Hi everyone,
What is the optimal wheel diameter for a custom west coast drivetrain?
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That’s going to depend on the game, in recent years with <1” bumps there’s not much read to go with larger than 4” wheels. In 2016 you’d want much larger wheels. There is really nothing to it other than size of bumps and packaging of your bot.
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It depends on the game and your gear ratio.
Bigger wheels offer more ground clearance and (depending on gear ratio) more speed.
Smaller wheels can offer more torque (again, depending on gear ratio).
I point out the speed vs torque thing because some COTS gearboxes are limited on gearing options, so wheel size may have to be used to get a desired speed/torque of the drive system.
Most of the time you’ll see either 4" or 6" wheels used. 4" are fine for games with flat floors (where you don’t need ground clearance) and 6" are good for games with mild ground obstacles (might have to go as large as 8" for some games, though this would be rare).
On a somewhat related note, I remember a few years back meeting a rookie team that had modified a KOP chassis to use like 4x 12" bicycle wheels for some reason, but they neglected to account for the gear ratio of the stock KOP drive system and that, plus the lack of a drop-center, meant they were cooking CIM motors (they were only using 1 per gearbox initially) any time they attempted to turn.
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Depends on the field, if flat, as small as possible to ~3".
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Small as possible on a flat field is gonna do a few things (albeit I am mostly thinking about 2 inches or less, 3 inches is not small enough to really cause issues beyond gear ratios):
- Tread may wear out faster
- Grip levels may be a bit different than a larger wheel, this depends on a lot of variables including tread → lesser contact patch but greater pressure
- Packaging, bearing size as a ratio of wheel diameter makes clearances tighter with standard WCD 2x1 tube construction (wall thickness doesn’t scale proportionally, may have sprocket or pulley clearance issues, etc. Just takes more planning).
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It depends on the game, in particular the obstacles on the floor. This year we used 6" wheels to make the Charging Station easier. Previously we used 5" (Colson) wheels. But we’ve always designed/built our own gearboxes so we could make the drive ratio whatever we wanted. (Next year, we will be moving to 4" Swerve).
This seems slightly counterintuitive. I know that angular speed and torque are inversely related, but shouldn’t the greater radius of the bigger wheels result in more output torque?
I know smaller wheels require less to rotate.
If the same gearbox powering a 4" wheel is used to power a 6" wheel, the gearbox may not have enough reduction to comfortably turn the 6" wheel.
Maybe that’s what @cbale2000 wanted to say?
Once you decide what constraints the field and/or field elements are going to place on wheel size, and once you decide what length of “run” is going to be typical to play the game (for example, in Charged Up, from the Substation to the edge of the Charging Station), then I would encourage you to use a tool like the ILITE Drivetrain Simulator, v2020 to determine what various combinations of wheel sizes, motors, and gear ratios do to the trade off between top speed and acceleration. If you are building your own gearbox, then many more ratios will be available to you vs. buying a COTS gearbox, but the principles remain the same.
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Yeah, basically for a given gearbox reduction, increasing the wheel size will increase speed and decrease torque, just the same as going from a 50t gear to an 80t gear in a gearbox would (in the case where the gear is driven from the hub and is driving another gear, like how wheels interact with carpet, effectively).
Also your average FRC COTS gearbox doesn’t have enough gearing options to make a 6in wheel drive as slow or with as much torque as a 4in wheel can, even using the same gearbox with a different reduction option (unless your driving on planetaries for some reason) or vice-versa. There’s some overlap in the middle ranges, but at the extremes you have to change wheel size.
For instance, on a VEXpro Single Speed, Double Reduction Drivetrain, you can go as low as ~7.25ft/s and as high as ~18ft/s on a 4in wheel with the 3 different gearing options, but on a 6in wheel with those same gearing options you can do ~10.75ft/s on the low end and ~27ft/s on the high (though you will probably brown out in the later case). My point was simply to point out that wheel diameter is not always interchangeable depending on your gearbox and desired speed.
Yes on everything except that you are using the word torque when you mean force, which might be contributing to the confusion. The wheel size has no effect on the torque but it has everything to do with the torque arm and thus the applied force of the the wheel. Torque = Force * Torque Arm, or, Force = Torque / Wheel Radius. Everyone be careful mixing your linear and rotational units and terms, it can be very important to be accurate and precise when talking about physics and engineering.
Again, other than the mixed up terminology I agree with your explanation totally.
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Now this makes sense. Same torque, bigger radius, less force.
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