How would you guys approach this...

General Composites Discussions Composites Talk
#1

After reading every post over the past several months, I felt it was time to solicit comments and/or advice about my project.

I’m building a bicycle. I will be mitering and bonding carbon tubes, then wrapping the joints with carbon to reinforce and strengthen them.

Using molds would be very difficult. At the area where the pedals and the seat attach to the frame there are several tubes that come together in different planes. So I’m planning on going moldless.

The picture below shows a simplified version of a joint that will be reinforced. On the right, I’ve shown a cross section. I will be building a fillet in order to ease the angle around the joints and (hopefully) prevent bridging.

I had originally thought about infusion, but I’m concerned about fraying, especially around the tight corners, so I’m going to try first with pre-preg.

The plan (in short):

  • Miter and adhesive bond the tubes together
  • Build up fillets
  • Wrap with pre-preg
  • Wrap with high elongation release film (perf or non-perf?) / breather / bagging film
  • bag & heat cure it

Strength is the primary consideration. While cosmetics are a close second, I can always fill, sand and paint the joint. But I really need to get a good, strong, wrinkle free layup.

Please feel free to poke holes in the plan, suggest alternatives. There’s an incredible amount of experience in this forum and you can rest assured that any advice you give will be strongly considered.

Thanks in advance.

Jim

#2

Have you considered building the entire frame out of foam core and then wrap carbon around it to create a unibody frame

#3

I have Evan, but I don’t want to go that route – I’ve had bikes like that and I really don’t like the way they ride.

#4

There is only one frame builder that joins CF tubes together, Calfee. http://www.calfeedesign.com/
http://calfeedesign.com/construction.shtml What he does is wrap the joint with carbon fiber and uses a mold to compress the carbon into the joint and also create a webbed area around the joint.

Trek makes a carbon fiber lug that the tubes are bonded to. There are a bunch of companies that bond cf tubes to aluminum, titanium, or stainless steel lugs. There’s also the monocoque design.

Damon Rinard has built a couple monocoque frames that he has detailed some of the construction process.
http://www.sheldonbrown.com/rinard/howibuil.htm
http://www.sheldonbrown.com/rinard/carbonTTframe.htm
http://www.sheldonbrown.com/rinard/betty.htm

And here’s one more from another guy.
http://www.sheldonbrown.com/rinard/joshpell/index.htm

Joining tubes together will not produce a better frame than a monocoque. The key in designing a monocoque frame is to know how you want to orient the unidirectional carbon fiber to produce the ride quality and stiffness that you want. By joining tubes, you don’t have much control. For instance, you can’t build a frame that is laterally stiff, but vertically compliant. With metal frames, all you do is change the shape of the tubing. But with composite, you can either change the shape or change the lay up orientation.

Keep in mind that you will be using 8+ plies of carbon fiber for the tubing, Specialized uses 17 plies. You will want the same number of plies over the joint since technically, there’s no carbon fiber between the two tubes.

#5

Scooter – thanks for the reply. I’m familiar with all of those builders and their techniques (plus a bunch of others) :slight_smile:

Bob Parlee arguably makes the best carbon frames right now:
http://www.parleecycles.com/

and Nick Crumpton makes some beautiful frames using a wet layup and bagging. http://www.crumptoncycles.com/

I’m a bike racer trying to learn composite construction techniques. I ride a lot and I know what I want, which is a traditional double diamond, tubed bike.

What I don’t know (and what I’m hoping to get people’s opinions about) is whether my plan has a chance of success.

Jim

#6

I used to race bikes, now I just ride for fun. I’m also a retrogrouch. Steel is real.

I think what you want to do is very difficult especially if you want to make it look good. Personally I don’t like the look of oversize tubes or oversized lugs. If you wrap a lot of CF around the joint, it just makes a big oversize and ugly joint.

I still prefer a lugged steel frame. Steel gives you much more feedback than the dead feeling of carbon.

#7

I used to work for the company that builds most of the carbon tubing for Trek and a few other companies. I would not build a bike the way you are thinking of doing it. I’m afraid your going to spend a whole bunch of time and money and get ugly,heavy resin rich joints that aren’t going to hold up. I would go with the lugs myself and just go buy stock carbon fiber tubing from where I used to work. The sell bike specific prepreg tubing for this exact purpose and they have the equipment to wrap or wind awesome carbon tubes. They offer multiple layups and winding schedules in bike specific lengths. I think if you are just testing out new ideas then try your out but if you just want to build a bike, buy some tubes and buy/make some lugs and do it. You just have to make sure you bond everything correctly. Use Hysol 9430 glue and keep the bond gap between the tube and lug to about .010"-.012"

I used to sell their stuff before I started my own business. Just hit the middle of this page to see their bike specific tubing, they quote lengths, weights blabla. http://www.macqc.com/orderForm.php
It’s priced pretty well. Actually there are a lot of places online to get carbon tubes but I have first hand experience with these. Lugs will be the real trick, I think you’d have to get them made. Carbon lugs would be cool if you want a roject to tackle. My $.02

#8

ARW – this was a very helpful post, thanks.

I actually already have some tubes from Maclean that I’ve been experimenting with – using them has been the plan all along – I didn’t really make that clear in the original post.

As for the lug building, well, that’s the idea – the only difference is that I want to try and build the lugs directly around the joint, as opposed to doing it seperately like Trek, Colnago, etc. This would allow me to customize the bike’s geometry easily. I realize it’ll never be a high voulme production method.

Using prepreg should avoid the resin rich joint problem. My main concerns are the layup schedule (I don’t think anyone can help with that – it’ll be a lot of trial and error) and the layup and bagging technique to build the lugs. I’m going to have to get good compaction all around and that’s the area I really need advice about.

#9

Thats great! As for your method you’re right the winding schedule will require a bunch of testing. I would say each joint is going to have its own schedule for sure. I would think the bare minimum you could lash and or layup the joint and then just bag and seal the tubes around the joint so you can pull vaccum. I wonder if you could use cellophane tape and wrap the joint very tightly after layup and then heat cure the joint so the cellopahne contracts and gives you external pressure as well as vaccum. Works great for tubes but you will have to experiment. Then just sand the joints to pretty them up and maybe even use some filler. I guess you would have to use some sort of jig to hold the tubes in the correct relative position to maintain geometry. I have heard Craig Calfees lashing method works pretty well but I’ve never ridden one of his bikes.