Cargo Rack, Part 3

In this series: Part 1, Part 2, Part 3, Part 4.

Fillets!

I finally got around to doing the fillets on the rack.  I like doing fillets.  You can see what you're doing a lot more easily than you can when you're trying to flow bronze under a lug.  Something about there not being a sheet of metal between you and what you're working on.

Here's one that I started to clean up.  You can see that the fillet forms a nice smooth transition between the tubes.  There were a few spots where I didn't build up quite enough filler for that, but they're inconspicuous.  Considering how thick I made the fillets overall, I'm not worried about it  being structurally unsound.  

If I'm feeling especially neurotic I'll fill the little gaps in with J.B. Weld.  Since that particular epoxy has a bunch of powdered metal in it, the powder coat will stick to it.

Sneak Peak

In our next episode, I'll get into how I'm making the wooden deck for this rack.  It's all lasers and power tools!

In this series: Part 1, Part 2, Part 3, Part 4.

Cargo Rack, Part 2

In this series: Part 1, Part 2, Part 3, Part 4.

Yesterday I started brazing the rack, which was pretty exciting.  In this step, I just laid down a bit of bronze to hold things in place.  Later, I'll build up fillets.  A fillet is a meniscus of metal that joins pieces together.  When done properly, the metal creates a nice smooth transition between the two.  

Tabs

I cut the tabs out of some sample sheets of chromoly steel that I had lying around.  They're basically little metal guitar picks.  When the bike is done and all of the attachment points are installed, I'll drill them.  That'll let me level it out if something weird happens.  

I'm pretty sure, but not positive, that the sheets will be thick enough to resist bending when I'm carrying things.  If I was using it for loaded touring, I'd double them up.  I still may add a bit of reinforcement.  

Once the tabs were done, I slotted the vertical tubes of the rack, lined them up with the tabs on my basic sketch, and then squeezed them with a pair of pliers just enough to keep them from shifting (much).  

Brazing

I started brazing by securing the tabs.  I did each of the four connections one at a time.  The hard part was dealing with the last joint.  It kept wanting to spring free, as the rack widens slightly near the rear (for aerodynamic effect, or something).   I solved that with the sheet-metal vice grips that appear in one of the pictures below. 

Next, I attached the rails, where a bag will hang.  I clamped the first on one end, rotated it level, then tacked it on the other.  Then I was able to remove the clamp and tack the remaining end.  

The second rail was about 100 times harder, since it had to line up exactly with the first.  At one point I probably would have hurled it across the room if there weren't other folks around working.  Thankfully, after quite a bit of trial and error, it fell into place and I was able to tack it down before it slipped and hit me in the eye or something.

Here we go!  I need to finish the fillets and grind the ends a bit, but the look and geometry work.  

Next time: finishing fillets, adding tabs for the wooden deck, and making the strut to connect it to the seat cluster.

In this series: Part 1, Part 2, Part 3, Part 4.

Cargo Rack, Part 1

In this series: Part 1, Part 2, Part 3, Part 4.

I started making a cargo rack yesterday.  My theory is that fillet brazing the rack will give me some much needed practice with my heat control.  Additionally, a lot of builders at NAHBS used custom racks to highlight aspects of their frame, and I think I may have caught a bit of inspiration.  

I'm using 5/16" diameter tubing, with .035" thick walls.  I have some 1/4" tubing in the mail, which I'll use for the struts that run to the seat cluster, since it's a lower stress area.  

I used a cheap, $10 tube bender.  On the up side, it works.  On the down side, the guide marks aren't really lined up with anything useful.  I had to bend it little bit by little bit until it matched my blueprints. 

The pieces already bent here will form the sides of the rack.  They'll connect in front, and hang down a bit in back.  

Here are the finished struts.  Each U will cross over the wheel.  

When I'm done I should have a light, narrow rack, slightly flared at the back.  I'm planning to bolt a piece of finished wood to the top.  Since I have a laser cutter handy, I'll probably engrave "Cedite E Campo", the  motto of my fake bike company, into it in ridiculous flowery script.  

In this series: Part 1, Part 2, Part 3, Part 4.

Ignore This Boring Post About Butts

One quick bit of advice for anyone else embarking on this whole frame building thing.  Always ALWAYS measure your tubing.  Most of the tubes we use to make a custom bike are butted, meaning that they are thicker on the ends and thinner in the middle.  This lets the thicker section soak up the higher stresses near the joint, while the thinner section makes the finished bike a lot lighter.

This instructive daguerreotype, found in my great grandfather's memoirs, should help to illustrate what's going on inside the tube.  

You have to be careful though, because you can cut too much off of one of the ends and end up with some of the thinner tubing forming the joint... which may then fold like a coke can.  Even worse, tubing often comes with the butts shorter or longer than the spec sheets tell you they should be, so you really have to measure for yourself to be sure.

I held my tubes up to a bike light and stuck the depth measuring tool on a caliper into it, set for the point where the transition was SUPPOSED to start, then made sure that it lined up with the actual change in reflection indicating the start of the slope towards the thinner spot.  It can be hard to see what's going on in those things, so I had to bracket the measurement (-10mm is DEFINITELY falling short, +10mm is DEFINITELY going over that line).

I found two discrepancies: my seat tube started getting thinner about 1.5 cm lower than it should have and my chainstays (which are ovalized) are about 2.5 mm too wide and a bit too short.  The first won't be an issue provided that I leave the tube as long as possible when shaping the end to fit into the bottom bracket lug.  The chainstays will probably need to get just a kiss from the vise to be the right shape.  I'm a little nervous about that, but it shouldn't be a big deal.

Now I have everything marked, so I will know EXACTLY when I'm in trouble as I trim stuff to length and shape the ends.  Next post will likely be about some more practice welds, which will hopefully be more interesting than all of this measuring.