July 8

July 8, 2005

Carbon chain stay dome

What is a carbon chain stay dome?

The chain stays are the names for the tubes at the back of your bike that hold the rear wheel on.

For the sandwich board frame, I have decided to build two domes that will cup over the wheel openings at both sides of the frame to hold the rear wheel in place. The prototype / proof of concept plywood frame below shows the simple approach - a horizontal square tube with dropouts:

Rather than the straight tube, picture two giant salad bowls on each side of the frame and the rear wheel axle going through the center of each bowl.

The reason I am doing this rather than a simply bar and drop-out chain stay, is because I want to fully isolate the spinning rear wheel from the inside of the fairing. Supposedly, a wheel with a disc spins with less resistance inside a tight fitting envelope, than a disc wheel spinning in the open air. To kill two birds with one stone, I decided to combine my chain stays with the wheel envelope.

To make the mold for the carbon dome, I decided to gravity form a PETG mold:

1. I built a frame to hold a 30" x 30" piece of 1/4" thick PETG plastic sheet.

The distance from the edge of the dome, down to the very bottom has to be exact. Two domes + the width of the carbon frame must exactly equal the dropout width of 100mm for the wheel axle and quick release.

2. The frame is built onto top a wheel which is held by a bench vice on the wheels axle.

This allows the frame to be rotated as the heat lamp (above the table) heats up the plastic.

3. The quickly spinning frame allows for even circular heating of the PETG, rather than heating in a line across the dome.

4. To get a good grip on the plastic, I had to sandwich it between two plywood frames with screws.

I spun the table as the quartz heaters started to soften the plastic. I let the plastic sag until a 3 to 5" round area was flat against the plywood square you see in the middle of the mold. The dimension from the top of this square base to the top of the edge of the PETG is exactly 30mm. 2 x 30 plus my frame width of 1.5" = 100mm which is the exact space between two drop outs for a front wheel.

5. This is the completed dome mold.
6. Now I will cover with two layers of 5.8 oz carbon and multiple layers of vertical and horizontal structure so it will be strong enough to support the wheel.


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