As I see things, the British motorcycle industry was running along nicely through the good times between the wars ,various company's produced V twins for speed, status or for dragging a sidecar along, and a range of singles from around 250 cc to 633 cc . The single's were built in side valve ,over head valve and over head camshaft format and a few small two strokes were on offer too .
You could get a big single to do whatever you wanted , small for economy , big SV for a chair , SOHC 490 for sport or a DOHC 490 for race [ if you need to set records in a outfit you could order a 596 OHC ].Just about all the company's made a single and they had tried all the gimmics and wierd stuff to improve the perfomance but there would be few real discoveries in technology after the end of the 39/45 war .AJS are generally credited with producing the first really sucessful OHV motor in the early 1920,s and Norton's single and double OHC motors were holding off the opposition on the race track perhaps by rider skill and reliability but they maintained the ascendancy won the races ,sold the bikes , paid the shareholders .
The big question is why did norton then go and design a parallel twin motor ???
Business mentality dictates that you need to be first in with the new ideas and be able to identify a winner early [ ask Kodak about digital camera's ], Well that sure wasn't Norton because Triumph had built a great little 500 parallel twin motor and bolted it into a rigid / girder fork frame in 1937, eleven years prior to the Norton twin hitting the market.
Norton was playing follow the leader on this one , around this time a small group of designers were working on projects for their bosses these men would be offered a better position with the opposition and would sometimes leave only to go back later . I think a designer could be briefed to work on a specific part of a machine and therby contribute to the design but in a minor way , another designer being given the credits greater part.
The blokes I want to write about are Ted Turner , Val Page and Bert Hopwood ,if these names don't mean anything to you they should ,here's why, these three designers appear to have all worked for Ariel at some stage both Turner and Page have worked for Triumph But only Hopwood had worked for Norton .I think this is how it goes, Turner does the design work for the OHC square 4 Ariel and hands over to Page, but prior to leaving he basically cut the motor in half and ran it as an experiment while at Ariel, he then moves to Triumph .Once at Triumph he Remembers the half of a SQ4 motor and how well it ran . From this came all the lovely Triumph derivative's ,Turner is without doubt the patriach of the Speed Twin , Tiger, T Bird ,Bonneville and so forth ,"on you Ted " he seems to have stayed a while and is credited with Triumph's success because of his styling input mouth organ tank badges chrome strips seats and stuff .Turner then moved upstairs and took on management roles it would appear he was good at everything !
Val Page completed the work on the SQ 4 and had done lots with their singles, once again sales rose after a styling lift involving the high level exhausts ,that Ariel red paint and judicious use of chrome the one I like is called a Red Hunter VH I think ?
Bert Hopwood had been employed as a draughtsman with Ariel and Triumph and had been under the direction of both Page and Turner at some time and would have been in the centre of the development work on probably the Triumph and the Ariel twin motors , guess what ? Hopwood takes a position with Norton
Norton had in the past employed Walter Moore and Arthur Carroll to do design work on the singles, especially the OHC ones but Moore got a better offer with NSU and left . Carroll was assisted by a bloke called Edgar Franks and I think that Hopwood and Franks came to an agreement over the work where Franks looked after the sloggers and hopwood rolled up his sleeves to play catch ups with all the other makers and get a twin motor in a test frame .The race stuff was controlled by Joe Craig and I mean controlled .