Part II — V5RC Advanced · Chapter 15
Advanced Drivetrains
Wheel size, gear ratio, and wheel placement. Nail these before anything else.
Three choices define every drivetrain: wheel size, gear ratio, and wheel placement. Nail these before anything else on the robot.
Wheel size sets your speed ceiling. Smaller wheels (2.75") spin faster for the same motor RPM. Larger wheels (4") cover more ground per rotation but need more torque to turn. Pick your target speed first, then let wheel size follow.
Gear ratio is where you actually tune speed vs. torque. Gear up (small driving gear, big driven gear) and you trade speed for torque. Gear down and you get the opposite. A well-known VEX Forum resource catalogs dozens of tested 6-motor gear layouts across wheel sizes, and its general rule of thumb is that drivetrain speeds under roughly 62 inches per second stay controllable on almost any build, while anything past 70 inches per second demands excellent driving and a very stiff robot to stay usable.
Wheelbase length trades stability for turning speed. Short drivetrains turn fast and fit tight spaces but skid and tip more under load. Long drivetrains push harder and track straighter but turn slower and cost more gearing length.
Traction wheel placement sets your turning point, not your speed. Center a traction pair on an omni base and the robot pivots cleanly around that line. Full traction adds pushing power but fights every turn.
6-motor tank drive, 450 RPM, 8 wheels, 6 omni, 2 traction centered. 450 gave us the speed our field strategy needed, the centered traction pair gave us pushing power at the goal without killing our turning radius.
6-motor drive, 320 RPM, 4" wheels, all 4 wheels omni. This season has a barrier obstacle on the field, so we geared down for torque instead of top speed, full omni kept us mobile crossing it instead of fighting for traction on the ramp.
One-line takeaway
Pick your speed range first, gear ratio and wheel size follow from that, not the reverse.
Try the gear ratio calculator
Model wheel size, gear ratio, and motor count together and see the tradeoff.
Try it yourself