VersaChassis with 3cim ballshifter driving rear wheels instead of center wheels

[roborb]

Our team has a working VersaChassis with 3cim ball shifter driving the center wheels, with #25H chain connecting the front and rear wheels. However attaching 3cim ball shifter to the center wheels created space constraints for attaching the elevator, mechanisms, and electronics. I am thinking of asking the team to attach the gearbox to the rear wheels so that there is more open space. Thanks to the Versachassis design, it seems doable even at this late stage. However, I am not sure if I am overlooking potential problems. We are a second year team and this is the first time the team built versachassis. If anyone has knowledge/experience with this configuration, can you please let me know if this works or not, and what to watch out for ?

[Anthony_Galea]

We are running that exact configuration this season (with belts instead of chain) and have not encountered any problems so far.

[Jon_Stratis]

In theory, it doesn’t matter which wheels you direct drive, so long as you run chain or belts to the others. In 2012, my team had space constraints that led to us driving the rear wheels and running chain to the others (This was on the old c-base KoP chassis, not that it makes much of a difference).

There are really two things to consider, I think:

1. When you drive the center wheels, you put a good chunk of weight in the middle of the robot. If you shift that weight to the rear, you could shift your center of gravity and increase the chances of tipping over. Obviously, there’s a lot more to the robot design that impacts that, so it’s not always an issue. In 2012, our robot could tip back 85 degrees and still fall forward onto its wheels, which was critical with the “bridges” that year.
2. You could be in a world of hurt if you break a chain. By driving the middle wheel, you ensure that, even with a broken wheel, your driven wheel is on the ground most of the time. With a drop center, the front/back wheels may not be on the ground the whole time (probably won’t be, if your center of gravity is managed well!). Our 2012 robot used #35 chain, which is beefier than #25 and less likely to break during competition (in fact, that robot still drives to this day as a demo robot, with no adjustments to the drive train since we competed with it!). I know many teams have used #25H chain on drive trains successfully… but personally, I’ve always preferred #35, and that’s all my team has used on drive trains, when we’ve used chain.

So by all means, make the switch… but make sure you manage the potential complications from doing so!