Here is a gearbox calculator that i wrote. It takes wheel size and the gears and calculates the ratios and the speed. Is the 18fps the theoretical or actual speed that you want. The actual is usually about 80% of the theoretical, and the actual is what is calculated by my app. 18fps actual is VERY fast and very hard to control. I would also try using a 12 tooth CIM gear, which can help give some better ratios. Those can be found at Andymark.
Also dont count out some of the smaller gears from andymark. From andymark you can also get 14,16,19, and 24 tooth spur gears, in addition to a 28 tooth dog gear. You can also get a 50 tooth aluminum spur gear from andymark as well which will give you a weight advantage.
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Teams 1510 and 2898 - Student 2010-2012
Team 4488 - Mentor 2013-2016
Co-developer of
RobotDotNet, a .NET port of the WPILib.