Super Shifter Gear Ratio's

[Slothbron]

Our team is looking to buy some Andy Mark super shifter's for the 2013 season. But we are trying to figure out what gear ratio to use. Last year a team told us to shoot for around a 2:1 ratio and have a low speed of 7 ft/s and a high speed of 15 ft/s. Is this a norm or more of a team preference?

[Jared Russell]

Depends on the game and your strategy. I have seen robots geared as low as 3fps (a "stump pulling" mode for heavy duty pushing)* and as fast as >20fps (especially for under-weight robots).

Remember that the faster you gear, the more current you will draw while accelerating, pushing, or turning. And the slower you will accelerate to maximum speed.

The slower you gear, the more torque you will have available. But at some point your wheels become the limiting factor (you are "friction limited").

I would guess that most robots with a 2-speed transmission have a low gear in the 4-8 fps range, and a high gear between 10 and 16 fps. Single speed robots tend to be in the 8-12 fps range. But again...depends on the game, depends on your robot, and there are plenty of exceptions to the above generalizations.

Keep in mind that for a given gearbox, you can vary wheel size and chain/belt reductions to achieve a wide variety of actual speeds.

* Definitely the slowest speed I have ever seen was the 2002 Beatty robot, but that's a special case...

[apalrd]

The general preference is, in this order:

-Use off-the-shelf components
-High gear should be fast enough to traverse the field quickly, but not too high to prevent running the majority of the match in high gear.
-Low gear should be able to break traction on carpet, given the choice of wheels and motors

Usually, this means a ratio spread of 2.56:1 or 4:1, since that's what AM sells off the shelf.

We usually design around a 15' or 20' sprint distance on a mediocre battery to determine the high gear, and make sure the low gear still meets the design objectives without obsessing over it too much. When thinking about sprint distances, remember to factor in acceleration, not just final velocity. This includes the motor power and any electrical losses.

There is no norm, aside from the 2.56:1 and 4:1 ratio spreads that AM sells. Many teams adjust the final drive ratio (chain or gears between the shifter and wheels) and wheel diameter to match their performance objectives, since it's significantly easier to change a sprocket than to machine a new shifter gear.