Tuesday, August 28, 2012

Martin's Ti belt drive 650b

Martin is looking on from afar, so I thought I'd shoot some step by step pics of day one of his fabrication.

This will be a Ti 650b belt drive single speed with custom EBB, post mount disc brakes, internal cable routing for geared use, and use a curved seat tube and top tube for funk.

The first hurdle of this build is insuring that with a curved seat tube, that the end saddle position remains the same as it would with a straight post.  It takes a bit of math and some datum point fixturing to insure that the intended overall cockpit length stays the same with the curved seat tube, as the design angles the post further back. 


The math done, I need to move the post forward .66 inches or about a total of 1.25 degrees to retain my saddle position.  To double check my math, I set up the fixture with the desired measurements using a standard post, attach a quick extension bar, and mark the saddle center point on it. Then the fixture is reset to the adjusted angle and checked to insure the intersect point at the correct saddle height stays the same.  (sorry for the rotated pic, can't figure out how to turn this one and make it stay).  Here's a quick pic holding the straight post up to show the forward offset to compensate...


With the fixture position set, I began making the seat tube by turning down the top mast insert, creating a shoulder to press in and then boring out the ID to allow for welding shrinkage...


The top insert done, it's time to cut the seat tube to accommodate, face the top edge, then press the insert into place and weld it in.


 
The weld is sanded smooth so that no indication exists of the coupling, then I begin setting up the binder.  The tube is fixtured and the top cut at a 12 degree angle, then the center is found and marked for the binder placement...
 
 
The binder is machined to fit the seat tube OD...
 
 
And then everything is cleaned and the binder welded in place, carefully leaving the soon to be slot open so as not to dull my tools by cutting through the weld...
 

Once she's all welded up, the binder slot is drilled and slotted with a slitting saw...

 
With the seat tube mostly done, I prepped the custom EBB, the stock is cut on the cold saw...

 
Then turned and faced to get our desired width dimension...


 
The ID is then bored to fit our custom insert...



The insert is a custom aluminum piece that allows for over 1/2" of throw and can easily be adjusted on the trail using your crank arm and an allen wrench, simply loosen the pinch bolts, put the allen wrench in the recessed hole on the EBB face, then use your crank arm to push against the wrench and rotate the insert to loosen and tighten...


 I then turn down the OD to take some weight off then clean it up, fixture the pinch binders in place, and weld it up...

The EBB shell is then cut on the cold saw and the edges filed smooth...


Once the EBB is on the frame fixture, I can then miter the bb end of my seat tube...


With the seat tube all nicely fitted up, it's on to the down tube.  I cut in the bb end first, then fit the head tube end...


The next cut is the st/dt junction...one of the most difficult on this build as the curved seat tube enters at an acute angle and falls away, so the cut must be precise the first time, as there is no filing this one in if you miss.  I begin by setting the down tube adjacent to the seat tube and using a square, transfer across the front edge of the seat tube...


I then mark around the seat tube, remove it, and place the down tube.  I use calipers and basically connect the dots to see where my miter margins should be.


The head is tilted and the miter fixture is tilted to match the entry angle for the curve...


One shot to get it right, so I double check everything and then make a cut.  You just see the remnants of my lines...


Spot on, whew!  I'll have to be very careful with my heat control coming through the crotch as the angle provides a tapering wall, getting thinner as it approaches the seat tube.  Will probably use a bit larger diameter filler there to fight the edge's desire to pull away.


The front triangle coming together...

 
cheers,
 
rody

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