So I’m wondering - how do top teams choose certain wheel sizes?
*Specifically for Omnis
My team last year used 4” Omnis, but after research and talking to other people, they all said that smaller wheels are more meta. We even noticed most top teams using smaller wheels (2.75 or 3.25).
Currently for this year, we are planning to use 3.25” Omni Wheels at 360 RPM (is this smart or not considering Spin Up?), but I want to know for general knowledge purposes -
What are the pros and cons of 2.75”, 3.25”, and 4” Omni wheels, and why do top team generally use smaller wheels.
Do 4” wheels have more drift? 2.75" + 3.25" wheels definitely have less speed than a 4” at the same gear ratio. What are the other differences?
360 on 3.25 is a perfect ratio imo. Our team uses 2.75 omnis because they are the most compact which allows us to mount our expansion mechs right above our drive, but that is just us.
There isn’t too much benefit to using any omni wheel over the other aside from the (now old) 4” and 2.75” having more traction than the 3.25s (traction rarely comes into play however) the reason to use different size wheels is for spacing or sizing or other build related reasons, and/or to achieve a linear speed that suits your robot and/or your strategy and/or your driver’s preference.
In my opinion and experience, the most well rounded drive setup for a 4 motor drive is 257rpm on 4” wheels (36t driving 84t driven with a 600rpm cartridge). For a 6 motor drive, the most well rounded is 360rpm on 3.25” wheels (36t driving 60t driven with 600rpm cartridge).
But other common ratios include:
-300rpm 4” (600 1:2)
-360rpm 2.75” (600 3:5)
-400rpm 3.25” (600 2:3)
-333rpm 3.25” (200 5:3)
-600rpm 2.75” (600 1:1)
It’s up to you to decide what’s best for your robot (weight and how many motors are on your drive), strategy (are you planning on playing defensively?) and driving style. (are you comfortable with it?)
Alright. So are you saying there are no major advantages or disadvantages of using certain sized wheels? Not anything like drift or anything like that for larger wheels?
It’s really up to preference and design choices. No drive layout is objectively best.
my personal favorite is 400 rpm on 3.25" wheels, both because I like 3.25" wheels (which is somewhat irrelevant now that the new 2.75 and 4" wheels are thinner and have mounting holes), and because the gearing on 400 is super nice and I like me some speed.
As you can see, with 400 rpm gearing I was able to use the minimum possible number of gears, eliminating all idlers. And using a sort of 36:36 compound to compact the 3rd motor on each side. Would not recommend going this fast with only 4 motors, but with 6 it was perfect.
of course, this was just the layout I enjoyed the most, and other people will tell you different things. Just go with whatever seems the most optimal to your particular design.
I slapped a single tracker in the middle, and used an inertial sensor for heading. You can make odometry with just one tracking wheel, but any amount of sideways drift will throw it off, and it wasn’t very accurate. Luckily this was for tipping point where all you needed to do in autonomous was move forward then backwards, and I didn’t even end up using the odometry for any sort of movement algorithms.
Also odometry is not an essential part of a robot. For the vast majority of teams, lack of sensor data is not the limiting factor in how good their autonomous can be. Using the built-in motor encoders is good enough in most cases.