Understanding WCP Drive Train Calculator

[InFlight]

I'm not a big fan of these calculators as you miss out on understanding the fundamentals equations of why you would want a particular ratio. These equations ignore any inefficiencies in the drive train, but give you a better insight into the tradeoffs.

Gear Box Selection

Calculating Wheel Speed and Gear Ratios

The distance covered by one rotation of a wheel is it circumference in feet.

Circumference = Pi * (wheel size in Inches)/(12 inch/foot)

Assuming the use of the normal CIM motors, 4455rpm is a typical design speed. The wheel speed in Feet per Second becomes:

Speed= (4455 rpm x Wheel Size x 3.14)/(60sec/min x Gear_Ratio x 12in/foot)

To identify the necessary gear ratio to achieve a desired speed, we can rearrange terms for-

Ratio= (4455 rpm x Wheel Size x 3.14)/(60sec/min x Speed x 12in/foot)

As an example, assume the use of a two speed gearbox with 8 inch tires.
We would like a high speed of around 16 feet/sec to move very quickly.

High Gear = (4455 rpm x 8" x 3.14)/(60sec/min x 16 ft/sec x 12in/foot)=9.7

We would also like a low speed of 4 feet per second to have a good pushing gear.

Low Gear = (4455 rpm x 8" x 3.14)/(60sec/min x 4 ft/sec x 12in/foot)=38.8

Limiting ourselves to commonly available to two speed gearboxes , the closest one to our desired specs is Vex 3 CIM Ball Shifter with the 20/64 output gearing that provides 9.07 & 33.33 output ratios. This gearbox option would provide:

High Speed= (4455 rpm x 8" x 3.14)/(60sec/min x 9.07 x 12in/foot) = 17.1 feet/sec

Low Speed= (4455 rpm x 8" x 3.14)/(60sec/min x 33.3 x 12in/foot) = 4.7 feet/sec

If we were selecting a single speed gear box for 8 inch tires

Assume a 14 ft/sec speed; which is a ratio of 11.1 using the methods outlined above.

There are three different single speed AndyMark Gearboxes that have 10.71 output ratio options, which would result in a predicted speed of 14.5 feet per second.

Torque and Acceleration are proportional to the Gear Ratio. Speed is inversely proportional to the gear ratio. In an open field you may favor speed. In a congested field with shorter travel distances, acceleration is more important. So choose your top speed for the gearboxes accordingly.