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Discussion Starter #1
Hi,

I've got an interesting question that I couldn't find an answer for.

I have a 09 LT with a CB Wing Screen. Normally, during tours, I take the trunk off, and put on one of those custom racks; then put two bags on.

My question has to do with Airflow and minimizing back pressure.

Right now, I've noticed when the screen is all the way up, I get some backpressure. The configuration is just with the luggage rack; not the trunk. .

Do you think I'll have less backpressure if I leave the trunk on and put a bag on the back seat, or if I take it off, and put two bags on..

http://geminiphotos.smugmug.com/Machines/BMW-K1200LT/9950577_PibuL#640614679_UCDhz

I'm thinking the trunk will change the flow so that it minimizes the flow....

Thanks

Dave
 

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Hi, Not for anything but, the beauty of an electric adjustable windshield is that you can adjust it on the fly for different speeds, visibility, and weather conditions. If you want less back pressure, just lower the windshield until it goes away. It is a matter of making the windshield adjust to your needs, with the top box, without the top box, with a bag on the back seat, what ever your needs are, you just adjust the windshield to suit. I have never had a motorcycle with an electric adjustable windshield before, and I absolutely love it!!! I would have never guessed what a great feature it is to have on a motorcycle. :bmw:
 

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You will find pivoting the wind deflectors also makes a big difference to back pressure.
 

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I've toured with all the configurations you've mentioned - and none of them seem to have had any effect on back pressure - which can be pretty annoying after a while.

For me, the V-stream windshield is the best of the bunch at reducing this. I also noticed that I don't "mess" with it as much - I tend to bump it up about an inch and a half off the bottom stops and just leave it there, except at low speeds when I appreciate the extra visibility and airflow.

What has worked for me when touring two up is to mount a fully stuffed "Mag's bag" on top of the trunk luggage rack. It really smooths the airflow - and especially for the passenger.

The airflow doesn't detach - it stays laminar. Pilot speak for no turbulence. ;)
 

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Discussion Starter #5
Thanks Ron.

I'm thinking about this all night, and regardless, I just bring too much stuff to rely on the trunk;

:))))

D
 

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Saw a GW trike this week with a small clear shield attached to the front of the top truck rack. I didn't get to ask why (may have been to shield the stuffed animal they all seem to carry up there) but figured it was to act like the curtain they have in some convertibles to reduce the back pressure.
 

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I've been playing with the airflow configuration a lot the past few weeks while travelling... have to when you've got 54 metre long road trains flying past at you at over 200kph (terminal speed for both of us) and I'm hauling a ~250kg camper-trailer...
Using a combination of windshield (V-stream in my case) and the side flipper vents to adjust the wind bubble for both my seating/leg position combination and the external wind direction....
Higher windshield puts more back-pressure on me, but then that feels less if I sit higher/straighter... than it does if I slump down in the seat. Moving feet from standard pegs to highway pegs (MOPs) or back to the passengers pegs also changes body position and hence the sense of the air-bubble effect.... I find each time I move I have to adjust the windscreen to get the most neutral position again.
Also varies whether I've got the visor up or down.. weird but it also affects the "apparent" sensation of where the air-bubble is.. possibly because of the wind-noise reduction with the visor down... with visor up I tend to raise the windshield again a little.
The final trick I've learned is that if there's a heavy cross wind pushing the bike a little sideways, keeping the windward side flipper out and narrowing the leeward(?) or opposite flipper closer to the bike seems to help right the bike by allowing more air to push around the non-windy side of the bike... it seems to stabilise the bike more.

And if I'm on the windward side of a road train I don't have to do anything but cruise past.... if he's on the windward side... I need to duck behind the windshield and hang on for dear life... the wind rush after they pass is amazing!

Oh, and this is with a trunk on.... :rolleyes:
 

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Discussion Starter #8
I spent the weekend testing, and well..I've got the answer.

The case deminishes the back pressure quite a bit. I did several configurations including with and without bags on rack, on rack / back seat; then put the case on, and ran with and without bags on the back seat.

Best configuration was with bags on the back in the same level as the top of the top case. There was literally no back pressure at all...

I wish I had done a mpg check too, but I didn't think of it till later...Either way, this week, I'm going to streamline the configuration with a tarp/net, and see if I can increase mpg anyway.

Meanwhile, testing was done from 60-90mph.
 
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