Tony wants a lot of interesting things for this frame, so I am doing a lot of (at least to me) fairly unusual fab work to make it happen. Here’s a brief photo essay of the first step – building the seat tube and pivot assemblies. Keep in mind, part 2 won’t be posted for a while, because I’m waiting for some head tube stock to finish the front triangle, and it may take a few weeks (I hope!)
I’ll explain the head tube thing (it’s pretty neat) in detail when I have the samples in hand from True Temper. Suffice to say it will be a big step forward for long travel bikes.
So, where to begin? In this case, at the bottom bracket shell. It’s an 83mm jobbie (Tony will use Saint cranks – it’s a long story but he needs a wide stance width/q factor, plus this will allow a Maxle rear end and proper chainline if one becomes available) from Paragon.
First I miter some hardware to hold the main pivot (4130, made on the lathe) and the BB shell. This stuff (the bracket) is provided by Sherwood at Ventana, and I don’t usually use it, but it made sense in this case, with some modifications (shown above) to get the spacing I wanted. So here are the pieces I use to attach the main pivot.
Next up, start welding things together. Here are some pictures of the process of attaching the pivot to the shell. This has to be very precisely mitered and joined, because even a tiny bit of misalignment will make the rear end line up wrong.
Checking the alignment here – if things aren’t fitting tightly, I still have a little bit of wiggle room to remiter and straighten everything out. Note that I am using Tony’s old rear triangle here, since it’s in fine shape (if a bit dirty).
Here’s the whole assembly welded up. Mitering the seat tube to this is a major pain (especially given that I’m using a weird 34.9mm seat tube) but it’s easier than mitering the tiny pivot tower parts to the seat tube…
Now on to the upper pivot. The tower comes mitered for a 34.9mm seat tube, which is great, but I don’t want to weld it directly to the .035″ wall seat tube – it would not be strong enough. So I have to make a sleeve that the pivot tower fits *into* which will slide onto the seat tube with it. Here’s a shot of that part (note that I ended up screwing this particular one up and had to make a new one, but you get the idea).
I weld in the pivot tower, with the sleeve and tower installed on the seat tube. The whole assembly is a tight slip fit, so I want everything in place before I do the welding – it would be extremely hard to relocate it after the welding distortion essentially locks it in place.
The seat tube assembly. I still need to braze in the sleeve at the top and bottom, as well as make the seatpost fitting. And of course, nothing is going to be finished until I have a head tube ready to go.
To make the 31.60 Joplin seatpost fit, I’m using some 4140 stock that I’ll turn down to slip-fit (and be welded into) the .035″ wall seat tube. More pictures of that next time. The Joplin is really neat, if heavy, and conveniently enough, 1.375″x.065″ is a near-perfect fit, so I don’t have to do any lathe work on the inside.