Building fairings

L

Lowell

100 kW
Joined
Jan 12, 2007
Messages
1,695
Location
Vancouver
Interesting video on large scale mold making:

http://www.instructables.com/id/EFPZ1HN4FWEZ439XF9/

I managed 57mph this morning on the downhill portion of the Knight St. bridge deck heading into Vancouver. I lowered the front end a little from it's off road stance by machining down the spring guide inside the front fork. Handling is much improved, and possibly a slight aero benifit.
 
I've built fairings before using various different techniques. One time I used clay and that worked well, but it took time for the clay to dry. Another time I used plaster and that dries fast, but once you sand it you can't reverse what you do. (so if you make a mistake you're stuck with it) I'm going to try using car "bondo" because that's very strong and sandable and if you need to add more you can very easily do so. You have to build a "base" first that is strong enough to hold the material and then once that's all smoothed out and perfected then you paint a layer of gel. On top of the gel you then start your fiberglass work until you have a completed mold. Finally you remove the mold and can then make a "final copy" from that.

57 mph downhill is fast... that tops my downhill speed by 5 mph...
 
Hi Lowell,

I've been ruminating on fairings all week. Many google searches, etc.

Many of the DIYers are using coroplast, some kind of thin plastic.

It is kinda remarkable how little is being done, considering the significant benefits aero improvements can make.

I'm starting to think seriously about fabric or mylar stretched over an alum or fiber frame.

The numbers are astounding.

(pix ripped from tonyfoale.com)
 

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For larger surfaces you probably want to go with those other types of fibers, but for things like a seat/rear fairing fiberglass is the ideal choice. The ability to vary the thickness of the fiberglass means that you can use it as a stressed part of the bike. The seat area can be five or six layers thick and the rest can be one or two layers thick. The thickness determines the strength and rigidity.

Fiberglass might weigh a little more, but the ability to control the rigidity and strength easily should be of higher importance than the weight.

Come on... we're talking about a few pounds difference... if it were for a human powered speed record it might be a big deal, but for an electric powered bike it shouldn't matter that much.

Fiberglass is about the most adaptable material..
 
I was thinking small carbon fiber rods and spandex or dacron sail cloth. Make it look like a Zeppelin. If the carbon fiber rods (or fiberglass) run parallel to the airflow, the spandex would keep the flow pretty laminar even though the surface would be convoluted. Certainly wouldn't add much weight and having a non-rigid fairing would be less prone to breaking if the bike tipped over.

I think mylar would be noisy and rips easily if punctured.
 
I might even try tyvek... also noisy, but tough.

BTW, Fiberglass tentpoles are cheap and easy to find.

8)

edit: the RC folks do interesting things with plastic/mylar stuff adhered to styro.
 
My past experience with fairings showed that even with some pretty thick and rigid material you still will get a lot of flex and FLUTTER over bumps and with the effects of the wind.

Think of it like the difference between wearing leathers verses wearing a nylon wind breaker. The windbreaker is lighter, but it flutters so badly that you actually increase wind resistance compared to not using it. The leathers remain stiff and so there is less drag.

If you were to go with the "ultra thin" approach I would advise building a very strong network of support members that you then stretch the thin layer over the top of. If you've ever done any RC airplane work you know about that thin plastic that can be heat shrunk to the web network.

You would be effectively building a "wing", only instead of wanting lift you want low wind resistance.

Fiberglass can be designed to not bounce and be rigid in the face of the wind. I can remember on my old road racer that at about 50 mph the relatively stiff fiberglass even began to flutter.

You will learn this once you try it... without rigidity the whole thing feels really lame... (been there, done that :wink: )
 
safe said:
I've built fairings before using various different techniques. One time I used clay and that worked well, but it took time for the clay to dry. Another time I used plaster and that dries fast, but once you sand it you can't reverse what you do. (so if you make a mistake you're stuck with it) I'm going to try using car "bondo" because that's very strong and sandable and if you need to add more you can very easily do so. You have to build a "base" first that is strong enough to hold the material and then once that's all smoothed out and perfected then you paint a layer of gel. On top of the gel you then start your fiberglass work until you have a completed mold. Finally you remove the mold and can then make a "final copy" from that.

57 mph downhill is fast... that tops my downhill speed by 5 mph...
Click to expand...

56.3mph was with a hacked together cardboard lower fairing on flat ground, 2 way average. 57mph was on a few percent downhill grade with no fairing. While not a real scientific test, there's no doubt that even an unobtrusive fairing is worth quite a bit at 50+mph.

I'd like to start with something like this front fender to streamline the fork tubes

http://www.rynofender.com/
 
Lowell said:
56.3mph was with a hacked together cardboard lower fairing on flat ground, 2 way average. 57mph was on a few percent downhill grade with no fairing. While not a real scientific test, there's no doubt that even an unobtrusive fairing is worth quite a bit at 50+mph
Click to expand...

:arrow: You have become one with the B.O.R.G.

(Bicycle Of Reduced Girth)

To all others... wind resistance is futile... you also will become one with the B.O.R.G.

I've been getting "in the mood" to create my own fairings and this just gets me more so. :D
 
While the top speed is nice to have, the goal of a fairing will mainly be to save watts at my average trip speed of 33mph, allowing for a longer range.
 
Lowell said:
While the top speed is nice to have, the goal of a fairing will mainly be to save watts at my average trip speed of 33mph, allowing for a longer range.
Click to expand...

Well, if you make it look cool and paint it with an attractive color it will also add to the overall effect.

There are no benefits to "pushing wind" unless you've been eating a lot of vegatables in which case it makes you feel better afterwards... :wink:
 

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Good thread. I've often thought of having a go at making a fairing myself, but the idea of messing about with fibre and gel doesn't appeal. Anyone have any experience with moulding polythene sheet (3–5mm)? Seems like a good material. It's tough, can be moulded at relatively low temperatures, and you can weld and reshape it with a heat gun. It's also easy to recycle. Not sure what the best way of making a one-off fairing would be though. Have to do some research.
 
Fiberglass isn't difficult to work with. I have a 3M 6000 respirator, nitrile gloves and painters coveralls for doing resin work.

Poly sheet would be ok if you want something rough, but to make anything decent looking you'll need a mold regardless of material. Marine shops are a great place to pick up supplies to get started.
 
If you create molds first, then the "final product" does not require any sanding. You just take the mold and fill it will a surface layer gel coat. Then after the surface coat dries you place the fiberglass and resin on top of that. Repeat as necessary until it's thick enough to do what you want it to do. Finally you just "pop" the "final product" out of the mold. You now have a completed product and no sanding was required. (the surface coat is ready for final paint)

:arrow: Sanding fiberglass is NASTY, NASTY, NASTY stuff.

If you do the female mold approach then you avoid sanding. It might seem like a waste to build a mold, but once you've done it you can now make all the "spares" you might need in case you crash and need a new one. This way you can "relax" about dropping the bike. (it does happen every once and a while)
 
I googled NACA duct (another thread) and found this guy... he sands, but his page is very thorough:

http://www.hellbentcycles.com/winter_trike_construction.htm


image009.jpg
 
Wow, that's a nice buildup. For some reason I'm not crazy about the way the final product looks though, but he's definitely a talented fabricator.
 
LMAO at the sanding on the carpet in boots with socks.. what a character!!

You guys should really think of coroplast, it's light, strong, cheap, easy to work with, readily available (politics season it can be free!!)
And with some effort you can build a damn nice fairing check out this guys page:
http://www.recumbents.com/wisil/pauljones/
P1010035.JPG

Mvc-002s-sm.jpg

Mvc-003s-sm.jpg


Obviously bents are a bit more conducive to fairings but you could easily build one from coroplast for less than $50 and if you did a good job come out with a nice looking machine.

~S

Lowell where is a picture(s) of your bike??
 
Recumbent racers are low and long, so they really benefit from any effort to clean-up laminar flow. Some well-placed corrugated plastic (coroplast) and lycra will make significant aerodynamic improvements on 'bents. It's cheap, light and easy to work with, but it won't take the shape of compound curves.

I've helped a few buddies lay-up shells of carbon/kevlar for their recumbent's while they had one of the later generation Varna molds on loan. The polyester resin is nasty, stinky stuff.. totally worth enduring to see the parts when they popped out of their mold's.

I think that ~3/4 of this fairing would be awesome on a 'lectric KMX.
This is a snap of the top.. or the bottom. Each part can be left or right, too.

PICT2544.jpg


http://www.easyracers.com/pod/ .. has an interesting pictorial of the entire mold-making process that led to the creation of this piece.


-S
 
For the upright bikes though, it's hard to beat Italian styling..

DUCSSS1C%7E2.JPG


http://www.airtech-streamlining.com/ducati/250350singleconv.htm

.. or you could build Batgirls' bike :wink:

DUCS2%7E2.JPG


Of course, you would need to trim for pedal clearance.. IF you're still pedaling.


-S
 
Somehow, in my view conventional bike fairings look as though they would be more effective.

http://www.zzipper.com/Products/prod_upright.php
 
Malcolm said:
Anyone have any experience with moulding polythene sheet (3–5mm)? Seems like a good material. It's tough, can be moulded at relatively low temperatures, and you can weld and reshape it with a heat gun. It's also easy to recycle. Not sure what the best way of making a one-off fairing would be though. Have to do some research.
Click to expand...

Here's a cheap first pass at a fairing from sheet-stock. Please print, cut, fold, tape, cut more, fold more, etc. etc.

I'm just messing about. I have a lot of clear sheet-polycarbonate and thought about cutting, heating, bending etc.

:?
 

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Tyler,
I think you're on to something.
Reducing air drag by just, say 20%, What will that gain you???
Quite a bit I think.
Anyone got numbers?

A small fairing should give you some more "power" but without getting blown sideways.

I just attached a "garbage can lid" as a fairing.
Crude? yes
Reduced drag? I'll go try it out.

:arrow:
 
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