VEX Pro Ball Shifter Ratios

This year we are using a custom 3rd stage on the new VEX Pro ball shifter gearbox so that we can tweak our ratios with much more flexibility. Due to nature of this years game we might need to cross the whole field to get to the feeder station, so we wanted a gear ratio that put us around 16-17fps. Because of the low spread on the ball shifters we found that if we geared to 16-17fps in high gear we didn’t have enough pushing force in low gear.

On the VEX website in the ball shifter description they state that they choose the 2.27 spread because it was better suited to a FRC robot than other off the shelf gearboxes. Unfortunately it seems that for our desired use that is not the case and a 2.56 or 2.66 spread from an AM shifter or custom gearbox is much more useful than a 2.27 spread.

For everyone using the new ball shifters: what speeds are you running? have you had to compromise your top speed for pushing power or is the 2.27 spread just right for you?

As a disclaimer, i’m not trying to bash on ball shifters. I think there great for teams that want to go 13-14fps max, but unfortunately that’s not us.

Why a custom 3rd stage, cant you tweak the ratio with differing sprockets?

The main problem with running 16-17 FPS is that you have a hard time getting a full weight robot up to that speed with only 2 CIMs per side. 254 was geared around 18 FPS in 2011, but they only weighed 100 lbs. Going that speed with a full weight robot and only 2 CIMs drains batteries quickly and has a slow rate of acceleration. Plus unless you have really nicely trained drivers, 17 FPS is fast, almost to the point of uncontrollably fast. So 16-17 FPS might look good on paper, but you have to take into consideration the amount of torque it takes to get to that speed, and how easily you can control that speed. That is why most teams for a fast speed gear for 13-14 FPS, and then low gear matches up nicely with the VexPro shifters.

EDIT: Have a look at this paper and it should explain a few things

How did you determine you didn’t have enough pushing force? Are you actually stalling your motors in low gear?