Friday, May 6, 2011

The Franks redesign of 1931

By Simon

The Franks redesign which resulted in the 1931 range of production Nortons was not an unmitigated success! I have already mentioned my late friend Jack Bindley's experiences with his new '31 ES2. He was as disappointed with the bike as he was with Norton's response to his complaints. Last week I came across a few back numbers of the Beaulieu magazine 'Veteran and Vintage' and found further criticism therein from Richard Chapman, who worked for Eric Fernihough (amongst other well known people) at Brooklands in the thirties. Richard bought a new 1931 Model 18 from dealers in Trowbridge, to which, after careful running-in, he attached a new TT Hughes sidecar. He goes on to say..

"This machine was the slowest thing on wheels and a great disappointment. With the light TT sidecar and a passenger, the maximum speed was about 50 mph and after a while I rode the machine to Birmingham to see the makers about her. A tester took her for a run round the houses and reported that she was up to standard, which drew from me the comment that their standards must be pretty low! After a lot of postal haggling, Nortons gave me a set of what they described as 'dirt track rockers' (ie cam followers) for the crankcase and these improved matters a little. I then got going on my own and raised the compression. I increased the inlet tract bore, fitted a larger inlet valve and a 10TT25 carburettor. This, with stronger valve springs, rebalancing and some work all over the engine, improved matters. The final result was that I could get 80 mph with a passenger in the sidecar, under reasonable conditions."

Richard must have had considerable tuning skills - doubtless enhanced by later experiences with Fernihough.

The reference to Dirt Track Rockers is intriguing as the dirt track model (above) was only made in 1930 - not '31 - and it would be interesting to know just how the rockers differed from the standard profile. And there is a possibility we can find out: a 1930 dirt track engine bottom end has surfaced in NZ and although rather hacked about, I am encouraging the owner to take it apart and see just what is inside! So watch this space!

The decline actually seems to have started with the 1930 enclosed rocker box set-up. The geometry of this rocker box is such that the valves open less than the lift on the cams while with the open rockers they open more...And why did Norton dowtune these engines? My guess is that they were seeking longer term reliability. What else could it be? From a marketing (doubt if the word had been coined then!) point of view it was perhaps no bad thing to have the OHC machines as real flyers and the over the counter bikes as just reliable moderate performance means of transport.


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