# 60 mph = 812 rpm on a 15" tire?



## DaveAK (Jun 28, 2009)

Assuming the tire on a 15" rim is 25" in diameter, then yes, those RPM figures are correct.


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## the1cyrus (Aug 6, 2009)

if the diameter is bigger will the rpms go up or down to be at 60 mph?


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## DaveAK (Jun 28, 2009)

To get an approximation multiply your diameter by 3.14, and then divide 63,360 by that number to get RPM at 60MPH

i.e 26" diameter

63360 / 26 * 3.14 = 776rpm @ 60mph

So to answer your question, as the diameter increases the rpm will decrease.


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## Bowser330 (Jun 15, 2008)

the1cyrus said:


> if the diameter is bigger will the rpms go up or down to be at 60 mph?


If the diameter is larger, the circumference is larger, which makes every revolution of that tire travel a farther distance. Therefore less revolutions would be needed to go the same speed.


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## the1cyrus (Aug 6, 2009)

great. id have to rethink some things then. im building a three wheeled ev and i plan to have a small motor on each wheel. two in the front and one in the back.

Originally i was thinking of using the 2 front motors for start up speeds (around 0-30mph) and using the back wheel for top speed. However i thought i needed much higher rpms from the motors.


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## the1cyrus (Aug 6, 2009)

So as long as i keep within the rpm that correlates to speed wanted i will reach that speed regardless of wind resistance or other things that are acting against my movement? Wind resistance is more of a matter of the power required to move at a certain rpm. is this correct?


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## Woodsmith (Jun 5, 2008)

the1cyrus said:


> So as long as i keep within the rpm that correlates to speed wanted i will reach that speed regardless of wind resistance or other things that are acting against my movement? Wind resistance is more of a matter of the power required to move at a certain rpm. is this correct?


If you have enough power at that RPM to maintain the RPM then the tyre will spin at that speed.
You will need to work out the rolling and air resistances to determine how much power that equates to to ensure that your motor will be sufficiently powerful and torquey to do what you need.

If you do not have enough power then the resistance will prevent the motor reaching the RPM you desire.


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