# Tip Speed & Drive RPM Calculator

**Tip Speed / Drive RPM Calculator**:

Select battery type:

cells

NOTE: **LiHV** will increase your tipspeed over standard **LiPo!**

Motor kV

kV

Spinning Diameter

mm

Output Pulley Ratio

**:**

Motor Pulley Ratio

**:**

If you’re unsure of your output & motor ratios instead in the same inputs you can enter:

Number of Teeth – **output pulley teeth : motor pulley teeth** or

Pulley Diameter – **output pulley diameter : motor pulley diameter**

###### Rotational Velocity

###### rpm

###### Max Tip Speed

###### mph

###### m/s

Note: this is **max theoretical tip speed** with a fully charged battery, it will spin slower than this in reality.

**UK beetle & feather spinners max theoretical tip speed is 250mph. ****If you are running a spinning weapon, it is your responsibility to ensure you are below this limit.**

**Kinetic Energy Calculator**

Moment of Inertia at Centre of Mass

gmm^2

MoI at CoM obtained from your CAD program. Ensure you’ve got the correct mass assigned and you’re using the correct axis (the letter of the axis you’re rotating around eg – horizontals would usually be Izz) For everything but drums/beaters just take the axis with the largest value – usually in the format 3.00E+05 g mm^2 (ballpark for beetles)

###### Max Rotational Kinetic Energy

###### J

**Robodojo operate a 200J max KE for spinning weapons at their Featherweight Sportsman events.**

###### Calculations:

Rotation Velocity (rpm) = max cell charge x cell number x motor kv / reduction ratio

Reduction Ratio = output pulley / motor pulley

Tip Speed (mph) = rpm x diameter(mm) x pi x 60/1609344

Tip Speed (mph) = rpm x circumference x rotatations x scaling for mph

Tip Speed (m/s) = (rpm x diameter(mm) x pi) / 60 / 1000

Tip Speed (m/s) = (rpm x diameter(mm) x pi) / scaling for m/s

Rotational Kinetic Energy (J) = 1/2 x MoI(kgm^2) x (rpm / 60 * 2 * pi)^2

Rotational Kinetic Energy (J) = 1/2 x MoI(kgm^2) x rad/s^2**Thanks to Rory Charlesworth for the inspiration and calculation help.**

**Disclaimer: Building, maintaining and fighting combat robots is dangerous and comes with risk that must be assessed on a person by person basis. We cannot be held liable for any personal injury, loss of property or money from partaking in building, maintaining, repairing, testing or fighting combat robots even when following our guides and rulesets.**