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  1. #1
    Senior Member Notso_fastLane's Avatar
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    Aero improvements on a (recumbent) bicycle.

    I've got some new goodies lined up in the near future to add to my recumbent.
    Recumbent:

    Aerotrunk:
    http://store.cycledifferent.com/mm5/...gory_Code=BAGS
    Fairing:
    http://www.hostelshoppe.com/images/p..._gx_bubble.jpg

    I'd like to design a good test of the aero improvement I get from this setup by coasting down a hill to a predetermined speed.

    So what's the best way to do that, and control for the added weight of the components?

    Here's my idea:
    1. With the bike in its current commuting configuration, Find a hill,
    1a. start the run at a predetermined speed (I'm thinking 10.0 mph, just below the speed where the aero starts to make a significant difference)
    1b. Stop the run at a predetermined speed (i'm thinking maybe 5 mph, or maybe back to 10 mph).
    2. Weigh all the components before adding them to the bike.
    3. Add the wieght to my current commuting setup and and repeat 1 - 1b. This shoudn't be too hard to just add stuff to the small bag I have on the back of the seat. I can use my camelback to add water and fine tune it to get pretty accurate.
    4. Add the new bag to the bike, and do it again. Note the difference in distance. (I wish my GPS app were accurate enough to really map the acceleration differences too).
    5. Add the fairing and do it one more time.

    I realize that the added mass of these items will negatively affect my uphill performance, but with all my commuting gear, I'm adding a significant amount of mass on a typical day, over and above my 'dry' weight, anyway. I just have to find a hill with a decent runout where I won't have to worry about traffic or braking for turns.

    I might even go find my arodynamics book and see if I can calculate the different coefficient of drag for each configuration.

    Any other ideas?

  2. #2
    Senior Member BlazingPedals's Avatar
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    Just my $0.02, but in my experience best bang-for-buck can be had by covering the spokes in your back wheel. Wheelbuilder covers are $89, but you can do it yourself for less by zip-tying some ABS disks to your spokes. The least bang-for-buck, at least for a moderately-reclined SWB like that, will be the front fairing.

  3. #3
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    I like your idea.

    I like free speed too; that's one reason that I like your idea.

    Like Mr. B. Pedals, I installed wheel covers on my rear wheel.
    The free speed boost is welcome, but the extra unexpected bonus,
    for me on my bike, was the added stability.

    I carefully cut and fitted my first wheel discs myself.
    Cardboard is free.

    As a retired motorcycle test rider, for a fairing manufacturer, I'd rather
    not have to deal with front fairings mounted on a monotrack vehicle.
    It'd be too much like work.
    But that's just me!

    Good luck!
    "We don't have to be mean because, remember, no matter where you go, there you are."
    -Buckaroo Banzai

  4. #4
    Senior Member BlazingPedals's Avatar
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    Cardboard is free and easy to get, but then you definitely want to go to ABS (for-sale signs.) Plastic is more durable, especially when you get caught in the rain. Cut a disk the right size, with a hole in the center for the hub and a slit so you can dish it. Zip-tie it to the spokes, leaving a small flap at the valve so you can air up your tires. (And make sure the overlap goes the right way so it doesn't open when the wheel spins.) The Wheelbuilder covers are nice, but they weigh 500 grams.

    The fairings I built for my V-Rex would typically cost about $40 to build. Lexan ones (including the frame) for a SWB cost $400-600 or more?
    Last edited by BlazingPedals; 08-23-12 at 07:11 PM. Reason: typos

  5. #5
    Senior Member LWB_guy's Avatar
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    How did you make a fairing without access to a large oven? Or did you have access to a large oven (large enough to hold the fairing) ?

  6. #6
    Senior Member BlazingPedals's Avatar
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    Mine were Coroplast.

    I had the best luck by building a frame from square aluminum. It was easier to keep straight! Like this:
    http://www.biketcba.org/TRICORR/work...workshop1.html

    The most effective shape with the least weight was putting this fairing on the square aluminum frame:
    http://www.biketcba.org/TRICORR/proj.../fairing4.html

    Those designs are for a SWB. I don't know how you'd best do one for a LWB. And obviously, if you're experimenting, use (cheap) cardboard first! Your design will evolve as you go.

  7. #7
    sch
    sch is offline
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    Air resistance is fairly negligible until you are above 20 mph, and realistically 25 mph, so unless you can average speed in the + 20 mph range your efforts won't gain you much
    The pivoting bar/stem will make it harder to wrap something around your body however, so you will be limited to fairings tapered from seat back
    or wheel covers. The bubble device from Hostel Shoppe, again not sure how well that will mate with the pivotting bar/stem. On my Rotator Pursuit, the bubble
    fairing provided a wind resistance reduction equivalent to sitting in behind another rider on an uprite bike. I eventually decided it wasn't worth the weight and
    took it off after the plastic began to split around the edges. It is not hill friendly either, adding about 3-3.5# to the bike. There were some articles on DIY
    plastic "bubble" fairings like the Hostel Shoppe jpg but their location escapes me now. Main problems were the mold and rigging the mounts off the bar ends and
    the bike front. These date back 12-15 yrs. Found it: www.recumbents.com/wisil/whatsup.htm

  8. #8
    Insane Bicycle Mechanic Jeff Wills's Avatar
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    Quote Originally Posted by Notso_fastLane View Post

    Any other ideas?
    The OHPV gang did tests like this a few years back. Here's the photos: http://www.ohpv.org/events/albums/coastdn05/ . Note that Terracycle now owns Windwrap fairings and makes mounts for all sorts of recumbents. Fitting a GX bubble to your TW-bent bike would be no problem.

    I used one of the WISIL bubbles to make a partial fairing for my P-38 Lightning about 14 years ago. It turned a fast bike into a rocket ship (10-15% speed improvement at 20mph):
    Jeff Wills

    All my bikes.

  9. #9
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    Quote Originally Posted by sch View Post
    Air resistance is fairly negligible until you are above 20 mph, and realistically 25 mph, so unless you can average speed in the + 20 mph range your efforts won't gain you much
    I always understood it starts to get significant at 15mph.
    But either way do not forget this is air speed, not ground speed: worst case you are on tour and spend the whole day heading into a 10mph headwind: you do not need a high ground speed to get to fairly significant wind resistance !! and if the wind is 20mph...!

  10. #10
    sch
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    I should have said net speed (bike spd minus air spd), which is why you rarely have a true tail wind, bike is almost always faster than air speed so it feels like a headwind, just less of one
    than you feel riding in still air. Also why the worst case scenario is going uphill on a warm day with a tail wind, no cooling effect at all!! Another reason I didn't pursue another bubble
    (still have two in boxes) is they are noisy on chip seal and I had to smear car polish on the top of the fairing (and not remove it) to dull the fairing so as not to be blinded by the winter
    sun in mid afternoons. The specular reflection was just right to hit my eyes. I was interested one day to note air flow was over the top of the fairing and then immediately downturn
    inside the fairing and out the bottom- an errant leaf followed this track, and not as expected up over my head or into my face.

  11. #11
    Senior Member BlazingPedals's Avatar
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    Quote Originally Posted by sch View Post
    I was interested one day to note air flow was over the top of the fairing and then immediately downturn inside the fairing and out the bottom- an errant leaf followed this track, and not as expected up over my head or into my face.
    That's one thing about front fairings that isn't well-enough advertised. Sure, they smooth the airflow in front; but behind them is a big wind shadow -a big area of dirty air that's worse than if the fairing weren't there. Sometimes there's a net gain and sometimes not. You have to really pay attention to the flow coming off the rear edges and make sure you the rider are out of the air flow. If the air comes over the top and hits you squarely in the chest, you've just lost all the advantage you tried to gain. In the case of your leaf, it probably didn't get blown down so much as it fell out of the airflow.

    So, my rule of thumb is that front fairings will help if you're sitting more upright. That's because to make the fairing work you need to be as close to the trailing edge as possible. As you get more reclined, the air will flow over your body better but still create a shadow in back. Then, tailboxes start making more sense.

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