My team is thinking of a rebuild before worlds, but we don’t know what size of wheels we should go with. Does anyone have tips on what size is the best for this challenge? We are between the 2.75” and the 3.25” on our decision.
there’s a lot more to consider apart from wheels size when designing a chassis.
All size wheels can work well, but the type of gear ratios that will work for these wheels varies. You also want to consider clearance with the ground, wheels grip, the simplicity of the gearing, motor and wheel placement.
I’ve been a fan of 3.25" wheels for a while now because of their slimmer profile and convenient mounting holes, but they do have less traction when compared to 2.75" wheels, not a problem for driving around on the grippy tiles but it can be an issue when trying to climb the platform, especially if the center of mass on your bot isn’t that great. Using traction wheels in the middle of the drive can solve this downside of 3.25" wheels, and it has the added effect of resisting pushing from the side, and eliminating drift, which may or may not be desirable for you. Good gear ratios for 3.25" wheels are 333 rpm (5:3 on 200), 360 rpm (3:5 on 600) and potentially 400 rpm (4:6 or 2:3 on 600) using the new gears. I don’t know if the 72t gears pose any clearance issues in size compared to 3.25" wheels yet, it looks very close but probably ok.
Gearing 333 or 360 on 3.25" can be a bit difficult with 3 wheels, since you have to add a considerable amount of gears to make the wheels spaced apart a good amount. Some people have solved this by not spacing the wheels far enough, but then adding an idler wheel in the front or back to prevent tipping issues, but I’m not a fan of this solution because if the robot puts a lot of weight on the idler wheels, it will lose traction with the powered wheels, which might happen when fighting for goals or climbing.
On the other hand, 2.75" wheels are smaller than 3.25" wheels, which can be both good and bad. good because more compact mechanisms are usually desired, but bad because it can limit the clearance with the ground as far as mounting bracing goes. It also limits the size of gears you can use because the gears have to be smaller than the wheels if they’re inline. I’ve heard that 60t gears are very sketchy when used on 2.75" wheels and are not recommended, because of the close size. But 2.75" do have superior traction, and you can get some nice gear ratios with them as well.
you could potentially get 400 rpm (4:2 on 200), 450 rpm (4:3 on 200), and possibly 600 (direct 600), but the latter is really pushing the limits of what is possible with v5 motors. I have seen some teams successfully using this ratio on 6 motors, but I personally wouldn’t try it. If you aren’t making a special robot with 8m drive, I wouldn’t recommend this ratio.
We have considered all the gear ratios that work with each size of wheel, and we pretty much know which one we’re going to use. We’ve just seen many of the teams we go against using smaller wheels while we still have the 4” ones. The only reason we are going with smaller wheels, however is just for the space to mount more things, because we were winning the race to the middle goals and tug of wars every time we had one.