Friday, October 24, 2008

CF Drop Outs


I'm working on CF dropouts. I want to use a new brand of rear triangles, but they don't work with any prefabbed DOs. This is because the chainstays taper all the way to the ends, which aren't parallel to each other. So, a round plug that fits into the end, is way to loose at its front. And, a Do that follows the axis of the stay, without a bend, won't form a parallel platform for the hub.

Anyhow, the first step is to make a plate from which to cut out the DOs. Luckly, I've come into some samples of very nice CF fabrics. The one that will be the base of the plate has 9 plys of UNI Cf in 0/90/45/-45 degree orientation. Three layers of this are used, creating a base 27 layers deep. On the outside, I'm using some 2x2 12K twill that is doubled layered, so the total sandwich will be 31 layers. Kevlar tape goes on where the axle mounts to protect the CF from the bolts.

So here are some pictures from my first plate. This is the backside - nice and flat with a stray piece of CF in the laminate. Unless this stray ends up in an exposed place, I won't worry about it. Note, the Kevlar hasn't been mounted on this side yet. That will wait until the DOs are cut out.

The next view is an edge of the CF that has been cut. All looks good from this slice in terms of bonding and compaction.

You can click on the pictures to enlarge them.

No indications of voids or dry spots. This whole plate weighs 65 grams. I expect that the dropouts will weight less than half this - say 30 grams. After adding the hardware for the derailer - it'll probably be 35 grams.

Typical aluminum DOs the come with many carbon rear triangles weigh around 110 grams (on my scale).

The stainless plate DOs that these are based on weigh in at 125 grams.

So, 35 grams for these sounds pretty good to me.

By the way, Blogger is turning my pix sideways for some reason - sorry.

Here's the front. I've laid the stainless DOs on top of the plate, then sprayed it with some gray primer - to show where to cut.

The yellow is the Kevlar. One piece of Kevlar picked up a spare piece of CF - but this will sand out easily enough.

Notice on the derailer side how there is a shiny rectangle. There was a bit of tape on the end of the Kevlar tape to prevent fraying. The rest of the material was pressed against a woven peel-ply - leaving a flat finish. Where the tape was left a glossy finish.

That's it for tonight. See ya next time.