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  • Carbon Fibre repairists
  • BearBack
    Free Member

    I’ve fixed plenty of things with carbon from my Westfield body when I chucked it in a ditch, crash damaged chain stays, seat stays, rim, front triangles even a future Olympians skate ski boot cuff.
    All of which have been layup or external compression with heatshrink so whilst often tricky, still straight forwards from a process point of view.

    Now, I’ve got a partially cracked BB shell (carbon bb sleeve to shell joint) that I want to add layers of carbon to the inside, but its a pretty complex shape with more than just 4 holes to plug. So, anyone had any experience creating internal pressure? I tried a balloon bladder once for a tube repair, but the sharp carbon ends from its original mold put an end to that and I ended up just creating an internal foam form to wrap around from the outside. A thicker bladder would be better, but then I don’t think it would conform well enough to fill the voids and press the carbon into place.

    Short of my best option of a mold release film/non-stick baggy filled with canned expanding foam to create pressur, then cutting it out afterwards, but I’d still need to plug all the expansion directions.

    This inside out process has be a bit lost for solutions from a DIY perspective.

    I welcome any suggestions

    Full Member

    Could you use something like an inner tube with sealant in to stop it from getting punctured? Maybe cut the tube and zip tie the ends so you can pass it through the BB hole.

    Free Member

    Might sound stupid but a Whoopee Cushion would be a thicker bladder and cheap.

    Full Member

    Maybe talk to the guys at Easy Composites and see what they suggest / can supply?

    Full Member

    I’ve done prepreg carbon but only with vac bagging and a home made autoclave.

    Do you have access to any form of pressure vessel? – I have a few spare ‘tanks’ in my workplace as an example.

    Without the above – it’s a bit tricky as you then need a box around the bottom bracket area of some description to contain a bladder. This is where vac bagging and bleed mats etc make life easy.

    Full Member

    Do you have access to a vacuum pump? A few guys at work repair bike frames on the side and mostly just use normal bagging material & tacky tape to locally bag around it with a valve, then oven cure under vacuum.

    Obviously that might not be kit you have kicking about at home!

    Free Member

    Appreciate the input above.
    No vac gear. I’m going to find the kids whoopee cushion to see how well it deforms. Nice idea 🙂

    I wonder about casting the Internal BB shell with silicon and having a tapered rod (mandrel?) that takes up as much volume through the BB axle path as possible to keep the silicon mold workable. Remove that and can then hopefully peel/fold the remaining silicon out once cured. Then, use an inner tube in place of the tapered rod to create outward pressure on the silicon mold and have it push against the carbon/epoxy that laid inside the BB shell?

    Full Member

    Professional job or new frame time ? Is it the main BB shell that’s cracked ?   I suppose, what happens if that part fails ? You are confident repairing other parts, but what’s the consequence of this bit failing, unseen ?

    Free Member

    Looking at the non-driveside bb face, there was a circumferential crack in the paint between 8pm-11pm. Paint removal and investigation suggests its nothing major and more of a telegraph through the paint and possibly minor delamination between the carbon bb sleeve and bb shell. Cant see any failure inside the shell but I appreciate I’ve not got xray.
    No idea how long I’d ridden it like that but I’d prefer to reinforce it
    Definitely recognise the concern the concern but I’m but sure its any more of a catastrophic risk than a rear rim, seatstay or chainstay going bang.

    Full Member

    That almost looks more like a wear mark than a crack (from one photo on the internet)

    You’re looking to reinforce it both inside and out? i.e. compress some pre-preg inside the down tube/BB junction and do the same across the ‘crack’ on the outside?

    Can you get at the inside for a good view with an endoscope or mirrors to figure out if it is a through crack or not?

    What make/model frame is it?

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