Drivetrains
We (usually) run swerve drive. Each wheel module independently steers and drives, which gives the robot full omnidirectional movement (drive in any direction while facing any other direction). This is the dominant competitive drivetrain in modern FRC and we don't see a reason to change that unless a game specifically punishes it.
The drivetrain is the first thing you build and the last thing you want to fail. It gets built in week 1, handed to software and drivers immediately, and everything else mounts on top of it.
SDS MK5i
We run MK5i modules from Swerve Drive Specialties. These mount directly to standard 2x1 aluminum tube frame rails with 10-32 clearance holes on 0.5" pitch.
Why the MK5i:
Fully gear-driven steering (no belt in the steering path, which eliminates a failure point that existed on the MK4i)
Enclosed gearbox housing keeps carpet fuzz, field tape, and other debris out of the gears
Quick-change drive ratio by swapping only the motor pinion gear, without disassembling the module internally
Sealed main steering bearing
No 3D printed parts anywhere in the module
Same footprint as the MK4i, so teams upgrading from MK4i don't need to redesign their frame
Our motor configuration: Kraken X60 for drive, Kraken X44 for steering. This fits naturally with our CTRE ecosystem.
Weight: About 4.1 lbs bare, about 6.1 lbs ready to run (with motors and encoder).
Gear ratios
The MK5i ships with all three drive pinion gears (R1, R2, R3), so you can switch ratios by swapping just the pinion on the drive motor. No internal disassembly required.
R1
Slowest, highest pushing force. Use for defensive robots or very heavy builds where controllability is more important than speed.
R2
The default. Good balance of speed and control for a full-weight competition robot. Start here unless you have a reason not to.
R3
Fastest, lowest pushing force. Only recommended for lightweight robots in games that heavily reward fast cycling. Harder to control.
For most seasons, R2 is the right choice. Verify with ReCalc using your estimated robot weight to confirm the speed and current draw are reasonable before committing.
Wheels
The MK5i comes with two wheel options. Both are wider than the MK4i wheel for improved traction and tread life.
Molded Spike Grip
Billet with Neoprene
If you're not sure, go neoprene. It lasts longer and the grip difference is not dramatic for most games. Whichever you run, bring spare wheels to competition because tread wears down over an event.
Bellypan
The bellypan is the flat plate that spans the bottom of the drivetrain frame. It serves two purposes: structural stiffness (prevents the frame from racking when the robot gets hit) and a mounting surface for electronics.
Material: 1/8" aluminum sheet. Light, stiff enough for the job, and provides a conductive ground plane for electronics.
Mounting: Bolt the bellypan to the bottom of the frame rails rather than riveting it. This lets you drop the bellypan for electronics access without disassembling the frame. Use #10-32 bolts with nylock nuts.
Plan the electronics layout with the electrical team before cutting the bellypan. You need to decide where these go before you start drilling holes:
RoboRIO
Power Distribution Hub (PDH)
Motor controllers
Radio
Battery (needs to be easily swappable between matches)
Pneumatic Hub (if applicable)
Leave enough clearance between components for wiring, and plan cable paths so wires route cleanly between components without crossing over moving mechanisms above.
Leave access to both sides of the bellypan. You'll need to reach electronics on top and fastener heads underneath.
Consider adding cutouts or access holes for areas where you might need to reach through the bellypan to adjust something below.
If the bellypan is large, add lightening pockets to save weight. The bellypan doesn't need to be solid aluminum everywhere since a lot of its area is just open space between electronics.
Etch component outlines or labels onto the bellypan using your laser cutter so the electrical team knows exactly where everything goes during assembly.
The MK4i (what we had before)
The MK4i is the previous generation SDS module that's still in our lab and still on many competitive robots. It uses a belt-driven steering system (150/7:1 ratio) instead of the MK5i's fully gear-driven steering. It has L1, L2, and L3 gear ratios, with L2 being the standard choice for most teams.
If you're working on or referencing a robot that uses MK4i modules, the main things to know are that the steering belt can skip or break under extreme conditions (which is why the MK5i switched to gears), and that changing the drive ratio on the MK4i requires more disassembly than on the MK5i. Otherwise, the mounting pattern and basic operation are very similar.
Tank drive (WCP 6-wheel drop center)
If swerve isn't an option for a given season (budget, software readiness, or a specific game reason), a WCP 6-wheel drop-center tank drive is a solid alternative. The center wheel is dropped about 1/8" lower than the corner wheels so the robot pivots on the center pair, which makes turning easier.
Tank drive can't strafe, so it's slower at aligning to targets and less maneuverable overall. However, it's mechanically simpler, cheaper, and has fewer software requirements. For a team running swerve for the first time, building a tank drive as a backup drivetrain during week 1 while the software team gets swerve code working is a reasonable risk mitigation strategy.
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