We would like to know how to determine the best wheel size for our robot based on the RPM of the CIM motors and the gear ratio. Speed and pushing power are our to priorities.
Uhhhh, do you make your own gearboxes, or are you using prefabricated ones like toughboxes? 1-speed or 2-speed? And in general, what are you capable of?
I can tell you that this year, some times teams might be faster and easier to push around thanks to the new CIMple gearboxes.
Assuming you’re building with the kit, this is what I would say:
Most years, a 6-wheel drivetrain with a basic 1/8" dropped center wheel using 4 cims and a basic toughbox (12.75:1) going to 6" IFI wheels (or plaction, for the budget-minded) would be plenty fast and be able to push around most robots. This year, you might want to go a little faster with a little less pushing power. This means either reconfigure the toughbox for a smaller reduction or get bigger wheels. Also make sure to reinforce the kit frame a little, they can be a bit flimsy, and add tons of grease.
You will have to find the optimum balance between the two. Unfortunately, you will have to give up pushing force if you want to gain speed and vice versa. (Assuming you’re using single speed gearboxes.)
Your best wheel size will depend on other things: what kind of gear reduction are you using through your gearboxes? What kind of sprocket reductions are you going to use? Is there physically enough space for the wheel on the robot?
I’m just going to assume you’re using Toughboxes because they are the most readily available gearboxes. Here are some additional ratios to the standard 12.75:1. Only one of them will slow you down and add more torque.
Here are some additional sprockets. You can change these out to tweak your speed/torque ratio to whatever you want.
I unfortunately don’t know your team’s exact goals so I can’t make any recommendations for your overall reduction.
Just do the simple math of CIM RPM -> through Toughbox ratio -> through sprocket reduction -> mulltiply by wheel circumference -> convert to ft/sec.
http://www.chiefdelphi.com/media/papers/2059
Makes it easy to just play with the numbers until you get what you want.
–Ryan
Do note if you use the ~5500 value of RPM, that will be the robot’s speed under no load. Unfortunately, the CIMs will experience some load. For a better approximate, use half the free RPM or plug in your preferences into JVN’s design calculator.
- Sunny
You’ll get far, far more than half the free speed with a drivetrain when you accelerate to full speed.
80 percent is ussualy the correct figure to use. 81 percent if you wanna get technical. (for CIMs)