Ok so I am new to this forum and hope that there is some expertise in details on carbon fiber techniques to join pre formed carbon bicycle tubes. I am familiar with the basic resin and bagging details but not sure on the type of fiber and direction of wrapping the tubes together. From what I have gathered I should use unidirectional tape to lash the tubes going perpendicular across the joints and maybe running the fiber down the tube about 1.5 times the diameter of the tube and just terminating the strands.And finally covering the joint with a woven fabric to match the final fabric of the tubes as merely a cosmetic finish?
You are on the right track, you will need to do some test samples and see what works for you. Make some test pieces and do some destructive testing, this will give you a definitive answer. By using a good epoxy like 3m dp420 to join and fillet your tubes you can keep joint layup to a minimum.
Do yourself a huge favour, go over to velocipede salon, go to the frame builder section, then best threads section, then carbon building thread by Nick Crumpton, there is a wealth of info there. Nick is one of the leading proponents of tube to tube bike construction.
I have built plenty and will endeavour to answer your questions as they come up but it is always best to go through the processes and discover what works for you.
Thanks Progetto,
The thread was good confirmation that I am on track. Nicks name has come up before but I did not want to call and bugg him about so many details. Looks like I was going to over build it a bit so now I have a better idea whats acceptable .
I have a real hankering to build my own bike frame, so have spent hours looking for as much info as possible.
A few decent links I found;
http://back40bicycleworks.blogspot.co.uk/2012/02/joint-wrapping.html
http://back40bicycleworks.blogspot.co.uk/2013_04_01_archive.html
http://www.bikerumor.com/2013/12/11/factory-tour-guru-cycles-part-2-building-a-carbon-bicycle-frame/
Doing a Google search of ‘tube to tube carbon frame’ brings up a fair few results.
This is something I"ve thought about for a while as well…
One of my coworkers just finished up a welding jig and I can use it as a bond fixture. I’m not sure if I should make my own tube or buy premade tube… I guess I should probably come up with a design and figure out necessary tube lengths. I would do a mitered tube frame that’s bonded together with adhesive, cured, then go back and wrap it for a bit of extra security. I would also use joint inserts if I can do it, so the bond has a lot of surface area as well as a place for good load transfer between the tubes.
Using the epoxy mold compound as a fillet isn’t a bad idea. I hate sanding though… 
My advice would be to use ready made tubing, you will have enough of a challenge getting everything else right on your first one without introducing more work for yourself.
Ive experimented quite a lot actually with frame building. Theres pretty much three ways of doing it.
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Bonded tubing with supporting plies added afterwards
Fine for road bikes that aren’t likely to take much abuse - I don’t think they would be good for any heavily loaded structure like an MTB - typically most adhesives used in bonding will be stiffer than the wrapped plies meaning the wrapped plies are unlikely to take much of a load other than perhaps providing a fail safe mechanism if the adhesive bond fails -
Foam core with wrapped plies - Does the job and was the way I made my first bike a number of years ago but heavens above the sanding was ridiculous. I hate sanding / trimming so this wasn’t enjoyable. But it worked and worked well - I took a slightly more unique approach by bonding in UD tubes for the main triangle, bonding a foam core acting as a sandwich as opposed to just a male tool and then wrapping with wet lay carbon. http://dirtmag.co.uk/news/hcomeragh-n56-carbon-hardtail.html
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Bladder moulded on female tooling - the option if you want a light product but for private builders, tooling costs or access to machinery typically prohibits this. You could do it other ways but would be rather labour intensive to say the least.
My advice - do anything to get the first one made and I mean anything - you’ll learn far more by doing it than planning to the nth degree-just get stuck in. I bonded BB bearings and headset shells in for example as I didn’t expect bearing life would be my limiting factor on the first frame-turns out it was!
Don’t agree with point no 1, joints won’t be a limiting factor if done properly.
yeah ok fair enough, just my thoughts would happily concede if convinced otherwise. I m thinking along the lines of the fact that if the adhesive at a joint is much stiffer than the overlying plies, will the plies ever be stressed anywhere near their capacity?
The adhesive at the joint is only adhering to a very small surface area, it is a butt joint, not a lap joint. Even with a decent fillet you are relying on it to have very good amount of peel strength. By applying the correct amount of laminate over the fillet, it allows you to transition the stresses over a much larger area rather than concentrating the stress directly to the fillet or butt joint.
have you considered internal lugs which are formed using a fiber wrapped bladder inside the premade tubs?
Do you mean like this?
Don’t over complicate things. Get some tubing, jig and glue, wrap joints, ride. Do some test joints first to get your technique down.
Are those titanium lugs?
Looks like… just a picture I got from the web. Not sure what is the best or most appropriate sorts of pieces required? Would be nice to just source a set of all the metal parts needed and then supply the carbon tubes and bond it together.