Author Topic: Douglas Dragonfly Designer  (Read 324 times)

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Offline Paul Taliesin

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Douglas Dragonfly Designer
« on: 11 Oct 2017 at 13:12 »
I have read a few articles on the Post War Douglas Range but have not seen any reference to who designed and Styled  the Dragonfly model ? It was George Halliday who is credited with designing the Mark Series with Walter Moore designing the forks.
Who was involved with the Dragonfly ? It seems such a modern design and styled ahead of it's time such a great pity it was not developed properly.

Offline eddie

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Re: Douglas Dragonfly Designer
« Reply #1 on: 11 Oct 2017 at 17:39 »
A Mark 5 was supplied to Ernie Earles/Renolds Tubes with a remit to update the design. The first prototype used the Mk 5 engine/gearbox unit and wheels. A second prototype was photographed with the engine modified to coil ignition and alternator electrics(but there are rumours this engine was only a mock-up). The Douglas factory then produced an engine that resembled the Dragonfly. Six machines were produced for the 1954 show at Earls Court, and badged as the Douglas Dart. Following protests from other manufacturers that the name 'Dart' was already being used, the bikes were rebadged 'Dragonfly'. Early production Dragonflies incorporated several modifications to overcome some of the shortcomings experienced with the 6 original 'Darts'. Later production Dragonflies incorporated further modifications. Evidently, production ceased some time during 1956 - the '57 machines being stock left over from the previous year - most of which were sold through Pride and Clarke.

  Eddie.

Offline Paul Taliesin

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Re: Douglas Dragonfly Designer
« Reply #2 on: 13 Oct 2017 at 10:15 »
Thanks Eddie great info on the Dragonfly Development. A great pity that it was not originally conceived to be a 500 from the start it would have had greater potential market in the 50's.

I've read on a number of places that Douglas used Alloy heads & Barrels on a number Plus 90 models which caused many problems.
There were problems in the alloy expanding more than the steel through studs holding the heads and barrel into the crankcases. What was the cause of the issue ? Was it not low expansion LM25 alloys used? or straight aluminum for the head and barrels which caused the problems?
All modern engines 1960's on run alloy heads so there must have been some poor calculations/design or specified  materials somewhere on the design. Surely today we can resolve this? Alloy conducts heat away much more efficiently than cast iron.

Offline Hampshirebiker

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Re: Douglas Dragonfly Designer
« Reply #3 on: 13 Oct 2017 at 17:15 »
On this site somewhere, I think there are some articles from Motorcycle Sport in the 60s by L J K Setright. He explains the development of the racing 90 Plus & the problem you mention. If not on the site, the article was reproduced in the club magazine. a few months ago.

Offline graeme

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Re: Douglas Dragonfly Designer
« Reply #4 on: 13 Oct 2017 at 22:15 »
It was indeed in the NCR a few months back - and very interesting reading too

Offline douglas1947

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Re: Douglas Dragonfly Designer
« Reply #5 on: 14 Oct 2017 at 11:42 »
It was not only Douglas with problems using  Alloy Heads + Barrels; for ex. Panther M75 too. They stopped the alloy heads and barrels and didi the same casting in iron again, with the shape of the "new" alloy parts.
Years later people know to get out the problems.
Michael

Offline Hampshirebiker

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Re: Douglas Dragonfly Designer
« Reply #6 on: 14 Oct 2017 at 17:26 »
The L J K S article is on this site, Nine down, under technical articles.

Offline Paul Taliesin

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Re: Douglas Dragonfly Designer
« Reply #7 on: 14 Oct 2017 at 19:26 »
Yes I'm sure it's always a problem when 2 materials of different coefficients of expansion heats up together. Today we see many engines with this very set up. Cast Iron block & Alloy head
I think it can be cured with cylinder studs that allow enough stretch to accommodate the different expansion rates. Possibly composite headgaskets will help also.
Improved steels that possibly were in not widely available or new  developments or expensive back in the early 50's