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Why do many velos have flat bottoms?

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runcyclexcski View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote runcyclexcski Quote  Post ReplyReply Direct Link To This Post Topic: Why do many velos have flat bottoms?
    Posted: 29 May 2017 at 6:07pm
As an inspiration on a DIY HPV build, I looked at the most popular velomobiles, like the Quest and the Milan. They all appear to have flattish bottoms, i.e. the cross sections are not round/oval, they are more liked 'rounded triangles', with the 3 wheels sticking out of the mostly flat base.

In designing my moulds, I am thinking that having a rotational symmetry would make making the moulds the easier from a DYIer standpoint (e.g. a cylinder, or an ellipsoid could be turned on a large lathe). I do not want to scpulture by hand too much, and it's OK for me to lose 10% in drag coefficient for the gain of the ease of manufactuting.

Is the flat bottom important? The Aptera, the most aerodynamic car that has never been, had a mostly round-ish bottom. Vehicles that set speed records for HPVs look oval-ish in cross-section, e.g. http://www.popularmechanics.com/technology/a22946/human-powered-speed-record-aerovelo/

In side wind, one the HPV starts to get lifted from the side, wouldn't the flat bottom pick up the wind and make the problem even worse?
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GeoffBird View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 29 May 2017 at 7:09pm
I'd be interested to see your finished design but when you try it you might find it is more difficult than you think - you end up with a series of 'lumps' (for wheels, esp. at full-lock, pedal/heel clearance, chain-run, suspension etc.) that are very close together and consequently in danger of creating interference drag. A flattish bottom is also easier structurally. A car is a lot wider.

I'm not saying it is impossible. It would be easier with a straight-line record machine - I did propose to Karl at Windcheetah using some old Arion moulds with added wheel nacelles for a three-wheeled endurance record machine.


Edited by GeoffBird - 29 May 2017 at 7:11pm
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runcyclexcski View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote runcyclexcski Quote  Post ReplyReply Direct Link To This Post Posted: 29 May 2017 at 10:01pm
Geoff -- assuming this can be done, is there any obvious *disadvantage* of having a rounded bottom? Would a round bottom be more, or less resistant to lift off of the front wheels at high speeds, esp when hitting a bump? Ditto about the sidewinds -- thumbs up/thumbs down? I agree, cars are wide and heavy (no lift-off), so it's the most obvious to have a flat bottom.

I will sure post my progress if I make any. E.g. about intermediate crash tests of separate small modules (don't want to crash test the whole thing). I tend to have crazy ideas about making something 'novel', spending months researching, then realising there is an obvious obstacle that it won't work, and it never gets anywhere.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RoyMacdonald Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2017 at 6:13pm
My own opinion for what is worth is that air directed under the bike will be compressed and that compression will have an energy cost. So avoiding directing air under the bike would make it faster.
Roy 
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runcyclexcski View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote runcyclexcski Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2017 at 6:33pm
Originally posted by RoyMacdonald RoyMacdonald wrote:


My own opinion for what is worth is that air directed under the bike will be compressed 


Roy, if this is the case, then a rounded bottom would save energy...

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Post Options Post Options   Thanks (0) Thanks(0)   Quote atlas_shrugged Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2017 at 9:30pm
Good question why velos have flat bottoms since all the fastest UK cycles e.g. streamliners like the Beano, Arion 1/2 all have rounded bottoms.
 
The velomobiles are all 3 wheeled machines and because of this they cannot lean into corners. This means the seat must be made as low as possible to get the centre of gravity as low as possible. I would guess this may be the reason. But can check with velo builders in Mannheim what the main reason is next week.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2017 at 10:55pm
You might imagine there would be room for big tunnels between the wheels and the 'cockpit' - that's what I thought until I tried it, but, as I say above, there isn't room. Here is a front view of a practical trike to illustrate. The remaining tunnels are very shallow and could potentially create more drag rather than less, especially if the air tries to flow across them rather than along them:


Right Time - Right Place - Wrong Speed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2017 at 11:02pm
You could just cut holes in the side of a big teardrop shape but then much of the front tyres are exposed to airflow and the wheel cut-outs would be large to allow steering:




Edited by GeoffBird - 30 May 2017 at 11:04pm
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GeoffBird View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2017 at 11:07pm
In design, many things become clear when you actually try them rather than just talking about them. That's why I hate it at work when people try to make design decisions in meetings! Meetings are good for brainstorming and evaluating alternative designs, but not for actual designing.

Edited by GeoffBird - 30 May 2017 at 11:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2017 at 11:23pm
This is what i ended up with, but it is slightly compromised by ease of manufacture. I'd like to try a more sculpted lower half as a Mark Two:




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