# Did some math for ya'll (RPMs)

M

#### mrsaxman99

##### Guest
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

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.

Thanks Tom. Yeah I'm really curious as well what the redline is on these engines...

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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