16
Sep
12

Eight Tubes, Part 9: Finger Rippin’ Good

We’ve taken a few casualties here in the Bike Bunker. I will spare you the details – and pictures – but I think that the bench grinder is plotting something…big. I’ve currently got it set up with a wire brush so that I can clean up parts quickly. For example, it does a really good job on removing spent brazing flux from tubing.

And removing skin from fingers.

Let’s just say that this isn’t the first time that the bench grinder has tried something. Its favorite party trick is to snatch whatever you are cleaning up out of your hands and wing it across the shop.

Or, if the mood strikes, at your head.

Despite the repeated attempts by my power tools to maim me, and the constant interruptions to go and find band-aids, we’ve also made a lot of progress on the frame.

1. My Head(Tube) Is On Straight: If you recall from the last installment, I screwed up the braze job on the head tube. I had come up with a dandy little device out of an old brake cable and a hose clamp to hold the tubes together while I braze. I got excited and tightened things down a little too snug. The head tube angle was supposed to be 74 degrees. It was more like 76+ degrees when I measured my work. Oops. This was an easy fix: I just grabbed the torch and sweated the joint apart ( i.e., got it hot enough to melt the silver solder) and started over. This also let me check my work and see whether my brazing skills are up to snuff. Things looked good, apart from the fact that I screwed up the angle.

2. Getting’ Down With The Downtube: Having fixed my mistakes, it is now time to attach the last of the four tubes that make up the front half of the bike. To do this, we will be attaching the bottom bracket – and setting the seat tube length and head tube length in the process.

Our seat tube will be 55cm, measured center of the bottom bracket to the top of the top tube. The easiest way to sort this out so that, when I am finished, the axle of the crank is at the desired 55cm from the top tube is to start by trimming the seat tube to 55cm, measured from the top of the top tube to the bottom of the seat tube. You will be cutting off more later, but for right now start with your target length.

I still haven't cleaned off my workbench.

When you measure “center to center” or “center to top” or “center to whatever” one of the tricky things it to sort out exactly where the center of the bottom bracket is. It is located somewhere in that open space where the cranks usually reside.

This is a machinist's center.  A cool old tool.

You could just eyeball it when it is time to measure where to locate the bottom bracket, but I figured out a better way. To make it easier to figure out where the “center” of the bottom bracket is located, I used a ruler, some tape, and a used spoke to help line this up. It is easier to show than to explain…

Slip the bottom bracket onto the seat tube, and work it up to the point where it touches the spoke that (if you did it right) is located in the center of the bottom bracket. When the end of the tube touches the spoke, the center of the bottom bracket is located 55cm from the top of the top tube.

I'm pretty good at coming up with Rube Goldberg devices.
Easy, huh?

I used my electronic angle finder/level to make sure that the bottom bracket was oriented so that the axis of the cranks is 90 degrees to the direction of travel. Or, viewed differently, make sure that the faces or “cheeks” of the bottom bracket are parallel with the top tube.

Okay, this is a weird picture that doesn't really explain a lot.

Get out your Sharpie and make some “witness marks” noting where the bottom bracket sits on seat tube when everything is lined up so you can locate it again without a lot of rigmarole.

Kinda zen looking.

Don’t put away the Sharpie quite yet. Use it to mark where you need to trim the seat tube. The was way to do this is to use the marker to trace the shape onto the tubing by running the pen around the inside of the bottom bracket and the as well as the opening on the front of the BB for the down tube.

Red Diagonal Lines: the univeral symbol for "trim this off"
After you trim it up, the end of the seat tube looks like this:

That discoloration on the tube is blood.  My blood.  The grinder claimed another victim.
Now it’s time to trim up the down tube. This is fairly straightforward. Like the top tube, the down tube is butted, so you want to trim it to length by taking an equal amount of metal from either end. Eyeballing it, I estimated the miter to miter distance along the bottom of the tube to be around 56cm. I found the center, marked it, and then printed one of those handy computer-generated patterns to miter the downtube/headtube joint. Using my 56cm estimate, I slapped the pattern on one using the same technique for cutting the top tube to length (take equal, smaller amounts from both ends, not one big cut from one end) and mitered the headtube/downtube joint.

To "eyeball" this I stuck the down tube in the bottom bracket before I cut anything.
At this point I located and drilled the holes for the bottle cage – it is much easier to do this before the tube is permanently attached to the bike. Because we’ve done both mitering and bottle mounts before, I will skip all the gory details here.

At this point, we need to figure out just how tall the head tube is going to be. On Old Number One, as on is bike, the length of the head tube is simply a function of where the angle of the lug on the bottom bracket and the angle of the lower head tube lug cause the down tube to intersect the head tube. It is easier to show you rather than to explain it with a bunch of words. As with Old Number One, the head tube worked out to 160mm. The fork that I built has a compatible steering tube length, so no worries. I knew to make it long…

This is not an optical illusion, the down tube is behind the head tube.

Her clothes are not OSHA approved for workers employed in light industry.Are you tired yet? The pretty girl isn’t.

Like the seat tube, you have to trim off the excess on the end of the downtube that intrudes into the bottom bracket. Get out the Sharpie, mark it off, and let the metal fly. When you do it right, it all looks like this:

Peering into the abyss....

Do a final fit, check all the angles, clean it up, and braze.

I need to find a better place to store my Christmas Tree stand.This picture looks like it was taken by a chimp.

The finished product:

Starting to look like a bike frame.

That toilet bowl cleaner sure makes things shiny....

A little trick: cleaning off spent flux from a brazed joint it not my idea of fun. The wire brush on the bench grinder (when it isn’t trying to kill me) is good for getting the big stuff, but it won’t reach into the nooks and crannies. The solution: toilet bowl cleaner. Yes, this is actually pretty nasty stuff. The Lysol brand that I bought contains hydrochloric acid. It can make a gnarly commode look like new, and strip baked-on slag from steel tubing. It will also remove skin and dissolve eyeballs, so be careful. I use a small steel brush to scrub the deposits. When I am done I rinse the the tube with lots of cold water, warm the frame with the torch to evaporate any trapped water, and then liberally spray the whole shootin’ match with WD 40 to ward off rust.

Checking my work, things turned out as planned. The head tube angle is smack on 74 degrees (74.05 according to the angle finder), seat tube is 73.4 degrees, and the joints all look like they have good penetration.

Next up: chain stays…


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