Über ßike part 3

7 08 2009

O.k.  got a little sidetracked.   Finished mocking up rear triangle.   Each piece started as thick stainless tubing and was bored to size, thinned substantially, mitered and fit up.

First the raw stainless tubing

Then machine to fit on my 1943 southbend lathe which came off a WW2 submarine.  Still wonderfully accurate after 66 years.

Then miter the chainstay ports to fit BB.  Just to give an idea.  A standard frame would have 10 miters.  This frame has 20 mitered tubes.  Essentially making a frame twice is what we call it.

Then same thing on seat stays.

final look

Throw in a little carbon fiber to the stew…..

Points from side.  I wanted these to line up.  I retain these points during the brazing.

So then I get it all mocked up and I discover something I did not quite think through.  Yes, we framebuilders are not perfect.   It may be one thing when you build the same thing over and over again.  You learn it perfectly.  You now how one change will affect something else.   In the case of mass produced frames.  They model every last scenario, then test.  That standard does not change.   Custom frames throw you a curve once in a while.

I wanted to build an extended seat mast bicycle frame.  I lowered the top tube, so far so good.   The rear triangle though is very small and this increases the angle of the seat stays as they attach to the rear of the seat tube.   In turn it decreases the clearance for a tire.  I did not fully consider this fact when I drew up the plans initially.   The tire would fit but only leave 1.5mm clearance on either side.   Not enough if one was to have an untrue wheel or a lumpy tire.   So what to do?  First, go have lunch…..solution…grind away interiors of seat stay.  Cap with leftover KVA headtube and voila 5mm of tire clearance.   Good custom framebuilding is often just finding good solutions to problems.

The carbon fiber previously will be cut down and a section added internally to the seat tube to help support the seat mast.   I designed the seat lug for this but it is paper thin steel tubing (.4mm) and I want this frame to last a LONG time so a little extra insurance here is great.   The same thing will be done to the downtube where it joins with the headtube to reinforce this high stress area.  Much the same idea as the exogrid stuff from Titus (well, without the grid)

Well, more to come!  😀

Dave Bohm
Bohemian Bicycles




One response

7 08 2009

Would’nt the heat from brazing affect the carbon tube on the downtube?

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