* * * Aerodynamics * * *

[Lowell]

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?

[xyster]

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

[safe]

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...

[xyster]

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.

[safe]

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

[D-Man]

Somehow, this doesn't look too aerodynamic from zzipper.

[safe]

Somehow, this doesn't look too aerodynamic from zzipper.

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

[xyster]

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?

[safe]

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!

[xyster]

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.

[rohorn]

The aerodynamics of modern race bikes, like the RS125 above, are horrible. Check out the Cd numbers on them some time.

It's the result of a combination of racing rules and market driven styling. The "dustbin" racers from around 50 years ago were more aerodynamic than modern GP bikes.

Check out some NASA publications on low drag bodies........

[Mathurin]

The aerodynamics of modern race bikes, like the RS125
above, are horrible. Check out the Cd numbers on them some time.[...]

Indeed it would seem:
http://www.gravitybike.com/forum/viewtopic.php?t=31



One day I found a way to get an aero advantage on my FS mountain bike
while commuting back from work. It was hot, a day where you sweat at
the least effort, and my back was getting wet. So, I stopped and carefully
attached my backpack onto my side of the handlebars, as there was
nowhere else to put it. It made 1-2 gears difference over the 24, with
400% ratio from first to last gear. So that's what, 15%? I dunno, but it
was a really pleasant surprise.



Some home-made fairings that make polycarbonate sheets +appealing:
http://www.gravitybike.com/forum/viewtopic.php?t=145


How to make wheel disks:
http://www.wisil.recumbents.com/wisil/w ... eldisk.htm

[safe]

The aerodynamics of modern race bikes, like the RS125 above, are horrible. Check out the Cd numbers on them some time. It's the result of a combination of racing rules and market driven styling. The "dustbin" racers from around 50 years ago were more aerodynamic than modern GP bikes.

In the early days people tried completely wrap around fairings, but people crashed whenever a strong wind hit them from the side. They realized that if they limited the front fairing to the center of the front wheel (vertically) and on the rear to the back of the rear wheel that you could mostly prevent this danger.

It's not a bad rule... it's accepting reality....

My opinion is that you want to get "good" aerodynamics and not seek after "perfect" aerodynamics. A little streamlining goes a very long way. The main thing is to get the "tuck" position (right there you cut your frontal area by about half) and get some smoothness in the front to keep the air out of the "pocket" that forms between your arms and chest area. That "pocket" generates a HUGE wake. It's like riding a "concave shape" (edges facing forward) into the wind. Really bad...


If we look at this chart again:



...and consider that a regular bicycle has the aerodynamics of a "cube" at about "1" if you can simply get to the level of a "half sphere" you can cut things in half. The fully streamlined shape can get as low as "0.04" which is WAAAAAAY better than the "half sphere", but it doesn't change the fact the we were starting from a "cube" shape.

Perfection is the enemy of "good enough".

[D-Man]

Safe, how much would you think a person would gain in mph given an upright e-bike and speed of 30 mph (no fairing) or more for each drag coefficient? Also, how much loss should there be from no load (wheel off the ground full throttle) to (load) actual mph for an aerodynamic e-bike set-up.

[safe]

Safe, how much would you think a person would gain in mph given an upright e-bike and speed of 30 mph (no fairing) or more for each drag coefficient? Also, how much loss should there be from no load (wheel off the ground full throttle) to (load) actual mph for an aerodynamic e-bike set-up.

Going to:

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

...and putting in 815 Watts after selecting "MTB" we get 30 mph. If you change from "MTB" to "Lowracer" you will go 44.5 mph. But those are "peak" wattage values which are NOT always the same as what will actually happen because the "peaks" don't always line up right.

The best thing about gearing is that you can make your motors peak just equal to what the aerodynamic peak can be.

A hub motor (5304 72 Volts) might "peak" in power at 40 mph, but if that same power was geared higher it "could" go 48 mph. Power drops off after the power "peak". (it's just like a car, you have to shift at some point if you want to go any faster with the same horsepower)