We are Building our Mecanum Drive for this years robot and are wondering if anyone knows what a good gear ratio would be if we are powering 4 wheels with 1 cim each. We want to be able to go across the bumps with our ~120 pound robot plus 6 totes. Thanks
It depends on your wheel size.
The recommendation I heard from buildblitz was to aim for ~10ft per sec. This pushes the gear ratio tradeoff more towards torque (controllability) and less towards top speed.
We used VexPro’s single reduction gearbox (11:72) with another 1:2 reduction via belt to drive 8 inch mecanum wheels. It puts us around 11fps, which is great for our applications.
If it’s single-speed it depends on the driver mostly. Can they properly position your drive, and quickly? I would recommend making a system with a number of selectable gear ratios so you can decide on the fly. A vex clamping gearbox with a secondary sprocket reduction to the wheels could work well.
Anything above fps can technically work, but I think we’re going with something like 14fps.
The robot + totes is only 180lbs, and you’re not pushing other robots (hopefully).
You should be able to drive the hump as long as you have enough ground clearance and any two-stage gear ratio. The lower you gear (that is, the higher the number opposite the :1), the more capability for acceleration, push, and fine control. The higher you gear, the faster you can go when you reach full speed. Maximum speed for a typical 80% efficient drive train is 18.5 fps * the wheel diameter in inches / gear ratio. To get 10fps (as recommended below) with 4" wheels (whether mecanum or traction) you’d want a target gear ratio of about 7.4:1. With 6" wheels, it’s about 11.1, and with 8", about 14.8:1. 10fps is rather slow for most of the FRC games, but with high-speed robots more likely to knock stacks over than build them up, this is certainly a defensible number.
I calculated it:
top speed = efficiency * free-speed-of-shaft * wheel-radius
For a 1 inch diameter wheel (1/2" radius), a CIM on a 1:1 gearbox, and an 80% efficient drive train, and converting all the units:
0.8 * 5310 rpm * (2 * pi radians/rev) * 0.5in (radius) / (60 sec/min * 12 in/ft) = 18.5 fps.
Take a look at this Build Blitz resource on Mecanum Drive Options. It gives you some good example ratios, as well as the calculations behind them.
We’ve elected to go for 72:12 from VEXPro’s clamping gearboxes, with an additional 42:24 belt reduction, for an overall JVN speed of 10.72 ft/s on 6" wheels.
We’ve also purchased 11t cim pinion gears in the event that we find find that we’ve geared too fast, which would reduce our JVN speed to 9.83 ft/s.
