05
Aug
12

Eight Tubes, Part 5: Get Forked

Now we’re getting down to business….

To recap: blah blah blah…I got a bunch of tubes and lugs that, when assembled, will hopefully form a lugged steel frame and fork…blah blah blah…Plans!? I don’t need no stinking plans….blah blah blah…because I am not working from a blueprint I need to build the fork first...blah blah blah…oh, I’ve never built a fork before….blah blah blah…so HOW do I do this?

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One step at a time works best. First things first: braze up the steering tube to the fork crown. Basic stuff here.

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The first photo shows a couple of things here. First, I wanted gravity to work with me when I started brazing this assembly (to help draw the metal all the way through the crown, from top to bottom) so I decided that I was going to position the piece with the steering tube in a vertical position. The fit between the fork crown and the steering tube was loose enough that it would not stay put so I pinned the two together using my special Ernesto Colnago Frame Building Pins (i.e a small finishing nail cut to length). The second picture shows the pin, a nice peek at the hollow crown on the fork, as well as photographic evidence that I possess feet.

These shots also show that I have a bit of the steering tube standing proud from the bottom of the crown. You need little bit of tube sticking out when you silver braze – it gives the molten metal a start as it wicks down into the joint.

So I got out the torch – a simple Bernz-o-matic MAPP gas torch (about $65 at Home Depot), cleaned the joints so things were all nice and shiny, slathered them up with flux, and did the deed.

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What you want to have happen is for the molten silver brazing rod to completely penetrate the joint. In a perfect world, you would feed the rod in from the top of the piece and it would eventually flow out the bottom of the joint as you worked it though with the torch. I must have had a good day, because that’s exactly what happened here.

I cleaned up the spent flux that was glopped all over the piece with Lysol Toilet Bowl Cleaner – it contains hydrochloric acid and really gets bare metal CLEAN. Be careful with skin and eyes, and rinse the piece with water. I then usually get out the torch and gently heat the piece to evaporate the water.

Next, I drilled the holes in the crown for the front brake. Because this particular crown is for a straight legged fork, the hole for the brake had to be drilled at an angle to match the fork legs. Luckily, the crown has little cast-in dimples to guide you. I set my drill press to drill at the necessary angle using a really cool tool that I just bought – a “digital inclinometer”. This thing kicks each of the 23 possible types of ass presently known to science, both actual and theoretical. It’s a little aluminum box with a digital readout that tells you (1) the angle of the piece from absolute level, or (2) the angle of the piece relative to a reference that you set. Mine is made by Beall Tools, and is made right here in the good ol’ USA.

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A little work with the die grinder and a file removed the portion of the steering tube that was sticking out of the bottom of the crown.

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Now it was time to trim the legs of the fork to length. As the come from the supplier, the legs are waaaaay too long. I was shooting for an axle to crown measurement of around 365 mm. I also wanted to trim the legs so that it positioned the wheel where the brake track would match up with the brake pads at about the middle of their range if adjustment in the caliper. (If that makes your head hurt, just look a the pretty girl. That’s what she’s there for.) I have the brake caliper that I am going to use, and a wheel, so I clamped the fork in the vice and did a trial fit. Uncut, this is what things looked like – off by about 4 inches.

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So I measured, got out the hack saw, held my breath, and trimmed it up. Against all odds, I hit the mark that I was shooting for.

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At this point I decided that I had used up all my good bike-builder mojo for the day and stopped while I was ahead of the game. All that needs to be done is to drill vent holes in the fork legs, clean up the fork ends, do a test fit in the jig that I built, and then braze it up.

NEXT WEEK: Get Forked, Part 2

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4 Responses to “Eight Tubes, Part 5: Get Forked”


  1. 1 Jonathan
    November 1, 2012 at 7:33 pm

    Hi there – I did bike #1 last winter using the Chimonas book (turned out amazing if I do say so myself) and am thinking of doing #2 this winter – more of a touring/ride to work bike. My question is whether you brazed up this fork just using the MAPP torch, or have you gotten a more serious rig for brazing bike #2. I think I’m doing MAPP again for the 2nd bike, but I can’t decide if I can appropriately braze a fork with only MAPP. Any thoughts/experience would be greatly appreciated! Glad I stumbled upon your blog.

    • 2 Anonymous
      November 2, 2012 at 12:13 am

      Hey –

      I used the MAPP torch for the fork with zero problems. The place where the most heat was needed was for brazing the steering tube into the crown, and it handled it without any problems. The crown that i used is hollow, and didn’t take too long to heat up.

      For my next fork, I am going to tweak the jig ta little bit. I am going to braze/weld a flat plate on the bottom to keep it from twisting, and I am going to add a support that will go under the crown to keep things from sagging.

      I’d love to see a picture of your Number 1.

      – Greg

  2. 3 Jonathan
    November 2, 2012 at 12:25 pm

    Thanks for the note – maybe I’ll be brave and try out the fork for my next build. Couldn’t you make your fork jig more solid and stop the twist by just bolting it to a piece of flatstock or even wood and not have to deal with brazing/welding?

    Here’s a thread on the framebuilders forum from a question I had in my build and I added some finished pics at the end:

    http://www.bikeforums.net/showthread.php/800643-Chainstay-alignment-help?p=14062421&highlight=#post14062421

    • November 3, 2012 at 1:08 pm

      Jonathan –

      Nice frame!

      I *could* just bolt the jig to a piece of sheet steel or wood, but I tried that with mixed success. I used clamps, and switched to bolts, and I still had to constantly futz around to get it right. I am looking of a nice piece of thick flat stock that I can just weld and forget.

      If you do a fork, pick a crown that has nice, long tangs that run down the side of the fork leg and have lots of surface area. That’s where most of the strength vis-a-vis avoiding a fork leg collapsing comes from. Inserting the steering tube into the crown looks scary if it fails, but it is actually pretty foolproof in terms of brazing skills. The legs…that’s where a bad braze will show up.

      Keep in touch – I would love to see your next project.

      Greg


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