I am wondering about the disadvantages to using 3.25 inch omni wheels, personally I haven’t used them (never have had access to them) but as I am going in to high school next year I should be able to use them on my bot.
The only difference I know of is the weight distribution and the traction difference.
Also, just thinking about it, wouldn’t 4 inch wheels be better due to them being able to achieve the same speed as a 3.25 inch wheels at lower RPM’s (which in turn gives you more torque). I could be wrong, but I would like to know before I start 3d modeling my early season bot.
Most teams would rather use a higher gearing 3.25 in wheel drivetrain than a 4 in one because the 4 in wheels slip on axles more than 3.25 in ones and you also can’t do a screw joint drivetrain or screw the wheels directly into gears with 4 in wheels
No. Torque doesn’t work like that. Yes, your motors would spin at a lower rpm, (excuse my lack of mechanical terms, but here’s my very un-knowledgable understanding) but they are spinning the same load over a longer distance per rotation as demonstrated below:
4pi inches * load (x) per rotation versus 3.25pi inches * load (x) per rotation
comes out to:
4pi*(x) > 3.25pi*(x)
(Meaning the distance load [aka torque needed] is higher per rotation on 4in wheels)
So yes your motors would have a higher torque, but you would need more torque to move your robot, and the effect cancels out.
Personally I didn’t think there was much of a meaningful difference between 3.25 and 4 in omnis. Aside from center of gravity, the only negligible difference would be a little bit of traction.
Not quite. The larger wheel radius lowers the torque, so it’s the same. If I were you, I would look at the catalogue of drive gearings and pick one based on the speed you want.
I will probably use smaller wheels this year so I can have a lower body. I’m probably going for 450 RPM on 2.75 inch wheels. Fast, but not too fast.
3.25" wheels package easier and lower your center of gravity (center of the wheels is .375" closers to the ground) it also means you can get 4 wheels, if that’s a a goal for you.
As to your speed and torque thing, here’s some math:
Power = force * speed
Speed = angular velocity (w) * radius
So power = force * w * radius. And power is the same between these two scenarios.
So with the same rpm (angular velocity), 3.25" give a lower radius, and there for a higher force.