Aerodynamics thread?

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fitek   1 kW

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Aerodynamics thread?

Post by fitek » Apr 01 2008 11:42pm

Tried doing a quick search but didn't find a thread dedicated aerodynamics, though I remember someone talking about mounting a trash can lid to their bike a while back.

Aerodynamics and wrist strain is why I build recumbents exclusively (mostly wrist strain to be honest). When I did my vehicle power calculations spreadsheet a year ago I noticed how much the aerodynamic component balloons out of control after 25mph or so. At some point it becomes cheaper to improve aerodynamics rather than add motor and battery power (not sure what point that is).

Lacking carbon fiber molding skills and a wind tunnel, what can ebikers do to build cheap fairings and quantitatively compare them to other fairings?
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Malcolm   10 kW

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Re: Aerodynamics thread?

Post by Malcolm » Apr 02 2008 2:02am

I don't think anyone's done anything quantitative like this on ES before, but it would be interesting to see how little power can be used to a maintain a constant speed (20/30/40/50) on level ground. A new challenge perhaps? :P

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Re: Aerodynamics thread?

Post by fechter » Apr 02 2008 8:57am

There were some interesting ideas in this thread:
http://endless-sphere.com/forums/viewto ... lit=faring

My favorite was the rubbermaid trash can lid.
"One test is worth a thousand opinions"

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Re: Aerodynamics thread?

Post by vanilla ice » Apr 02 2008 11:41am

Fun thread there. I'm off to mount a GIANORMOUS bag on my handlebars now...

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Re: Aerodynamics thread?

Post by tomv » Apr 02 2008 3:41pm

There's a nice web calculator that lets you have an idea of the drag that will be generated:

http://www.kreuzotter.de/english/espeed.htm

I think their numbers are a bit optimistic though.

The thing is, it's extremely difficult to improve Cd. You have to maintain laminar flow all the way to the back of the bike to recover drag energy. Trash can lid, transparent fairing (zzipper.com) with fabric sides, etc will most likely make things worse, as you increase drag area more than you decrease Cd.

Or look at it this way - at 60mph car is more energy efficient than bicycle. I've decided to just ride under 20mph and be happy :) I've never seen a practical fairing yet. Close but no cigar:
http://www.ihpva.org/battle/WHPSC2001.html
http://www.velomobiel.nl/

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Re: Aerodynamics thread?

Post by Mathurin » Apr 02 2008 6:36pm

tomv wrote:http://www.kreuzotter.de/english/espeed.htm

I think their numbers are a bit optimistic though.
I've found it pessimistic for road bikes, unless I'm some kind of natural born athlete who somehow becomes ~50w more powerful when the road gets flat, and consider I wear jeans/tshirt to ride, old bike with box section rims, non-aero levers, bottle cages on the handlebars and so on... Then again, that simulator is made to sell Quest VM's.


Anyways one way I had improved aero on a bike without thinking about it was to strap my backpack on the handlebars one day it was ridiculously hot out. Made a difference in pulling one extra gear on the flat. But it also made my crotch area get wet from lack of air circulation.

Tho not long ago I rode with a MEC backpack that was all rounded and could comfortably eat up a 12 pack of pabst. It was better then my old backpack cause it had elastics that kept it nice and round and stuff, the top part didn't sag out and the back peice was made of i dunno what that made sweat not bad. Riding around with nothing in it was neat because I didn't even feel it once up to speed, but it only had a chest strap, no stomach strap to prevent it from raising up when popping the bike over potholes and stuff. By comparison my mess bag is a drag, but then it can swallow a 24 pack of cans with room to spare...
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Re: Aerodynamics thread?

Post by lawsonuw » Apr 03 2008 12:07am

I've done a little thinking on farings. Aerodynamics is a pretty complex subject. At the speeds we're interested in, I think we'd be best off coping the aerodynamics of a dimpled golf ball. I.E turbulent surface flow but a smaller wake (less drag) behind the bike because the surface turbulence makes the flow "stick" better. Anywho, data collection is going to be key for any project like this. A smoke pencil and someone riding parallel with a video camera would be one good tool. Short sections of bright thread taped to the surface with a free end can also show surface flow. I've also seen old wind tunnels equipped with banks of [urlhttp://en.wikipedia.org/wiki/Manometer#Liquid_Column]U-tube Manometers[/url] a precise cheap way to monitor 30+ pressure taps, some way to record the positions would be needed. (point a video camera at them!)

Marty

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Re: Aerodynamics thread?

Post by lawsonuw » Apr 03 2008 1:38pm

Another crazy thought. Since any full fairing other than a round torpedo is going to act like a wing in cross winds, why not take advantage of this? Enter the magic of Google http://home.swipnet.se/ansar/s.html. Dang that guy goes fast on ice, wonder if a similar fairing would fit over a bicycle? Such a fairing would need to be able to rotate on a vertical axis relative to the bicycle frame. 10 degrees left and right might be enough. Gusts would still be an issue. (although regulating the fairing/wing angle of attack with a tail could reduce buffeting from gusts) Worst case with a direct head wind it would still function as a great wind fairing.

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Re: Aerodynamics thread?

Post by Mathurin » Apr 03 2008 6:04pm

Yeah some dude on this forum did that, what's his name... It's a white home made recumbent trike with an odd shape, like teardrop if you look at it from the top, apparently it takes advantage of sidewinds. But his name escapes me...

Riding with tight fitting shorts & t shirt tucked in the pants lets me go faster then with jeans and baggy shirt. Taking the 3 inch wide headlight off my helmet made it pull back noticeably less on my head, but I couldn't feel a speed difference. Related, I'm pretty sure I'm more aero with the helmet then without it, but idk. Cola can thick (the old, narrow cans) tires make an xbox hueg difference next to thumb thick tires, may not be just aero tho. Dual suspension bikes are definitely a drag at speed no matter what, also mine make a kawaii whistling when going ~35km/h with a slight crosswind. I've seen a paper saying bottles mounted on the bars hurt aero next to downtube/seattube mount, I haven't noticed that but I find it more comfortable that way. Kinda like this:

Image


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Re: Aerodynamics thread?

Post by paultrafalgar » Apr 05 2008 5:10am

fitek wrote:Tried doing a quick search but didn't find a thread dedicated aerodynamics, though I remember someone talking about mounting a trash can lid to their bike a while back.
....
Lacking carbon fiber molding skills and a wind tunnel, what can ebikers do to build cheap fairings and quantitatively compare them to other fairings?
Fibreglass is too heavy (velomobiles weight ~ 90kg). Coroplast might be alright
(see: http://sports.webshots.com/album/556581441oVNuFf)
Carbon fibre is too difficult to work for hobbyists. Wind tunnels are too expensive for hobbyists.

I've often wondered if you could make a fairing by having a double skin of, say polypropylene or polyethylene, and pumping it up to a high pressure. The weigh would be low and it would be rigid to promote lamellar flow. What d'ya think?
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Re: Aerodynamics thread?

Post by TylerDurden » Apr 05 2008 10:00am

paultrafalgar wrote:I've often wondered if you could make a fairing by having a double skin of, say polypropylene or polyethylene, and pumping it up to a high pressure. The weigh would be low and it would be rigid to promote lamellar flow. What d'ya think?
I ruminate on inflatables everytime I use my van... it is shaped like a brick. Semi-trucks could use inflatable tailboxes. PMTG uses plastic tailboxes:
http://www.pmtghome.com/about.aspx
Image

Inflatables can be quite strong, but I'm not sure they'd be any easier to DIY than rigid materials.
:?
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Re: Aerodynamics thread?

Post by fitek » Apr 06 2008 2:11am

Probably just about everyone on this forum has done some mobile smoking. Just the smoke comes from using cheap lamp cord from the battery pack...

I'm not an aerospace engineer, not even a mech e, and I suck at math so I wimped out on all the fluid stuff (hail Matlab). There must be ways to improve aero without resorting to big huge fairings. Most ebikes out there look like a rats nest of wiring, battery bags, and enough headlights for a semi, which somehow doesn't seem like it's good.

I wonder how much would cleaning up the exterior of a bike help? (remove protrusions, replace battery bags/boxes with streamlined boxes or hide them inside the frame) Or is the rider just so much more frontal area that these little things don't really matter?

And whats up with the full suspension speed hit? Is it real? I swapped from a 20" kids bike to a FS frame (with the same wheels) and my max speed dropped by about 25%. That seems like a lot to lose from energy lost to the suspension bouncing around. The frame is heavier and probably less aero but the top speed before suspension was only 21mph anyway-- the aero component hadn't crept in very much yet. I'm gonna keep working on this bike, maybe a springy seat and front suspension is good enough.
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Re: Aerodynamics thread?

Post by paultrafalgar » Apr 06 2008 3:37am

fitek wrote: ...
Or is the rider just so much more frontal area that these little things don't really matter?
I seem to remember the statistic that at above 15mph 80% of you energy is spent overcoming wind resistance. On a delta bike, that is - one answer is to get lower (recumbent) :wink:
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Re: Aerodynamics thread?

Post by paultrafalgar » Apr 06 2008 3:52am

TylerDurden wrote:
paultrafalgar wrote:I've often wondered if you could make a fairing by having a double skin of, say polypropylene or polyethylene, and pumping it up to a high pressure. The weigh would be low and it would be rigid to promote lamellar flow. What d'ya think?
...
Inflatables can be quite strong, but I'm not sure they'd be any easier to DIY than rigid materials.
:?
O.K. Let's brainstorm - Borrow a Quest or Mango (velomobile) spray with silicone to make a release material - lay a sheet of polyethylene film over it - spray on glue - stick a layer of bubblewrap to that - spray on glue - stick another sheet of polythene to that - allow to dry - cut lengthways - remove velomobile - duct tape the 2 halves together - swathe around own recumbent trike - fit tight fitting collar from neck to cockpit rim - inflate - Viola! One boil-in-a-bag cyclist!
What d'ya think?
:D
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Re: Aerodynamics thread?

Post by TylerDurden » Apr 06 2008 9:38pm

paultrafalgar wrote: O.K. Let's brainstorm..
I'm ruminating on an enclosed delta that echos the styling lines of the SR71.
Image
(Turbines are optional.)
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Re: Aerodynamics thread?

Post by Link » Apr 06 2008 10:10pm

TylerDurden wrote:(Turbines are optional.)
That's a matter of opinion.
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Re: Aerodynamics thread?

Post by monster » Apr 07 2008 3:35am

what about simple things that people can do without having to build a dorky fairing?

e.g. i have a wire mesh basket on the front. does that help my aerodynamics? -like the tail gate up vs down on mythbusters?

there must be other things like rucksacks or slick tires?

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Re: Aerodynamics thread?

Post by kyakdiver » Apr 07 2008 8:55am

I was thinking about making one of these.... :mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen:
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Re: Aerodynamics thread?

Post by vanilla ice » Apr 07 2008 2:39pm

The best non-dorky way to go is the aero shaped carrying bags. I bet you could get a lot of benefit without even having one on the front, just a smartly shaped rear bag to close up the hole behind the rider. But also I bet a clear thin plastic front fairing could be pretty stealthy if you did it right. Maybe combine the two.

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Wind tunel on the cheap

Post by tomv » Apr 07 2008 3:12pm

Some tests showed that for partial fairings - rear fairings did as much good as front fairings, and neither did nearly as much good as full fairing.

I'm thinking of a way to actually measure drag. I need to know drag force. That would be

ma = (F_motor + F_pedal + F_downhill) - (F_air_drag + F_tire_drag + F_bearing_drag)

To simplify this, if I find level loop course, wait for day with little wind, accelerate to max speed and coast, then the equation above becomes:

ma = -F_drag

I can measure

a = dv/dt

And m is found on bathroom scales holding the bike :D . So all I need is a good way to measure dv/dt. I can think of:

- Use wheel magnet sensor plugged into data logger. Good: cheap. Bad: low sampling frequency

- Use hall sensors in the motor. Good: no extra parts. Bad: only works on e-bike, not other bikes. What kind of signal can i expect out of hall sensors? Is it discrete three pulses per rotation or is it continuous so i can know motor position more precisely?

- Strap an optical mouse to the fork and let it roll on the rim. Record digital signal going out. Good: Super high precision :twisted: Bad: not very sturdy and some work to setup.

- Use accelerometer sensor like in Wii. Good: works to measure drag of your car too! Easy to do. Bad: probably not very precise.

Any more ideas? It's so simple I wonder why nobody does it. I've read about some guy estimating drag by going down hill from the hill of known height/length and measuring times on the stopwatch. That's pretty much the same idea, but computer based logger would improve accuracy.

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Re: Aerodynamics thread?

Post by fitek » Apr 07 2008 5:17pm

I just wrestled a set of 40ah SLAs into my next bike, and I thought about how the batteries hang. Typically I just attach boxes under the seat on both sides. I measured the area of those boxes and its about 9 inches tall x 5 inches wide for each box. I realize this is bad for aero. With my full suspension frame, I tinkered with the idea of cutting the lower half of the diagonal frame member and replacing it with an L shape so that I can hang the batteries in line. This would present a frontal area of only 9 tall x about 5 inches wide. That said I wasn't crazy about cutting the frame that way so I found a way to hang the batteries inline where they present 9 inches by 7 inches. Little bit worse but a lot less welding. From the front the bike now looks a lot like a motorcycle, with a cage holding batteries rather than a motorcycle engine located between the riders legs. I'm thinking a stop by Tap Plastics for some coroplast might be in order...

I also thought about under seat steering but that would mean lifting the rider up higher. Since I haven't settled on a recumbent seat yet, I might still give it a shot. But it looks awfully complex to make it work and be comfortable.

It might be interesting to build up a basic bicycle and then start throwing crap on it and do a test run to see how max speed is affected. Well, I'll give some of this a shot with my next bike..
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Re: Aerodynamics thread?

Post by monster » Apr 07 2008 6:43pm

you might need a wind speed meter

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Re: Aerodynamics thread?

Post by Mathurin » Apr 07 2008 6:55pm

fitek wrote:And whats up with the full suspension speed hit? Is it real? I swapped from a 20" kids bike to a FS frame (with the same wheels) and my max speed dropped by about 25%. That seems like a lot to lose from energy lost to the suspension bouncing around. The frame is heavier and probably less aero but the top speed before suspension was only 21mph anyway-- the aero component hadn't crept in very much yet. I'm gonna keep working on this bike, maybe a springy seat and front suspension is good enough.
Yes, but from 21 to 15 mph sounds like something else's amiss. I'd check to see if the coaster brake's gotten too tight, easy to mess that up when installing such a wheel. The cog should be able to rotate pretty pretty freely in the slack between driving and braking. Otherwise a chain too tight steals power, maybe a rubbing brake? I assume you took pretty much the same position as on the other bike?
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Re: Aerodynamics thread?

Post by shinyballs » Apr 07 2008 8:07pm

Farings from zipper - http://www.zzipper.com/Products/prod_upright.php
Does the shape helps against wind drag?
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Re: Aerodynamics thread?

Post by lazarus2405 » Apr 07 2008 10:31pm

The simplest way to reduce drag would be to simply get in a better position. That means a recumbent, or for us on mountain bikes, an aerodynamic tuck. A "superman tuck" with aero bars would work wonders. Perhaps when my setup runs again, I'll check out what my lbs has.

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