Making a part with an 'Inside/outside' surface

I know the title hasn’t explained it very well at all about what I mean, so here is a link to help;

http://www.sicklines.com/gallery/data/534/evil-undead-prototype-neon.jpg

also - http://www.sicklines.com/gallery/data/1073/evil-undead-2013-fox-rc4.jpg

I’m wondering how you’d go about making that ‘tunnel’ section for the rear suspension - especially making sure that the inner surface is the correct width for bolts & spacers?

Does anybody have a link to pics or videos that show how this type of shape is achieved in the molding process?

Would the ‘walls’ of the frame that go around the shock be solid carbon or have a very thin layer of foam inside?

Finally, how do they know that the ‘walls’ of the ‘tunnel’ are not only have the correct inside dimension, but also the correct outside dimension with proper fabric layer compaction?

This type of process has had my brain working over time to figure out, so any slight insight/info would be much appreciated.

Regards,

Kyle.

That frame is made using a multi-part bladder mold. It’s not something you will be able to replicate very easily at home. You will be better off wrapping a foam core.

To get the pivot points right you have 2 options. The first is making fixtures that hold the inserts in the correct position. They need to be stable enough not to move but small enough not to get in the way. The alternative would be getting the mounting points as close as possible in the foam (1 - 2 mm off max) and bonding them in later.

One thing you need to consider more than the location of Al inserts is that this type of frame is very highly stressed under normal riding conditions. If you don’t want it to break landing a 2 m drop you either need a proven layup or you should massively overbuild.

Thank you for your reply.

I was thinking that overbuilding the frame would be necessary as I don’t have FEA that the big bike companies use. I know that the layup schedule would require a uni directional mix of 0, +/-30, +/-45 and 90.

I suppose consulting with a professional composites company for advice wouldn’t be a bad thing to get ideas and recommendations.

I’d love to make my own bike frame - well a carbon version of the aluminium one I intend on buying. That way a mould could be taken from the original aluminium frame.

Although you could use an Al frame as a plug, you will have problems where the brazons are. You will also see the weld beads in your CF frame. You will likely need to make everything twice. First a mold from your frame then a plug from that. Sculpt it to how you want it to get rid of the weld beads, logo print through (the letters on the downtube etc. sit slightly higher) and bumps caused by cable stops and the like. Finally, make a new mold from the modified plug. All things considered, I think you would be better off wrapping a foam core instead if this is a one-off deal.

The physics behind this type of frame are much more complex than a road bike or a hardtail. Even the big names have problems where Al meets CF at the pivot points. I’ve had a few frames with cracks that developed in that area. As such, your layup with be more complex than those 6 fiber angles you mentioned.