Tribot swerve drivetrain

My friend and I are working on a 3-wheel swerve drivebase, inspired by 3-wheel platforms from 1425/1640/16 pre-2011. The drivebase isn’t intended for competitive use.

This is our first time developing a swerve drivetrain. Is it significantly more difficult to implement 2CIMs/swerve module (6CIMs total) vs. 1CIM/swerve module (3CIMs total)?

If you get over the initial swerve hurdle, adding an extra motor might not be as hard but you should examine if essentially doubling drive base weight is worth the extra torque. Don’t mean to discourage the effort, just something to think about if your team intends to use the design during build season.

I would guess that it would be fairly simple to add another motor (disclaimer I have never designed a swerve module). But I am not sure why you would want that extra torque. The way I have always veiwed the swerve driving style is to drive fast, and evade defense.

You prolly don’t need it, but it’s easy to add another.
EDIT: I was a little rushed, let me go in a little more depth.
If you are doing a coaxial swerve drive, then it’s easy to add another cim. If you are using chains to the pivot shaft, then wrap the chain around both cims. Same for belts. Just make sure you have enough wrap around the pulley/sprocket that the teeth don’t skip.
If your cims are turning with your module, it’s still not too complex. You just end up with a much larger module, and as a result I don’t recommend going 2 cims if you do that.
Doing a cim-in-wheel swerve will be nigh on impossible to implement well with 2 cims.
Another choice could be using 2 minicims per module. That gives you more power than 1 cim/module, but not so much as to easily cause brownouts.

I have not designed or built a gearbox, but I have been following a number of the threads for the past year or so, and reviewed some old ones. There are basically three classes of swerve drives as far as I have noticed.

  • Drive motor below the pivot - this means that you have to limit the steering angle or use slip rings. An example is CIM-in-swerve. Here, you would be adding a bunch more weight and probably volume to the rotating module. Poor choice for dual-CIMming.
  • Axial, with both drive motor and at least one reduction stage above pivot: this should be the easiest to implement a second CIM - just find a place where its pinion can engage the same gear that the other CIM’s pinion engages, and provide the necessary support.
  • Axial, with drive motor above pivot, but all reduction gearing below pivot: not easily viable for a dual-CIM swerve.

And good luck finding a slip ring that is not terribly priced and weighs as much as a midsize car (hyperbole) that has 4 wires that can handle that current.

Why add torque?

Well… if you expect to travel short distances (when compared to how far you must travel before reaching top speed), more often than you expect to travel long distances, you might want to accelerate as fast as possible.

In the extreme, in a 1/4 mile drag race, a device that can reach a 200 MPH top speed after accelerating (linearly) for 5 miles, is unlikely to beat a device that reaches a 50 MPH top speed after accelerating (linearly) for 1/8 mile.

Is a VRC/FRC/FTC match (or demo) more like a series of 1/4 mile drag races, or more like the Bonneville salt flats? And during the match, which movements (short or long) need to be completed more quickly in order to score/prevent the most points?

Strategy(s) --> Requirements --> Designs --> Build --> Test --> Decide


You might want to look into 148’s robot from 2008

Not always, and maybe not even mostly. A swerve or crab drive can apply all of the friction force of all of its wheels in any direction, making it great at pushing. The triangle base reduces stability in pushing matches, but not too drastically. The archetypical tri-wheel crab robot, Tumbleweed (148 Robowranglers playing 2008 FIRST Overdrive), spent most of the matches I saw (on video; before my time in FRC) getting in the way. With nine bumpers, they could push within 10 degrees of whatever direction they wanted. Also, because they could then go sideways to allow a pass, it let them push for all but a fraction of a second of the duration allowed under the obstruction rules.

You want to make sure that the COM is both low and near the middle, this is true for all swerve drives, but especially true for a tri-module design. This would help prevent tipping when hardstoping or quickly changing direction.

In tumbleweed’s (148 2008 robot) reveal, you can see that it easily totters while turning, however, this is mainly because of the small footprint.