Gear Ratio Troubles


I’m from team 4405a, and am relatively new to vex. I’ve never had much luck with anything other than a 1:1 ratio for my drive base (gearing it any faster caused it to overheat). my last robot weighed about 20 pounds, and I had to go with a 1:1 yet again.

My question is this: Is there a way to calculate gear ratios ahead of time?

(i’ve seen this done in a tutorial, using weight, friction, gravity, torque, and wheel size as variables. It seems as though the angle (having to do with friction) at the beginning was measured in degrees and the tangent was calculated using radians)

This went over my non math-oriented head. The alternate question is this:

Are there general sets of rules for how heavy a robot can be before it can no longer sustain a faster gear ratio on its wheels.

using regular motors and omni wheels i would say if your bot is over 15lb do 1:1

My team’s robot is about 20 pounds, and we use 1.6:1 internal gearing with 4 393s. This works pretty well, aside from the fact that be break a lot of motors. (we are working on this). There is a handy chart which I have used here, basically if it is in the green it will work, and the yellow may or may not work depending on the weight of your robot and your motors. The red will not work unless your robot is very light and you have many motors on your drive.
Good luck.

1:1 with 4 torque-393s works for our 25lb robot, and we see no reason to go any faster. Our speed hasn’t been a very big disadvantage, because during the last 20 seconds of the match, the faster NZ-style bots bow to our immense pushing power.

We had an extremely heavy robot last year that ran 1:1 with 4 269s on the wheels. It overheated occasionally, but once we added green clutches to the wheel motors, all overheating ended. Adding clutches really is an effective way to prevent overheating, which might allow you to gear your wheels a little faster.

Im not sure what video you saw but here is the one I made. I can e-mail you the pdf file if it is difficult to follow.
a 20lb robot ran at a 1:1 should work just fine.

Indeed. That helps quite a bit. I’m trying to make my next robot quite a bit lighter, so I can use a faster ratio, but again, this helps A LOT.

lol you are at the fine line
try gearing it down to 1.4 at least?
you may have tripping issues at worlds because the mats are new and they have very high friction (or maybe its the spray they use)

As you mention, the tutorial inputs (motor type, motor count, wheel type) are critical. I assume everyone not mentioning wheel type is using 4" omniwheels, since that seems to be the most common.

My understanding is that the Vex wheel chart applies primarily to 2x 3wire motor squarebots, but it clearly shows how wheel size affects gear ratio. Owen’s reference point doesn’t mention wheel size, but he points out how the colors on the chart depend on weight and motor count (which are not on the chart).

No wheel size again, but its a reference point for motor count, motor type, motor weight, and overheating avoidance with clutches. Note that clutches wear out, so replace them every few? hours of drive practice. There are other threads mentioning how programming to do slew rate control on motors can minimize overheating. Also spreading motors across cortex ports can be important.

There was an old thread that suggested that gears have smaller pitch than sprockets, so it possible to have more gear ratios that sprocket ratios, in theory.
In practice, I find that it is much easier to switch sprockets and chain length, than to switch motor placement and gears. The “Gary” practice robot drive frame on the gallery is an excellent prototype to experiment with different sprocket sizes, since the sprockets are cantilevered and easy to access. Theory is fine to get a starting point, but practical experiments will provide real world results.