4" 200 rpm 1:1 chassis or 3.25" 600 rpm 5:3 (geared down)

The first chassis is pretty much classic, however, I’ve been seeing the second one popping up here and there (for example: 1961z). What r the pros and cons of each. Also, why use 3.25" wheels instead of 4" wheels.

the first one give 800’’ per min and the second one gives 1170’’ per min. Dont do excessive gearing to minimize the risk of gears breaking or just to minimize the building work

3.25" wheels are lighter.

Personally, we used 3.25" wheels because of the different ratios you can utilize with them. 3:5 600RPM on 3.25" wheels is roughly 360RPM on 3.25", which is comparable to ~280RPM on 4" wheels. This is a pretty excessive speed, so unless your driver is able to fully handle the robot at such speeds, I would not try such a high ratio.

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400 rpm on 2.75" wheels is around the equivalent of 270 rpm on 4" wheels, so I’d say from using it for the last little bit it’s not a bad ratio either.

There’s a few things that you need to consider when you make this decision.

  1. Can your driver handle the speed?
    This is super important. if you’re driver isn’t able to handle the extra speed, then the extra speed will be a bigger detriment than an advantage (you’ll spend more time correcting than you’ll save driving in a straight line). Don’t know if you can handle it? You’re probably good to stick with 200rpm on 4", it’s not too slow, not too fast, and has a good amonut of toruqe. Essentially, a great all-around starting place.
  2. How does your driver drive?
    Does your driver prefer to drive around defense, or drive through defense? Their driving style is going to determine which ratio you choose at the end of the day. Some drivers might be more comfortable with the way certain drive bases handle. Don’t know? Think a bit about how you want to play the game and take a shot anywhere between 200-250 rpm on a 4" wheel, or 200-300rpm on a 3.25" wheel.
  3. Are these your only ratios?
    With the different options available to you, there are a host of other gear ratios that you can consider. 15 - 12 chained is good, 18 - 15 chained is good, 7:5 is pretty nice, 3:7 on 600 is also really nice, 3:1 100, 5:3 200, 3:5 600, etc etc. Each of these is going to be slightly different in terms of drive speed, depending on which wheel you choose. The easiest way to compare is to take the ratio and turn it into distance/time - and then you can compare them against each other. More speed = less Torque.
  4. What’s the design requirement?
    Whether or not you have space for these ratios can also impact your choices. Chain might take too much space, or be easier to run. Big gears might have clearance issues, while small gears may not. Consider how the gearbox will be designed so that you can still service your motors in case one breaks, and where that puts you in terms of mounting options. Pick one that will integrate with the rest of your robot nicely. Consider some other things you might encounter (gears breaking, chain snapping, etc). And how each base would respond to that.

At the end of the day, if you’re unsure, starting with 4 200rpm motors on 4" wheels direct driven 1:1 is always going to be safe. This is the least likely to fail, is the easiest to build, and has a good amount of torque and speed to get around the field. After you guys do a little bit of driving and matchplay, your driver might decide that 200rpm is a good speed to keep as it gives them the pushing power they need, or they might realize that they’re driving at full stick all the time just to get around the field, that probably means it’s time to speed up the drive.