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46 model 18 gearing question?

Submitted by nathan_stuch on Fri, 22/09/2023
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Gents,

I am curious as to what some of you are running for gearing on your bikes? I live in the country side (south of Omaha Nebraska) and need to hit a few miles of 65 mph road to get where I usually ride…traffic sucks! The bike has stock gearing (19/42) and seems a bit stressed at 50-55…not expecting much but would like to lessen the load a little on it when having to travel down these roads.  I have read some folks change the primary sprocket and then some do both gearbox and primary sprocket…what would you recommend for such a situation? I can find the primary sprockets from RGM but what about the gearbox? Would one from a Commando work?

Thanks much.

 

 

Submitted by richard_payne on Fri, 22/09/2023

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Good practice is to change the engine sprocket and speed the gearbox up. Large gearbox sprockets increase the load on the gearbox bearings. The only reason that there is such a range of Commando gearbox sprockets is that there are no alternative triplex engine sprockets. This practice contributed to the premature gearbox failures that many suffered.

What size do you have on the crank ? A 500cc in good condition with an average weight rider should be able to pull 21t on the engine.

65mph is quite fast to "cruise" an eighty year old 500 single.

Submitted by nathan_stuch on Fri, 22/09/2023

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Hi Rich,

Well I had blind moment (non-readers on) about rear sprocket…42T not 41. Crank sprocket is a 20 tooth and gearbox is 19.   

Im not looking to cruise at 65 just keep these people off my a&& for a couple miles until I can slow down…traffic has increased 300% out here.   

My weight is 220lbs…the bike moves out fine with me on it.  

Thanks.

Submitted by david_cooper on Sat, 23/09/2023

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Bacon lists engine sprocket of 21 for Model 18 up to 1937.  Then 20 from 1938.

I believe most of our bikes are undergeared.  They were geared to give maximum speed at peak power in 4th gear.  And they were judged by acceleration (which needs low gearing) plus the smallest possible "no snatch speed in top gear".  There was an almost paranoid fear of changing down gear in cars with crash gearboxes, and it seemed to be carried over by motorcycle journalists into bike reports.  And British roads were mostly terrible before the motorways arrived in the 60's.

Having said that...we are so used to silent modern machines that we get unnerved by the racket made by an air cooled bike.  If you fit a rev counter, you'll probably find it's not revving nearly as fast as you think it is.  Ear plugs give me at least 10mph cruising speed, as well as delaying deafness.

Submitted by john_doig on Sat, 23/09/2023

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I’ve always run a 21 tooth engine sprocket on my ‘52 ES2 - it pulls up hills ok, and cruises comfortably at about 55 mph. 

John Doig

 

 


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