 # Converting from MPH to RPM?

Dear community,

I have a value of 400 MPH, and would like to convert it to RPM for use in Blender and I really don’t know where to start. I would want it’s horizontal spinning bar to spin at a speed of 400 MPH.

The object is the robot Tombstone from Battlebots, built by TzHaar-Ket and is available here

How long is the spinning bar? What is its radius?

The circumference of a circle is equal to 2 * pi * radius of the circle. The distance it traverses per rotation is that circumference.

I believe it can be seen in this picture. I’m not 100% sure if that’s the correct dimension though, as the bar is rotated and still it’s axis is wrong, but approximately a meter long?

I don’t need to see it. I need to know how long it is. From the center of rotation to the end (diagonal or middle, whichever part you want travelling at 400mph)

Well that would be 50 centimeters.

Then the distance it travels in a full rotation is 100 * pi cm.

So if we want to solve for RPM, let’s use “x” to designate that unknown.

In this case, 100 * pi centimeters * x / minutes = 400 miles / hours.

There are 60 minutes in an hour and approximately 160934 centimeters in a mile (according to Google.) So let’s get everything in the same units:

100 * pi centimeters * x / minutes = 400 * 160934 centimeters / 60 minutes

Now our centimeters and minutes can cancel out. We have

100 * pi * x = 400 * 160934 / 60

To solve for x, we can divide both sides by 100 * pi:

x = (400 * 160934) / (60 * 100 * pi)

which is about equal to 3417 revolutions per minute. (But you can use a calculator or something for more precise value.)

1 Like

Alright then. I will try to get this as accurate as possible in Blender. Thanks for your time!