Home-built Custom Coupled Carbon Fiber Touring Tandem Bike
 

My wife and I have been into riding tandem bikes for the last 5 years. We ride at least four times a week or whenever we could get the time. We have four other bikes, but they rarely get used anymore. We love tandeming so much that on most of our vacations, we would look for places to rent them. For us, there is no better way to explore than on a tandem. Unfortunately, most of the time, we can only find poorly made or ill fitting tandem rentals if at all. More often then not we end up renting single bikes.

Our current tandem bike which I built from the frame is perfect for us, but there is no way to pack it for air travel. Its irregular tube shapes also prevents SS coupler conversion. So I have been calling the frame builders searching for a new travel tandem frame for our upcoming trips this summer. Seven cycles, Calfee, Zinn all want about $10k for their coupled frame. Co-motion wanted $3.5k for their aluminum one, but they all required a 3-4 months build time.

I am also an avid DIY'er, so I am thinking about combining my hobbies and build the bike and frame myself. I have been researching on the internet for the last month including a pretty thorough read of Sheldon Brown's and Damon Rinard's sites.

From what I've gatherer, I have decided to build a home built, custom coupled carbon fiber touring tandem. Home built because I have to build it with woodworking equipment in my garage. I will have to design a new custom couple for linking bike frame sections because SS couplers are not sold to home DIYer. I chose carbon fiber because it is easy to work with and no welding is required. This bike will be a touring tandem built for durability and comfort over weight saving and speed. It must be able to withstand the rigor of packing, travel, and unsupported touring. And it must also be pack-able in two airline friendly 26x26x10 cases.

Any one here with previous frame building or carbon fiber laminating experience they care to share? Love to have other inputs, advice, suggestions, and critiques as I attempt this project. Always better to have more than one view. For those experienced tandem riders, if you have a clean slate, how would you design your travel tandem? Thanks in advance for any help.

CJ

The frame builders forum will likely be a better source.

However, I've got to say you're jumping into the deep end of the pool. Have you built a bicycle frame before? It's one thing to build a steel frame, It's another thing to build a CF frame. (while there's no welding, there's also no opportunity to cold set it to correct any alignment issues) Then it's another tihng to build a coupled frame, and another thing to design and build your own couplers.

Taking on all that it pretty ambitious. But all that and a tandem for a first frame build sounds pretty much insane. Buliding a good tandem frame is another order of magnitude of difficulty from a single. Many very good custom builders have found its not worth their while to build tandems, and a number of customers have found that just because a frame builder builds a good single bike, its no guarantee they can build a good tandem.

Personally, I'd start with something a little less ambitious, like a non coupled single frame, and work my way up.

+1 to merlin's comments.

My own experience: I bought a tandem tubeset from Santana in the 80's, built a fixture table, and bronze welded a nice frame that we continue to ride today.

From my own experience I'd speculate -
- it'll take a good deal more than 3-4 months to learn about, design, and build your own frame. As noted, carbon fiber is much harder to work with than steel, and tandem frames are much harder than singles.
- you will need to invest a lot of time and work into making sure the details of your frame design "mate" correctly with the components you select- seat posts, stems, brakes, bottom brackets, wheels, etc etc etc. This consideration makes it necessary to select and buy most of these components ahead of the frame build. I had to take all sorts of measurements from them, in order to design and complete all sorts of details on the frame such that the components mounted+worked correctly, provided clearance for movement, avoided interference, etc.
- be VERY careful, and wary, of safety. If the frame were to suffer a failure at anything above a crawling speed, or in/near vehicle traffic ... well that's gonna end very badly for the tandem team. Carbon fiber is more prone to catastrophic failures -- that is, suddenly and completely -- than steel.
- I've never worked in carbon fiber myself. But in addition to bicycling I've invested a good deal of time in small airplanes. Many newly designed small planes use carbon fiber and composite construction. Small plane and kit plane forums will have information about working with those materials. Molds are a big deal. Curing correctly is a big deal.

Whatever you decide, be safe and have fun.

Agree with above statements.
Unless you have previous framebuilding experience + c/f specific experience, it is not a DIY project.
We have a custom built c/f tandem built by an engineer that worked on space shuttle and rockets.
Plus, it will take lots more time and $$ than you anticipate.
Having said that, have seen two single steel bikes welded/cobbled together to make a homemade tandem.

A boatbuilder acquaintance of mine built his own CF MTB. It came out a little heavier than what one can buy, but it was stiff and worked fine. He is very experienced in working with epoxy, and doing accurate setups. He was less experienced with CF, but had used it some in boatbuilding. So he had quite a few skill sets he could build on. He used the common technique of making tubes, then joining them together. I suggest that approach to you: build some tubes first and test them. See how that goes. If you have an engineering background and access to a good machine shop, the couplers shouldn't be all that hard to duplicate, just time-consuming to manufacture.

I sell plans for amateur boatbuilders. My universal advice is: don't look at this as a way to get a boat, or in your case, a bike. Look at the project as an end in itself. If you wanted a bike, it'll be a lot cheaper to buy one, whatever the cost, if you count your hours for anything at all. In the case of my boat plans, one gets a better boat than one can buy. But you're not starting with bike plans. You're where I was 35 years ago, learning to design. But thinking along those lines, have you considered bamboo? At least the tubes are ready made.

And why do you need couplers, anyway? Many people, including me, have flown with uncoupled tandems just fine. Once in a while, one runs across a used coupled tandem for sale.

Another thing about a CF tandem for touring - all the "braze-ons" and attachment points will be tricky. For some reason, there are no manufactured CF touring bikes. Trek built one for a little while, but discontinued. If you could find one, you could look at it to see how they did the braze-ons.

I'm not an engineer, but have wrenched all my life and have a great appreciation of what an engineer does, I am also a realist and think Chris SS is right,"it'll take a good deal more than 3-4 months to learn about, design, and build your own frame." Please keep safety in mind if you pursue your project, our best

Rod & J

'Braze-ons' is a bit of a misnomer for c/f 'glue-ons'.

chojn1 - It wouldn't hurt to talk to at least one more builder before you embark on your ambitious project. I had a bike built about 5 years ago by Bill Davidson http://davidsonbicycles.com . I don't think he builds with CF, but he does work with titanium and steel. My steel tandem weighs just under 30 lbs. but I don't think weight will be a big priority for a tourer. He raced many years ago and has been building bikes for a long time, many tandems, many coupled bikes, bikes of all kinds really. It would be worth talking to him. I see on their web page that a steel tandem frame goes for $2399. This would be for a well fitting, custom sized, beautifully finished steel frame. Obviously S&S couplers will add to the cost. If it seemed like a good option, a trip to Seattle for a nice weekend and a fitting would be a blast. If you are looking to build the CF bike as a hobby, that is one thing - but on the other hand you could spend that time riding.

I have to agree with the posts other have offered; Basically the time line is way too short to go from scratch skills to the stage of being ready to start let alone complete a CF tandem.

Fully agree with your desire to develop a new hobby (and delighted that you want to add to the tandem side of the framebuilding community as we always need more folks), but to meet your timelines feel you have either of four paths that should be viable;

1) Look for a used quality tandem which has the couplers already. I have seen 2 or 3 for sale recently at not to bad prices.
2) Rent a coupler equipped bike ... this should be possible. Might want to troll the shops which support user tandem tryouts for brands such as Santana, CoMotion, Rodriguez, etc., as then tend to keep several in stock.
3) Borrow one from someone... betting that someone in the group ownes one about the size you need and is going to be tied up during the required time and thus can't use his/her bike at the time. Of course buy full replacement insurance and beg sincerely. For many 2bike folks the bike is nearly as well loved as their children...
4) $3,500 is a pretty good price for an AL coupled 2bike or even for the frame itself frankly. I'd recommend taking the offer and seeing if you can chip in a bit more or beg sincerely to bump up the schedule (no guarantees there)! The actually difference in ride or performance between AL and CF is unlikely to be very significant and I suspect the AL might be a better ride (but not as good as a well crafter modern butted-steel bike; imho). At a minimum, there is at least enough history on AL tandems to not be that worried about them holding up for the long term.

Hope this helps ... but whatever it takes to make it happen to get the wheels rolling is the right answer

K

If the OP is considering a trip to Seattle, be sure to check in with R+E Cycles. This time of year, they can bang out a coupled tandem in about a month. Heck, it only took six weeks or so for them to build my coupled touring tandem and I chose the peak season to have it done. Of course, you can always make a virtual visit unless you're jonesen for some rain.

ksisler said this "The actually difference in ride or performance between AL and CF is unlikely to be very significant and I suspect the AL might be a better ride (but not as good as a well crafter modern butted-steel bike; imho). At a minimum, there is at least enough history on AL tandems to not be that worried about them holding up for the long term."

Please, before you make statements like the one above ride a cf tandem, itis obvious that you have not, because if you had you would not make that statement. Our Calfee CF tandem is extremely comfortable and I am confident that everyone who owns one will say the same thing. There is a significant difference in the ride quality of different materials!

We ride with a guy who has a really old Giant CF bicycle that has CF tubes and aluminum lugs, it is probably at least 20 year sold. I believe that it is Cadex 9800 model that was cutting edge at the time. It probably has at least 200,000 on it. He has ridden over 21,000 miles this year alone and most of those mile on the Giant. So please do not keep preaching about the long term life of carbon fiber bicycle frames.

Wayne

I think your desire to build the bike is awesome.

My opinions as a person that works with Carbon Fibre almost each day, with simple care taken, it will be easier to fabricate from carbon then any of the metals. I assume you plan to build as a tube construction vs formed / shaped. This makes things much easier. Pretty simple to miter the tube ends and joints, tack it together with super glue and then wrap the knob style joints like a Calfee. You can ensure straightness while superglued.

The downside of working with carbon is the dust, abrasiveness and chemicals. Carbon dust is bad, it is conductive and a serious health hazard. Any amount of cutting will require specialty carbide or diamond grit tools. Not bad to obtain but tooth type tooling will cause a lot of fibre damage. Chemicals and fumes, the list is not long...Epoxy, Hardener, solvents. Put this all together to fabricate your bike and without realizing it, you can pollute your entire workshop and home if you work in the garage or basement.

I won't say don't go for it, just realize the situation you create is far worse than the internet explains.

I know nothing about the details of you or your shop. Bottom line, don't kill yourself in the process of building your bike.

Carbon is really a lot of fun to work with and scientific to understand. Your timeline, while tight seems doable if you don't work full time.

Visit MTBR.com frame building forum, look for topics / posts from MRMagura. That may offer some insight.

If you have more questions, ask, I'll do my best to keep you straight on carbon construction.

Best of luck with it.

PK

Having ridden and raced (in ultra-distance races) steel, Al and CF tandems I can say that none of the steel and Al tandems I've owed and/or used come anywhere close to the ride of my current CF tandem. Don't know if you've ever ridden a quality CF tandem but you might want to give it a try.

As far as the OP goes, if you're wanting to do this for the fun of it I say go for it. Just be careful and plan on blowing it at least once before you have a respectable and safe product. Tandems have a few more stresses than a 1/2 bike so your "engineering" will be important. I don't see that there is anything "magical" about the technology as long as you do your homework and go into it realizing the difficulties.

It occurs to me that you could do a bang-up job of putting a frame together, but if you screw up one aspect, the rest of the effort is pretty much wasted, too.

When someone's riding a bike with me, I try to be more careful and more prepared than what I might be on a single bike. I wouldn't get a stoker on a bike I built myself unless I dang sure knew what I was doing.

Thanks PK and Homey for the replies and encouragement. I do have a well equipped woodworking garage shop. While this won't help much with building the bike, it will help me build the jig for the alignment. More importantly, it has a very good dust evacuation system and a HEPA filter. All I have to add is a vacuum machine for bagging the carbon layers. Construction of the frame will be from prefabricated round tubes joined to custom made lugs. Joints will then be overlapped with Kevlar and carbon fiber layers.

Regarding the links for the alternate frame builders, I will definitely look into those, if this frame build fails. This project is a fun hobby for me. I am under no delusion that it will be easy, or that I can build a better or cheaper bike than what I can buy. But, I do get satisfaction from building things, and this will be a great project if I can get it to work.

Regarding the safety issues, I am not cavalier enough to put my wife or myself at risk. I will thoroughly stress test the frame before use. I may have to go through several iteration, but it will be strong and safe.

Regarding starting with a single bike or a non-coupled tandem, I already own four singles and a perfectly good tandem. I would not know what to do with yet another one of those frames. I do see your point about starting from a simpler project and progressing with more experience to more complex jobs. To which I would like to introduce my first carbon fiber build:
http://bikeforums.net/attachment.php...hmentid=289019
This will be the captain seat post. So far, I've molded a fiberglass tube and epoxied it on a clamp cut from an old seat post. The next step would be to wrap the tube and clamp in carbon fiber. This will give me the practice for carbon fiber laminating techniques and well as a test bed for the strength of the parts I can produce.

First set of questions for PK:

1) How bad of a problem is galvanic corrosion at the junction between aluminum and carbon fiber? Is it worth isolating the metal parts with fiber glass? Which is better: layering aluminum with fiber glass then carbon fiber, or layering aluminum with Kevlar then carbon fiber.

2) I am using Hysol epoxy cement to join the parts. Does the super glue that you mentioned provide a stronger bond?

Thanks,
CJ

Another builder
 

Check with Todd at www.davincitandems.com on his delivery timeframe. In the middle of winter like this I'm almost positive it would be weeks and not months. After touring his plant more than once I'm convince that they are well-built (CF, steel, Ti, Al or whatever) tandems with couplers available. Hand built, great workmanship.

I just noticed there is a seller who has listed a tandem framebuilding jig and some tandem frame parts (BB shells, etc.) on Ebay. You might want to check it out: http://www.ebay.com/sch/jma6610/m.ht...p2047675.l2562

Understand that carbon dust is not sawdust, it is super fine, abrasive and conductive.

If you are building with prefab tubes, there is minimal if any need to vacuum bag the parts. In fact, bagging that type joint will often pull air into the joint weakening it. Best to understand resin content and methods / techniques to achieve the resin content you desire.

Galvanic corrosion. You should never place carbon or graphite in contact with aluminum. Sometimes you must, such as an eccentric, but it is not good practice. Why would you even need to use aluminum on the frame in a bonded joint? Also the proper prep for adhesive bonding aluminum opens up another prep can of worms to accomplish.

If you use a carbon seatpost into a carbon frame it works, no need for insulating. If you can acquire headset bearings in SS, make the bearing seats from molded carbon epoxy.

Hysol EA products are stronger than super glues. May suggestion of super glue was to tack the frame at small points, allowing it to be removed from the tool if needed to align or prep for filament winding by hand or laying up fabric joints.

If you plan to make your own tubes, that would be where I would suggest you might consider buying vs building. The way to build a molded bike is to sculpt a pattern (from wood most likely). Make it swoopy, sporty and sexy. From that build a mold. In the mold, make the frame. Honestly, at that point you might be better served to attend a class at Brew or a similar school where they teach you how to weld and your project is to leave with a frame.

http://www.brewracingframes.com/id56.htm

You can see this is popular but better yet, they are having a special for two...bring your stoker.

PK

CJ, from your photo it looks like Maguiers wax for releasing. I assume it is mold release wax. A word of caution, always by high temp process wax. There are times when the exotherm of the curing epoxy can exceed the temp rating of the wax and destroy the release properties.

PK

I agree that you are considering an ambitious project. Nick Crumpton has some information on Velocipede Salon Frame Forum (scroll down.)

http://www.velocipedesalon.com/forum...ing-18058.html

Sheldon

Hi Sheldon,
Thanks for the link. Nick Crumpton's site is fantastic. A lot of great information. Thanks.

PK,
Thanks for the tips. The wax was just to release the fiber glass tube from the aluminum tube I used as a mold. No problem with the separation.

My bottom bracket shell, essentric shell, and head tube are all aluminum. Hence the need to insulate from the carbon. I am experimenting with Kevlar instead of fiber glass hoping for a better bond and more shear resistance. I like to overbuilt this thing for safety.

Here is the seat post so far:

http://bikeforums.net/attachment.php...hmentid=289131
This is after 8 layers of carbon fiber wrap. Still a little rough, but I'll sand it once I get it attached to the stoker stem which is my second build:
http://bikeforums.net/attachment.php...hmentid=289132

The stem is made to the length and angle of my current setup which we are happy with. Eliminating the adjustment tubes and bolts should save a little weight, but this is more about testing the carbon fiber build and the joint strength Once the two parts are joined, I'll finished them with a clear coat and begin to test them out on my current bike.

CJ

I see the path you plan to follow.

This doable but a lot of work. Yes for the methods of fabrication you are using getting a good amount of pressure is a must to build it strong and light.

As far as insulating the metallic from the composites, in aerospace we have access to a lot of "stuff". I don't recall ever seeing Kevlar utilized as an insulator for carbon / aluminum bondings.

Have you ever worked with Kevlar composites before? Realize that for your application, you will probably have a fuzzed edge that won't sand out easily. Also, Kevlar is known to absorb moisture so I personally would have it low on the list to prevent corrosion between to other materials.

I'd be more impressed to see a photo of you working with materials in your safety gear. The internet has unfortunately led many to work with carbon composites as if they are harmless. Be careful.

PK

For mine, building the bike out of CF is not an issue, especially based on the process intended.

However, everyone seems to have overlooked the most important part of all -- the frame dimensions and angles. Just what design is the frame to be based on? Has the OP got himself some frame-design software, and has he banged in the numbers he is envisaging?

The lugged method is relatively outdated in terms of production CF bikes these days, but I do know that it can produce a very stiff frame, to the point of being tough on the butt to ride. The sample point for me is my Merlin C110 Works. It's not just my opinion; others have expressed it, too.

What head-tube angle will there be? Fork trail and/or rake? Dimensions of the head tube (conical, 1-1/8th) What configuration of diagonal tubing will there be? How long will the chainstays be? What curves are there to be in the seat stays? Is there a compensation to be made in any of these things because CF is being used? Can an alloy or steel frame design be applied to CF construction?

And as someone has alluded to already, be prepared to make a major mistake. The further the build goes, the more difficult it becomes to make good any mistakes, particularly in terms of geometry.

Because there are so many tubes being dealt with, there is a critical need, in my opinion, to have the lugs made to very accurate tolerances for (a) longevity of the frame and (b) so that the opposite ends of the tubes end up where they should be without having to stress them back into position (again, as someone said, you can't coldset a CF frame, even a couple of millimetres like you can a steel one).

Likewise fabricating the couplers. Plus, in my estimation, every couple introduces a potential failure point on the frame. Hence S&S' reluctance to allow anyone except fully authorised fitters to add them to frames. The couplers have to be done exactly right. On a Co-Motion tandem we saw last week, there were four to enable the bike to be broken down adequately for travel. Is that the number to be incorporated into this design? And where are they to be placed?

Expense and time will be a factor. Carbon cloth is quite expensive, much more than I thought. Epoxy resin, from my understanding, needs to be carefully chosen, with additives at least to avoid deterioration from sunlight. The couplers would, I expect, be as expensive as the rest of the bike.

Has the OP been to BikeFriday and tested ridden their steel tandem? We were there the day before yesterday, and a white tandem was sitting on the demo floor. We didn't ride it, but I know others who own BFs, and they love them. They aren't exactly the prettiest tandems in the world, but can break down pretty small, I think, without the need for fabricating couplings from scratch.

Now, having said all that, one of my ambitions is to build a CF high-wheeler recumbent. There are several sites dealing with this, and it's not all that difficult because there really is only one significant tube to deal with. Bonding the chain and seat stays seems to be the only significant issue, apart from alignment. But it's a project I am giving long and deep thought to before committing myself...

(Also note that the OP probably has thought of all these things already, but this is not evident in the postings so far. I am really interested in the progress and outcomes, too...)

PK,
No time to take pictures of myself, but imagine an open 2 car garage workshop with dust collector and HEPA filter. I have goggles, respirator, gown, and glove. I don't take chances.

I've never worked with Kevlar, but I've never worked with carbon fiber before either. I just know that it is much harder to cut into Kevlar and it is harder to sand than CF which is more reassuring to me. Kevlar is also non-conductive which would make for an insulator as well. I am a little worry about the carbon fiber-Kevlar bond but I think the epoxy should take care of that. Also, if the Kevlar layer is completely enclosed in the epoxy, should it not be water tight?

Here is the progress of the stoker stem:

http://bikeforums.net/attachment.php...hmentid=289243
It seems pretty solid. I've put 250 lb on it and it has not budged. I'll be riding on that thing for the next few months while I'm building the frame. I'll report any changes.

Rowan,

Thanks for joining the discussion. I don't have any special bike design program. I've tried Bike Cad but could never got it to work properly. I am using plain old Sketchup now. Here is what I have so far:

http://bikeforums.net/attachment.php...hmentid=289245http://bikeforums.net/attachment.php...hmentid=289246
To save time, I've just copied the geometry from my current mountain tandem bike. It is a medium/small bike. As drawn, it has 700c wheel but it can easily accommodate 26 in wheels. Both with disc brakes ofcourse. I'm not quite sure which I'll use yet. I've also shortened the front top tube to accommodate a drop bar. But, I can also revert back to a straight bar with a longer stem. Head tube and seat tubes are at 73 degree. Chain stay is ~440 mm. The four couplers are on the rear top tube and boom tube where I've drawn the notches. Their location is dictated by the size of the disassembled frame. It must fit a 26x26 in. case.

What do you think so far? Any suggestion?

CJ

I am curious why S&S won't sell you couplers. You see their couplers in many small builders like in the hand made bike show and all those builders started somewhere with their first frame. Calfee and the other carbon builders use the S&S aluminum couplers for standard tubes so if you use standard size tubes that would make the process much easier. Your design is the same as our coupler design on our Calfee and it fits into the 26" cases easily with fork removal.

What dimensions will be the beam and other tubes be (ie diameter and wall thickness)? And as an aside, what effect does changing from 700C to 26" wheels have on the handling and BB height and will those be important to you?

My feeling is that you need to track down someone who owns something like a DaVinci or Calfee or Precision CF frame and go over it with a fine toothcomb, calipers and camera and record every dimension you can. Then translate that to your plan.

Check the thickness and diameter of the CF where possible (eg, seat tubes). Look carefully at their dimensions and angles. If you happen on naked CF with the frame, check the lay-up at the junction of the lug and tube for any clues on directional changes in the cloth. Check the weave and quality of the cloth that's used.

You probably won't be able to imitate everything you see, mainly because of the different technology used to mould everything together, but a good lugged tube replica should turn out OK. You might also need to consider additional diagonal tubes in either or both of the diamonds to compensate for any weaknesses in your own frame design and construction.

Again, these are just thoughts that would run through my mind in planning a venture such as this.

You can use carbon and kevlar together. It is done all the time. When done it is called a hybrid. Hybrids can be interply or intraply depending upon if the two materials co exist in one ply or are two separate plies.

As I mentioned previously, your concern with the kevlar will be in finish sanding and working with it. Sanded kevlar is very difficult to make pretty along the edge.

In regards to the materials you are using, I assume you are using Kevlar 49, and guess you are buying either 281 plain weave or 285 4HS. Kevlar is a hollow tube, and is made from nylon. Nylon is known to absorb moisture, not to become dripping wet, but can absorb. Kevlar is similar and being a hollow tube, you will never coat the insides. You may seal some ends, but good luck sealing all of them.

I realize also you plan to use the Kevlar to insulate, therefore exposure to UV should be nil. Then again, any exposed kevlar (no painted with solid color) will lose up to 40% strength.

Fiberglass is a much better choice. My suggestion is not to use a light layer, but rather go with a heavier layer you can trust to accomplish the task AND have enough material thickness if minor sanding is required. One ply of 7781 style will be strong, is woven tightly and is easy to work with.

Since it appears you are going to build a lot of this as a wet layup, build yourself a resin degassing machine. This will take very little time during each layup, and will save time producing a better final product.

All this talk of building CF bikes reminded me, besides the day job of aircraft stuff, I have three more broken CF singles (two broken top tubes and a broken seat stay, all crash damage) to get fixed for people and the extra money will be handy buying stuff for our two aluminum tandems.

In your next reply, if you are willing to share, post your materials being used and techniques in using them. That was one reason for the request for photos, from that info, I may be able to steer you around "stuff" that can cause you grief.

PK

AK, FWIW, a while back I spoke with S&S about buying couplers for a project I wanted to build. Plan was to build a Cannondale off-road tandem. Going further, I planned to take a late model Super V or Jeckyll, and cut of the rear frame / suspension. Basically this would have been welded to a Cannondale main frame and heat treated. I also wanted to make it coupled. I had a couple of emails and phone calls with S&S. They informed me of what they expect from a builder. At the time, no one had installed the couplers by adhesive bonding. After explaining the techniques and process we use for these type assemblies in aerospace they would work with me provided I worked with one of my engineer friends to provide data showing it would not fail. The cost was prohibitive is why I never followed through. Ironically though, I was surprised to see a short while later a Paketa that was coupled. Not sure how they secure the joint, but S&S was all ears when I let them know it was easy but required care to properly accomplish each step.

For the project here, while S&S would be nice, I believe that if the builder understands his materials, AND understands how to build a structural joint, he will be able to surpass the S&S design for all aspects of tension, compression, and twist loads while being lightweight and pretty.

PK

PK,
I am not an engineer and have no experience with carbon or Kevlar, so your help is very much appreciated.

I am using prefabricated cf tubes raging from .5 to 1.75 in OD with wall thickness of about 0.06 in. I am purchasing these pre-made to save time. Carbon Fiber Cloth Fabric are the plain weave 3K - 5.7 oz and plain weave 50" 12k 300gsm.Kevlar is a 5oz/sq yd, 0.01" thick plain weave. Adhesive is Hysol E120-HP. Epoxy resin is the 2000 system from FibreGlast. I am also using their #180 clear gel coat for some UV protection.

Technique is wet cf lay up over joints after they are glued to the lugs and aligned. Vacuum bagging (29+ inHg pressure) with heat lamp (140 degree) for curing the resin for about 6 hrs. Any hotter my vacuum bag and tubing will melt. Since I am using vacuum bagging technique, do I still need a degassing machine?

This is my attempt at the couple joint so far:

http://bikeforums.net/attachment.php...hmentid=289400http://bikeforums.net/attachment.php...hmentid=289401
It is still unfinished. Once attached to the frame, it will get two more layers of Kevlar and several more of carbon fiber. It is essentially a compression joint with two parallel barrel bolts connected by an M6 screw. One of the barrel bolt is not threaded and acts as a washer. The idea is to distribute the pressure throughout the length of the barrel bolt rather than concentrating it at the screw.

I thought about extending the washer bolt 5 cm out and permanently attach that to the joining section. That will eliminate any torque, pushing or pulling force. But I think the compression joint alone should be sufficient. I just don't think with the frame structure, toque is that big of an issue. Any rotation on the boom tube will be controlled by the top tube and vice versa. The pushing force is controlled by the tube ends butting against each other. The pulling force is the only other concern of the joint. The slight difference in angle of the top and boom tube will transfer some of that force to the sidewall of the tubes. And the timing chain will naturally pull the two sections together. Adding the washer bolt extension is pretty easily done if needed later.

CJ

First, let me say, neither link is working for me. No idea why, but I get a default message to contact the admin.

From your writings, and understand I am try to visualize your build, the idea seems sound. The requirement for special couplers, while pretty and efficient, is not mandatory. Especially if the frame is to be uncoupled for a trip vs the need to always transport.

As for the techniques you employ for building...be forewarned that while vacuum bagging is a great method to gain pressure, and optimize resin content to achieve a high fibre ratio, it can also be a disaster when working with hollow structures.

The need to have a good seal is apparent. Unfortunately, with a tube structure and accompanying joint, when bagged there is a good chance that air can be drawn from inside the tube and pulled through the wet layup.

One method to avoid this is envelope bagging the entire part. This can create it's own problems. Crushing a hollow part is easily done with 14.7psi, and distortion is also a concern.

I have had discussions with people that repair carbon frames (not Calfee), and was setback by how often the repairs were promoted as high quality on account of them being vacuum bagged. When I questioned the idea of drawing air through the repair, obvious dancing occurred.

The first carbon frame I repaired, I went to great lengths to seal the tube and prevent a porous repair. The volume of air within the tube was enough to create problems. In process I decide to pull vacuum on the tubes internal side also. Better but a lot of work. The repair turned out well, but every repair since has been done with no vacuum, but rather much more careful detail work in the final steps of placing and working the layup. Just last night I repaired a broken top tube on a Specialized Tarmac, next week I have another completely broken top tube on an Argon time trial bike. After that, a broken seat stay on an Orbea. None were or will be vacuum bagged.

Back to your build. I was surprised to see you are using a 12k carbon. This would be fine for straight sections, but wrapping 12k can be tedious. 4 plies of 3k would be easier to work and similar strength. Additionally, why use plain weave. Harness styles are typically stronger and more easily placed. Again, I don't know your build but seeing you are using Fiberglast products, I assume you buy carbon there also.

This would be more easily worked than the 12k PW, it's a 6k 5hs they list.

http://www.fibreglast.com/product/6K...rbon_fiber_all

Are you planning to use carbon dropouts? If so, let's see if we can find some titanium to clad the carbon and minimize wear. I have the materials you need to properly treat the ti for bonding. The 12k sandwiched between ti, cured to a controlled thickness and flatness would work well.

I am excited to see this. No I am not motivated enough to build myself. If I did, my patience would be done fabbing the pattern from wood.

PK