July 10

July 10, 2005

Carbon chain stay dome - almost finished

This dome is fastened to the frame where the rear wheel goes, and holds the rear wheel allowing it to spin inside a tight fitting envelope. I mounted the wheel with a quick release to check for clearances and all looks good. The chain stay dome isn't quite strong enough yet. I'll need to add more reinforcements. When I push in the middle, it dips a bit. This is probably because the compound curve flattens out at the top. For the ultimately strong dome, it must curve all the way to the center point. The carbon dome consists of 4 layers of unidirectional tape on two layers of 5 oz. weave. It weighs slightly more than a piece of paper.

Here are the sorted details:

1. The blow molded PETG mold is securely screwed to a flat piece of plywood. I discovered that as the PETG cools after blow molding, it expands quite a bit.

The critical dimension is the distance from the center of the disc to the bottom, so I cut an exact sized block of wood and screwed the top of the PETG dome down to it to ensure that the height of the bubble was exact.

2. 2 layers of 5.8 oz. weave over the entire dome, then 4 layers of really thick unidirectional tape across the middle and directly up from the axle hole.
3. The unidirectional carbon tape

4. OH NO!!! Disaster strikes. The PETG dome collapsed from the vacuum pressure.

5. Ben saves the day! Luckily Ben was on his way over and he picked up some expanding foam spray. We drilled a hole in the plywood base, and sprayed the foam into the dome cavity. Then we plugged the hole with a cork.

Note: These photos are of the wheel disc mold - NOT the chain stay dome. We did exactly the same thing with the chain stay dome mold, but the carbon, peel ply, breather blanket, and even vacuum bag were ALL STILL INTACT. We drilled the hole right through the vacuum bag, then patched the bag up.

6. The foam quickly filled the void - hopefully, there would be enough pressure inside the dome to stop it from collapsing.
7. It worked!
8. The completed chain stay dome.


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