Friday, November 12, 2010

Huret Dropout Adapter

Here's my first pass at an engineering drawing for the adapter I made for my 1972 Schwinn Sports Tourer.  This simple little part installs on the derailleur mounting/pivot bolt, and sits squeezed in between the derailleur and the dropout.  Its purpose is to fix an incompatibility between modern derailleurs and an old Huret dropout.  The Huret dropout has the derailleur adjustment screw flat maybe 30 degrees farther forward than does a modern dropout, and the adjustment screw on a modern derailleur won't connect with the flat at all.  This adapter provides a new flat in the right place for a modern derailleur.

All dimensions are in mm.  Start by taking a little scrap of 1-1.5 mm sheet metal (I used chrome plated brass that started life as some sort of washer), and cutting it like so:

If you could get it to scale right, it would be great to be able to print this image on a laminating sheet and just stick it onto your sheet metal as a template.  I'll see if I can figure out a way to post a file that's 1:1 scale.

Anyway, once you have the thing cut out, file the edges smooth so you don't cut yourself.  I'd even round off those pointy tab corners a bit.  If you use brass like I did, filing takes no time at all.  Then drill a 10 mm hole up at the top, there, and file its edges too.  The hole is oversized to accommodate adjustment.

Then the next step will be to fold the part on the three lines toward the bottom (sorry, I couldn't figure out how to dash the lines) using a pair of needle-nose pliers.  The two tabs at either side fold towards you about 110 degrees or so -- past vertical anyway (you'll need to fiddle the correct angle during installation, so don't sweat that too much).  Then the rectangular flap with the (now folded) tabs folds up 90 degrees so that the tabs now sit along the body.  The tabs' angle should be about radial to the hole.  The little box section formed by the three folds is installed facing away from you, fitting up against the dropout's adjustment flat.

The drawing is nothing fancy, yet.  But I'm used to freehand graphics packages, not tools that expect accurate measurements.  Next up, I want to make the drawing 3D, and figure out how to fold it in the software to give a preview of the finished part.  And I'll take a picture of the installed prototype adapter so you can see how it fits.

Should be fun!

All for now,



John Ellsworth said...

Here's a link to a photo of the original handmade part, pre-filing, so you can see how it folds.

Anonymous said...

This may have just saved me. I have an older Motobecane Grand Jubile w/ Huret dropouts. After asking several bike mechanics for advice I was resigned to having the frame modified. But I think that I should be able to get this to work.

John Ellsworth said...

Hey, that's great! Let me know how you made out and if there's any way I can help!