Layup overlap?

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#1

Hi Folks,
I’m new to the forum, but have read many of the posts and have learned alot. So thank you for that!

I am working on building a carbon fiber bicycle with pre-made tubes mitered(coped) and bonded together(and fillets build up). Then carbon fiber prepreg is wrapped around the joints for reinforcement and then frame is placed in a vacuum bag and baked in an oven at 200F for 1.5hrs at around 29in/hg. (Prepreg is debulked every 4th layer.)

In terms of the layup of the reinforcement plys on the joints, should individual(single) plys be butted or overlapped? On some of my test joints, I did small overlaps and they come out rather bulky and lumpy looking at the overlaps and wondered if butting the edges of a single ply is the way to go? If I do that it looks better but am I losing any strength characteristics in not overlapping? Do the overlaps create stress concentrations(risers) and more opportunity for voids? I have not found any good books or reference materials that discuss the butting/overlap issue that might apply here.
(For clarity, I’m just talking about overlaps of a single ply(the 90deg, slightly(1/4"-1/2") overlapping itself around a tube, for instance. Subsequent plys are then laid over that, being careful to offset the seam.)
Any suggestions at all would be most welcome. Thanks in advance!

#2

I’d imagine overlapping would be the absolute best way to join the tubes together. At least that way you have more flexural strength. Correct me if I’m wrong :).

#3

Your question has no straight forward answer. Your first concern should be structural integrity. Look at how forces are being applied and work out a layup schedule to match. The HT DT joint for example, is mostly in tension with some torsion thrown in so you want more in the 0° direction (relative to the DT) with something around +/- 45° (depends on your HT angle)over the top, followed again by 0°. The 45° pieces should and will overlap but the 0° pieces won’t. Once you have the design sound you can add more material for sculpting which will then be sanded off.

BTW the above example is overly simplified. There is not joint on a bike frame that is that simple.

#4

Mcuddier,
I would echo the advice that Kevin gave above. You mentioned that you go 4 layers of prepreg at a time and debulk between them. It might be possible for your last lamination to use a flexible female “glove” mold made of thin silicone that you can place over your prepreg before inserting it into your bag. This might help smooth out the overlapping layers of prepreg, Parlee does something similar to this. There is a video tour of their factory on this forum.
I did something similar when I repaired a broken carbon frame several years ago. it involved re-wrapping the head tube with low temp prepreg that cures at 170f in 4 hours. I did not vacuum bag it. I made a mold of the original ht/tt/dt junction, created a temporary plug from that, then made a silicone glove around that plug. I ground away all the damaged carbon and replaced it with new low temp prepreg, slipped the silicone glove mold around all of it and wrapped glove with 4 layers of dunstone shrink tape. and baked it at 170f for the 4 hours with a very slow cooling cycle. I did not know how the tubes would react to the 170f sustained temps so I made a box that surrounded the entire molding area and very closely monitored the temps. It required minimal clean up after the repair other than some clear coating and the repair is still holding great.

I chose shrink tape over vacuum bagging because of the low molding pressure that bagging offered. I don’t have an autoclave and the shrink tape could develop so much more pressure without having to go to an autoclave. The localized nature of the repair made bagging seem completely unnecessary. Carbon tubes are made using shrink tape.
I figured it would work for this as well and it did.

#5

I don’t do these, but a friend of mine makes CF bike frames at a factory in Europe, they use bladder moulding to create the tube structure and have slip-fit style joints, OD into ID with a titanium “connector” between the two. That doesn’t help you I know, but the overlap method you mention, well, I don’t think will provide the asthetics you maybe after, is this a one-off bike for yourself or is this a development thing which will progress into production.

From what I have seen in the past, the 100% answer is in the tooling, basically a small fortune.