less power to go fast.

And here’s why that gives meaning to all of those “…saves 50 seconds over a 40km time trial at 40 50km/h…” claims we see on so many products’ sales sheets. Because the speed doesn’t matter, the benefits are basically the same. Do the math and you’ll see that the differences in time saved over 40km varies very little whether you’re doing 12mph or 30mph. Your total time longer, for sure, but your time savings will likely be within a few seconds of the faster rider. (UPDATED: Corrected to testing at 50km/h, per followup with Specialized)

Specialized Win Tunnel tour

If you want to plug your numbers in, Specialized’s aerodynamics engineer and road product manager Cameron Piper (who also manages the Win Tunnel) says an average sized fit cyclist has a CdA of 0.28 to 0.35 m², and an “average” day has a Rho around 1.15 to 1.18. You’ll have to put your other numbers in to figure out power…or velocity. He says that, roughly speaking, for every 0.001m² reduction in CdA, you can expect to save about 4 seconds per 40km.

So, if no one outside of the pro peloton is really holding a 50km/h average, why do they test at those speed? Piper says it delivers better data. The higher speeds amplify the inputs, providing higher resolution data that paints a clearer picture. For Specialized in particular, it’s done to match up better with data coming from high speed wind tunnels that other brands are using. In the Win Tunnel, 50km/h winds are more than half it’s top speed. At tunnels used for testing F1 cars and airplanes, 50km/h is barely above idle, and about the minimum speed those facilities are able to use to produce good data. (UPDATED: Corrected from 40km/h to 50km/h)

The takeaway: Next time you see those talking points here on Bikerumor, just know that you’re likely to see similar time savings whether you’re a pro or not.

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Adam
Adam
5 years ago

Not sure why they are using v^3. The drag equation is relative to v^2. At least that’s what I’ve generally used in my 15 years as an aero engineer. Anyone explain?

Derek
Derek
5 years ago
Reply to  Adam

Power, not force. And power is force dot velocity.

Adam
Adam
5 years ago
Reply to  Derek

Yep, my bad, they’re talking power. P=Fv.
Late night last night…

Shane
Shane
5 years ago

Why velocity cubed? For aircraft its always velocity squared.

Shane
Shane
5 years ago
Reply to  Shane

nvm, since its power not lbs force

Tyler Durden
Tyler Durden
5 years ago

Exponentially more power? That there is a third order polynomial, not an exponential.

joe
joe
5 years ago
Reply to  Tyler Durden

Not only that, but a cubic polynomial should be referred to as ‘the cube of power”….cause sh*t sounds badass when it’s cubed.

Frank
Frank
5 years ago

Nice article, but I would like to make two remarks. Firstly about the testing speed: for many years wind tunnel tests of cycling stuff were made at 50 km/h (in Europe) and 30 mph (ca. 48 km/h) across the pond, as the applications then were in track and professional time trialling. With interest recently extending into amateur riding the testing speeds have come, but this is the first time I read of 40 km/h as a ‘standard’. I believe the most common speed used is still 45 km/h.
It would seem at first that testing at lower “real-world” speeds would be preferable (especially because converting measurements taken at a given speed into a different speed entails some errors due to scaling or “Reynolds number” effects). The problem with using low testing speeds is that the signal-to-noise ratio worsens quickly to the point were no significant information is being generated. Aero testing has repeatability issues even under the best of conditions, so using very low testing speeds is unwise. A tunnel custom-made for low-low speeds can of course get away with procedures that a general-purpose low speed tunnel cannot. If this is the case of the Spesh tunnel cannot say.
My second remark is about the statement that drag “doesn’t matter”, only power. In fact drag, power and CdA give exactly the same information and they all require knowledge of the corresponding test speed in order to be interpreted correctly. Even CdA changes significantly with speed due to the above mentioned Reynolds effects.

JK
JK
5 years ago
Reply to  Frank

It’s a marketing tool, of course drag doesn’t matter.

Tom
Tom
5 years ago
Reply to  Frank

Long-winded way of saying exactly what the article summed up with “The higher speeds amplify the inputs, providing higher resolution data that paints a clearer picture.”

zigak
5 years ago
Reply to  Frank

Thank you for a very insightful comment.

joe
joe
5 years ago

Oh, and there’s great simplification going on here. Because that power equation does not have a lot of constants. The wind tunnel is a highly ‘idealized’ place. In the real world, your velocity is not constant, neither is your Cd, and neither is your Area. Technically, even your rho is variable, although quite negligible compared to the others.

The most valid tests, are over long distance trial with a motor with a given wattage and batter power (known values) are placed on bicycles with different aerodynamic configurations. The ‘riders’ may pedal, but are not allowed to actually influence the power output. If you set all these bikes out on the same course at the same time, you need only wait to see who is the winner to find the best aerodynamic solution (in a real world scenario). Because the motors only output a set amount of power, the bicycle to travel the longest distance would automatically be the best aerodynamic configuration.

Inspector Gadget
Inspector Gadget
5 years ago
Reply to  joe

That sounds like a more valid test if you could find motors and batteries of the exact same power, efficiency etc. The differences measure between individual LiPo packs alone are generally greater than the 0.11% that “4 seconds saved over an hour” represents. Or were you proposing power meters be used (1-2% at best)?

Rich W
Rich W
5 years ago

220, 221, whatever it takes.

Marin
Marin
5 years ago

This is all irrelevant for non pro time trial rider. Bike and wheels make for such small percentage of drag compared to rider.
Far better gains can be achieved with fitness, clothing, position than any “aero” bike or wheels especially fitness because amateur riders ride slower and aerodynamics have far less impact on them than a pro time trial rider.

Even if gains were as stated at lower speeds, what’s the point to an amateur rider saving several seconds over 40km or so…

This test doesn’t account for difference in comfort and endurance. What’s the point of saving few seconds if you’re going to be losing minutes in the real world because of riding position that’s less than optimal for one’s physique or riding on actual roads that can vary significantly and “aero” bike can be slower due to broken or slippery roads, hills etc.

Renny
Renny
5 years ago
Reply to  Marin

Totally agree with the point that for most people, body position is where most of the time saving can be achieved. If you can reduce A by 5% or so, by sitting in the drops or just by bending the elbows, then you would save so much more time compared to ‘fast’ wheels, frames, etc. But not everybody can physically do that. Hot tip; (power) yoga. Makes you strong and flexible, without making you heavier.
Tight apparel would be my next step in minimizing drag. Relatively cheap and very effective.

Imho, these time savings by choosing aero bike components is for those who already have optimal position and can maintain that position (without creating faster fatigue of course!). And maybe for those who wanna look fast, to get the psychological advantage over the competition! Don’t underestimate that brain factor 😉

Enjoy the yoga

Garth Magee
5 years ago

For an open-wheeled vehicle, the system equation is far more complex than the simple drag equation, with many more terms involved.

As taught in my now allowed patent application (to issue this summer) US2015/0346051, traditional floor-mounted pedestals measuring translational drag force are appropriate for airplanes—rather than vehicles—where all surfaces are exposed to the same wind speed.

Wheel surfaces move both up and down, as well as in translation, dissipating power in many directions. Lateral force sensors do not capture the vertical power dissipation. And tying the vehicle to the ground changes the force amplification mechanics of drag on the wheels as they affect propulsive efficiency. (Drag on upper wheel surfaces is magnified, and drag on lower wheel surfaces is de-magnified against propulsive forces applied at the axle.)

Instead, the vehicle should be freely mounted and self propelled on a moving belt, as on the road itself. Then external drag forces are countervailed by forces transmitted through the bottom of the wheels. Measuring the power needed to propel the belt then becomes a direct measure of the power being dissipated in drag on the vehicle.

Using this new method, there is no need to inaccurately model how drag on various wheel surfaces actually affects vehicle propulsive efficiency. It can be directly measured.

Garth Magee
5 years ago

And it is worth noting that contrary to common assumption, the relative concentration of drag between the wheels and the frame changes dramatically depending on external headwind conditions. As one faces a stronger ground headwind, the concentration of drag shifts to be more highly concentrated on the faster moving upper wheel surfaces. For this reason, the vehicle drag coefficient is a function of vehicle operating conditions—and not a constant.

For minimizing vehicle drag, the mechanically-magnified upper wheel should always be shielded and the mechanically-demagnified lower wheel always exposed. My now allowed patent application for upper wheel fairings teaches this novel principal: US 2014/0265431.

This principal has now also been recognized in related issued patent US 9,567,016, for a new Truck Trailer Skirt design, where the upper wheel is shielded and the lower wheel is optimally exposed.

alvis
alvis
5 years ago
Reply to  Garth Magee

patent is not validation. just sayin’

Dude
Dude
5 years ago
Reply to  alvis

Yeah, there better be some actual rigorous testing data there. Otherwise… Alex Simmons did a good job on that one. http://alex-cycle.blogspot.com/2014/08/the-null-hypothesis.html

fred
fred
5 years ago

i wear a skin suit all the time, not just cycling. aero is so cool.

duder
duder
5 years ago

I’d like to see an analysis of mtb handlebar width.

bbb
bbb
5 years ago

“…Do the math and you’ll see that the differences in time saved over 40km varies very little whether you’re doing 12mph or 30mph. Your total time longer, for sure, but your time savings will likely be within a few seconds of the faster rider…”

This is the most irritating type of misleading manipulative marketing bs. I’ve ever come across and it’s been repeated hundreds of times by jurnos forum posters etc. without anyone questioning it.

Specialized is really good at marketing and uses itheir wind tunnel to bombard consumers with factually correct but completely irrelevant and misleading (to overwhelming majority of consumers) numbers and statements.

Gains should be expressed as percentage (of power or speed) , not in nominal time saved. Drag at 15mph will be dramatically different from that at 30mph and so will be savings.

Cameron Piper
Cameron Piper
5 years ago
Reply to  bbb

BBB – appreciate your comment here. Before digging too deeply into explaining more on why we use Time Savings over 40km (which can really be Time Savings over any distance we choose), I’ll link to you a video we created simply explaining this choice: https://www.youtube.com/watch?v=O-7g1kqYJAY&index=15&list=PLcmaLnqmqDnmn_bCR0RJ-soSCDKCKR97t

We don’t show gains as a percentage of power or speed, largely because those percentages would change depending on the rider’s speed/terrain. Sure, the time savings (as a percentage of the time you are riding the specific distance – 40km) is a different percentage depending on how fast you are riding, but the actual time you are saving is roughly equivalent.

Additionally, this video explains why percentages are a bit more complicated, but we could theoretically still express the gains (or savings) in these terms: https://www.youtube.com/watch?v=MSAHa8brcCM&index=7&list=PLcmaLnqmqDnmn_bCR0RJ-soSCDKCKR97t

skip
skip
5 years ago

Has anyone here seen a big bang where Penny wants to participate in a conversation but has nothing worthwhile to say around the other guys….?
That is this moment.

Garth Magee
5 years ago

One of the more extreme examples of overlooking the proper system drag mechanics of open wheels on a vehicle is the original rear wheel fairings on the Bloodhound SSC. At the time of the invention, the rear wheels were forwardly shielded to well below the axle. (Recent photos now show the fairings removed altogether, likely to avoid infringement.)

As originally designed, the car then had a supersonic lower wheel fairing portion shielding a slower-moving sub-sonic lower wheel surface. Moreover, the slower-moving lower wheel always enjoys a mechanical advantage over frame drag forces, and therefore should never be shielded.

Simply propel the wheel at the axle while resisting on the lower wheel, versus resisting on the upper wheel, and this effect becomes apparent. However, for some reason, this effect had not been recognized before. Indeed, there exist many examples in both the prior patent art and at least two examples in post patent art which argue the opposite faulty principal: that shielding downwardly more of the wheel necessarily decreases the effective drag on the vehicle.

In fact, for any given operating condition, there exists an optimal minimal fairing configuration shielding the upper wheel which will yield a minimum in drag on the vehicle; Extending the shield surfaces any more, or any less, both produces an increase in the effective vehicle drag. This principal is embodied by the present invention.

Robin
Robin
5 years ago
Reply to  Garth Magee

Sounds like a sales pitch.

Dude
Dude
5 years ago
Reply to  Robin

… from someone who 1) can’t point to empirical data collected with good methodology and 2) doesn’t understand how to pitch

Ben
Ben
5 years ago

More super techy mathematical reviews like this please!!!!

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