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geoff birds new trike

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robh24 View Drop Down
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    Posted: 01 April 2015 at 1:35pm
Not just his trike but general faired bikes. The cfd and all I have ever seen is always straight into the incoming air. Reading solar car books its clear some shapes perform significantly better than others when not heading straight into the incoming flow. It must be worse for bikes as the wind vs bike speed is closer than cars. Does anyone have any details of how knee blisters head bubbles etc effect drag?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robh24 Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2015 at 2:46pm
*sidewind would have been a better term
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2015 at 11:25pm
Hi rob, you can do CFD with the model at an angle to airflow - If I had unlimited access to CFD facilities then it is something I would definitely try. Graham Sparey-Taylor and I were plotting to do some CFD runs on different streamlined bike shape a few years ago to research crosswind stability but nothing came of it.

Blisters and bubbles aren't the ideal aerodynamically - a homogeneous shape is better as there is less parasitic drag, but if you are designing a circuit racer (or road machine), as opposed to a Battle Mountain machine, then it has to be practical for use in a wide range of conditions otherwise that theoretical speed will never, or rarely, be realised.

In terms of side wind stability as opposed to efficiency, a profile that maintains attached flow in a side wind will probably be less stable as it will act like a wing or sail to create a larger side force.

New smoother nose below.




Edited by GeoffBird - 02 April 2015 at 11:40pm
Right Time - Right Place - Wrong Speed
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robh24 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robh24 Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2015 at 7:39am
It looks lovely and I understand the importance of being able to see where your going. I was just wondering how much the drag increase therefore the speed drop? I suppose the competition slows as well so it's not a problem. Once you have a cad model how long does a cfd simulation

Hopefully I'll make it to at least one event this year

Edited by robh24 - 03 April 2015 at 7:46am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote blogwat Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2015 at 9:32am
Good luck with the new build Geoff it looks amazing can't wait to see it in real life and watch how it performs.
Jeff.
when your up to your waist in aligators they forget to tell you you've got to clear the swamp
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2015 at 11:22am
Thanks guys. I don't know how long it takes to do a CFD simulation cos someone else (very kindly) did it for me. You have to create a mesh from the CAD surfaces first and then let the computer whirr away.  the initial run suggested as much as a 25% drag penalty for a lumpy shape, but it was a fairly crude shape, so don't know without doing more tests.

Edited by GeoffBird - 03 April 2015 at 11:27am
Right Time - Right Place - Wrong Speed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Yanto Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2015 at 4:49pm
i'll be following this project, oh yes!

Looks rather good by the way Big smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2015 at 7:41pm
I was very much inspired by your variants of Bubble & Squeak/Wobbly Bob, Ian.
Right Time - Right Place - Wrong Speed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Yanto Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2015 at 8:02pm
Too kind, at least you will be able to do it properly, i won't be offering my technical skills.

I hope I fit in it though, wouldn't mind a spin in it, not literally of course!

A couple of thoughts about it though; with the height and length of the foot/knee bubbles seeing the apex of a bend might be a problem, of course if you take a nice safe line it won't matter, but if you want to running with a tyre on the edge of the grass it might, also i think you might have overestimated the gap between knee bulges. However without seeing scale drawings it's hard to tell, it also depends on the width of your hips and what width BB you are using of course, just sit and stick your legs out and see what width you have between your knees and then allow clearance for the bubbles, believe me knees and bubbles don't go together, and if they do blood will soon flow.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2015 at 9:39pm
I'm not using a particularly narrow bottom bracket for that reason. The ergonomic mock up should ensure everything clears. It looks like I might have quite a good sight line behind the bubbles. It shouldn't be any worse than the Milan.
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