i would prefer 5 inch wheels, but settle for 6 inch wheels for several reasons. most of the omni wheels from andymark smaller than 6 inches are very bumpy, also 4 inch mechanums don’t exist for FRC. i think 6 inch wheels are the most versatile, and that the tiny benefit of going to 4 inch wheels is not worth the flexibility trade-off.
one reason is nonadrive or octomanium or any other number of names for the same basic mechanical idea, the drives usually use 6 inch mechs and 6 inch traction wheels. i prefer to use a very standard, modular design strategy, with multiple options. there are many more options for 6 inch wheels than any other wheel set. closon, andymark, pneumatic, good omni wheels, mechanum. with 4 inch wheels many of these types of wheels must be custom made. the 6 inch wheel size is the most versatile. and while it is true that some sacrifice is made in order to gain this flexibility, it is such a small advantage that it doesn’t really impact performance too much.
I already designed a frame that will accommodate any drive-train. one of the sacrifices of that frame, was giving up 4 inch wheels of any type.
The 4" dualie omnis on 2175’s robot this year gave a smoother ride than 6" omnis we have used in the past IMO. Admittedly they are a bit heavier and are more expensive than a single 6" omni, but the weight and cost both typically are gained back by knocking out a stage of reduction.
Wouldn’t this vary somewhat significantly based on the material of the wheel as well as any lightening? I’m sure we could calculate worst case scenarios (solid wheel of aluminum billet) but I’m not sure it would actually be terribly useful. Perhaps assign real materials to the AM plaction/performance wheels and use your CAD program of choice to computer the moments?
Roughly speaking, it will increase with the square of the radius. (I say roughly, because the different sizes aren’t scaled versions of each other; some things, like hub size and rim thickness, tend to be driven to a large degree by other design constraints.)
The major reasons have been stated, but the biggest for me is the redundancy in gear ratio of having a big wheel. Gearing down then using a big wheel is redundant. The wheel size is part of your gear ratio. If you have a 2 stage reduction gearing down 12:1 with an 8 inch wheel, you could achieve the same ratio with one stage 6:1 and a 4 inch wheel. The weight savings add up, lighter wheel, less reduction, therefore fewer gears etc…You will also get better acceleration (relative to the gear ratio) with a smaller wheel because, in a general comparison, smaller wheels have a smaller moment of inertia.
(Big wheels do have some advantages in tackling rough terrain, but we don’t see that too much in FIRST).
Worst case scenario, solid wheel made of 6061 Aluminum 1 inch thick. (I picked a type of Aluminum)
D (inches) Moment (in^2*lb)
I used (m*r^2)2 for the moment of inertia.
Once you start lightening it could get different since the amount of material you have to leave on the edge of the wheel doesn’t scale with size so we would start treating it as a tick walled tube + however many support spokes you have. This starts getting a little more complicated but it is still doable. I am attaching an excel spreadsheet that (unless I did something stupid) should compute a rough estimate of the rim and spoke style wheels.
Edit: I had originally forgotten the width of the spoke so I was finding the area instead of volume, this has been fixed.