back40 bicycleworks

Two New Road Frames

2012-01-18T18:54:00.000-08:00

It's been too long since I updated the blog, my appologies. I've had over 10,000 page views from 34 countries so far. Apparently the interest in carbon frame building is larger than I thought when I started posting to keep a friend updated on his frame build.

I received a box full of ENVE Composites ( http://www.enve.com/ ) tubing and frame components today so it seemed like a good time to start posting again. I have two new road frames to build over the coming weeks. Check back and see the build process changes I have planned.

To start off we have the tubes and rear end parts for the first build.

Here is a close up of the in-molded chain stay cable stop that ENVE offers.

Included in the tube order was some custom spec 44mm ID head tube and 41mm ID tubing for use with BB86 press fit bottom brackets. There are a number of 90deg layers used to tolerate the press fit forces.

Here is something new for me, a 0.25" titanium tube that I'll be bending and using to route the rear brake cable through the top tube.

Here is a close up detail for the ENVE seat stay brake mount. The first frame will use this while the next one will use individual stay tubes and a custom brake bridge.

I design each frame using 3D CAD software. From here I can generate dimensions for cutting each tube and how to set up the frame jig.

That's it for now. Dropouts are on the way and I'll be ordering some 300g/m^2 uni-directional and some 10.7oz/y^2 8HS carbon prepreg shortly for the joint construction.

Frame #2 Four Months of Testing

2011-07-04T14:08:00.000-07:00

It's been far too long since I last posted. In the mean time I completed frame #2 about three weeks after the first one and I've been test riding it ever since. It has about 2700 km on it on mostly paved roads, rollers, some gravel roads and some less than gravel twin track roads in the Appalachians of North Carolina near West Jefferson.

I built the bike with an EDGE 2.0 fork, Shimano Dura-Ace Di2 groupo, Ritchey WCS stem and post, a Pro carbon bar and Shimano 7900-C24-CL wheels.

It has internal Di2 wiring and an under down tube battery mount. The frame has a "lugged" look to the joints rather than the smother finish on the first frame. The finish on this is spray can automotive clear coat. I have an interesting finish idea for the next frame and it will have the proper logos etc (now that I know the frames work).

It's been ridden hard and has held up well. No frame related issues of any kind. At first I though the back end was a bit too harsh but I think that was just in contrast to the True Temper S3 frame I'd been riding for years. I used the BB86 or press fit Shimano standard for the BB.

I'm planning the next version of this frame with a lighter chain stay assembly and thinner individual seat stays. This should bring the weight down from the current 1060g for a 58.5cm frame. I'd like to do internal rear brake cable as well as Di2 wiring.

Finished Carbon Frame #1

2011-02-18T18:02:00.000-08:00

Neil's frame is finally finished. It's the first full carbon frame and it turned out well. Still need to work on the cosmetic finish etc and see how test riding goes. Without further ado here it is with integrated seat post, PF30 BB, ChrisKing 44mm Inset compatible head tube and a final weight of 990g.

Frame Load Test

2011-02-13T04:40:00.000-08:00

I took the first trial frame and loaded it to failure. It took a lot of force to get it to fail. In most cases it took almost double the load I've applied in the past to cold set OX Platinum frames and accidentally destroy an S3 frame. For example, the side load test where the head tube is supported, I was suspending my full body weight on the end of a 4' bar and bouncing slightly. Is the frame "strong enough"? It's hard to say yet but this does give me confidence.

More Cable Guides

2011-02-12T19:15:00.000-08:00

I finished the cable guide through the chain stay yoke of Neil's frame. It turned out well, see for yourself.

A little more finish work and it should be ready to clear coat. Here are the down tube mounted derailleur guides. I'm going to work on carbon versions that bond on to avoid the drilled holes in future frames.

Next up, a real treat (here is a hint).

Cable Guides

2011-02-11T19:21:00.000-08:00

The next and approaching final step was to position the cable guides and drill the holes. Here is the frame with the derailleur and one brake cable guide in place.

I put a modified 1/2 cable guide on the chain stay next.

The next step was tricky. I had to position and drill a hole through the frame behind the BB for the front derailleur cable. After placing a Shimano BB guide on the frame I marked the entrance hole on the bottom.

After drilling through the far side guided roughly by eye I glued in a 1/8" dia brass tube.

I'll trim and sand the ends flush once the glue cures. Here is another photo for Neil, the weight prior to cable guides.

I have to modify my rivet gun to reach down into the cable guide pockets. Once that's done the guides will be glued and riveted in place.

Sanded Finish

2011-02-11T19:04:00.000-08:00

Here are a few photos after the joints have been sanded. First I rough sanded the joints to get the high spots and bag joint wrinkles off trying to remove as little carbon as possible. Next I painted the joints in a thin layer of laminating epoxy. This was followed by another sanding and another coat of epoxy etc. The end result looks quite nice when wet or I assume under clear coat.

Unwrapping and Sanding

2011-02-07T17:41:00.000-08:00

When the frame comes out of the oven you have to strip all the bag, bleeder and release cloth off. Sounds like no big deal except the excess epoxy has soaked through the release cloth and into the bleeder. It forms a lovely crust around the joints that does not come off willingly, visible as the dark areas around the joints.

Next we have a guest frame building assistant, in this case Neil. He's been tasked with peeling off the above mentioned crust which he does with the maniacal glee of a 4 year old at Christmas.

Look, you can see the mania in his eyes I swear!

Anyway, with the bag and crust removed it was time to inspect the joints for good compaction etc. In this case everything turned out quite well.

After a bit of sanding to take the high spots and wrinkles out it looks like this. With a few thin coats of epoxy and sanding it will look quite nice. Neil requested that the lugs blend into the frame which these do nicely.

Last up is a "baby picture" of Neil and his new bundle of carbon joy.

Next up is the joint finishing, cable guide attaching etc...

Bagging and Baking Neil's Frame

2011-02-07T17:16:00.000-08:00

With all the joints wrapped in Carbon it was time to "bag'n bake". First step was to close the frame openings with the UHMW plugs. The plugs also serve to keep the BB and head tube "round" under heat and vaccum. The UHMW actually expands in the oven and are tight when hot.

Next step was to carefully cover the carbon wrapped joints in release cloth. I use an Airtech product that has a very fine weave. I try to make it smooth and wrinkle free but it turns out it does not matter that much.

After that it's time to cover each joint in bleeder material which serves to distribute the vacuum over the joints and soak up excess resin pulled out of the prepreg under vacuum during cure cycle.

The final step before baking it to apply the vacuum bag. I choose to enclose the frame with a fold between the stay. It's a bit more work to get the bag sealed but does not apply extra side load to the stays.

Here it is post cure in the oven. It was hung from the top of the oven but it fell down during cure. Note to self, get a right angle air fitting for the vacuum seal. I think the bend in the air hose pushed the frame off when the door closed. Next oven will have a window!

In the above photo you can see the heat dissipating plate I installed after the first test frame stays failed under vacuum due to localized overheating. This time there were no issues.

Carbon Wrapping Neil's Frame

2011-02-05T15:13:00.000-08:00

With the joint fillets sanded and the frame thoroughly cleaned with isopropyl alcohol it was time to start wrapping the joints in carbon. I start with a few pieces that will wrap along one tube and around the other. This should set a good solid foundation for the joint.

Here is the first piece going on. It looks nice and easy but this stuff is sticky and fragile and it's being draped over 3D contours. Working in a cool shop (+12C) the prepreg often needs to be heated lightly with a heat gun to make it sticky enough.

After many overlapping layers you end up with this which I hope turns out cosmetically pleasing (and of course structurally sound) after the oven.

The BB wrapped.

And the Seat Tube Junction.

Tomorrow I hope to get it at least in the vacuum bag if not in the oven.

Alignment Check and Fillets

2011-02-05T14:50:00.000-08:00

Once the epoxy had cured I check the alignment of the frame. I have a fixture that holds the frame horizontally by the BB. I use a height gauge to check everything. First I check the seat tube comparing one end to the other. This is how much it was out by...in other words almost nothing (0.008" over 23").

Next I check the head tube using a straight steel rod and cones to center it on the head tube. I compare the ends of the rod 16" apart to magnify any error on the head tube. It was not out by much, 0.012" over 16", so I am not complaining.

Next I added the fillets to the frame joints. I use mixture of DP-420 and glass micro spheres that forms a thick paste. Here they are prior to and then after sanding on the head tube.

Since this is the first frame using a Press30 size BB I had to make new UHMW plugs for vacuum bagging.

Prior to adding the fillets the frame weighed 894g. With the fillets sanded it was 906g.

Gluing Neil's Frame

2011-02-03T20:30:00.000-08:00

The time has come, Neil's frame is taking shape. I mitered the rear end and prepared all the tubes for gluing. They've all been sanded with 80G and wiped down inside and out on the ends with isopropyl alcohol. First step was to install the water bottle bosses. I applied a little epoxy to the inside of the tube around the hole before inserting the Rivnuts.

Here is the BB joint ready to cure over night.

The dropouts plugs were sanded rough, cleaned and coated in glue. I also coated the inside of the stays after roughening them up. I used flash breaker tape to keep excess epoxy off the ends of the stay.

The seat tube junction turned out well. I used some tape to lightly hold the joint while it cures. It's important that everything sit at rest in the jig, anything forced could lead to misalignment.

The head tube joint is critical. I'll check alignment tomorrow and if it's not good enough I'll have to break the front triangle apart and try again.

Finally the whole frame.

Tube Cutting and JIG Fitting

2011-01-31T18:24:00.000-08:00

I promised ever excited Neil an update so here it is. Below I'm showing part of the tube mitering process and how they fit in the JIG when complete. First is the marked downtube ready to cut. The BB30 sized mandrel is installed in the mill. You can see a fine black line scribed in the silver area on top of the tube. I sand up to that line.

Next is the finished cut.

Here is the BB joint with all the tubes fitted in the JIG ready to glue.

Same thing at the head tube junction.

And the water bottle boss holes ready for the Rivnuts.

And finally the whole front triangle in the JIG showing the integrated seat post extension.

Next step is to install the bottle bosses, sand, clean and glue the joints together with DP-420...

While Waiting...

2011-01-29T15:03:00.000-08:00

While waiting for Neil's seat tube to cure I marked the rest of the tubes for cutting and mitering (the fine black lines scribed in the silver marker).

Still had time so I modified a cable guide to better fit the chain stay diameter (and took off one of the rivet holes).

More time left so I set up the JIG and placed the head tube and BB.

I had to make new mitering mandrels for the sander so I cut and turned the one for the BB30. Oh look...Neil's seat tube is magically finished and the post mount fits!

Neil's Frame Begins

2011-01-29T14:51:00.000-08:00

I've started on the first "real frame" that will be ridden. It's for Neil (and he's terribly excited) and here are his tubes ready to start marking and cutting.

This frame will have an integrated seat tube and the post head Neil provided had a 33.9mm ID. I built up the top end of the seat tube to fit by wrapping in carbon (being carefull to keep it straight).

Next I put the tube in the lathe and inserted the end plug.

Then I wrapped it in flash breaker and shrink tape.

Then it's off to the oven for 2+ hours of baking. The oven now has the heat dissipation modifications (fingers crossed that it all works).

Water Bottle Boss Test

2011-01-25T18:57:00.000-08:00

I did a quick water bottle boss test. I used AW Rivnuts designed for "softer" materials in an M5x0.8 thread. After drilling the correct size hole I installed the Rivnut watching it collapse on the back side as I went.

Here it is installed.

And here it is ripped out after applying considerable force. Enough force to delaminate the tube wall.

I'll use DP-420 in behind the Rivnuts when I install them in a frame for added security.

Head Tube Finish

2011-01-25T18:50:00.000-08:00

I decided to see what sort of finish I could achieve without hours of work. Below is the head tube joint. It was rough sanded to get the ridges and high points off the carbon. Then it had a thin layer of epoxy "painted" over it. It was then sanded, "painted" and sanded. Other than a few small holes and scratches it looks pretty good.

Doh!

2011-01-09T19:09:00.000-08:00

I did have something go wrong in the oven. Apparently the cheap carbon rear end I used had poorly cured chain stays. I noticed when I mitered them that they gummed up the abrasive belt a little. Didn't think much of it at the time. When I pulled the frame out of the oven I noticed this.

OOops...apparently 250F was more than this stay could handle under vacuum. Good thing it as A)Cheap and B) Only on a test frame.

The odd thing is the at the seat stays from the same set were not affected?

UPDATE: I've determined that there was uneven heating in the oven which caused the stay to go over temp and collapse. I've modified the oven with a plate to diffuse the heat from the elements and increased the fan speed with a larger motor.

Out of the Oven and into the...

2011-01-09T18:58:00.000-08:00

...Workbench I guess. Ok, here is the post cured head tube joint. Lots of resin pulled out of the joint under vacuum which could be a good sign (or a horrible failure, time will tell).

Even the convoluted BB looks good.

After stripping off the "easy" vacuum bag material I ended up with this mess at each joint!

After much gnashing of teeth etc...

You get this...which actually looks pretty damn promising so far, at least good enough to unwrap the rest.

Head Tube joint looks good with a small "flash" line where the bag joint was. This will sand off easily.

Even the BB looks like it had full compaction under vacuum, no shiny parts where the release cloth was not compressed against the carbon.

And the important part (for some), the weigh in!

It's in the Bag!

2011-01-09T18:37:00.000-08:00

Before I put the frame in the vacuum bag, I added a few layers of carbon as bottle boss reinforcements. One I surrounded with flash tape, the other I did not. They both got wrapped with shrink tape.

Here is the head tube wrapped in release cloth. I tried to keep it as smooth as I could to help make for a smoother finished joint.

After wrapping the other joints I covered them in bleeder and "connected" them with some more to where I put the vacuum fitting in.

This is a shot of the frame in the bag with vacuum pulled to test for leaks and to make sure there was enough slack/stretch to conform to all the joints.

I folded the bag back on itself inside the rear triangle to avoid stressing or crushing them or pulling things out of alignment.

I managed to get around 25-26 in Hg and it was consistent so in the oven it goes.

Here is the oven and compressor running and of course a pile of crap on the handy "shelf" conveniently located on top of the oven.

Not it's time to pace back and forth staring at the vacuum gauge for 3 hours. Actually I checked it once the oven reached 250F then left it to cook.

Layup

2011-01-08T16:22:00.000-08:00

It took about 4 hours to lay up all the carbon on the frame. Below are a series of photos of the layering.

I start each joint with a long continuous layer along the joint.

Lots of layers later...

Again, one long layer to start.

The seat tube junction had a few fewer layers.

The flash breaker tape should help with maintaining the "lug" edge after curing.

A lot of cutting little pieces of carbon. I think I need some woven cloth or 300g/m^2 uni.

Sanding Joint Prep

2011-01-08T16:15:00.000-08:00

I sanded the fillets and installed the UHWM plugs in preparation for layup. Everything was wiped down with isopropyl alcohol prior to layup.

Filleting The Frame

2011-01-05T19:48:00.000-08:00

No, not cutting it up, that comes later. Below is the glued frame with DP420 and glass microsphere putty applied to create fillets at the joints. The tiny 50 micron hollow glass spheres make the epoxy thick and easier to work with. These will be sanded smooth prior to layup. The blue tape is flash breaker tape and marks the end of where the "lugs" will be for this test frame.

First the head tube joint.

Then the BB...

And the complete frame...

The frame currently weighs 957g. It was 452g as a finished front triangle and 952g prior to adding the fillet paste. I think 5g is worth it for smoother flowing joints. The frame alignment out of the jig is pretty good as well. The head tube is out by 0.012" of the 6.5" head tube length (should be better). The seat tube is dead on at 0.004" over 20". The rear wheel sits straight and the dropout spacing is 129.25mm which is a little under 130mm, I'd rather be 0.5mm over.

Glue Time!

2011-01-02T15:59:00.000-08:00

I skipped over gluing the front triangle together. My gloves were too sticky to touch the camera. Below are the dropouts sanded rough, cleaned with isopropyl alcohol then slathered in DP420. The same thing was done to the inside of the chain and seat stay ends.

Here is the finished glued BB joint. The tubes around the joints were sanded to bare carbon and cleaned with alcohol in preparation for wrapping with prepreg.

Same thing at the seat tube joint.

The shop was about 10C today so I have the rear end "curing" in front of the heater. It only blows warm air...nothing catching fire or melting.

Next step is to fillet the joints with fairing compound, sand and shape them.

Mitering Stays

2011-01-02T15:44:00.000-08:00

I could not use my normal stay mitering fixture with the carbon rear end. Luckily whoever designed it was thinking and made the top and bottom edges parallel for the first 6" making it relatively easy to hold in the mill vice.

Here is another shot showing the belt sander attachment and shop vac clamped in place to suck away the dust.

Test Frame

2010-12-30T18:28:00.000-08:00

It's time to build a test frame. I collected some appropriate tubes and substituted some 1.75" OD for the BB and head tube. Step one (well, actually about step 17) is to miter the tubes to fit in the jig.

Here I fit the head tube to the jig with the new oversize locating cones.

Below you can see the mill mounted belt sander in action cutting the DT to HT joint. I hold a shop vac with a fine particle bag installed next to the belt/tube to catch the carbon and epoxy dust.

I built the mitering fixture so the tubes could be removed with both the holding blocks locked in place, one on each end of the tube. This allows me to test fit tubes in the jig and return them to the mill in the same location if further cutting is required. Here you can see the DT with it's holding block in the jig.

The finished BB joint with the other miter block shown.

The front triangle fit in the jig. Next step will be to miter and fit the rear end.

Break Time II

2010-12-12T13:32:00.000-08:00

After the side load I tested the in plane bending (trying to pull the joint apart in the the plane of the frame). I used split blocks to support the ends of the tube and a 4' solid steel pry bar to apply the load.

I applied almost 1/2 my body weight to the joint about 4 feet from the support block. So, the joint was under 300+ ft-lb of torque. After alternating this load in opposite direction a few times there was a cracking sound and this small flap of carbon pulled free. Also, a small crack curved under and along the joint fillet..

One good hard heave (probably over 400 ft-lb) and the joint cracked further.

Eventually if failed completely...

Break Time

2010-12-12T13:12:00.000-08:00

First you'll notice the custom patent pending "distributed loading system" employed for this test. I stood on the test joint suspended between two frame tube blocks. This resulted in 170lbs being applied. Nothing happened. I then pulled up on the nearby work bench and applied more load...and more until I could not pull any harder. Other than a tiny deflection there was no damage. Repeating this later with a bathroom scale sent the needle off dial over 340lbs. So, 350lb sideways load test passed.

Finished Finishing?

2010-12-05T07:54:00.000-08:00

I hand sanded with 240grit emery strip then used a cloth buffer wheel to smooth out the surface. Not perfect but good enough to prove I can produce a good looking joint with practice.

When it's wet you can see the uni-directional carbon finish. It would look good if it was clear coated.

Epoxy "Painted" Joint

2010-12-05T07:50:00.000-08:00

Here is the test joint after initial sanding with a coat of clear laminating epoxy "painted" over the surface. I will sand this down until it's smooth or I hit carbon (when the sanding dust turns from white to black).

Sanded Joint

2010-12-03T04:25:00.001-08:00

I quickly sanded the joint to take off the epoxy flash and the high points etc. I didn't take too much off so I would not cut away fibers.

I'm going to "paint" the joint with clear epoxy and sand that to get it cosmetically smooth. The top side of the joints turned out well with little sanding. The underside had a void in one area that will have to be filled.

Once I make it all pretty it's time to break it...

Unsanded Joint

2010-12-03T04:17:00.000-08:00

Here is the joint after pulling off the vacuum bag and unwrapping the release cloth and tape. Seems to be good compression this time with a just a few small voids at the root of the underside fillets.
This side (that was facing up in the oven) did not seem to be as compressed. It lacks the even pattern from the release cloth you can see on the other side. It appears well consolidated though.

Vacuum Bagging

2010-12-02T18:26:00.001-08:00

After reading about Nick Crumpton's methods here
http://www.velocipedesalon.com/forum/f26/carbon-building-18058.html
I decided to try to bag the whole joint this time and not seal off on the tubes. You can see little wood caps taped on the tubes I put there to keep the bag from sucking in and breaking.
This is at 20 inHg and it went to 26.5 inHg before it went in the oven.

Here it is resting on a block of wood ready to bake.

Seat Cluster Test

2010-12-02T18:18:00.000-08:00

I'm trying larger fillets (Aeropoxt Light fairing compound) over the glued joints to see how the carbon layup works with them.

Here are the first few layers of 150g/m^2 unidirectional prepreg being applied.

And here is a the finished joint. I tried to be aware of the final cosmetics of the joint as I went. Time will tell how it looks and performs after it come out of the oven.

Out of the Oven

2010-11-21T16:52:00.001-08:00

Here is the joint out of the oven. It sat at 200F for 2 hours after ramping up for 30 minutes. The bag, release ply etc peeled off fairly clean but I'll use some flash break tape next time to keep the tubes clean in the first place. You can see in the photo below I had some bag pressure issues. A small hole developed right at the inlet tube and the pressure was 15-20inHg (or about 10pis) when I it took it out of the oven. The shiny area is where the release ply was not being pressed into the joint.

On the top side though, the joint is well compacted and solid. I found that tapping on the joints tells you a lot. You can tell the uncompressed areas by sound alone.

While not pretty yet (these are un-sanded) and not fully compacted the main tube joints feel strong and stiff. Destructive testing and cutting it apart will tell the whole story though.

Joint Test Vacuum Bag

2010-11-21T16:45:00.001-08:00

Here is a BB test joint I've prepped for the oven. I used 3 pieces of Stretchlon 250 bag material and 350F rated bag tape. Under the tape is release cloth with a very fine weave, then the carbon prepreg layers. You can see the UHMW plugs I use in the BB to run the bag and seal tape onto.

It took about 10 minutes to get all the leaks sealed and I was able to pull 26mmHg on the pump. The bag did it's job and stretched nicely into the fillets. The release cloth appears to be enough of a bleed layer on a bag this small, everything was pulled down tight.

Head Tube Cure and Finish

2010-11-05T14:59:00.000-07:00

After applying 10 layers on the mandrel I used the lathe to hold the it in place and slowly turned it by hand to apply the release coated shrink tape. I'm carefully overlapping 75% with the previous turn for better compressive force and a smoother ridge free finish.

Into the oven it goes...

Here it is straight out of the oven with the shrink tape removed. Notice the left hand end, that's where the masking tape holding the shrink tape in place let go and allowed it to slip. Good thing I made the tube a little longer than required.

Back in the lathe I hand wet sand the outer surface and square the ends with the lathe tooling. Wet sanding eliminates the dust issue (as long as you protect the lathe from the splashing).

The finished headtube. It is ridiculously stiff and weighs 50.3 g.

Head Tube Layup

2010-11-05T14:47:00.000-07:00

After calculating the thermal expansion of UHMW for the temperature delta of the cure process I decided to layup a headtube for Chris King InSet headset press fit ID of 44mm.

You can see the layers below, 10 in total, alternating between 0 deg and 90 deg.

The next two photos show the wrapping process as the carbon sticks to the previous layer and the backer is peeled away. Working in the shop around 50F I had to use a heat gun to increase the tack level of the prepreg in order to get the wraps to stick and conform.

The finished layup ready to shrink tape.

Tube ID

2010-10-07T19:19:00.000-07:00

The tube ID turned out nicely. The fine groove from turning on the lathe was clearly reproduced as well. It will need more layers to be an effective BB shell tube. This process may even work for custom layup frame tubes as well.

Press30 Tube

2010-10-07T19:16:00.000-07:00

Here is the finished tube (test) for a Press30 BB. I need to adjust the mandrel OD to account for the thermal expansion of the UHMW but other than that, the process worked as planned. I might even be able to use 90deg fibers and not have it stick to the mandrel.

UHMW Mandrel

2010-10-07T18:59:00.000-07:00

I tried unsuccessfully to lay up a head tube on a turned polished aluminum mandrel. Even with 3 layers of high temp release wax, post cure the carbon would NOT come off the mandrel. Back to the drawing board. Next I conducted a test using UHMW as a mandrel. It has a higher coefficient of thermal expansion than aluminum and nothing sticks to it (at least not epoxy). It turns up in the lathe nice and smooth and the 250F cure temp has no lasting effect on it.

(un)Finished Product

2010-09-19T19:07:00.000-07:00

I got the joint unwrapped, pulled off the melted vacuum bag the removed the peel ply. Where it had been compressed the layers were well compacted and had a fine woven impression in the surface from the peel ply. Where the vacuum had failed the fibers were not fully compacted during cure. Looks a little rougher than I'd like but with practice I should be able to tidy it up. Lesson learned, try cure at 200F for 2 hours instead of 250F for an hour.

They Really Mean 250F

2010-09-19T19:02:00.000-07:00

Ok, lesson #1 the hard way. Apparently when they rated Stretchlon 200 for 250F they really meant it. I had the joint in the oven with the pump running steady at 25mm Hg. The temperature was ramping up slowly and everything looked good. I noticed the pump speed change right around 240F, glanced over and saw the needle falling on the vacuum gauge. I opened the oven door and head a hissing sound. Doh! The bag had failed where it was most highly stretched in the joint fillet areas. I left the joint in place to fully cure.

Vacuum Bag

2010-09-19T18:59:00.001-07:00

This is the joint wrapped in Stretchlon 200, a high stretch vacuum bag. I chose this because it makes it easier to achieve smooth compression, avoid pleating etc. I was able to get 25mm of Hg or about 12psi.

Peel Ply

2010-09-19T18:55:00.000-07:00

To vacuum bag a joint you have to apply peel ply over the carbon. It's a fine release coated fabric that can be removed from the cured epoxy. I was trying to minimize wrinkles under the vacuum bag.

Layering

2010-09-19T18:52:00.000-07:00

Here is an example of the layering process for a joint. I still have to work on making them neater and record the actual layup schedule etc. Trying to get most of the support fibers to be as long as possible and wrap around the head tube.

Prepreg Test Layup

2010-09-19T18:48:00.000-07:00

Completed another test of prepreg layup. This time it was used to test vacuum bagging the joint while in the oven curing. Below shows the first few pieces of uni-directional carbon being applied. The lower one wraps all the way around the head tube.

Tube-to-Tube Fillets

2010-09-14T18:10:00.000-07:00

For these test joints I'm going to use a light weight, high strength fairing compound to add small fillets to the tube joints. The theory is that the carbon won't be making sharp radius turns when it's compressed against the joint. I know "filler" is frowned upon so we'll have to see how the joints perform with it. The photo below is as it was applied, prior to sanding. When finished the fillets will be less than 1/4" radius and the tubes clear of excess material.

The filler used in all the joints depicted below weighs less than 12g prior to sanding.

Preparing to Vacuum Bag

2010-09-14T18:00:00.000-07:00

In order to vacuum bag the joints I need to extend the ends of the exposed head tube ends as well as the BB width. That way I can extend the vacuum bag compressed area beyond the ends of the tube. I turned some plugs from UHMW plastic rod since most glues just don't stick well to it. Any expoxy that might migrate onto them during cure "should" (he said hopefully) not stick.

In the background is a turned aluminum tube mandrel. I'm going to attempt to lay up a head tube for use on a frame with 44mm Chris King In-Set headset cups. It was turned with a very slight taper, polished and coated with high temp release wax. In theory a quick dunking in ice water when it comes out of the oven should let the carbon tube slide off...time will tell.

The Shrink Tape Experiment

2010-09-14T17:52:00.001-07:00

I was impatient and wanting to test the oven curing process. Instead of carefully setting up a vacuum bag around the test joint I decided to see how the shrink tape would conform to a tube intersection when it was heated. I can say that on round straight sections of tube it works very well. On curved tube joints it conforms not at all leaving large wrinkles, pockets of epoxy, uncompressed fibers and generally a nice mess.

After a little sanding (and only showing the least embarrassing angle) the first wrapping test looked like this...

[Trust me, the underside is not so pretty]

A Quick Test

2010-09-14T17:44:00.000-07:00

I did a quick test of the prepreg curing process by wrapping a test joint. I found out that 150g/m^2 uni-directional carbon prepreg is very sticky and hard to work with by hand with gloves on. The fibers come apart very easily. I quickly determined that I need small tools to carefully push and lay the fibers neatly.

A good example of how not to do it...

Carbon Frames

2010-09-14T17:37:00.000-07:00

I started experimenting with carbon frame construction in the last few months. After a couple of years of reading and research I settled on using tube-to-tube construction and wrapping the joints in prepreg carbon. In order to use prepreg I built a temperature controlled oven. It's a well insulated sheet metal box heated by two oven elements running on 120V for a total of 1250W. There is an aluminum fan blade driven by an external motor to circulate the hot air. It's all controlled by an Omega Instruments controller to +/- 1 deg at 250F.

Finished Bike

2006-08-03T14:11:00.000-07:00

Above are photo of the finished assembled bike. I got a little too immersed in the welding and finishing stage to take photos. I'll be sure to get some next time.

2006-08-03T14:06:00.000-07:00

Here is another photo of the tube belt sander mounted on the mill. The belt is tensioned by two springs acting on the left hand roller frame. The left hand roller is crowned as well for better tracking. The black knob is to adjust the tracking which is required when the belt heats up etc.

Tube Fitting

2006-04-26T20:12:00.000-07:00

The sanded joints make for very accurate tube intersections and fit into the jig perfectly to maintain exact geometry and post weld straightness.

Tube Cutting

2006-04-26T20:04:00.000-07:00

The next step is to lay out the dimensions on the tubes. I use a blue layout spray ink and mark the cuts with a fine scribe. Next I cut the tubes to length with a cut off disc. To miter the tubes I do things a little differently now. I used to use bi-metal hole saws to miter tubes on a lathe using a tube holder in place of the tool post. Now I use a mill based belt sander I built to work with very thin walled True Temper S3 tubes. It makes for very accurate cuts in any diameter required. You just need to make a simple round spindle for each size.

Tube Drawings

2006-04-26T19:57:00.000-07:00

From the CAD model of the frame I make a drawing for each tube and proceed to cutting them to final size etc. These drawings need to created only once per tube (mtb downtube for example). All changes made to the CAD model are automatically reflected in the drawing.

JIG Drawing

2006-04-24T18:17:00.000-07:00

The JIG model is used to produce a set up drawing. Once adjusted it is used to position and hold the frame tubes in place for tacking/welding. It only takes a few simple settings to align the JIG.

JIG Model

2006-04-24T18:10:00.000-07:00

When I first started building frames I designed and built a frame JIG. The CAD model of the JIG is mated to the frame model and the JIG adjusts itself. From this model comes a drawing that tells me where to set the JIG to fit the tubes for the frame.

Frame Model

2006-04-24T18:02:00.000-07:00

Using the frame layout sketch I add appropriate frame tube models. I've previously modelled each tube available from the supplier, in this case True Temper. The tubes are aligned with the lines on the layout sketch and the intersecting between them cut in the CAD model. This creates a model of each individual tube that is linked to a drawing, which i will discuss later.

I also add models of wheels, tires, bars and stem etc to check for clearance.

Frame Layout

2006-04-24T17:52:00.000-07:00

The first step I take when building a new frame is to create a CAD drawing of the geometry or in most cases modify an existing CAD file (for a previous frame) to reflect the new geometry. Using this sketch I can capture the required geometry and fit for the rider. I can also check things like toe overlap.

2006-04-23T18:09:00.000-07:00

back40 bicycleworks frame building blog

Over the next little while I will be posting here and documenting the build process I use to construct custom steel bicycle frames. I build as a hobby and have constructed about 20 frames to date. The build blog will chronicle the steps I take in building a 29" wheeled rigid mountain bike frame and fork. Stay tuned for photos and descriptions...