* * * Aerodynamics * * *

General Discussion about electric bicycles.

* * * Aerodynamics * * *

Postby safe » Tue Jan 16, 2007 11:06 am

I know that this is an "Electricity" based messageboard, but when you go about designing an electric bike you have to decide whether aerodynamics are going to be a factor or not. The simple fact of the matter is that the legal limit for most electric bikes is 30 mph or in many cases 20 mph. At those low speeds aerodynamics have almost no effect. Above 30 mph the effects of wind resistance predominate to such a degree that you can almost factor out other issues. (like losses due to gearing, tires, etc..)

Let's see if there are ideas or opinions about this:

:arrow: Should the electric bike designer ever concern themselves with aerodynamics?


Maybe designing for aerodynamics is a confession of guilt for breaking the law?
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Postby Lowell » Tue Jan 16, 2007 3:24 pm

I guess it depends on the inteded purpose of the bike. I think all of us are aware of the speed potential of designs like this:

http://www.seriouswheels.com/2006/2006- ... 24x768.htm
http://en.wikipedia.org/wiki/Image:SamWhittingham.jpg

And while it's nice to see people push the envelope, most people would have little use for something so impractical. I'd like to see peoples aero enhancements that keep the every day usefulness of the bike.
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Postby D-Man » Tue Jan 16, 2007 3:37 pm

Is there any low cost bolt on aerodynamics that a person could use on a regular bike?
Last edited by D-Man on Sat Feb 24, 2007 10:57 pm, edited 2 times in total.
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Postby safe » Tue Jan 16, 2007 4:11 pm

D-Man wrote:Is there any low cost bolt on aerodynamics that a person could use on a regular bike?


The problem with the "bicycle" is that you have to be able to pedal. Without that restriction you can do much more with aerodynamics and electric motors because you could have a realistic expectation that the rider would accept a tighter riding position.

There are really two factors in aerodynamics. One factor is the total frontal area and the other is the shape of the object. There are factors in streamlining that have to do with "eddy currents" that are very hard to figure, but frontal area is a real "no-brainer". So if an electric bike (without pedals) can cut a path though the air with half the frontal area then you will get the corresponding effect in lowered wind resistance. This is why "bents" can beat most upright bikes.

Apparently in the early 1900's someone tried to enter the Tour De France with a "bent" and they banned him from riding. Lance Armstrong has acknowledged the value of "bents".

And yes, you can buy fairings for "bents".

But the area of the "Electric Bicycle Road Racer" (my personal interest) doesn't exist yet to my knowledge. (it's "virgin" territory)

An electric bike can go FASTER if the rider is in a tuck (no pedals) THAN if they pedal assist (causing increased frontal area) because wind resistance dominates that much...
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Re: * * * Aerodynamics * * *

Postby Geebee » Tue Jan 16, 2007 4:19 pm

safe wrote:I know that this is an "Electricity" based messageboard, but when you go about designing an electric bike you have to decide whether aerodynamics are going to be a factor or not. The simple fact of the matter is that the legal limit for most electric bikes is 30 mph or in many cases 20 mph. At those low speeds aerodynamics have almost no effect. Above 30 mph the effects of wind resistance predominate to such a degree that you can almost factor out other issues. (like losses due to gearing, tires, etc..)

Let's see if there are ideas or opinions about this:

:arrow: Should the electric bike designer ever concern themselves with aerodynamics?


Maybe designing for aerodynamics is a confession of guilt for breaking the law?


The statement that aerodynamics have almost no effect at these speeds is so wrong, at speeds above 30 kph wind resistance is 80% of the resistance to forward motion.
Using the http://www.kreuzotter.de/english/espeed.htm a MTB at 160 watt input will achieve 16.3 mph, whereas the same wattage on a velomobile (Quest) will achieve 25.6 mph.
Put another way the MTB will require over 3 times as much power (over 500w) to achieve the same speed.
Real world my GT3 trike which is much, much lower drag than a MTB, but putting a shell on it increased speed from 48 kph bare upto 63 kph on a control hill that I can easily replicate a no pedal speed, and my shell is optimised for practicality rather than speed.
Oh and if you throw in a 20 mph head wind it is barley discernable in a velomobile, how about on a bike?
Velomobiles are slowly starting to increase world wide as people realize all the benefits, speed being one of them.
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Postby Lowell » Tue Jan 16, 2007 4:34 pm

http://www.freepatentsonline.com/4066290.html

http://damonrinard.com/aero/aerodynamics.htm

This page has a lot of good aero info. Who knew taping over your shoe laces could be worth 7 seconds on a 40km time trial.
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Postby xyster » Tue Jan 16, 2007 5:18 pm

Ok, so who here is going to test this?
http://damonrinard.com/aero/aerodynamics.htm

For the first trial shave their legs, tape their laces, wear a one-piece skin tight suit, riding for an hour in a super-tight tuck while measuring power used. Then for the second trial, grow their hair like a hippy, wear super-baggy clothes, attach every implement on that list to their bike, riding for an hour standing on the seat bent over at the waist holding the handlebars.

Then post the results to YouTube.

Since this aero stuff is Safe's topic, I vote he does :-)
Or perhaps we should write Mythbusters.... Jamie will do it in the name of Science should Safe refuse :-)
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Postby safe » Tue Jan 16, 2007 5:20 pm

"Also the idea to achieve a more favorable frontal area and thus a smaller aerodynamic drag by assuming a different seating position had already emerged before the turn of the century. Mochet's recumbent set several new hour world records. Starting from 1913 records were broken with aerodynamically faired racing cycles (5, 6). However, the governing body of bicycle racing, the Union Cycliste International (UCI), did not view these as regular records and tried to prevent any possible technical advantages to individual racers by changing the regulations. Racing should serve as a comparison of athetes, not a comparison of technology. Because of that the most important incentive to aerodynamic improvements to the bicycle was omitted going forward."

Amazing isn't it. (that was from Lowells link) They basically made illegal what could have been a technological race towards a better machine about 100 years. We've essentially been living with a decision that someone else made.

Aerodynamics is the single biggest factor on a bicycle, but the rules in racing don't allow very much experimentation on it.

They should run a special "Tour De France" equivalent race and allow any bike the rider chooses for each segment. ANYTHING. Then see what works...

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Postby safe » Tue Jan 16, 2007 5:26 pm

It might be interesting to combine all these things:

:arrow: Very Small Motor - 500 Watts or less.

:arrow: Very Light Battery - Lithium pack.

:arrow: Very Many Gears - Expand torque options.

:arrow: Very Slick Aerodynamics - So it can use the gears.

And find out just how far this can all go... I would suppose people have already done that... anyone know some good links?
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Postby rohorn » Tue Jan 16, 2007 5:49 pm

A 26~mph standup scooter of mine ended up going 34~mph after it was turned into a lowracer - with no drivetrain changes at all.

If speed, range, and weight matter, then aerodynamics is very important. There is no good reason to mimic horseback riding ergonomics anymore.
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Postby xyster » Tue Jan 16, 2007 5:55 pm

A few months ago during a blustery ride I noticed that wearing a padded windbreaker created noticeable drag and increased power use.
Ebike: 5304/20", 72V 35A controller, 33AH 80V 20s15p (18650 sized cells) DIY lithium-ion pack
http://endless-sphere.com/forums/viewto ... 47&start=0
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http://endless-sphere.com/forums/viewtopic.php?t=148
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Postby safe » Tue Jan 16, 2007 6:36 pm

Okay, here's a little bit of a mind warping bike. If you use all the standard numbers that I've been using, 185 lb rider , 100 lb bike, low wind resistance and then take a 750 Watt motor and run it in "super efficiency" mode by limiting it with a 20 Amp controller to produce a peak of just 564 Watts and add all this to an 8-speed internal hub transmission (which is definitely not going to break with only half of the power the hubs can stand) you get a bike that can climb just about any hill and hit 30 mph in three separate gears. That means that if you have a slight downhill you can exploit that to your advantage and get up to an rpm "hard" peak of 44 mph.

What's nuts is the range. If you use a 36 Volt 40 Ah pack (120 "D" cells for $600) you can expect to get a "worst case scenario" of 50 miles and a best case about twice that. The runtime is in the order of a few hours. You could literally go some real distances with this thing!

Spend more money and it gets even crazier...

This could be a really cool bike to own. Not the fastest, but it would open your mind to range possibilities that you never would have imagined.

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Postby xyster » Tue Jan 16, 2007 7:01 pm

Don't know if this qualifies as "crazier" exactly....If I keep current draw between 5-10 amps (20mph -25mph) by watching the ammeter, I get 3+ hours of runtime regardless of aerodynamics. My 33ah lithium pack cost about 50% more than the NiMH pack above, but has ~60% greater capacity.
Ebike: 5304/20", 72V 35A controller, 33AH 80V 20s15p (18650 sized cells) DIY lithium-ion pack
http://endless-sphere.com/forums/viewto ... 47&start=0
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http://endless-sphere.com/forums/viewtopic.php?t=148
Ebike: '06 Currie Mongoose, 32V 35A, 32V 22AH hybrid SLA/Li-ion pack
http://endless-sphere.com/forums/viewtopic.php?t=1010
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Postby TylerDurden » Tue Jan 16, 2007 7:27 pm

safe wrote: 100 lb bike, low wind resistance and then take a 750 Watt motor ... you get a bike that can climb just about any hill and hit 30 mph in three separate gears.

If you use a 36 Volt 40 Ah pack (120 "D" cells for $600) you can expect to get a "worst case scenario" of 50 miles and a best case about twice that. The runtime is in the order of a few hours.


Sounds good! I'm even thinking of a lighter bike, a smaller pack, like 30AH, and 35 mph max speed, using a bottom-bracket setup. The resulting range and speeds are darn near perfect for the daily-driver I plan to build.
Have a Nice Day,

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Postby Jay » Tue Jan 16, 2007 8:41 pm

Has anyone tried one of these Zzipper fairings on an upright bike?

http://www.zzipper.com/Gallery/DF/index.htm

I'd like to, but they're out of my price range.

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Postby Lowell » Tue Jan 16, 2007 9:10 pm

Jay wrote:Has anyone tried one of these Zzipper fairings on an upright bike?

http://www.zzipper.com/Gallery/DF/index.htm

I'd like to, but they're out of my price range.

Jay


Looks like the right idea, I'd like to see test data on an upright bike. Price seems fine to me for a low volume piece?
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Postby xyster » Tue Jan 16, 2007 9:15 pm

Fairings can be made on the cheap by cold forming polycarbonate (Lexan):

http://www.onlinemetals.com/merchant.cf ... its=inches

I've been planning to buy a 24x36" sheet of this stuff for a fairing, and fab clamp-on framework from PVC pipe.

The ATV fairing here is polycarbonate, which I also understand is easy to cut as well as form without special tools:

http://www.nationalcycle.com/catalogue/fairings.html
Ebike: 5304/20", 72V 35A controller, 33AH 80V 20s15p (18650 sized cells) DIY lithium-ion pack
http://endless-sphere.com/forums/viewto ... 47&start=0
Scooter: '06 Stealth s1000, 48V 30A, 4x10ah SLA
http://endless-sphere.com/forums/viewtopic.php?t=148
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http://endless-sphere.com/forums/viewtopic.php?t=1010
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Postby safe » Tue Jan 16, 2007 9:22 pm

The shape of a fairing is very important. If the front is too flat it will actually create a wake that is worse than no fairing at all. If the fairing is too curved back onto itself you risk an "eddy current" that will develop as the wind smashes back onto the surface of your body. Ideally the shape should very closely follow your own outlines so that the least amount of turbulance is created. The "extreme" is to actually encase yourself in an aerodynamic shape from front to rear.

I suspect many people tend to make a mistake and create too flat of a fairing. You want the front to be nearly flat and the sides wrapping backwards.

This guy got it right... the shape of the sides is more important than the front...


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Postby xyster » Tue Jan 16, 2007 9:39 pm

Safe: if I'm cold-forming a 24" wide sheet of polycarbonate into a fairing for my bike, what degree of curvature would be optimal?

Or put another way: in it's final form, how many inches ahead of the edges should the middle be pushed forward?

I'll probably just bend it into one, simple arc with one curvature value.

I've contemplated constructing a polycarbonate fairing for some time also to protect from flying road debris, and biting wind chill. So it needn't be 100% as aerodynamic as it could be...but I certainly don't want it less aerodynamic than no fairing at all.
Ebike: 5304/20", 72V 35A controller, 33AH 80V 20s15p (18650 sized cells) DIY lithium-ion pack
http://endless-sphere.com/forums/viewto ... 47&start=0
Scooter: '06 Stealth s1000, 48V 30A, 4x10ah SLA
http://endless-sphere.com/forums/viewtopic.php?t=148
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http://endless-sphere.com/forums/viewtopic.php?t=1010
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Postby safe » Tue Jan 16, 2007 9:43 pm

Get as close to an aerodynamic shape as possible...


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Postby D-Man » Tue Jan 16, 2007 9:44 pm

Somehow, this doesn't look too aerodynamic from zzipper.
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Postby safe » Tue Jan 16, 2007 9:48 pm

D-Man wrote:Somehow, this doesn't look too aerodynamic from zzipper.


The flaired ends are just plain wrong. What where they thinking? :roll:

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Postby xyster » Tue Jan 16, 2007 9:51 pm

Get as close to an aerodynamic shape as possible...


Can you quantify that statement so I can mold it as close to aerodynamic shape as possible?
Ebike: 5304/20", 72V 35A controller, 33AH 80V 20s15p (18650 sized cells) DIY lithium-ion pack
http://endless-sphere.com/forums/viewto ... 47&start=0
Scooter: '06 Stealth s1000, 48V 30A, 4x10ah SLA
http://endless-sphere.com/forums/viewtopic.php?t=148
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http://endless-sphere.com/forums/viewtopic.php?t=1010
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Postby safe » Tue Jan 16, 2007 10:30 pm

Image

Just be sure to be a little like the "half-sphere" or the "streamlined body".

It should wrap to close to flat (parallel) on the sides...

Look at those numbers!
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Postby xyster » Tue Jan 16, 2007 11:20 pm

Thanks. Since it'll only be 24" wide to begin with, I'll probably try to bend it into a half-sphere -- better than my cube-ic chest and head anyway.
Ebike: 5304/20", 72V 35A controller, 33AH 80V 20s15p (18650 sized cells) DIY lithium-ion pack
http://endless-sphere.com/forums/viewto ... 47&start=0
Scooter: '06 Stealth s1000, 48V 30A, 4x10ah SLA
http://endless-sphere.com/forums/viewtopic.php?t=148
Ebike: '06 Currie Mongoose, 32V 35A, 32V 22AH hybrid SLA/Li-ion pack
http://endless-sphere.com/forums/viewtopic.php?t=1010
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