I am a newbie to Vex. Just putting that out there, so if this is dumb please understand.
My team has settled on building a 4 inch wheel drivetrain with 2 11 watt motors on each side and one 5.5 watt motor on each side, with a red cartridge motor on a 2:1 gear ratio for 400 RPM. Note that we will also be using a transmission to switch between a 2:1 ratio and a 1:2 ratio. From all that I have seen, all 400 rpm drives have been on the smaller wheels. Before we go through with finishing it, would the motor counts/gear ratios and wheel sizes actually work? Or do we need do switch to smaller wheels if we want 400 RPM?
Sorry quick edit I am a newbie to High School Vex. I have 7 years of vex iq.
It is my first year in VRC as well, so my advice may not be accurate, but 400 rpm on 4in wheels feels very fast for a drivetrain that’s only 55 watts. My team is doing 333 rpm on 3.25 inch with 6 motors. If you are prepared to deal with motors burning out regularly, go ahead, but I can’t say I recommend such a fast drivetrain for a game that requires a lot of push power
With drivetrains - in almost all cases - you want to avoid using red cartridges. They are slow, annoying to gear, and you lose a lot of power to friction. Greens are fine - though if you want to be a great team your first year I would suggest 4m or 6m drive with blue carts. If your team wants to be competitive I would highly suggest not doing a transmission at all unless you have a lot of prior knowledge with them. They are usually annoying to build correctly and control while driving under defense.
For gear ratios:
The way you calculate them is the driven gear over the gear being spun.
So… if you want a 400rpm drive on 4 inch wheels - you want a 48t gear driving a 72t gear.
48t/72t * 600rpm (type of cart) = 400rpm
(This only applies if you are using blue carts)
With the 2 11w and 5.5w you could do:
72t/36t * 200rpm = 400rpm
We had 6 motor drive last year with 400rpm 4 inch wheels…our robot drive motors overheated super fast: would not recommend
Correction, sorry! We are using Green cartridges.
400 rpm on 4" wheels is around the same linear speed as using 2.75" wheels with 600rpm direct drive, which is pretty much the upper limit teams have pushed for in terms of drive speeds. It really only works well with 66W on the drive, however 55W could work if the bot was lightweight enough (13lbs max IMO). but don’t expect to be playing too much defense. That being said, I wouldn’t recommend that speed at all for this year’s game. You aren’t travelling too far in any given straight movement, and because of all the objects and stops on the field you’ll be making, acceleration is favored over top speed. I recommend 300rpm on 4" wheels personally (2:3 200rpm). It has plenty of acceleration and all the speed you will ever need for this game. But don’t get me wrong, slower speeds can absolutely be viable if you see fit. The slowest I would go would be around 257rpm on 4" (7:3 600rpm).
Thanks, we just had a meeting and settled on 3:2 300 rpm. Thanks everyone for helping!