Forum

[Midge]

To answer Dean's original question, have a look at page 54 of the catalogue. This lists the number of chain pitches for each sprocket size. Mr H comes up trumps again!

[Martin C]

Changing from a 19 to a 20 tooth gearbox sprocket was quite straightforward when I did it some years ago.
However, I doubt a 500 would pull it very successfully - unless it is seriously performance-tuned and also you're not excessively heavy? I only went to 20 tooth after upgrading to 612cc and fitting a 5-speed gearbox. My HMC GT Cafe-racer is highly entertaining to ride at all speeds up to a genuine (proven) 100+ mph, but it definitely isn't a docile thumper. Fifth gear is usable for gentle cruising only above 40mph, and for useful acceleration only above 50 mph, whereas at 70 mph it's quite handy to drop down a gear for blasting past the obstructing peasantry.

[John M]

Gearing the bike down will shrink the big gap between third and fourth gear. I have a road going 350 Bullet that I went down one tooth on the gearbox sprocket which makes it more rideable. I also have a 1956-ish 350 Trials Bullet that has a 14 tooth gearbox sprocket and a larger rear sprocket to gear it down for off road use. This bike does not have the big gap between third and fourth and is still rideable on the road. I did 125 miles on last years Beamish Trophy Trial, although it does get a bit revy at 50mph. (You don't bother about top speed too much when you have a 6" front brake!)

[MadMike]

Alan Dean did explain that his use in traffic made lower gearing highly desirable. John the gap between 3rd and 4th will remain the same if you lower the gearing at the back wheel. Unless you cahnge the gearbox internals then you do not affect the gap. However by lowering the gearing the engine torque will in fact make the bike more responsive to throttle openings in top gear, albeit at the expense of fuel consumption and top speed. HTH.

[PeteF]

Mike's right, if you just change the sprockets,the gap will be the same "size" but at a higher rev range. Another effect of the close ratio conversion (which does shrink the gap) is that it raises the 1st gear ratio which is perhaps not what you want for town riding. Ideally I suppose you would do both. It's always been a mystery why they selected these ratios - perhaps they suit Indian road conditions. Certainly the CR conversion makes my 350 a lot more "ridable" but then I avoid town riding whenever possible so the higher 1st gear isn't a problem.

[Dean]

I did order the higher compression piston, alloy cylinder, competition valve kit, amal carb, boyer bransden ignition and am in the process of installation of same. also got the sealed bearings for the gearbox and a new clutch. it was after I tore it apart that I started thinking about changing the primary gear from 16 to 14. I do live at 5500 ft elevation and always seem to be riding up the side of a mountain. if you guys really feel that the gear change isn't necessary, I will comply (as previous recommendations on this site have been "spot on").
thanks for the lively discussion.
cheers, Dean

[John M]

Mike, you may be right about the gap in theory, but in practice the gap is not as noticeable and in the case of my trials bike not there at all.

[MadMike]

John the difference between the 3rd gear and 4th gear internal ratios remains unchanged by altering the gearbox or rear wheel sprockets. What changes is the overall final drive ratio. Now by simply lowering the final drive ratio, in all gears, by changing the sprocket(s) you allow the engine to rev more freely thus creating the illusion that the gap between 3rd and 4th has disappeared when in fact it has simply been masked by the faster reving engine at any given speed.