A note on aerodynamics.

"Grindintosecond" (Grindintosecond) 08/02/2017 at 12:53 • Filed to: None

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Believe it or not, folks, but this THULE arrangement is horrible for best mileage. Disregarding the fact a bike is completely in the airstream, we will ignore it and focus on the box. Backward mounting would be best and here’s why.

Simply put, the turbulent air flow is what adds drag and kills mileage. It comes from the separation of air from the surface along its travel. Yeah yeah, we know this. We study race cars and we see drafting and slipstreaming all day long in NASCAR (if you care to watch that) so why do the aero-box users persist in mounting the sleek looking “ aerobox ” in the promoted blunt end back direction?

The ability to keep the airflow smooth as it leaves the rear of the body makes for less turbulence and less drag. Airplanes rarely have pointy noses unless they’re splitting the sound barrier. The most efficient Boeing/Airbus products have a rounded nose and knife edge or pointy tails. Spin it around and you have a blunt front (actually good) and a smooth teardrop shape for the air to flow with, leaving a smoother less turbulent/low drag ending behind.

Especially in the truck case, backward would be perfect as the airflow off the roof wouldn’t even touch the blunt end very much at all and just conform to the box.

Better mileage for you, and if you’re the judgy sort, you can now look at everyone else doing the top picture idea and eye-dagger them while smiling, knowing, as a superior motorist in a sea of depressing Luddites.

....what a terrible way to live, all judgy like.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Grindintosecond 08/02/2017 at 12:57

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But if you don’t have a truck, that’s the correct way to install it. Though for the same reasons, the wrong way to design it.

Nibby > Grindintosecond 08/02/2017 at 12:58

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we should just put shark fin shaped camper caps

MonkeePuzzle > Grindintosecond 08/02/2017 at 12:59

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what if, and hear me out, what if they made a vehicle that had like an enclosed back?

like a sort of truck like thing, but if you didn’t need to carry a pile of dirt and rocks every day you could instead just fit things inside the climate controlled extended cabin and it would probably have room for all the things that fit inside that roof box AND MORE!

and I bet some sort of “roof rack” (patent pending) could be arranged to go on top to hold the bike!

Grindintosecond > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/02/2017 at 12:59

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Regardless of the vehicle, blunt end forward is most efficient. The designers of these things are probably slapped around and bullied by marketers every day in their cubicles.

For Sweden > Grindintosecond 08/02/2017 at 13:04

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Disregarding the fact a bike is completely in the airstream

Is this my undergraduate aerodynamics courses?

Ash78, voting early and often > Grindintosecond 08/02/2017 at 13:06

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1. Thule and Yakima need to build full, symmetrical airfoil boxes instead.

2. People need to stop letting the box overhang the windshield by a couple feet, creating a massive air dam (it’s so they can open the tailgate, but IMO it’s a lazy compromise).

3. Roof bags are awesome. Most people who use roof bags treat them as afterthoughts and just toss their remaining junk in them. Pack the roofbag first, not last.

/rant off

RamblinRover Luxury-Yacht > Grindintosecond 08/02/2017 at 13:06

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Wunibald Kamm says hi. As do the 1934 Chrysler aero team.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Grindintosecond 08/02/2017 at 13:07

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I knew about the streamlined tail thing, but I’d have thought if one end had to be squared off and had nothing in front of it blunt back would be best.

That said thinking it over again... Air will create its own cushion of relatively still air to flow around, as shown (for example) by the guys that tested a pickup with and without bed covers and got better MPG’s without one—so it makes sense that it would do the same around the squared-off front of the carrier and be preferable to creating a vacuum behind it (such as old galleon-style ships would create—one of the reasons clippers were so fast was doing away with the square ass-end).

Grindintosecond > MonkeePuzzle 08/02/2017 at 13:07

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You, sir, have super geniusing .

I can only imagine this person either has multiple needs and events in his life, or is dreadfully single with a lot of stuff because a “truck topper” (trademark now pending) weighs enough that two not entriely drunk buddies are required to lift it off and on in case of outsized cargo (mom or GF said move out)

But really, an aft-sloped bed topper would be pretty slick..

Grindintosecond > RamblinRover Luxury-Yacht 08/02/2017 at 13:09

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Well hello! How are they?

Wacko > MonkeePuzzle 08/02/2017 at 13:09

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you and your Crazy ideas...

Grindintosecond > Ash78, voting early and often 08/02/2017 at 13:10

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yup yup

Perhaps I’ll build my own. I’m getting into fiberglass mold making soon.

Wacko > Grindintosecond 08/02/2017 at 13:11

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put your shit in the box, not in the box on top of the box. Problem solved.

Grindintosecond > For Sweden 08/02/2017 at 13:12

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Omit! disregard the problem! Solve what makes no difference to the overall issue presented.

You are now employable in government.

Patrick Nichols > Grindintosecond 08/02/2017 at 13:12

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Personally I would’ve chosen an example of good aero that didn’t have a swastika on it...

diplodicus > Grindintosecond 08/02/2017 at 13:12

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The real question is, what’s in the bed?

Grindintosecond > Wacko 08/02/2017 at 13:13

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But this dude’s box is full of his other stuff he needs! Where else will he put the bodies he “finds” while “camping” and “biking”?

RamblinRover Luxury-Yacht > Grindintosecond 08/02/2017 at 13:16

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Mostly, still dead. The unguents and candles for the necromancy were expensive , yo.

But outside the joke, you *do* need some front end streamlining to avoid a large dispersion area, preferably something like an ogive shape, and past a certain point (heh) on the back there’s a lot more to it than “GIT U SUM TAPR”. That was the lesson of the Chrysler Airflow: a reduced cross-section in back and/or managed turbulence are often enough, and getting away from sharp edges and a large initial cross-section up front are important.

The Thule box’s little quasi-spoiler may not be completely useless, and would do more hellacious things running backward. Probably.

Grindintosecond > diplodicus 08/02/2017 at 13:16

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People asking questions....

Wacko > Grindintosecond 08/02/2017 at 13:16

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So torch’s Truck frunk doesn’t sound too crazy anymore

MonkeePuzzle > Grindintosecond 08/02/2017 at 13:17

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pfft, who hasn’t owned a truck topped and immediately set up a system of ropes and pulleys in their garage to aid in removal?

Grindintosecond > Patrick Nichols 08/02/2017 at 13:18

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I chose an Audi. Audi chose the wrong team.

Grindintosecond > MonkeePuzzle 08/02/2017 at 13:18

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Obviously this guy isn’t as geniused .

Sovande > Grindintosecond 08/02/2017 at 13:21

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Was the only aero-car you could find a picture of the one with the swastika?

Do you think Thule ran any aero tests, or did they just draw a shape and call it good?

MonkeePuzzle > Grindintosecond 08/02/2017 at 13:22

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obvi

Grindintosecond > Sovande 08/02/2017 at 13:24

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Edited the picture.

If we think about the shape, people love a pointy front “fast” looking jet fighter appearance not thinking about actual aero effects, so I’m sure THULE minimized what they could but still kept the design “appealing to the consumer” so sales would flourish.

RamblinRover Luxury-Yacht > Grindintosecond 08/02/2017 at 13:26

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Kind of appropriate, I guess, in a roundabout way - company named Thule, Thule Society...

https://en.wikipedia.org/wiki/Thule_Society

TheRealBicycleBuck > Grindintosecond 08/02/2017 at 13:28

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Everything’s a compromise. They are dealing with aerodynamics versus carrying capacity (primarily interior volume) versus consumer perception, or appearance.

Aerodynamics are important and all of the manufacturers do testing of some sort or another.

Thule’s most aerodynamic box has been tested to 186 mph.

It sure is fun to be an armchair aerodynamicist, but I’ll leave the modeling and testing up to the experts.

Grindintosecond > RamblinRover Luxury-Yacht 08/02/2017 at 13:31

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THULE company founded by the Thulin family in Sweeden. True lovers of the outdoors! Ah, I dug trying to find out more. I wonder if they were fans.

TheRealBicycleBuck > Ash78, voting early and often 08/02/2017 at 13:31

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Sometimes we don’t have a choice!

:)

Jason Spears > Grindintosecond 08/02/2017 at 13:34

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This leads me to wonder how much of this applies here, and whether Thule tested and measured during the design process.

https://en.wikipedia.org/wiki/Kammback

Grindintosecond > Jason Spears 08/02/2017 at 13:35

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I would suppose this example would be a form of large scale, low aspect ratio “gurney flap”?

Either way, the THULE design would need a slope down to that abrupt cutoff. It does not exhibit that trait.



TheRealBicycleBuck > Sovande 08/02/2017 at 13:36

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Yep, they tested them.

Sovande > Grindintosecond 08/02/2017 at 13:37

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Ha! That made my day.

Censored > Grindintosecond 08/02/2017 at 13:47

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Yep. Doesn’t matter as much how you split the air, it’s how you put it back together.

Ash78, voting early and often > TheRealBicycleBuck 08/02/2017 at 14:34

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No worries, you’ve got ample airflow going under it. I was more thinking of the people who flush-mount them (or nearly) to the roof and then you get to watch them buffeting on the highway, begging to take flight.

Ash78, voting early and often > Grindintosecond 08/02/2017 at 14:38

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For years, I’ve had an idea to do one from Marine-grade plywood. You could have a single 4'x8' top (arc) with a 4'x6' base, give or take. The sidewalls would dictate the shape of the curve and the top could be “tension fitted” and could spring open on a hinge, exposing the entire interior. Seems like it’d be pretty easy to do, but getting the structure and mounting points reinforced enough might be tricky. You could even stain it for a nice vintage wood look (eg old-school teardrop trailers)

BaconSandwich is tasty. > Grindintosecond 08/02/2017 at 21:39

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I’ve been wanting to do the same, but out of aluminum. Make it look like part of a WWII bomber.

Jason Spears > Grindintosecond 08/04/2017 at 18:50

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Yeah, that’s why I was wondering if they tested, or if this is just “make it shaped like a bullet train.”