Assembling the Frame's Rear Triangle

The rear triangle is a lot more difficult to assemble than the front triangle (quadrangle). Of all the aspects of frame building, the rear triangle is the most stressful to do, so I took a lot of pictures. 

The first task is to braze the dropout to the chain stay. The plugs didn't quite fit, so I had to file them thinner. Also, I rounded the top of the plugs so that the silver ring I placed inside the tube would drain around the plug better. 

We used a dummy BB and an Anvil jig to hold everything in place. A 130mm axle keeps the lugs aligned. The actual braze is done with the lugs pointing down and the BB up, so that the melted silver braze flows down. We know the braze is complete when we see it leaking out of the dropout.

To figure out how to cut the chain stays into the BB, we take the ideal chain stay length, 435mm, and subtract that from the actual length, 495mm, to get 60mm. I filed the excess 60mm of chain stay away with my 12" bastard file. All rear triangle miters, cuts, and adjustments are done by hand.  

Chain stays are tricky because they both need to be the exact same effective length. Also, the chain stays have a tendency to move up and down, so we had to use a guide pole, picture here. So, while brazing in the chain stays we found that sometimes they slide too far into the BB, or while cooling they'd contort and move around. We had to re-do the braze a couple times to get it right, and there was still a little filing I had to do to the dropouts.

To make sure the chain stays are perfectly even, we first check the dropouts with a cup gauge. Then we measure the distance between the two dropouts, and bend them to 130mm. Finally, we take a perfectly dished wheel, and a frame alignment gauge, and we bend and bend until the wheel's rim is equidistant from the inside of both chain stays. Then, we use the frame alignment gauge to bend the chain stays into the same alignment, relative to the front triangle. Sorry, it's complicated and I failed to take a picture. 

The Henry James dropouts I used come at a stock angle that isn't exactly the same as what I want for my seat stays. So, here Doug is using a lathe to drill a hole in an iron rod that should fit the dropout plug. We'll use it as a lever to bend the dropout. 

Here, Doug is bending the dropout.

Here, Doug is checking the angle of the dropout with the seat stay.

I wanted to do a "Fastback" seat stay attachment because I like the name. To do this, Doug had me file the end of the seat stay to match the back of the seat mast lug. 

We connected the seat stay to the lug using brass brazing, which is a little trickier than silver brazing. The key differences of braze brazing and silver brazing is that with brass you want the torch frame to be a little closer, and you wait until the steel is cherry red before you add the braze. 

Here, Doug is brazing a seat stay to the side of my classmate's lug. Note the angle-iron used to weigh the seat stay down. 

Here, he's heading the inside of the tube. 

Here is what the fastback looks like. I'll file the brass down to make it look a little smoother with my Swiss file. Also, that extra seat tube you see there took about 6 hours to file away. The slit between the seat stay attachments was especially tricky. A seat post slot should be about 1 1/8" long, by the way. 

Before brazing, we measure to make sure the distance between the dropout is exactly 130mm, and the dropout plugs fit perfectly into the seat stay tubes. In my case, I had to do a few hours of filing. Also, we measured the seat stays to guarantee they were both exactly 496mm long. 

Because the dropouts are made of stainless steel, I used 50N silver braze. You can see here that I filed the inside of the dropouts. What isn't so clear is that I did a lot of filing between the chain stay and the drop out so that they will look like 1 piece after painting. You can tell they aren't by the color difference in the picture. 

It takes a lot of muscles to pull the chain stays down to fit the seat stays in the plugs. Needless to say, we did a lot of double-checking all our measurements. Minor variations between final seat stay lengths, after brazing, were made up for by filing the dropouts. There was lots of bending involved here, and again, we used a dished rear wheel to make sure the two sides of the rear triangle were perfectly even. 

This is the braze immediately after  heating. You can see just a little silver coming out of the joint between the tube and dropout plug. I'll file that away later. The hole above the joint is for exhaust, but I'll drill it out wider to fit a threaded boss in for rack attachments. 

In this blog I cover a lot of ground quickly, but the rear triangle took the better part of a week to finish. The most stressful thing was that on Friday, right before the end of class, we brazed the chain stays into the BB, but as they were cooling one of them slipped into the BB about an inch. I was afraid we'd made a big boo-boo and freaked out about it all weekend. On Monday, Doug fixed it in about 20 minutes by heating the joint and smacking the dropout with a hammer. It's good to have somebody who knows what they're doing around.