Thursday, July 29, 2010

The next hurdle, the seat tube

Where as the head tube has a lot going on from a machined specification perspective, the seat tube really is the spine of the build and needs a lot of loving.
The seat tube is the priority of the main triangle, as it in important to keep it oriented and in phase to build the rest of the main frame off of.  With a ti frame, the work necessary to prep the tube before it enters the fixture is a bit more involved, as there are no "ready to go" butted tubes that have the proper inner diameter for your standard sized seat posts.  So, how do you get a tube that is right size for your desired seatpost and still light and responsive for the characteristics you are shooting for in the build?  You use two tubes.

Let's cut the first tube, a 3/2.5 1.25x.089 ti tube that will be our seat tube top portion. I'll square up the parting tool so we can get going...

And are our two pieces that are destined to become one single seat tube...

With the piece successfully parted off, I'll now face it, bore it, and then remove material from the OD to allow a press fit into the 1.25 x .035 seat tube material. 

I've accounted for 2.25" of internal overlap, which will give sufficient material for the top tube intersection to prevent post weld distortion as well as offer a rock solid interface for the seat post.

Pressed in with about 500 pounds of pressure...

With the top insert in place, let's cut our 12 degree angle cut for the top of the seat tube...gotta be precise :)

pretty darn close, eh?  No wasted length of the tube here :)
and the finished cut...

After a good cleaning, it's off to the weld table to sew these two pieces together, place the binder bolt, then ready for the mill to cut in the bottom bracket miter.  The shop was like Grand Central Station this afternoon, with emergency after emergency popping in; a broken spoke here, help with a motorcycle design issue there, a little fire department business and a crashed bike that needs to be fixed by tomorrow morning as Jeff is traveling towards Philly for a race this weekend.  Crap, the seatpost is gonna have to wait...this is the last repair I wanted to do right now.

Racing in Alum Creek on Sunday, Jeff hit the terra firma hard, and learned why it's called "FIRMA".  He's pretty bruised up, ego tweaked, and his bike is a mess.  He twisted and fractured his derailleur...

and broke his Paragon dropout, fracturing it in the web while twisting the dropout inward.  Given the time restraints, the best approach was to realign the dropout, machine a steel insert to fill the window, weld it around the periphery and silver braze in the last portion that abuts the silver filled stay end...

Not real pretty, but hopefully functional enough to get him through the rest of the long as he stays upright :)

Back on the Ti frame in the am..


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