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Author Topic: How to determine RPM change with rear sprocket change?  (Read 802 times)

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Offline TimLoon

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How to determine RPM change with rear sprocket change?
« on: September 18, 2019, 05:20:17 PM »
One project for the off season is to lower my cruising rpms. Right now my stock '81 cb650c is doing 4500 rpm's at 55 mph. How do i calculate rpm change for 1 or 2 tooth change on rear sprocket.
Thanks, Tim
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: How to determine RPM change with rear sprocket change?
« Reply #1 on: September 18, 2019, 06:29:04 PM »
Increase the front 1 tooth. Better results for what you want plus it's easier. Probably save you ~ 500rpm??
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Offline 754

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Re: How to determine RPM change with rear sprocket change?
« Reply #2 on: September 18, 2019, 06:35:25 PM »
 1 on the front, usually like 3 on the back..and really easy to change..
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Offline Deltarider

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Re: How to determine RPM change with rear sprocket change?
« Reply #3 on: September 18, 2019, 10:59:58 PM »
Quote from: TimLoon on September 18, 2019, 05:20:17 PM
One project for the off season is to lower my cruising rpms.
Why? If your mirrors are vibrating and become blurry at your favourite cruising speed, I can understand it. When not, I would leave it like it is. You might end up with your mirrors vibrating and blurry at your favourite cruising speed...
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Offline robvangulik

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Re: How to determine RPM change with rear sprocket change?
« Reply #4 on: September 19, 2019, 02:18:32 AM »
www.gearingcommander.com lets you try all possibilities and does the math for you.
« Last Edit: September 19, 2019, 02:22:20 AM by robvangulik »
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Offline Little_Phil

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Re: How to determine RPM change with rear sprocket change?
« Reply #5 on: September 19, 2019, 04:00:28 AM »
If just changing the rear sprocket the new rpm = Original rpm (4500) x New no of teeth/Original no of teeth. Therefore if changing from 36 to 34 teeth new rpm at 55 will be 4250rpm.
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Offline 1976cb750f836

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Re: How to determine RPM change with rear sprocket change?
« Reply #6 on: September 19, 2019, 04:59:47 AM »
Id also recommended the front, its cheaper,easier to go back if wrong, and no chain modification .
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Offline BenelliSEI

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Re: How to determine RPM change with rear sprocket change?
« Reply #7 on: September 19, 2019, 06:41:36 AM »
You can also do the math. For an easy example. Say you are running 12/42. The overall ratio is 42/12=3.5. If you go 13/42 the ratio becomes 3.23 so there is a 7.7% drop (3.5/0.27 X100). Instead of 4500RPM, you would be turning 4154RPM.
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Offline TimLoon

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Re: How to determine RPM change with rear sprocket change?
« Reply #8 on: September 19, 2019, 05:27:47 PM »
Thanks folks for all the good advice.
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