Sunday, November 23, 2008

Cable Runs

Something that new framebuilders should learn sooner rather than later is that planning and installing small bits (cable guides, brake mounts, H2O mounts, etc.) are better done up front rather than after the frame is built.

This is doubly true with carbon fiber frames where small bits present some special challenges. I don't like to put lots of holes through my tubing, and I like to keep the holes as small as possible. The biggest challenge comes when a rider requests internal cable runs.

Routing of internal gear cables is dependent on the crank axle design. Some, like cartridge BBs, are hard to fit a cable around while staying within the BB. For these, I exit the cable at the base of the down tube and use a normal BB cable guide.

Where more space is available, however, its possible to do some interesting things. There are still some challenges however.

Many external cup BB's use a sleeve running around the axle and between the two cups. This isn't compatible with having shift cables inside the BB's. I've developed a split seal for inside the BB, where each bearing cup is sealed separately from the other - leaving a nice smooth axle around which the cable can turn on its run.

This op
ens up some neat possibilities. A large hole in the front of the BB allows the cable to run inside the down tube and directly into the BB. Then a whole at the rear of the BB has a cable tube running part way down the chain stay where it exits. The cable continues to a pre-formed housing stop - and through the housing to the rear derailer. This makes a very tidy looking setup.

For guides, I like 0.125" O.D. brass tube. Th
is is easy to handle and requires a minimal hole through the tubing and chain stay.

Here are some pix of the chain stay tubing exit that I'm working on. To begin, I build up a filet around the tube as it exits the chainstay.

The filet is basically a thickened epoxy which: 1) doesn't run while curing; 2) has better compression strength than plain epoxy. As you can see from the picture, its hard to get a smooth filet - it's too sticky.

The first thing we have to do is drill out the end of the tunnel and make sure that the cable passes freely through it.

Then we borrow some techniques from steel frame building. Out comes the file, and I file the filet into a smooth shape. It seems to be looking pretty good here, so now we can reinforce this area.

Frankly, this is probably not necessary, but this bike is being built to last and the chain stay encounters pretty significant forces on a regular basis.

In this case, I'm going to put on two longitudinal plys of uni-CF. Ea
ch will be split in the middle, along its length, for about half its length. One will be laid from each direction, splitting around the exit hole. The splits allow me to better flatten the CF down, where I want it. These plys will wrap about 2/3 of the way down each side of the stay.

Next comes a uni-CF ply running directly around the stay (at 90 degrees to the prior stays). This ply won't be split, so our exit hole will be covered. Not to worry, it's easy to see where the hole is under the CF.

Finally, I'm putting a full wrap of my double layer 2x2 12K twill. This has a toughening l
ayer between the two plys of CF, making it good for reinforcing a chain stay. The total reinforcement is 5 layers of 150 Gr/SqMeter CF - 3 uni, 2 low-crimp. When it's cured we'll need to drill out the exit hole and make sure all is smooth. Then this guid will be done. A similar arrangement will be used for the front derailer, but we can't finish it yet for reasons that will be obvious later.

Meanwhile, we need to squeeze our CF layers together, and remove the excess epoxy. This is a complicated area to work with. The part is small enough to easily fit in a vacuum bag. Given the small diameter of the chain stay, holding the plys together with good alignment, along with peel ply and bleeder felt, is a bit of a challenge. So for this situation I prefer a different approach - wrapping with electrical tape.

Before I begin wrapping plys of CF, I put a ring of electrical tape around the chain stay, just beyond each end of the section to be reinforced. The tape is wraped upside down - so the sticky side is out. These sections of tape will be used to start and end the compression wraps of tape. Mounting them before things are wet with epoxy makes it a much easier and cleaner step.

Once the CF sandwich is in place, I take off my gloves (so I have clean dry hands), and begin winding the tape around the stay. This too gets wrapped upside down - the inner (usually outer) side won't stick to the epoxy and by pulling the tape tightly, we get good compression.

Ultimately I wind from one end to the other, then reverse and wind back to the original end. Along the way, it's importan
t to keep the tape as free from wrinkles as possible. Every wrinkle will cause a pocket of epoxy that needs to be later sanded down. And a big wrinkle could actually lead to a hump in the CF which would be nasty.

With this done, its time to poke lots of holes in the tape. Actually, in the course of wrapping, excess epoxy was driven ahead of the wrap and out of the layup. But we want to do more and the holes will allow more epoxy to migrate up and out of the CF. The whole setup gets heated by a nearby incandescent lamp to thin the epoxy and facilitate it's flow. If you click to enlarge the picture, you should see lots of little bubbles of epoxy on the surface of the tape.

I won't be able to return to this before tomorrow - but hopefully then I can show pictures of the raw finish.


Friday, November 21, 2008

Done Baking

Here's the main plate. This is viewed from the top and looks much like the surface of the last piece. However, when we turn it over, we'll see a different finish.

Unfortunately, there are a couple of defects on the bottom. Most obvious, a corner got folded over, apparently while placing the layup in the vacuum bag.

While not visible in the pictures, a close observation shows the defect visible
through the top layer as a triangular corner which is lower than the rest of the plate.

There is also a stray fiber that got caught - which is strictly a cosmet
ic defect. But there are also some depressions in the bottom - apparently the table had some defects in its surface which I didn't detect.

All of t
his leads me to consider a different approach for the next time. Rather than using a bag on a table, I'm going to try laying a piece of plate glass on the table. This should provide a smooth lower surface. The layout w/ peelplys and breathers will be laid on this. Then a lay piece of bagging material over this and seal its edges to the table with a special caulk-like tape. This will avoid the difficulty of sliding the layup into the bag.

Having said this, the majority of this plate looks good, and should make some fine dropouts.

All up this plate weighs 310 grams - so 40 grams of epoxy was squeezed out in the vacuum. That said, my ideal weight was 250 grams and my realistic expectation was 290 grams. So we're just a bit pudgy. It seems clear that I started with too much epoxy to hit my targets. Also, I need to check my pressure next time to make sure that I'm pulling as much as I think (possibility of excess leakage in the bag). Nonetheless, this plate came in with a lower epoxy weight than the last plate so we're making progress. And there doesn't appear to be so much epoxy as to reduce the strength of the plate.

More later.

More Carbon Plates

Pictures! Yea!!!!
Sorry for the quality, its a very old digital camera and the color is off because these were taken without a flash.

Anyhow, these are some more carbon fiber plates under vacuum. Last time, some heavy Kevlar was applied where the axle nuts rest. Two problems with this: 1) It doesn't cut; 2) It doesn't cut. A $35 ceramic tile blade died in the jig saw trying to cut Kevlar. Note, it didn't have any problem getting through the CF - slow steady cuts worked great. Moreover, the edges of the Kevlar end up fraying where the saw tore through them - very unsightly. Meanwhile, some of the CF elders suggested that I experiment without and protective layer over the CF - so that's what I'm doing here.

The last plates were cut up into smaller pieces and showed now voids or soft spots - clearly the epoxy soaked through just fine. After some careful weighing of my raw CF stocks, and some further calculations, it appears that I added nearly 70% epoxy by weight to the final product. That's fine, but not as light as it can be. It may be that for this sort of heavy layup, and infusion technique might be best. Something to explore down the line.

Back to the pix (you can click on them to enlarge), you can see the rough outline of the larger plate being vacuumed. In the one picture, you can also see a 2x4 clamped down on top of the other plate - squeezing it to the work table.

This time my layups looked much dryer than last - but after weighing everything, the main plate has 175 grams of CF and had 175 grams of epoxy added. Once its cured, we'll weigh the plate and find out how much epoxy remained in it. I'm hoping that this will be lighter (by volume) than the last.

The hidden (by the board) plate is just a lever about 1" x 7mm (finished thickness) x 12". It'll be used for some deflection testing.

The main plate will be used for some sample dropouts and the remainder will be used for a variety of tests, including impact & tensile strength.

For this set of plates, a release film was used on the bottom side instead of a peel-ply fabric. This should lead to a layer of smooth epoxy - although nothing like a jell coat. So it's just an experiment. Just as the clamps on the narrow plate are an experiment.

You can see the dark spots where the epoxy is oozing through. Apart from the edges, there's not too much. I gave the edges an extra drink just because I apply epoxy from the center and they looked a bit dryer. Maybe next time we'll do with out the extra.

Just FYI, there is a bleeder felt under the whole setup
(inside the bag of course), and one over the top. The long edges of the big plate got an extra layer of bleeder on top - you can see the ridges running across the top of the bag where these end. You'll also note that the bleeder is much wider than the plates - so there should always be an unobstructed path for the air to reach the outlets for the vacuum pump.

That's about it for this time. See ya around.