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Thread: A discussion in aerodynamics..rain on my windshield

  1. #1
    It's a way of life! oldnslow's Avatar
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    A discussion in aerodynamics..rain on my windshield

    I'm riding my 98 R1100RT with CeeBaileys' #2 3 inch taller windshield. So the girlfriend and I are riding home the other day and it was pouring down rain, (later heard it was about 2-3'' in an hours time) and we were riding 60 mph on the 4 lane highway with little to no traffic. Wind was calm, it was just rain, not a 'storm'.

    The observation was this; the rain drops that struck the windshield did not have a tendency to blow off of the shield unless I lowered the shield to its lowest point, at which time they would slowly roll to the top of the shield and promptly blow on to my face shield and then blow away from my face shield. If the windshield was raised to say half way up, and creating a quiet pocket of air for me and the girlfriend, the water drops would remain stationary on the shield! I would still be looking over, not through, the shield at this point.

    The bikes aerodynamics as a whole kept us both dry, with all water being pushed up, over, or around us we travelled. I was very pleased with that and with the bike as a whole. Our ride was for at least an hour in the down pour.

    The windshield thing has me puzzled.

    Subject posted for discussion, knowledge, and conversation purposes only!
    Mike Davis
    "Old n Slow" It's a way of life!
    1985 K100RT

    1998 R1100RT

  2. #2
    Brick Pilot der ziegelstein's Avatar
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    Short simple version:
    As any shape moves through a fluid (air is a fluid) it will have a layer of that fluid "attached" to it. That layer is called the boundary layer. With the windshield in a more vertical position the boundary layer of air is thicker. As you lower the angle of the windshield this boundary layer decreases to the point that the water droplets stick through the boundary layer. When this happens the droplets are whisked away by the air as it rushes past.

    Long technical version:
    http://www.grc.nasa.gov/WWW/k-12/airplane/boundlay.html

    On my RT when I lower the windshield to its lowest position the water on my helmet's visor travels up and away.

    On my RS the water stays on my visor unless I turn my head to either side to clear it. That is the same principle you are seeing with your windshield.

    Rain-X works by changing the surface tension of the water allowing it to form larger droplets that can then protrude through the boundary layer and roll off of your windshield.
    Jim McGill
    '87 K100RS
    '04 R1150RT

  3. #3
    Registered User
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    I just bought an adjustable w/s bracket to hopefully "find that special location"(alluded to above) where I'm afforded the best protection. As for winglets, maybe, maybe not for me, depends on how well the 1st thingy works...
    "If I had my life to live over, I'd dare to make more mistakes next time...I'd relax,I'd limber up... I would take fewer things seriously...take more chances... take more trips...climb more mountains...swim more rivers...eat more ice cream." Jorge Luis Borges at age 85.

  4. #4
    Cage Rattler wezul's Avatar
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    I was just gonna say "Duuuude!" and be done with it.

    Have been caught out in the wet a few times running both the Cee Bailey lip and the OEM screen.
    I sit tall in the saddle although I am constructed like a Weeble, I've found it best for me to go OEM about halfway up and let it skim over top of helmet.
    I do like more air than not.
    Cee Bailey is collecting dust, pricey experiment.

  5. #5
    na1g
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    Then sometimes you get the rain running up the back side of the 'screen.

    On my old Honda ST1300, with the 'screen most of the way up, the rain would go over my head and wet my back. Strange at 50 mph to stay dry on the front but get wet on the back.

    Aerodynamics can be strange indeed. Look at a Formula 1 car.

    pete

  6. #6
    It's a way of life! oldnslow's Avatar
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    Quote Originally Posted by na1g View Post

    Aerodynamics can be strange indeed. Look at a Formula 1 car.

    pete
    I've heard that F1 cars are actually 'flying, but in the wrong direction', in other words, they create more than 1g of downforce, and that at speed they would actually stick to the roof of a tunnel! ?

    F1 cars are absolutely amazing in every aspect! Have you ever seen the various controls on a an F1 steering wheel!!? Holy cow!
    Mike Davis
    "Old n Slow" It's a way of life!
    1985 K100RT

    1998 R1100RT

  7. #7
    Registered User 36654's Avatar
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    Quote Originally Posted by der ziegelstein View Post
    Short simple version:
    As any shape moves through a fluid (air is a fluid) it will have a layer of that fluid "attached" to it. That layer is called the boundary layer. With the windshield in a more vertical position the boundary layer of air is thicker. As you lower the angle of the windshield this boundary layer decreases to the point that the water droplets stick through the boundary layer. When this happens the droplets are whisked away by the air as it rushes past.

    Long technical version:
    http://www.grc.nasa.gov/WWW/k-12/airplane/boundlay.html

    On my RT when I lower the windshield to its lowest position the water on my helmet's visor travels up and away.

    On my RS the water stays on my visor unless I turn my head to either side to clear it. That is the same principle you are seeing with your windshield.

    Rain-X works by changing the surface tension of the water allowing it to form larger droplets that can then protrude through the boundary layer and roll off of your windshield.
    I think you're confusing a stagnation point was a boundary layer........
    Cave contents: 99 R11RS, 2013 Toyota Tacoma, 03 Simplicity Legacy, 97 Stihl FS75, Dewalt DW625 & DW744

  8. #8
    Brick Pilot der ziegelstein's Avatar
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    Quote Originally Posted by 36654 View Post
    I think you're confusing a stagnation point was a boundary layer........
    Yes I am, in regards to the helmet reference. Thanks for pointing that out. By turning my head to the side the stagnation point is moved to a different location on the visor.
    Jim McGill
    '87 K100RS
    '04 R1150RT

  9. #9
    advrider.com
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    Quote Originally Posted by wezul View Post
    Cee Bailey is collecting dust, pricey experiment.
    In 2004, I bought a brand new, fancy Givi windscreen. I have yet to mount it.. but I still ride year-round.

    Cool thread. Y'all must be rocket surgeons!

  10. #10
    MOA,RA,ABC,AMA,TT,MOAL brownie's Avatar
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    Angry Rain X

    Have never heard anything good about Rain X on PLASTICS!!!!
    Heed NEAD: No Egos, Attitudes, Distractions!!!!!
    Shep Brown MOA 27510
    "Igor" 82 RS "Inga" 04RT
    Pensacola, Flairider

  11. #11
    Registered User miairhead's Avatar
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    This is from RainX web site

    Avoid using Rain‑X? Original Glass Treatment on any plastic surfaces, including motorcycle windshields, ATVs and solar panels
    Tom
    '84 R100RT '04 CLC(gone) Honda NT700V
    BMW
    Beer Motorcycles Women

  12. #12
    iscream-stop
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    I too have been caught in the 3" hour rain showers. On my LT I have a Cee Bailey with the wings and will generally play with the adjustment untill it works just right for the conditions. I like it very low to get air most of the time but in the rain it depends on the speeds and how heavy rain is. Of course I learned the hard way that first time about tucking gloves under sleeves. The funny looks I would get from cars I was passing when I was holding arm up dumping them out was worth it though. I did find that if I put windshield too high the gap at bottom would allow to much water running up inside and it ended up on me. Then there are the trucks going by and the buckets of water that get splashed into your lap that no angle can help.

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