In a Slowtwitch post about 2 years ago, I wrote I'd try to build a deep fairing out of polyester/fiberglass for my not-so-aero front wheel. Oh well, I finished that fairing last automn, and rode it for about 1500km so far.
But this is Slowtwitch, so let's start with some pictures ...or nothing I will be saying has actually ever happened ;-)
Here you see the front wheel and the finished fairing, before installation:
The wheel is a Mavic Ksyrium SSC anno 2000, with the original stickers removed / replaced with reflective tape.
The fairing is composed of two rings that are posed onto the left and right side of the rim, then fastened together using 9 small screws. The fairing can be removed & re-installed in a matter of minutes.
The cross-section of the 'tyre + brake surface + fairing' approximates a teardrop shape.
I chose that shape because I trust the birds when it comes to aerodynamics of something of similar size and speed-range. After all, they have successfully used and refined that shape over the past 150 million years or so!
(In all that time however, birds do not appear to have figured out dimples yet! I'm confident they will when they learn to play golf.)
This is how it looks after installation on the bike:
People usually do not realize or believe they are looking at a fairing until they are told so, or until I point out the recessed attachment screws and some of the, ehr, 'production defects' to them.
Now the gory bit - how was it built?
Dear Slowtwitcher, be warned! You'd better skip the following lengthy post if you do not fit into the maniac wrench-it-yourself category.
You may however still comment about stuff like seat height or tilt, the wussy triple groupo, the bike pictures not meeting ST standards because the chain is not on the big ring, the Bento box, the SPD pedals, the shabby appearance of my garage...
The first step, and also the most difficult one, was to construct a mold for laying out the fiber. The mold would have to be ring-shaped, and provide a negative imprint of the desired teardrop shape.
I decided to use plaster for the mold, because plaster is resistant to the polyester resin's aggressive chemicals. It is also non-toxic, and a cheap and easy material to work with. You can make it thicker/more fluid by adding more plaster/water, and form it to your desired shape while it has not yet set. And you can still file and sand it afterwards. That gives you a comfortable margin for errors.
A mold for the mold
But first, I had to build a mold for the mold, something to pour the liquid plaster into:
The first picture in the series shows the materials: The greenish stuff are styrofoam sheets which are normally used for thermal/noise insulation. You also see a stick of wood, tape, steel wire and pliers - used to assemble a makeshift compass (...the one I had left from highschool was too small!) - and a cutter knife.
I measured the distance from just below the rim's braking surface to the wheel's axis, and added about 15mm to it - that would be outer radius of the disk I'd cut from the foam. Subtract the depth of the fairing plus about 30mm, and you've got the inner radius of the ring.
Then I cut 30mm wide stripes of the foam and taped them to the outer and inner circumferences of the ring, ending up with the U-cross section mold shown in the second picture.
I tried to work as accurately as possible with the ring's outer circumference, as that would serve as a reference guide for the fairing's outer diameter.
The plaster mold
See that yellow thingy on the third picture!
It's a bit of plastic I cut to the teardrop cross section shape I wanted to give to my fairing, with a notch that fits snugly against the outer edge of the foam mold.
With that stencil, the plaster was then formed, scraped, pulled, forced into the shape shown in pictures 3 & 4.
And that's where, in retrospect, I made two significant mistakes:
Laying out the fiber
Another mistake seen in the other pictures is to use household items like enamel paint (that yellow stuff), vinyl glue or wax as mold release agents, and hoping that it will work out. It won't!
I used the same mold for both fairing halves, and had to do a lot of plaster repair work after struggling to separate the first fairing from the mold. Further I had to scrape/sand a lot of paint and plaster remains from the cured fairings ...next time I'll get a mold release agent specifically designed for the task.
In the remaining pictures of the first series you see the messy, stinky, but really the most fun part of the process: Laying out the fiber and wetting it with polyester resin.
I've used two layers of fiber. First a layer of light, woven fiberglass cloth for a better surface finish, then a heavier layer of chopped fiber strand for strength.
On the last two pictures in the series, you see small rectangular bits of fiber that I applied at specific markings, between every second spoke. The purpose was to make the fairing thicker at those spots, so I could countersink the attachment screw heads, to make them flush with the fairing's surface.
I'd like to add a few remarks concerning polyester resin, for those who have never messed with that stuff before:
Finishing the cured fairings
The next series of pictures shows some work done on the cured fairings:
Pic 1: cutting away the excess fiber.
Pic 2: filing the slots for the spokes and the valve stem.
Pic 3: checking the fit on the rim
Pic 4: cutting and filing small, wedge shaped pieces of wood that are glued to the inner side of one of the fairing rings, facing the holes for the screws on the other fairing. The wooden wedges not only serve as an anchor for the screws, they also center the fairing onto the rim.
I've used the same polyester resin to glue the wood to the fairing, which results in a very strong bond! (...got that great idea from an article about boat construction, where it was mentioned that resin is used for bonding wooden masts to fiberglass hulls)
On the remaining pics the fairing is almost finished. I didn't make pictures of the rather boring & repetitive 'apply bondo / sand' procedure, neither of the airbrush painting process.
The only worthy remark concerning the 'apply bondo' bit is how to make a smooth rim/fairing joint:
Apply vinyl tape (electrical tape) to the rim's whole circumference, where the fairing contacts the rim. Fasten the fairings to the wheel, then apply bondo to the rim/fairing contact patch and smooth it out. Remove the fairings after the bondo has cured (fiberglass bondo won't stick to vinyl tape), lightly sand the fairing's circumference - et voila, you've got a perfectly smooth rim/fairing joint!
I've used matte black paint for the fairing, as glossy paint would emphasize too much any surface imperfections :->
But this is Slowtwitch, so let's start with some pictures ...or nothing I will be saying has actually ever happened ;-)
Here you see the front wheel and the finished fairing, before installation:
The wheel is a Mavic Ksyrium SSC anno 2000, with the original stickers removed / replaced with reflective tape.
The fairing is composed of two rings that are posed onto the left and right side of the rim, then fastened together using 9 small screws. The fairing can be removed & re-installed in a matter of minutes.
The cross-section of the 'tyre + brake surface + fairing' approximates a teardrop shape.
I chose that shape because I trust the birds when it comes to aerodynamics of something of similar size and speed-range. After all, they have successfully used and refined that shape over the past 150 million years or so!
(In all that time however, birds do not appear to have figured out dimples yet! I'm confident they will when they learn to play golf.)
This is how it looks after installation on the bike:
People usually do not realize or believe they are looking at a fairing until they are told so, or until I point out the recessed attachment screws and some of the, ehr, 'production defects' to them.
Now the gory bit - how was it built?
Dear Slowtwitcher, be warned! You'd better skip the following lengthy post if you do not fit into the maniac wrench-it-yourself category.
You may however still comment about stuff like seat height or tilt, the wussy triple groupo, the bike pictures not meeting ST standards because the chain is not on the big ring, the Bento box, the SPD pedals, the shabby appearance of my garage...
The first step, and also the most difficult one, was to construct a mold for laying out the fiber. The mold would have to be ring-shaped, and provide a negative imprint of the desired teardrop shape.
I decided to use plaster for the mold, because plaster is resistant to the polyester resin's aggressive chemicals. It is also non-toxic, and a cheap and easy material to work with. You can make it thicker/more fluid by adding more plaster/water, and form it to your desired shape while it has not yet set. And you can still file and sand it afterwards. That gives you a comfortable margin for errors.
A mold for the mold
But first, I had to build a mold for the mold, something to pour the liquid plaster into:
The first picture in the series shows the materials: The greenish stuff are styrofoam sheets which are normally used for thermal/noise insulation. You also see a stick of wood, tape, steel wire and pliers - used to assemble a makeshift compass (...the one I had left from highschool was too small!) - and a cutter knife.
I measured the distance from just below the rim's braking surface to the wheel's axis, and added about 15mm to it - that would be outer radius of the disk I'd cut from the foam. Subtract the depth of the fairing plus about 30mm, and you've got the inner radius of the ring.
Then I cut 30mm wide stripes of the foam and taped them to the outer and inner circumferences of the ring, ending up with the U-cross section mold shown in the second picture.
I tried to work as accurately as possible with the ring's outer circumference, as that would serve as a reference guide for the fairing's outer diameter.
The plaster mold
See that yellow thingy on the third picture!
It's a bit of plastic I cut to the teardrop cross section shape I wanted to give to my fairing, with a notch that fits snugly against the outer edge of the foam mold.
With that stencil, the plaster was then formed, scraped, pulled, forced into the shape shown in pictures 3 & 4.
And that's where, in retrospect, I made two significant mistakes:
- Working with plaster in the kitchen is admittedly a bad idea!
My wife gave me hell about it, and I was banned into the garage for all subsequent work.
- The foam wasn't rigid enough! It deformed a bit while manipulating the plaster with the stencil. That resulted in imperfections (slightly undulating surface) that I could not entirely remove by sanding. The slightest imperfection in the plaster mold would later show up in the finished fairing.
Therefore, if I were crazy enough to attempt something like that again, I would probably first build some kind of oversized potter's wheel, with a robust fixture for the stencil - for producing a more accurate mold.
Laying out the fiber
Another mistake seen in the other pictures is to use household items like enamel paint (that yellow stuff), vinyl glue or wax as mold release agents, and hoping that it will work out. It won't!
I used the same mold for both fairing halves, and had to do a lot of plaster repair work after struggling to separate the first fairing from the mold. Further I had to scrape/sand a lot of paint and plaster remains from the cured fairings ...next time I'll get a mold release agent specifically designed for the task.
In the remaining pictures of the first series you see the messy, stinky, but really the most fun part of the process: Laying out the fiber and wetting it with polyester resin.
I've used two layers of fiber. First a layer of light, woven fiberglass cloth for a better surface finish, then a heavier layer of chopped fiber strand for strength.
On the last two pictures in the series, you see small rectangular bits of fiber that I applied at specific markings, between every second spoke. The purpose was to make the fairing thicker at those spots, so I could countersink the attachment screw heads, to make them flush with the fairing's surface.
I'd like to add a few remarks concerning polyester resin, for those who have never messed with that stuff before:
- Always remember that polyester resin, acetone and especially the MEKP hardener (methyl ethyl ketone peroxide) are nasty, flammable, unhealthy stuff! Avoid contact with skin and eyes, avoid inhaling the fumes, and keep your work area well aerated!
- It is better to mix 'a little too much' hardener into the polyester resin, rather then not enough! More hardener will make polyester resin gel and cure faster ...meaning that you will have to work faster!
Not enough hardener however and the resin will never fully cure. It will stay tacky, and you won't be able to sand or paint it afterwards, meaning that you will most probably have to ditch the part.
- Prepare small quantities of resin at a time (I used half a cup). Resin always gels faster than you think!
- When mixing resin with hardener, mix thoroughly but slowly, to avoid formation of air bubbles.
- Stains of resin can be removed with acetone.
You can also add a bit of acetone (use common sense!) to the resin to make it more liquid, so it is easier to wet the fiberglass. It won't interfere with the curing process, but the resulting part will be of lesser quality / less strong. For a non-structural part such as this fairing however, this is a non-issue.
Finishing the cured fairings
The next series of pictures shows some work done on the cured fairings:
Pic 1: cutting away the excess fiber.
Pic 2: filing the slots for the spokes and the valve stem.
Pic 3: checking the fit on the rim
Pic 4: cutting and filing small, wedge shaped pieces of wood that are glued to the inner side of one of the fairing rings, facing the holes for the screws on the other fairing. The wooden wedges not only serve as an anchor for the screws, they also center the fairing onto the rim.
I've used the same polyester resin to glue the wood to the fairing, which results in a very strong bond! (...got that great idea from an article about boat construction, where it was mentioned that resin is used for bonding wooden masts to fiberglass hulls)
On the remaining pics the fairing is almost finished. I didn't make pictures of the rather boring & repetitive 'apply bondo / sand' procedure, neither of the airbrush painting process.
The only worthy remark concerning the 'apply bondo' bit is how to make a smooth rim/fairing joint:
Apply vinyl tape (electrical tape) to the rim's whole circumference, where the fairing contacts the rim. Fasten the fairings to the wheel, then apply bondo to the rim/fairing contact patch and smooth it out. Remove the fairings after the bondo has cured (fiberglass bondo won't stick to vinyl tape), lightly sand the fairing's circumference - et voila, you've got a perfectly smooth rim/fairing joint!
I've used matte black paint for the fairing, as glossy paint would emphasize too much any surface imperfections :->