Comparing Tank Drivetrains

Key Features to Remember

  • All wheels must be powered for full pushing potential
  • Using lower traction wheels to ease turning will reduce pushing potential
  • Long wheel bases are difficult to turn

Physical Features

  • All tank drivetrains have
    • Pairs of wheels
    • Left & Right wheels driven independently
    • All motors used to propel robot
  • Key variances
    • Number of speeds
    • Type(s) of wheels
    • Number of wheels
    • 2, 4, 6, 8, or more

2 Wheel Drive (2WD)

  • 2 wheels are driven
  • Unless it is a segway (self-balancing), other methods of supporting the drivetrain are required
    • Casters
    • Slide blocks
    • Low traction, un-driven wheels
  • Advantages
    • Turning is relatively resistance free (no scrub)
  • Disadvantages
    • Since there is no scrub
      • It is difficult to control
      • Easily defended
    • Weight is distributed to supports that do not provide propulsion
      • Lower pushing force
    • Depending on setup, may not turn on centre (may be advantage)

4 Wheel Drive (4WD)

  • Long wheel base
  • Requires one set of lower traction wheels and/or skewed CoG in order to turn smoothly
  • Advantages
    • ???
  • Disadvantages
    • If lower traction wheels are used to aid turning
      • Lower pushing force
      • Will not turn on centre (may be advantage)
    • If CoG is skewed to aid turning
      • Higher potential for tipping
      • Will not turn on centre (may be advantage)

      6 Wheel Drive (6WD)

      • Long wheel base split in two short wheel bases
      • Middle set of wheels lowered
      • Advantages
        • Great turning with high traction wheels (assuming sufficient drop for surface/wheel and a centred CoG)
        • Full pushing force potential
      • Disadvantages
        • Not great at traversing some types of obstacles
          • Could be said for any drivetrain, very dependant on specific situation
        • Extra set of wheels adds weight

      8 Wheel Drive (8WD) and beyond

      • Advantages/disadvantages are specific to game
      • Advantages generally involve ability to traverse obstacles

      Standard Kit of Parts Drivetrain

      • 2011 KOP Drivetrain
      • 6WD with dropped center wheel
      • 2 out of 6 wheels are driven
      • Uses 8" FIRST Kit of Parts Wheels
      • Outer wheel holes spaced perfectly for 3/8" chain
        • No tensioners required
      • Has holes for 8WD if required
      • Uses CIMple boxes
        • Geared to drive very fast (>18 ft/s)
        • Gearbox sprocket = 12T
        • Wheel sprocket = 26T
      • Overall, very good drivetrain

      Kitbot on Steriods

      • Chain all 6 wheels together
        • Full pushing potential
      • Replace 8" KoP wheels with 6" AndyMark or VEX Pro traction wheels
        • Higher traction
        • Smaller diameter
          • Requires less sprocket reduction
          • Lower CoG
      • Add base plate for rigidity
      • Change gear ratio to something a bit more manageable
        • Wheel sprocket
          • 36T → 9.9 ft/s
          • 32T → 11.1 ft/s
          • 30T → 11.9 ft/s
          • 28T → 12.7 ft/s
          • 26T → 13.7 ft/s
      • Other potential upgrades
        • Replace CIMple Boxes with 2 speed gearbox
          • Will need to recalculate wheel and gearbox sprockets