Gear Ratios. Which gears are fitted?

b'knighted

Well Known and Active Forum User
VOC Member
From your new figures the ratio between first and top is 2.586:1. The book figure is 2.6:1 for a Rapide so you have a Rapide first gear not the original Shadow one. If your speedo and rev counter were absolutely accurate then the revs in top with normal gearing would be 2,875. Your figure of 2,900 is well within the 10% error that many chronometric devices have so it seems likely that you also have a standard ratio on the rear chain.

Sean's first post states " Rear wheel is 19" and the larger 48T sprocket is in use. It also carries a 46T sprocket on the left side. " would this account for the slight difference?
 

timetraveller

Well Known and Active Forum User
VOC Member
Well spotted b'knighted. I had not gone back and looked at the first posting again. The use of a 48, as opposed to a 46, toothed rear sprocket will increase the revs by 48/46. This means that my figure of 2,875 rpm should be 3,000 rpm. Probably well within the measuring errors for this kind of task. I often notice that people worry about what wheel size is used but it is not the wheel size but the rolling circumference of the tyre that is used. For example the choice between a standard twin with a 20 x 3.00 front tyre and a touring spec. bike (D) with a 19 x 3.50 should be negligable, i.e. they are both nominally a 26" wheel. The problem arises with modern tyres which often do not have a circular cross section so one needs to know the radial thickness of the tyre, not its width. It is easy to check if a mark is made on the tyre and the bike wheeled forwards on a piece of flat road an exact number of turns and then the total distance measured. Divide the total distance moved by the number of wheel rotations and then divide by pi, 3.14 is near enough for this, and you have the rolling diameter of your wheel. Sorry if this seems condescending but there might be people out there not used to old bikes and old tricks.
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Moderator
Well spotted b'knighted. I had not gone back and looked at the first posting again. The use of a 48, as opposed to a 46, toothed rear sprocket will increase the revs by 48/46. This means that my figure of 2,875 rpm should be 3,000 rpm. Probably well within the measuring errors for this kind of task. I often notice that people worry about what wheel size is used but it is not the wheel size but the rolling circumference of the tyre that is used. For example the choice between a standard twin with a 20 x 3.00 front tyre and a touring spec. bike (D) with a 19 x 3.50 should be negligable, i.e. they are both nominally a 26" wheel. The problem arises with modern tyres which often do not have a circular cross section so one needs to know the radial thickness of the tyre, not its width. It is easy to check if a mark is made on the tyre and the bike wheeled forwards on a piece of flat road an exact number of turns and then the total distance measured. Divide the total distance moved by the number of wheel rotations and then divide by pi, 3.14 is near enough for this, and you have the rolling diameter of your wheel. Sorry if this seems condescending but there might be people out there not used to old bikes and old tricks.

If you go to www.vintagetyres.com and look at the motorcycle tyres you want to know about, in the info for the tyre it tells you the tyre diameter. Handy for comparing tyres. ie a 300x20 speedmaster has a diameter of 665mm, a 325x19 speedmaster is 658mm and a 100/90 19" AM26 Roadrider has a diameter of 671mm.

Cheers
Simon
 

Seán Ó Maoildeirg

Forum User
Non-VOC Member
If you go to www.vintagetyres.com and look at the motorcycle tyres you want to know about, in the info for the tyre it tells you the tyre diameter. Handy for comparing tyres. ie a 300x20 speedmaster has a diameter of 665mm, a 325x19 speedmaster is 658mm and a 100/90 19" AM26 Roadrider has a diameter of 671mm.

Cheers
Simon
Very interesting. I hadn't noticed that info was on the website http://www.vintagetyres.com/ and it shows something that surprised me. The 3.50x19 has a larger circumference than the 300x20. 682mm for the rear as opposed to 665mm for the front. I went with my string and measuring tape to verify it was not a misprint.
 
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