Ivadating 2016 jelsoft enterprises ltd
The easiest way to walk teams through these calculations is the JVN calculator.
It has tabs set up for commonly used gearboxes, rotary mechanisms, linear mechanisms, and much more.
The 81% value is "about right" for the robots that team 148 builds. The calculations end up being "about right." "About right" is totally okay for a FIRST Robot." Stuff in quotes are excerpts from an old JVN post.
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This is the "JVN Gearing Spreadsheet" which can be used to help design robot mechanisms & drivetrains which use DC motors. Older versions: loss constant accounts for the difference between actual speed and free speed of the motor.
You can pretty much leave it at 81% for most FRC applications.
For "weight on driven wheels" if you have a 4WD robot with all wheels being driven then you would set that at 100%.
It also has motor specs for many of the commonly used FRC motors pre-loaded.