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| When I made the drawing of the bike (next page), I designed a fork that would accept a 700c wheel with long reach caliper brakes at full extension.. I measured everything carefully in the drawing, since any error in fork length will affect frame angles and bottom bracket height. |
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| I decided to use a Henry James 3 degree bend fork crown. The 3 degree bend in the crown would put the blades 20mm forward. The blades would need to be bent another 20mm forward to get to the 40mm rake I wanted. This would result in a unique, short rake. I used blades with 1.02mm wall thickness. |
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| First, after a thorough sanding and cleaning of the parts to be brazed, I silver brazed the crown to the steerer. I used 55% silver and a large #3 torch tip I took my time heating it up with a soft, neutral flame. I fed the silver into the crown from above, and made sure it went all the way to the end, where I saw a thin line of silver when I was almost finished brazing. Afterwards, I drilled a hole for the brake caliper in the steerer, through the casting. |
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| Here is how I bent the blades. I got a wheel from the junkyard and brazed a small piece of tubing to the top edge. I bolted it to a heavy board, and put a wedge under the wheel (to keep it from moving). |
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| Then I slipped the blade into the other small piece of tubing I brazed onto the wheel, and bent it down, using my weight, and holding onto the end of the fork blade. It took a few pushes, and all of my 165 lbs, to get the bend right. I went back and forth to the drawing after every push to check the progress. After I was satisfied, I bent the other blade to match. I was concerned that the blade would turn sideways during bending, but that didn't happen. I would have liked to have a formed channel to bend it against, but the flat surface of the wheel seemed to work just fine. |
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| Here is the top of the wheel. The short piece of tubing brazed onto the wheel comes from a middle section of a junk fork that I had. |
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| I cut away the first inch of the tip, to get to where the bend started. That made the rake look more graceful, carrying the bend all the way to the end. |
| In this picture, I'm grinding a slot in the fork blade tip. |
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| Here, the tip is fitted to the end, prior to brazing. Not much rake was required here, since the crown was already bent 3 degrees. Notice how the curve extends to the tip. Most forks are bent with the dropout already attached. This leaves a straight section before you get to the curve. It makes the fork look "kinked". It's a small detail that most people don't notice, sometimes even when you point it out to them. |
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| Here the fork dropouts are brazed on with brass, held in place by a bolt-on front axle. I used 3/32" brass rod to bridge the gaps next to the dropouts. On the rest of the frame, I used 1/16". The lower ends of the fork blades are slipped into the crown, but not brazed there yet. Brazing the dropouts like this assures me that the wheel will fit well when it's all finished. |
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| I cut the tops of the blades until I got a good fit with these long reach brakes and pads at full extension. This will give me lots of room for fenders. I need them since I live in Portland, Oregon now. Notice that the blades are not yet brazed to the crown, but just sitting in the slot. |
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| Does it look straight? |
| I am about to braze the fork blades to the crown. I probably should have made a jig for this step. The wheel has to be absolutely in line with the steerer, and the fork dropouts and crown have to be perfectly parallel. Also, the intended rake (of 40mm) has to be right. This is a very important step! |
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| Well, I went ahead and brazed it up without a jig anyways. Extra silver can be seen lumped up along the side of the joint. The experts can braze this up and have a nice crisp edge. I am no expert, but the joint is sound. I made every effort, using heat, to move the silver deep into the joint. It's difficult to tell when you have enough silver in there. I would have extra on the edge, trying to move it into the joint, but the space was probably already filled. Since I had practiced a lot with tubing already, I felt confident that it had good penetration. However, the only definitive way to tell is to wait till it cools, and cut it up with the Sawzall, and I don't want to do that! |
| The rake ended up being 42 mm, a tad longer than intended, but that should be fine. . Unfortunately the blades were about 2mm to the left of the centerline. This made the bike pull to the right slightly when I rode it. It was noticeable at first, but easily forgotten unless I rode with my hands off the bars. I could do this, but I had to lean a bit for compensation. Of course, I had to fix it... |
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| How I learned to Silver-braze: |
| I got a 1 1/4" .058" cromoly tube from Aircraft Spruce. Then I cut 1" off it, and cleaned it out on the inside with sandpaper. Then I put it over an externally sandpapered 1 1/8 .035" tube. The amount of space between the tubes (about .004 on all sides, - a sloppy fit) was about right for this. I put silver flux on both tubes, where they met.. Next, I heated it up with the #3 tip, until the flux looked clear. I placed the 55% silver wire against the joint, and watched it flow inside, between the tubes. I alternated the flame between the joint and the silver wire to get the flow started. I know it made it through, when I saw it come out the other end, 1" away. I learned how proper movement of the torch made this possible. There is no substitute for practice. I made every effort to avoid getting the joint too hot. If I see any red at all with my goggles on, it was too hot, and the flux might stop working. When the flux gets black, it's cooked. Ideally, the work proceeds quickly and the flux stays clear. Afterwards, I cut it up with the sawzall, and looked for gaps between the tubes. |
| Silver can also be used to braze narrower gaps, like .003, or even .002. Brass seems to need at least .004 to flow into the joint well. A .058 wall tube that you may buy form an aircraft supply company like Aircraft Spruce can accept a tube inside of it that is 1/8" smaller in diameter (this inside measurement of tube is actually .009 larger than the tube you insert into it), and you will have the .0045 gap all around, that you will be able to to braze with silver or brass. |
| Brass melts at about 1600F; silver at about 1100F. Both will make a strong connection, but silver is more appropriate for using in lugs, and small part braze-ons. Building up silver into a main tube fillet has been done, but is not a reliable joint for an amateur to make. Jalon Hawke of Desperado Cycles makes beautiful frames using silver fillets, however. Silver fillet brazing must be left to the experts. |
| It is easier to use silver for lugged construction, since it flows so well, and the joint doesn't have to get so hot. The area can be heated to a temperature just before it starts to get a dull red, and the silver will start to flow in and fill the joint. With goggles off, a dull red may be seen perhaps, but with the eye protection, you are unlikely to see any red. Brazing without any goggles can be damaging to the eyes; causing cataracts and blindness. Clean surfaces are more important for a silver joint, since the lower temperature will not burn away as many impurities, like brass brazing will. |
| Brass can also be used to braze lugs, and is preferred by some builders. The joint has to get RED HOT, and you can see the shadow of the brass flow into the joint as you heat it and apply the rod. |
| Small parts like cable stops, are best brazed on with silver, since silver is plenty strong, and chances of overheating the thin wall of the tube are much less. On small parts brazed to thin tube sections, I frequently use my hardware store propane torch. I don't need to use goggles, so I can see better, and not as much heat is needed. Other silver wires, like 45n, melt at slightly higher temperatures, and are better for filling bigger gaps, like at the dropouts. |
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| My friend lent me his Park fork alignment tool. I put it in the vise, with the fork in it. I slipped a 1 1/8" .058 tube over the steerer. On each fork blade, I wrapped a towel and put a 2" diameter "cheater bar" over it for leverage. I sat on a stool, put my foot on the vise, gripped both left and right bars, and pulled hard. It took a few tries to bend it 2 mm. This was one strong fork! I had straightened many forks in by bike shop days, and this one was the hardest. The crown is very strong, and the blades seemed stronger than they need to be. Next time, I will get blades with a .9mm wall. I repeated the maneuver on the other blade, and checked it with the gauge. Now it was in line. |
| However, when I put the perfectly dished wheel in, it was slightly off center. The center of the tire was just to the left of the brake center bolt. I have arrived at the point I dreaded: there was no other way to achieve alignment than to file the dropout a bit. I used a round file, and it only took 10 strokes, but it's kind of like admitting failure and cheating. Next time, I will make the effort to construct a fork jig. |
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