History reasons for gearbox ratio's

History reasons for gearbox ratio's

> From: Rod Dittmar <hdrdr05@charter.net>
> Subject: Wide/Close/Std Ratio Transmissions
>
> Hello again Brit-Iron,
> Over the years, I have seen references to different ratio transmissions,
> but never understood the rationality behind them. Now I want to know.
> What are the differences between a wide and close ratio gearset vs. a
> standard ratio? And, what would be the advantages or disadvantages of
> using such things? In other words, when/why would someone need to use
> them?
> Regards, Rod


Dear Rod & Brit-Iron,

The following list of ratios found in 1932 - 1950 Sturmey-Archer ( later Norton) gearboxes shows the various wide/standard/close gearsets and the sort of bikes/purpose for which they were used.
The Broughs were fitted with 1,100 cc side valve v-twin JAP 60 degree engines with massive torque and could easily pull from a stop - even with a sidecar attached on the bottom gear of about 11 to 1 (overall gearing was 4.2.1 in top but they could use 3.8 solo) They had a closer ratio between 2nd, 3rd and 4th compared to the standard ratio S-A Heavy Weight gear set intended for smaller sized touring bikes 500 -600cc designed for lower speeds and which had lower torque available. The Norton Trials ratios are an extreme example of a vintage wide ratio set designed for getting the torque to the ground at slow trials speeds with a broad band of power from a 500 cc engine. The extreme of close ratio in the Norton racing International (Manx Norton) shows a first gear almost the same as the 3rd gear in the trials box (too tall to allow a kick start to work) and ratios to take advantage of a very narrow megaphone-determined power curve.

Early Brough 11-50 Special - 2.54, 1.46, 1.13, 1.
Standard Sturmey-Archer HW - 2.97, 2.17, 1.21, 1.
Norton std. & 16H - 2.97, 1.77, 1.21, 1.
Later 11-50 (Norton-type) box - 2.5 , 1.7, 1.14, 1.
Norton trials (500T) - 3.68, 2.37, 1.62, 1.
Norton Inter. (With K/S) - 2.33, 1.33, 1.1, 1.
Norton racing International - 1.77, 1.33, 1.1, 1.

Allan Johnson

Most of the late Brough Superiors actually used Matchless V-twins, built to Brough specifications. J.A.P. Motors were used in many applications before the war, including Morgan 3-wheelers. George Brough, however, found the Matchless motor had some advantages over the Prestwich motor.

J A Prestwich engine production was taken over by Villiers after the war, And J.A.P. Speedway engines were put in speedway bikes well into the '60s

After the war, the new Formula 3 racing specifications called for 500cc motors. The Norton Manx put out almost double the horsepower of the JAP Speedway motor, and many Manx engines found their way into Formula 3 cars, like Coopers. Since Norton would not sell engines, racers would by Manxes, take the motor out, and sell the chassis - sometimes with the transmission - to recoup some of the cost. Thus was born the original Triton.

Interesting that the company that built the Manx and the company that built the Speedway motor would end up under the same corporate umbrella later.

Brilliant. Just last night we were musing about why folks were pulling Norton engines off Nortons and substituting them with Triumph engines. Now I know. Thanks

Milo of Vt.

Most 4 speed close ratio gearboxes have the first gear up where second is in a normal road gear box. In push start road races this is not a problem, and the close ratios provide better usa ge of the torque band - you can keep the motor on the boil better. It has been said that if you have a torquey motor, you don't need a close box - that is a load of bull. If you are racing , it is essential to be smooth in your gear changes. If you have to give a heap of revs on the downchange, it is really not so good. If you have really sticky tyres, it might not matter so much, but with the old T1 compound triangulars we used in the old days, it could easily crash you. I used a close box from the US in my Seeley 850, and it was excellent, except that these days we vhave clutch starts. It meant smoking the clutch to get a decent start while the guys w ith 5 speed Japanese bikes did not have that problem. My bike now has a 6 speed TTI gearbox with first gear slight ly higher than the standard commando. I have yet to try it , but I will be increasing my overall gearing until I get a bit of lag in the revs after each gear change . It is all about going faster , and riding better (smoother).

Milo
If you are going to build a bike with a featherbed frame, a Jawa speedway engine is a much better option than a Triumph engine. You would have to fit an oil return pump, and lower the compression ratio. Unless the Triumph engine is mounted as far forward as possible, the bike won't handle. And the power characteristics don't resemble those of the manx anyway. With a Jawa engine, the bike will be lighter and just as powerful, and much more fun to ride .