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The Most Aerodynamic Shape For a Recumbent Bike Shell

07/14/2010 12:18 PM

I was wondering, what would be the most aerodynamic shape for a recumbent bike shell would it be better to make a larger shape with a smooth surface, or a smaller shape with an protruding "pod" for your head to fit into?

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#1

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/14/2010 1:21 PM

This is an example of my question: Would this design be better: or this : or this

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#6
In reply to #1

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 4:30 AM

I wouldnt risk putting any kind of all enveloping fairing on a two wheeled Recumbent, unless you only intend to use it on a sparsly poulated racetrack. The stability problem would orevent you using it on a public road and the efeect of sidewinds are bound to induce serious wobbles. The golden rule of design iis 'form follws function' so at the start of your project be very honest with yourself as to what the function of the fairing is.

Why not concentrate your undoubted talent on a practicle fairing for recumbent Trikes (i.e. 'Tadpoles'). These are inherently much more stable and less sensative to sidewinds. The existing commercial fairings have drawbacks; either too expensive , or not very effective. There is definately scope for improved designs with the emphasis on Practicle designs that can be used for everyday riding on public roads.

I have a trike (ICE Trice Q} which I am electrifying and will subsequently devise a fairing to make it an allweather vehicle. I have ideas for how to do it but am not yet ready to publish.

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#8
In reply to #1

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 6:23 AM

I think the top photo is your best bet for speed, if that is your major design consideration. I would change the very rear though. Instead of a vertical profile, I'd go with a elliptical.

Or at least put winglets on the top corner to reduce the drag. No matter how perfect the design is, the "construction" will have flaws and one side or the other is going to have more lift than the other, and little vertices will form at the squared off rear corners. These vortices could create as much as 5-15% additional drag from what I have read.

The tuna is the most efficient aero shape in nature (sub sonic) ... tuna on wheels?

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#14
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Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 12:48 PM

"No simple answer"

You are correct. The most efficient shapes will be found swimming in the water. Certain species of Sailfish have been clocked at 110 km/h (68 mph) (The Tuna is given as being able to move at 20 m/s, which I calculate to be 72 km/h ?)

Air is 0.1275% as denes as water, and therefore objects should be able to move ~8 times faster in air, which would allow the sailfish to move at 880 km/h (544 mph)

When I was a kid, I remember hearing that a Loon could swim 60 mph underwater catching fish... but now they just hang out on cr4!

Chris

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#17
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Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/17/2010 2:17 PM

I missed the 'elliptical' idea.. I like that one.

It is kind of hard to believe that "vertices" (I think you mean vortices) cn create 5-15% drag... but I'm not an aerodynamicist.

but again, a very insightful comment.

Chris

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#2

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/14/2010 5:35 PM

side winds will still hit like a ton of bricks.

Is there really a "true" advantage to a bike pod shell?

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#3
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Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/14/2010 8:00 PM

It would seem there is, because you can go signifigantly faster on them and wouldn't your design try to minimize sidewinds?

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#4

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 1:02 AM

No simple answer. Air drag is

drag = V2 * da * A * cd (which is a force in [kg-m/s2] or [Newtons])

is dependent on:

V2 = velocity squared [m2/s2]
da = density of air in [kg/m3]
A = cross-sectional area [m2]
cd = drag coefficient [dimensionless]

The drag coefficient is dependent on SHAPE, not size. While the air density and velocity are the same for both of your cases, the cross-sectional areas AND the drag coefficients are different. The goal is to minimize BOTH. Without knowing the actual size and shape of both designs, you cannot know which design will provide better (lower drag) results.

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#7
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Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 5:49 AM

Also apparent drag ... from the surface of the road & drag from any lift created.

Trike designs have more induced drag, because of the greater surface area closer to the ground & the greater lift induced.

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#5

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 2:20 AM

You're on the right track and side wind pressure isn't realized as much as perceived.

Checkout Craig Vetters designs:

1981: Jim Knaub checking out the possibilities of a Human Powered Vehicle at Ontario Raceway

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#9

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 6:50 AM

Make yourself a model of each and also a mini wind tunnel and test the theories for yourself. Allow enough wind tunnel size to turn the model through 180deg. My first wind-tunnel was made with a household fan, shoebox, clear plastic sheet window, garbage bag to funnel the air from the fan around a wire frame from the fan to the end of the shoe box. A bunch of cards slotted together to normalise the airflow. I used wire rods to support my models of air foils stood on pieces of balsa wood with a scale on the side, floating in a basin water. I managed to develop my own air foils for model planes I designed and built. Also used it to develop fairings for race motorcycles. Nothing fancy, but enough to improve on a set of base readings and to determine crude drag co-efficient and lift, induced drag and torque figures of the "primitive" air foils. Bottom line, the models flew very well.

A wind tunnel is a wonderful tool that sure can bust your crazy perceptions/understandings of aerodynamics. Unless you are going to get into full on analysis don't waste money on fancy electronic load cells etc, use what you have. Once you are comfortable with your concepts you can spend some cash down at the local university's wind tunnel for a small fee to define accurate values if you need them.

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#10
In reply to #9

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 8:39 AM

In order for the test results to be meaningfull on a full size vehicle or model in a different speed regime I would think it necessary to ensure that the tests in the improvised wind tunnel are at a similer Reynolds number to the full size application. Otherwise the flow characteristics may be different to that occuring on the full size application

As an alternative thought, how about devising a three axis force measuring mount of some kind with the model mounted on the end and then fixing this on the bonnet of a car or some way to one side where the flow is reasonably straight. At least with this method you could get an accurate measure of air speed.

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#12
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Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 9:31 AM

Hi Cadman, sorry I had not logged in when I posted the above....grill me. To develop the project fully would require a lot of work, the writer has not stated what his intentions with the project are. Your comment on R'no's is just the tip of the iceberg. As long as his two models, with or without the bubble, are on the same scale as his control model of an open recumbent he can use a simple balance connecting the two to establish which design has the lower drag coefficient. He will not be able to predict the full scale figures accurately, but can certainly compare the different efficiencies of design at scale...

By setting the models at angles to the air-stream he will also be able to compare side-wind effects on the different models. His Q was; which is better, not by how much. A 3D cad model could also be run on an aerodynamics program and get very realistic results.

At the end of the day he will have to do the sums and test the results at full scale. I sounds like a typical undergraduate team project to me and I have therefore not been toooo specific as to do all his homework for him. This is one of those been there, done that, ones and if he chooses not to use a wind tunnel there are a few potentially nasty surprises coming the way of the pilot.

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#16
In reply to #12

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/17/2010 1:00 PM

This isn't a undergraduate team project. I just turned 15. Also, this would be for longer distance riding. I have a question about how much it would cost to make a shell? I plan on making a recumbant from a two old mountain bikes and some steel tubing. Also for a shell, what thickness/weight fiberglass should be used?

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#18
In reply to #16

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/18/2010 12:08 AM

I have a question about how much it would cost to make a shell?

At 15 years you don't have name recognition of C. Vetter, he invented the motorcycle fairing, a very down to earth guy.

You can find the necessary materials on Craig Vetter's site...the components are inexpensively available in your town.

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#11

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 9:29 AM

What is the purpose for your Recumbent Bike, is it for long-distance riding or a short sprint for setting a speed record?

Remember the rider is expending work and will sweat. If the bike will only be used for a short sprint the rider will tolerate getting hot, but over a long haul sufficient venting will be required to cool the rider.

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#13

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 9:37 AM

I'm afraid the only way to answer your question is to conduct full-scale wind tunnel testing of various bike shells in order to determine the most aerodynamic efficient design.

Hope that helps...please have a great day!

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#15
In reply to #13

Re: The Most Aerodynamic Shape For a Recumbent Bike Shell

07/15/2010 12:52 PM

Yes, you are right. Hydrodynamics are great for figuring 'streamlining' but Aerodynamics is not the same... air does not behave the same as water, so for figuring out control surfaces, I think that what you say is true, and that reality works best. He can certainly get in the ballpark less expensively by starting with the fish shape.

chris

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