Meca Dural Duralumin Bicycle Frame Construction

Meca Dural bottom bracket shell

From the 1930’s through the 1950’s, the French were enamored with aluminum bicycle frames, even though steel was the material of choice for most builders.  A number of examples still exist today, and after disassembling and cleaning this 1940’s/50’s Mercier Meca Dural frame, I can see why.  The bottom bracket shell is a work of art, looking as if it had been machined yesterday, rather than more than 6 decades ago.

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I became curious about the method of joining the aluminum tubes with aluminum lugs, using what I had read were internal steel expanders.  Apparently, at the time there was no method to “glue and screw” the aluminum tubes, a method that was pioneered by ALAN beginning in the early 1970’s.  The only joining methods which were available then were gas welding the tubes – a process used by Nicola Barra; connecting octagonal aluminum tubes into aluminum lugs with connector bolts – a process used by Pierre Caminade; joining the tubes with aluminum lugs and wedged internal expanders – the method used by Meca Dural, and other other hybrid methods involving pinning the lugs, and using a steel rear triangle.

When I passed my magnet over the frame, I picked up no attraction, except for a very faint pull near the lugs.  You will note that the chain stays and seat stays are connected with a combination of bolts and aluminum sleeves, and that the bottom bracket shell is held in place with two large bolts connecting the lug to the chain stays.  The aluminum sleeves do double duty as the brake bridge and chain stay bridge.

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The seat tube/seat stay lug is pinned, as you can see above.  But what about the main tubes – how do the internal expanders work?

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As it turns out, it was fortunate that the Meca Dural headbadge was missing, which allowed me to peer into the head tube lug to examine the expander inside.  My magnet told me that the expander is steel, and the method to accomplish the expansion process seemed to involve a steel tab which was probably manipulated with a special tool.  When you think about it, the same idea is used for quill stems inserted into threaded steerer tubes.  That seems to have worked pretty well, so why should these lugs be any different?

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At first I wasn’t sure of the purpose for the large holes underneath each of the two headbadges.  Upon closer examination, you can see that the head tube and head lugs are actually machined as one piece.  The holes are necessary so that the expanders can be inserted to join the top tube and down tube, necessitating a hole for each tube.  And that is why there are always two headbadges on every Meca Dural frame -to cover these holes.  That’s one mystery solved.

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One thing that seems true of older aluminum frames is their flexibility, relative to steel frames.  My ALAN is a very flexible frame, but not too flexible.  I guess you would say it is flexible in the right kind of way.  To satisfy my curiosity about this frame’s flex characteristics, I squeezed the rear dropouts to test the amount of flex.  Then, in my unscientific experiment I compared the amount of flex on this frame, to all the other bare frames hanging in my shop, all of which are steel, and some of which are Reynolds 531.  I was able to flex the dropouts on the Meca Dural about 7 or 8 mm, using my weaker left hand at full force.  On several mixte frames, I could barely move the drop outs 3 mm, and on a diamond vintage Reynolds 531 frame, I could flex the drop outs about 6 mm at full force.  That’s a significant difference in flex, and it will be interesting to see how this frame rides once I have it restored.

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With just some minor polishing with a wadding cleaner (I use NEVRDULL), the frame shines beautifully.  I need to source some 650b wheels from the period, because at some point someone tried to install 700c wheels on this bike, and that is how the bike was configured when I acquired it.  The spacing at the rear dropouts is 115 mm, so it would be hard to find the vintage hubs to build a wheel set, even though I have a nice vintage set of rims.  Instead, I am on the hunt for a donor vintage bike from the 40’s or 50’s which can give me a decent 650b wheelset, and maybe a few other parts to add to my collection.

 

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