Carbon Fibre Dropouts

General Composites Discussions Composites Talk
#1

Hey everyone, I am here to ask for your advice regarding making rear dropouts for my bamboo bike out of carbon + kevlar layering. Ok so this is what my current dropouts look like.

Disc dropouts.jpg
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Now my plan is to make the same thing out of 8mm biax carbon + the outer layer of kevlar to make it abrasion resistant. The reason behind this is that these dropouts are nearly 300g and won’t be very practical for my lightweight bikes.

I have never done any moulding before so I absolutely have no idea where to start.

I currently have the following equipment.

  1. Vacuum pump (2 phase)
  2. Gauges
  3. Catch pot
  4. West system 105-205
  5. Mould release

Stuff I don’t have and need help with

  1. Cheap vacuum bagging material
  2. Peel ply (not sure if I need it?)
  3. Other stuff which don’t know about yet

I literally have no idea what to do when it comes to making moulds and vacuum bagging? The only reason I got the vacuum pump was to use it for vacuum drying my bamboo but now I know I can use it for other cool stuff.

All the videos I have seen online for carbon fibre moulding is all for very thin carbon layering. I seriously have no directions on how to make a mould for something this thick?

Now the idea I have in my silly head is to soak my steel dropouts in mould release and coat the working surface with some release agent too. After this I get stuck I don’t know what material to use to form the mould? I will be filling the mid section of the dropouts with some blu tack and leave rest of the holes as it is. But say even after the mould is finished I still don’t know how to layer the carbon and kevlar so that I get the finish with one side with variable depth and one side which is perfectly flat. Can someone please help me out choose the best materials and give me some basic advice as how to achieve this sort of finish?

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#2

I don’t think using carbon for this dropout part is good idea. The leverage that this part will get if/when something hits the derailleur can break composites much easier than metals. This is why they are usually metal. I have recently seen a Look bike that I was asked to repair where their carbon version was broken, this means the whole $4000 frame was a write off.

Maybe 3D printed or machined Titanium??

#3

I think you won’t have to mold anything since these parts are just flat pieces… Molding them would require a different setup than just trying to copy the piece with a splash.

If you made a flat sheet and then machined it you could get an exact copy. I have some BMI flat sheets that are cured in the autoclave at 100PSI and that material is pretty damn strong. I’ve machined pieces similiar to yours from material like this. Of course by machining a piece of flat sheet you don’t get optimal fiber lengths and orientations but if the piece is strong enough maybe it wouldn’t matter?

I don’t know if just 14PSI from vacuum would be enough? Maybe.

I just searched and saw these from alchemy bicycles

looks like they use metal molds with silicone mandrels and a press. Looks like a two piece deal that’s bonded together after cure. I guess that method works for those guys. I’m sure that would be the lightest way to go about it rather than a solid piece of carbon. the page said they use woven, as you can see in the picture. UD would be a pain to layup in a part like that and the woven would give better isotropic properties. Also it’s all carbon. I don’t think you’d need kevlar so much.

as for your other questions.

Cheap bag material… if your doing room temp cure, than plastic sheeting is fine. High temp requires something else, like nylon.

Peel ply helps to give a more even surface and may be useful for bonding, but you don’t ‘need’ it unless you do. It’s a choice. But for a small piece like this it might be difficult to use depending on the mold.

Other stuff… well… there are many other things but, it’s one of those things where you figure stuff out as you go. :smiley:

#4

Oh in reading those molded pieces i pictured are solid, or so the caption from the web site says.

http://www.bikerumor.com/2014/02/07/alchemy-bicycles-factory-tour-gorgeous-carbon-frames-100-made-in-colorado/

which is what I would think would be best for this sort of a part. Especially a part that has a bolt through it, unless you use some metallic hardware for the bolt through hole.

#5

The beauty of making bikes out of Bamboo is that I can individually repair or replace the parts which can get damaged due to an accident etc. It is usually the dérailleur hanger that gets bent or damaged in these situations to be honest that’s why all ali frames and carbon frames come with their own version of dérailleur hangers (they are all made from aluminium alloys). I might be doing the same with these dropouts too I think as it will be easier to replace just the hanger rather that a whole dropout. Most of the carbon frames have their dropouts made out of carbon still but they attach a dérailleur hanger separately

Scott_Addict_dropouts.jpg
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#6

Yeah well at least the derailleur hanger should be metal.

For an easy drop out you could just layup or buy flat carbon sheet about 6mm thick and have the profile water jet cut?

#7

I have just concluded that machining any carbon fibre part will make it weak and cause problems. Instead I am going to get my moulds machined and start from the perhaps?

#8

It wont make it substantially weaker if its machined properly so I wouldn’t worry hugely about that: I don’t think it will be the weakest part of a bamboo bike that’s for sure, particularly if its 8mm thick - the joint to the rest of the bike or the bamboo itself will likely be weaker. By far the easiest method is to get the flat sheet and machine it: easiest and cheapest and will be plenty strong!
Fancier method is obviously moulding it to spec if you want to go down that route - has its advantages of course but will be trickier.
Have you a method for mounting the derailleur? ie a threaded insert or something?