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Old 07-07-2011, 11:23 PM   #1 (permalink)
dcb
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sort-of armchair sprocket changes

If you want to think through your sprocket changes a bit for efficiency (the only concern I am addressing), here are some notes. Of course they are not terribly expensive and relatively easy to change, but anyway.

first go to jtsprockets.com ( JT Sprockets: Catalogue ) and looked up your vehicle in the catalog and see what the typical range of teeth choices are for front and rear sprockets for your bike, then look in the buyers guide to see some other models to google/ebay for.

for my american 1975 cb125s it was:
Front sprocket available in: 12, 13, 14, 15, 16, 17 teeth
Rear sprocket available in: 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 49, 50, 51, 52, 56 teeth


Then go look at your bike and confirm how many teeth are on the front and rear (I used a flashlight and a bit of chalk to count the teeth on the front sprocket quickly)
I had 15 teeth on front and 40 in the rear

then figure out your approximate current gear ratios. I drove around a little in each gear at 5000 rpm and noted my mph on the 125, dont crash

gear,mph@5000rpm
5,32
4,27
3,22
2,17
1,12


ok, now search/phone/ebay around for the biggest front sprocket and smallest rear sprocket you can find, possibly someone has a kit with a chain also.

I was able to locate a 17 tooth front for a few bucks stateside, and a 35 rear from thailand (apparently the cb125 is alive and well there)

So now for some fudge, try starting your bike in 2nd gear. Lets say this will be the "worst case" scenario, for launching. Give yourself plenty of tries to get used to it and see if it is something you could live with or at least get a feel for how close it is to something you could learn to live with. For this exercise we won't select a new combination of sprockets that would make the "new" first have less than the "current" second gear launch. Of course if your conditions allow less launching torque compromise for, say, less rpm at cruise plus aeromods and minimal stop and go or whatever, feel free to select any ratio you want.

to see the effect of the front sprocket change, take the tooth count of the largest one you can find and divide it by your current tooth count.
mine is 17/15 or 1.133...

then multiply the mph figures by that number, i.e.
5,36.3
4,30.6
...
1,13.6

since my new mph in first is 13.6 and my old mph in 2nd was 17, and launching in 2nd was barely tolerable, I should have no problem getting going with just the 17 tooth front sprocket change.


to see the effect of just the rear sprocket change, take the tooth count of your current tooth count and divide it by the tooth count of smallest one you can find.
mine is 40/35 or 1.14

and again multuply the original mph figures by that number, i.e.
5,36.5
4,30.8
...
1,13.7

And again, the mph/rpm ratio in first is not terribly close to second (13.7 vs 17), so I should be fine with just changing the rear and not the front also. Also note that 4th gear is ~1.5mph below the old 5th gear mph when we change either sprocket, so 4th is basically the new 5th and we have some noticable overdrive in 5th now.


Finally we want to consider changing both sprockets, so we take the original mph figures and multiply by both numbers, 1.133... and 1.14 here.
5,41.35
4,34.9
...
1,15.5

So we can see that in this case after changing both sprockets first gear would still have more torque than launching in 2nd had, so we should have a better feel how the low end will be affected, and we have added about 10mph at 5000rpm. Even 4th is higher than the original 5th. Of course this is a dramatic change and you should expect the bike to feel very differently, and you might need to shorten your chain too, but easy sprocket changes is one of the charms of motorcycles.

FYI, I have a 17 tooth front on order for my 125, will consider the rear especially if there is one stateside (don't like waiting weeks).

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Old 07-07-2011, 11:42 PM   #2 (permalink)
dcb
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and of course this does not apply to shaft drive...
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Old 07-07-2011, 11:59 PM   #3 (permalink)
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You could also aim for 1000-1200 ft/mn piston speed at the desired cruise speed...
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Old 07-10-2011, 01:43 AM   #4 (permalink)
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k, did some figuring with piston speed. Formula is stroke in inches * RPM / 6.

my 1.95" stroke hits 1200fpm @ 3700 rpm.

Of course the question is how much power do I need in top gear at whatever speed 3700 rpm represents. Which would be 27mph with the new 17 tooth sprocket.

So I think I will try centering my shifts on 3700 rpm and see if that is tolerable anyway since I'm otherwise flying blind, and with the new sprocket and traffic allowing, I should be in 5th by 27mph. Just gonna have to take my time, not a problem

With a little "luck" my power demands will just be met at 27mph and I can just cruise there all day at uber peak efficiency with both sprocket changes I hit 1200fps (3700rpm) at 30.5mph
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Old 07-11-2011, 05:47 PM   #5 (permalink)
dcb
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ok, got the 17 tooth front sprocket installed, chain tightened/lubed. Doing the front was a lot easier than the rear It is liveable off the line and noticably less "angry" at speed. It takes a tiny bit more clutch before I can let it out completely but still nothing like a moped.

I was shifting at 3900-4000 rpm all around town, it seems like it is just above the "bog" point, and I would probably wind it out more if I had to join traffic, but if things are clear then its putt-putt-putt all the way up to cruising speed
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Last edited by dcb; 07-11-2011 at 06:02 PM..
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Old 10-17-2012, 11:18 PM   #6 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
You could also aim for 1000-1200 ft/mn piston speed at the desired cruise speed...
Hi Frank, what is the significance of the 1000 to 1200 ft/min piston speed? How does that relate to fuel efficiency or even engine longevity?

Thanks in advance,

Tom

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