December 18, 2007

Ventana pt. 2

So as soon as I got better, Sarah got sick with whatever it was that I had, so now I’m taking care of her, and things are, you guessed it, moving kinda slowly on the bike building front. What can I do?

In any case, when I left off last time, I’d just made the shim/sleeve that reinforces the seat tube around the upper pivot. Here’s a picture of what I’m talking about. The back side is cut out to save a few grams (10, to be precise).

I’m not positive that this reinforcement is really needed, but it’s 30 grams of insurance, and I have to shim the pivot mount to the seat tube anyway, so it seems worthwhile. In some cases (albeit not this one), this also allows me to use a butted seat tube, since the sleeve reinforces things enough that the thin section of the tube can handle the strain. In this case, though, the seat tube is too short (14.5″ center to center) to use a butted tube.

Next up I braze the sleeve on using bronze – I do this instead of TIG welding it mostly because it’s faster, and partially because it creates fewer stress risers than 2 TIG beads really close together. I feel that this is a safer/stronger way to make a sleeve joint like this. It’s pretty much the same as brazing in a lug.

Attaching the actual pivot mount comes next. In this case I use the Ventana hardware to hold everything in place, tack, and then finish weld. People sometimes ask me if it’s a problem to TIG on top of a bronze-brazed sleeve like this, and as far as I can tell, it’s not a problem. I’m sure some of the bronze under the sleeve is melting as I weld, but not enough to allow the sleeve to move or to weaken the joint. Here’s a close-up shot of the welded/brazed sleeve and pivot. You can see some of the flux still gunking things up, notice however that the area where the pivot is welded is cleaned up (prior to welding) to avoid any contamination problems.

Finally, I miter and tack the front triangle, then weld it up. Here’s a shot of the frame prior to getting finish welds. All that’s left is mounting the shock – but that’ll have to be another post.