A question to teams that choose to use mecanum wheels, Why is it that they are commonly 6 and 8 inch wheels instead of 4in wheels?
It depends on clearance, gearing and overall speed of the robot. We are going 4" this year.
That is what I assumed. That said I have been seeing teams that are using massive mecanums this year.
An unfortunately large number of teams choose wheel size because of what’s laying around, price of components, and what wheels are available by various FRC vendors.
A unfortunately small number of teams do some hard questioning and calculations to decide the optimum wheel size for their role.
this is all anecdotal based off what I’ve seen at competition, no hard surveys or numbers to back it up.
On a flat field, is there any reason to not go as tiny as possible (supposing the wheels can handle the weight & wear)?
Assuming your drive system isn’t vulnerable to rolling over obstacles (gears, etc etc), small is generally advantageous.
However, let’s be sure keep in mind that acceleration is often just as valuable as top speed, so overall reduction and wheel sizes must be balanced with current draw, weight, etc.
Basically, anyone who’s interested should dig around here on whitepapers for how to best choose a drive system, and then go with Michael Corsetto’s advice, and build a WCD anyway. (unless you’re us, in which case you disregard logic and make a swerve because zomgswerve)
And most teams who are headed down the path of vectored intake wheel drive that do some hard questioning and calculations end up ruling out mecanum drive entirely.
Which is a very legitimate conclusion. There’s use cases for a [strike]Mecanum [/strike] Vectored Intake Wheel drive base, and there’s also legitimate arguments against them.
In my opinion, as long as good decision making was demonstrated, and everyone learned and was inspired, the competitive value is entirely secondary.
Because mecanum have a lower effective coefficient of friction (wheels will roll before sliding) you need to be really careful choosing gearbox ratios. You can get good FPS speed with small or large wheels, but the acceleration will change.
We went with 6" wheels to balance between “pushing power” and acceleration.
If we take a 7:1 gearbox ratio to 4" Mecanum wheels and change the setup to 14:1 with 8" Mecanum wheels, what about acceleration changes? Is there something about Mecanum that makes this is not a standard drive train problem?
Theoretically, nothing.
But the 14:1 gearbox will likely be less efficient, resulting in more losses.
Accelerating wasn’t the correct word. I should have said something like “ease of accelerating the wheel”. Bigger wheels have higher rotational inertia (not to mention regular inertia!), so takes more to get them spinning. Larger gear reduction will give you the higher torque needed.
But in the end, it’s convenience. We’ve used 6" before and had spares to test with. We were happy with the performance and stuck with them.