Frames Failure and Repairs - Douglas Mark Series.It is generally well known that a manufacturing fault occurred during the production of frames for the Post-War Mark Series Douglas which lead to the replacement of many frames by Douglas Motor Cycles Limited under warranty provisions.
Australian distributors of Douglas acting on behalf of Douglas Motorcycles Limited repaired many frames of the bikes imported to the country during this period. Presumably this also occurred in other countries about the world to where the bikes were exported. Factory made replacement frames were provided under warranty provisions to the home market in the UK.
Many non-factory repairs have been carried out by owners and restorers since those times with varying degrees of success. It is therefore not at all uncommon to find a Mark Series bike otherwise seemingly in top condition but in need of frame repair as an initial step in restoration.
Owners are well advised to keep a weathered eye on their bikes for signs of any movement in the frames during use as obvious danger to life and limb are ever present. Periodic inspections are recommended.
Breaks can occur in any tube frame member at any location but most commonly they occur at points where working stresses are most concentrated such as the forward and rear ends of the bottom horizontal tubes (which take the direct and indirect loads and torgue forces of the engine) and often at the top and bottom of the forward downtubes, particularly near the headstock connections.
The fractures in the frame tubes occur mostly near or immediately adjacent to the cast steel lugs (or knuckles) that occur at the frame intersections.
Although written essentially as a narrative about repair of the frame of my own 1948 T35 carried out late in 2006 and not intended as a 'Technical Article', the document has been deemed appropriate to this Board where it will be readily accessible and more easily located by those seeking advice about frame breakage and repairs.
There have been several discussions and accounts of repairs undertaken that have been published within this forum. The more prominent of these have been linked to the Article for convenience of reference.
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Alwyn
