Did some math for ya'll (RPMs)

M

mrsaxman99

Guest
#1
Details towards the bottom...the good stuff right here.

If you have a 44 tooth sprocket, and a 26" rim with average sized rubber, then use this equation to find your engine RPM:

RPM = 226.7885 * MPH

This is what I did:

Rear sprocket is 44, front is 10. On the flipside of the engine there is a 20 tooth crank sprocket running an 82 tooth sprocket. So:

(#1) 44/10 * 82/20 = 18.04 (This is your ratio of engine rpm to rear wheel rpm)

1mile/hour * 1hr/60min * 5380feet/1mile = 88feet/minute (per MPH...read on, you'll use this later)

I measured how far my bike travles in one rotation, pretty much exactly 7ft (7 feet = 84inches, and 84in = 2(pi)r, r = 13.36inches, just about right)

(#2) 2(pi)r = x feet/rotation

Therefore:

(#3)]88ft/min (for 1mph) / 7 feet/rep = 12.5714 tire rpm (for 1mph) (your tire will travel 12.57 feet in one minute going one mile per hour)

and

(#4)12.5714 tire rpm * 18.04 (from equation #1) ratio = 226.7885 engine rpm per mile per hour

so if you have a speedometer like I'm sure you all do, just multiply out whatever speed you want to get your engine rpm (ex. 20mph * 226.7885rpm = 4535.77rpm @ 20mph)

To get more accurate, measure the circumference (or diameter and use eq #2) of your wheel and plug it into equation #3 to get an accurate wheel rpm per mile per hour.

For a different sprocket size, just use equation #1 to adjust the ratio. Check this:

a 22 tooth rear sprocket would yield a 9.02 ratio, so instead of 4535.77rpm at 20, you'd be at 2267.88rpm.

THAT ALL BEING SAID...does anyone know the max rpm a broken in engine should be run at? what about while breaking in???

Ciao for now,
-Chas
 


Tom

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Aug 4, 2006
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#2
Wow. My engine is running at 6k-6500 at max speed... I wonder what the "redline" is. I am hopefully going to get a 32t sprocket for my bike so I am not running the RPMs that high all the time. Or at least I am going much faster when my RPMs are that high....

Good math. It would have taken me forever to figure that out.
 


srdavo

Active Member
Joined
Nov 4, 2006
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#4
I read somewhere that redline was 5800 rpm's. according to your "math" I've gone over 7000 rpm's. these lil 2 strokes like to run wide open.
 


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