# Which gear ratio is ideal? deciding gear ratio

so we plan to buy 3 CIM ball shifters from west coast products, but the problem was unlike the dog cim shifters (which are more expensive) they don’t have the gear ratio to speed chart under this project. If you are reading this you are probably thinking “why don’t you look at the options of ratios and decide based on that?” Sadly, I don’t understand how to pick the proper gear ratio and I would like to know for the future. So to clarify, I would like to know which gear ratio to pick, and how to understand what i am selecting. Sorry about the noobish question, I am more of an electronics person. Thanks for the responses! 1 Like

JVN Design Calculator is your best friend

We choose a higher gear ratio due to the fact that we could out maneuver and still hit really hard and if you are in tank drive they can’t push you sideways. To calculate output speed- output speed= input speed/ gear ratio, output torque= input torque* gear ratio* efficiency. What gear ratio you choose depends on what you want, the lower the number the faster it is with less pushing ability and vise versa for higher numbers.

There are a few decisions you need to make, and then you can work backwards to select the gear ratios that will provide what you need.

What size wheels are you using? 4”, 6” 8” or something else?

Are you able to direct drive one wheel and then run belts / chains from it, or will you be using some speed reduction between the gearbox and the wheels.

And, how fast do you want the robot to travel in high and low speed?

I will make some assumptions and then you can calculate for what you are designing.

Let’s say you want to use 6” wheels and you want a low speed of 9 feet per second and a high speed of 14 feet per second.

First determine the circumference of your wheels - 6” wheel = piD = 3.146 = 18.84”. So for every revolution of the wheel, your robot will travel 18.84”.

For the high speed of 14 feet per second:
18.84 inches per revolution / 12 inches per foot = 1.57 feet per revolution
14 feet per second = 14 x 60 = 840 feet per minute
840 feet per minute / 1.57 feet per revolution = 535 RPM (Revolutions per minute).

So the wheel needs to turn at 545 RPM.

If you are direct driving the wheel (connected to the gearbox output shaft or with a 1:1 sprocket set), then the reduction you need is (assuming you are using CIMs)

5310 RPM (CIM) / 545 RPM (Wheel) = 9.7:1 gearbox reduction.

If you are not able to direct drive the wheel, then you need to factor this reduction by the tooth counts of the sprockets (belt or chain) of your drive.

So if you have a 20 tooth sprocket on the gearbox and a 30 tooth sprocket on the wheel, then you are slowing down the RPMs going from the gearbox to the wheel, so that 1 revolution of the gearbox is only 0.66 revolutions of the wheel (20/30). So you need to reduce the gearbox reduction (so it has a higher RPM) by the ratio of 20/30. So instead of a 9.7:1 reduction you need a 9.7*.66 = 6.4:1 reduction.

You would do the same calculations for the low speed values, and then try to find a ratio combination that is close to the pair you need.

Remember the calculations are theoretical assuming everything is perfect in the system. Most people use a 95% efficiency number, so you can make this adjustment to your numbers to reflect this.

1 Like

As mentioned, the JVN design calculator is an excellent way to pick your ratios. To makes things easier, we have also incorporated the math that both JVN and Ether have discussed here on chief into a web-based drivetrain calc. This already has all the VEXpro/WCP gearboxes pre-loaded into it, you don’t have to go searching for the exact teeth numbers and copy them elsewhere.

The calculator outputs a simple speed, the output speed * one simple efficiency number, plus a loaded speed, calculated by running an acceleration model over a certain distance, factoring in multiple drive train and electrical losses.

When picking your ratios, you’ll want to take into account your robot and your drivers. Faster high gear speeds (~18FPS) allow you cross the field quickly, but are more difficult to control. Slower speeds (~14FPS) give you more control, but its going to take longer to get places. You can play around the “Test Distance” variable on our calc to see how different speeds affect how long it takes to get places. Low gear ratios depend mainly on whether you favor pushing, or speed. Slower speeds are great for pushing other robots (less current draw), high speeds once again favor quick movement.

Let us know if you have any more questions

0_0 That calculator is GOLD! thankyouthankyouthankyou!   