Vintage Workshop
Services for Brough Superior motorcycles and their contemporaries

SS100 frame replica page last update: 15-6-2002

Well, I have obtained a set of unmachined lugs, but I am none too happy with them.

I understand these lugs were originally moulded off a late-twenties 680 o.h.v. frame that had been sawn up for re-tubing. While people have built SS100 frames from them, I can see the following problems:

  • They are cast from SG iron. Originally, white malleable iron was used for the lugs of brazed frames.

    I am afraid I will have to give a small lecture on ferrous casting materials here in order to explain the problem.

    There are five basic types of material for ferrous castings:

    material easy to cast welding properties ductility strength
    grey cast iron

    ++

    --

    --

    o

    white malleable iron

    ++

    ++

    ++

    +

    black malleable iron

    ++

    -

    ++

    +

    spheroidal graphite (SG) iron

    +

    o

    +

    +

    cast steel

    -

    ++

    ++

    ++

    Grey cast iron has a high carbon content which makes it flow very nicely into the mould. During the solidification, the carbon forms small graphite flakes, which make the casting very brittle, but give good damping and sliding characteristics. Cylinder barrels are made from grey cast iron.

    White malleable is produced by heat treating grey cast iron bits at 1050 C for 4-6 days in an atmosphere that does burn the carbon in the iron. Thus, a tough and malleable iron with low carbon content is produced, having good welding and brazing properties. Unfortunately, there are almost no foundries left that still do this treatment.

    With black malleable, the carbon is not burned altogether, but transformed into a kind of coal. This makes the material ductile, but it cannot be welded or brazed.

    In the 1970's, spheroidal graphite (SG) iron has been invented. In this type of iron, the carbon does not form flakes like in grey cast iron, but very small graphite spheres, which is achieved by adding small quantities of magnesium into an otherwise very pure iron. The lower strength varieties of SG are quite ductile (15 .. 18 % elongation), while the stronger types have only some 5% elongation.
    While this would be ok, the main problem is that welding and brazing it can produce unpredictable results, as the delicate equilibrium of the structure may be disturbed quite drastically due to the heat of the process. Brazing and welding can be done, if the very process with its specific temperature gradients has been proven to be acceptable by destructive material testing. But everybody who has yet brazed up a frame lug with an open flame knows that you have a very poor control over the heat range and time gradients, usually you are happy if you have got the braze well into the joint and not burnt the bit!

    Cast steel finally combines high strength with high ductility and very good welding properties, but it is very difficult to cast due to the higher temperatures and greater shrinkage.

    From the above you can conclude that cast steel is the only fully-fledged alternative to white malleable iron, which, unfortunately, is almost unobtainable these days.

  • Then, the above mentioned lugs have been cast from the original ones 'as is'. This means, they have neither machining allowances nor any allowances for shrinkage. This means that they become a bit thin in some places if you bore them to accept the original tube sizes and you get quite thin rear fork ends etc...

  • Lastly, it is debatable whether or not the same lugs were originally used for the 680 and SS100 frames. Some people say there is not one common lug on both of these frames, some say it is only the front bottom lug which is different. I will have to find out about this...

    5/2005: I have now made many investigations into this matter. I have taken measurements from a number of bikes, and have spent many hours consolidating these in CAD drawings. My conclusions are the following:
  • Before the arrival of the Alpine Grand Sports SS100 model, the 680 ohv, the SS80 and the SS100 shared most of the frame lugs, with the SS100 frames just being built a bit higher to accommodate the ohv engine.
  • The AGS frame, which appeared in the end of 1926, was a quite radical re-design. It used a different headstock casting, having twin reinforcement webs running from the top tube to the front down tube (the so-called box-type headstock). The centre of gravity was set a bit lower, and with most of the lugs, the angles included between the tubes are different from those on the pre-AGS frame. This is certainly true for the rear fork lugs, the seat lug, the gearbox bridge, the seat down tube bottom lug and the front down tube bottom lug.  So these two frames have very few lugs in common, if any.
  • The late 1920's SS80 frames are still pretty similar to the pre-AGS SS100 frames, my guess is that G.B. used up the old lugs for these. With the 80-100 frames I don't know. I have heard of one which has a box-type headstock, and another one that hasn't.

    My previous plan, to use some of the better and less critical lugs of the above set, is thus more or less frustrated.
    I have now got a very nice steel headstock casting, and a few other lugs, but it will still require a lot of pattern making until I have steel castings for all the lugs...
     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

I will keep you updated when there is any news...
meanwhile I am happy to hear from you if you have any comments!

Any kind of feedback to is appreciated
(sorry, this is not a clickable 'mailto:' hyperlink. If you want to write me, please type my address in your mailer. )

Back to HOME page - Back to SS100 Replica Project main page