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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 12-20-12, 09:19 PM   #26
Rowan
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Originally Posted by chojn1 View Post
PK,
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:


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
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.
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Old 12-21-12, 05:50 AM   #27
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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:


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:


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
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
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Old 12-21-12, 06:07 AM   #28
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Originally Posted by akexpress View Post
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.
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
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Old 12-21-12, 08:29 AM   #29
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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:


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
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Old 12-22-12, 04:39 AM   #30
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Originally Posted by chojn1 View Post
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:


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

Last edited by PMK; 12-22-12 at 04:44 AM.
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Old 12-22-12, 04:57 AM   #31
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Wanted to add, skip the gelcoat, it's polyester based and you may have adhesion concerns from flexing the structure. After you get the bike frame finished, accomplish your wet sanding with something around 500 wet.

During your building process, attend a few street rod runs at local food places. Find out who in town can paint and talk with them. You want a uv barrier type clearcoat. A good painter will know what you desire and can deliver the goods. Many carbon bikes look like SH until they see a lot of smoothing and then a lot of clear on top of the carbon.

Most repairs I try and sand the paint away by hand. Last night, after some initial hand sanding to repair the Tarmac, I got out the DA and removed the many layers of paint and clear. Suffice to say there is enough paint on the frame that if the owner elects to repaint, the paint will be thicker than the repair plies.

Nothing of quality high performance made with epoxy is gel coated. (Saying that I'll likely get a counter post about a Bugatti Supercar that is 100% gel coated.

Have Fun and be Safe.

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Old 12-22-12, 07:19 AM   #32
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Here are the missing pictures:

As you can see I'm using mostly 3k weave. You are right about the 12k, but I'm using it mostly in tow form for building up bulk fast. I used it on the coupler to shape the area around the two smaller tubes for the barrel screw. Then I layered 3k on top.

Here is another view of the seat tube/stoker stem I am testing:

It is made from two hollow tubes. I cemented the joints with epoxy which I was hoping would be air tight. Vacuum bagging of the additional layers is in stages so each successive stage I presumed will have less and less problem with air. Additionally, I stuffed the hollow tubes with news paper which gets compacted in with the vacuum for support. I did put the entire thing in the bag.

I used a clear glossy gel coat which I thought looked good. I was planning on doing the same for the frame leaving the glossy nude carbon look. Spec sheet on the gel coat stated that it is UV stable and crack and chemical resistant. Powder baked would have been a better technique, but I like the idea of doing it myself rather using a professional painter. This is a DIY project after all.

CJ
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File Type: jpg couple 1.jpg (51.8 KB, 43 views)
File Type: jpg couple 2.jpg (92.4 KB, 42 views)
File Type: jpg stoker stem 3.jpg (103.8 KB, 44 views)

Last edited by chojn1; 12-22-12 at 03:47 PM.
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Old 12-22-12, 07:31 AM   #33
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PK,
These are my dropouts:


They are titanium from Paragon. How do I prepare them for carbon bonding?

Thanks,
CJ
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Old 12-22-12, 11:04 AM   #34
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Cool thread. The best way to learn is by doing! Keep us posted on your progress.
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Old 12-23-12, 08:24 AM   #35
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Hi PK,
How do I surface prep the titanium parts?
I've read that I need to etch with chromic acid and prime with BR-127. All I have in my shop is ferric chloride for copper etching and muriatic acid I used in my pool. Is either of these strong enough to replace the chromic acid? Also, where can I find BR-127 in small quantity? Can I skip this step for my application?
Thanks,
CJ
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Old 12-23-12, 08:49 AM   #36
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This is my working drawing for the frame:

I do like the look of a lat-less design. This design will save weight and reduce the coupler requirement to four instead of six. It is a medium/small frame so I think I can make up for the reduced stiffness by overbuilding the joints.
Changing wheel size from 26 in to 700c will change the stand over height by about 2cm and obviously alter the riding characteristic inherent in each wheel. I can experiment once the frame is complete.
For now, anyone with any suggestions, recommendation, critiques, or alteration to this frame design? Again, this is my first frame build, any help is appreciated. Stupid mistakes in design are easier to correct in the planning stage than during construction.

Thanks,
CJ
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Last edited by chojn1; 12-23-12 at 02:11 PM. Reason: New link
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Old 12-23-12, 11:54 AM   #37
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Sorry, but your link isn't opening for me. I think you are missing some technical bits in there somewhere.
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Old 12-23-12, 12:19 PM   #38
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For aerospace we use a special process. I'm thinking you need to have some sent to you. Very easy and environmentally friendly. You'll want to prep all the metallic parts at the similar time, since once mixed it only last for hours.

In one of your previous posts, you mentioned envelope bagging, this is how you are minimizing air into the joints. Not sure if you realized it or not. Bigger parts become more difficult.

BTW, I still have issues opening those links.

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Old 12-23-12, 02:39 PM   #39
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I've reduced the size of the attachments, hopefully they are view-able now. Let me know if you are still having trouble with them.

I've also started with my homemade jig:


Essentially, I just printed my plan on wide format paper. I taped it to a 3/4 in plywood where I trace the axis of the frame's tubes. I am going to level the plywood top on some legs to make a table.
To this top, I'm going to attach my wood jig blocks along the axis to the tubes.

These blocks are cut from a single 4x4 attached to a single 2x4. All the grains are thus running in the same direction. This is important because any movements, expansions, or contractions of the wood would affect all the blocks in the same way. The blocks will have holes centered at the intersection of the 4x4 and 2x4. The diameter of the holes in each block is dictated by the diameter of the carbon fiber tube it is designed to clamp. I'll post more pictures once I get it setup. This is a very simple inexpensive homemade jig. Hope it works.
CJ
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Old 12-23-12, 03:06 PM   #40
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I forgot to ask:
Does anyone know the diameter and width of the Gates CDX 69 teeth pulley with belt in place? Also what is the distance between the pulley and the center line? I want to make sure it will clear the left chain stay.
Thanks,
CJ
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Old 12-23-12, 04:31 PM   #41
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Originally Posted by chojn1 View Post
I've reduced the size of the attachments, hopefully they are view-able now. Let me know if you are still having trouble with them.

I've also started with my homemade jig:


Essentially, I just printed my plan on wide format paper. I taped it to a 3/4 in plywood where I trace the axis of the frame's tubes. I am going to level the plywood top on some legs to make a table.
To this top, I'm going to attach my wood jig blocks along the axis to the tubes.

These blocks are cut from a single 4x4 attached to a single 2x4. All the grains are thus running in the same direction. This is important because any movements, expansions, or contractions of the wood would affect all the blocks in the same way. The blocks will have holes centered at the intersection of the 4x4 and 2x4. The diameter of the holes in each block is dictated by the diameter of the carbon fiber tube it is designed to clamp. I'll post more pictures once I get it setup. This is a very simple inexpensive homemade jig. Hope it works.
CJ
The files you are attaching have an extension of php, Try attaching jpg files and they should be viewable.
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Old 12-23-12, 05:37 PM   #42
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That is odd. I don't work with php files. These jpegs come straight off my phone. I did re-size them thinking that was the problem. Usually there is no problem with the upload, and it display fine. But when I recheck later, I can't them either. Is there a limit to the number of photos I can post because I have been posting a few lately? Anyway, here are the last two attachments again. Are the others visible?
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File Type: jpg jig blocks.jpg (38.9 KB, 44 views)
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Old 12-23-12, 09:05 PM   #43
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You may already have these as a reference to building up jigs, but it doesn't hurt to post them (they are for recumbents, but still make provide some ideas if you still looking for them):

http://www.wisil.recumbents.com/wisil/jig/bike_jig.htm
and
http://www.ericksonracing.com/frame_jig.htm

This one is on building a CF stickbike, and might have something useful for CF work:

http://www.wisil.recumbents.com/wisi...ke/default.htm
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Old 12-24-12, 07:08 AM   #44
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That is odd. I don't work with php files. These jpegs come straight off my phone. I did re-size them thinking that was the problem. Usually there is no problem with the upload, and it display fine. But when I recheck later, I can't them either. Is there a limit to the number of photos I can post because I have been posting a few lately? Anyway, here are the last two attachments again. Are the others visible?
That worked well.
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Old 12-25-12, 12:16 AM   #45
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This thread started out amusing, but suddenly sounds prototypical of an emerging bike company - an enthusiast toying with new ideas. Some of us may be riding frames from this fellow in 5 years. You go chojn1!
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Old 12-25-12, 08:10 AM   #46
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This thread started out amusing, but suddenly sounds prototypical of an emerging bike company - an enthusiast toying with new ideas. Some of us may be riding frames from this fellow in 5 years. You go chojn1!
A day prior to your post, I had the thought of wondering if he is keeping a good record of hours and materials spent. Not so much in regards to building, but just for comparison.

I found the topic and post entertaining, the OP came in with a plan and a mission, myself, I think it's pretty cool he is giving it a shot. I doubt he just decided one hour to build and the next hour started, he has too much base knowledge to be jumping in.

PK
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Old 12-25-12, 08:16 AM   #47
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PK,
These are my dropouts:


They are titanium from Paragon. How do I prepare them for carbon bonding?

Thanks,
CJ
CJ

Do some homework then let me know.

http://www.chemical-supermarket.com/...20Features.pdf

Scotchbrite the mating surface with AC130 treatment. Better than Chromic Acid Anodize (CAA).

When you read BR stuff that is adhesion primer, they also show tests with film adhesive as we use in metal bonding and sometimes composite pre-pregs.

PK
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Old 12-25-12, 03:26 PM   #48
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Quote:
Originally Posted by PMK View Post
A day prior to your post, I had the thought of wondering if he is keeping a good record of hours and materials spent. Not so much in regards to building, but just for comparison.

I found the topic and post entertaining, the OP came in with a plan and a mission, myself, I think it's pretty cool he is giving it a shot. I doubt he just decided one hour to build and the next hour started, he has too much base knowledge to be jumping in.

PK
Hobbies (and this is a sophisticated one) can't really be measured in terms of dollars and time. Otherwise, we wouldn't have hobbies.

It starts with research and self-education. And you can't put a value on the skills and knowledge gained by the end of a successful project.

Could the pros build one faster, more technically detailed and for cheaper? For sure. But they had to learn and experiment, too, and combined, it's a cost that, while written off in the accounting books ages ago, remains the most intrinsically valuable.
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Old 12-26-12, 07:53 AM   #49
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Hey,
Thanks for the words of encouragement. Seriously though, I am blessed with a very rewarding day job that I love. This is just a fun little project. What little I know on building carbon fiber bike or any bike, I learned from the internet over the last month. Isn't the internet amazing?

Here is my jig table so far:



Again, I transferred the axis from my drawing to the table. Then I align the wood blocks and screw them in from the bottom. I used my self leveling laser to make sure the blocks are leveled and lined up accurately.



I have notched a mid-line grove on each block with my router table to help with drilling and alignments. This jig is to be used for aligning the front triangle and the mid section. I will make another jig for the rear triangle.

PK thanks for the link. That is exactly what I needed. Now I am trying to find a source where I can buy those chemicals.

CJ
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File Type: jpg Jig Table 3.jpg (95.7 KB, 36 views)
File Type: jpg block alignment.jpg (72.2 KB, 32 views)
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Old 12-27-12, 11:56 AM   #50
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PK,
Thanks for the lead. I can find the AC130 prep, but the primers (either BR127 or BR6747) are giving me trouble. Coastline can order them for me, but it will take four weeks. That's a little beyond my timeline. So the search continues.

In the mean time, here are the completed couplers:

Here is one with the titanium nuts and bolt in place:

Nice and light! Hope it will hold up.

CJ
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File Type: jpg coupler weight.jpg (93.0 KB, 47 views)
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