Often on West Coast Drives I see four traction wheels on the back and then the front two wheels are omni wheels. How would this help the drive train if it can’t take advantage of the omnidirectional capabilities of the wheels if the back four wheels are traction, and what is the point of it? Thanks!
Reducing turning scrub and adjusting the center-of-rotation of the robot. In some cases it may also allow for the elimination of the need to run a center drop and assist with getting out of “friction pins.”
^^ What Sean said.
Amplifying: How much scrub force you want on your robot is often a balancing act - you want enough to stay on course and not be too easily spun by opponents, not so much that you can’t turn, and not so much that you can be easily pinned from the side. I’ve seen teams do omnis on front, omnis on rear, and omnis on opposite corners to reduce scrub without eliminating it.
Another compromise tactic I’ve seen (and used, in 2016) is to use pebbletop tread. On carpet it acts something like a paddle wheel or rear tractor tire, and has a higher effective CoF in the direction of wheel rotation than parallel to the axle.
Hi Will, good questions! In this scenario, you’re spot on that the design objective doesn’t include omnidirectional motion. However, the omni wheels are useful to significantly reduce the “sideways” tractive force that would oppose the robot’s rotation as it tries to turn. It is used as an alternative (or in addition) to the drop-center wheel feature seen on many all-traction wheel west coast drives, or even on the standard kitbot chassis.
By using omni wheels instead of the drop center, it can make the robot seem more stable, as it doesn’t rock between the front or rear wheel bases. It can also allow the robot’s center of rotation to be manipulated to suit a team’s desired handling. You could put omnis in just the front, just the back, all four corners, etc. Team 148’s 2018 robot is an example of this, where they had variable wheel spacing and omnis strategically located to tune in their bots handling.
Another possible advantage is that having omni wheels at all 4 corners and traction wheels in the center could mean that the wheels no longer need to “skid” for the robot to turn (as they would in a traditional skid-steer drive with all traction wheels). This could be considered advantageous for sensory feedback on distance based on wheel rotations. Team 254’s 2018 robot featured this style of drive train.
As mentioned before, it all depends on preference in what you want in terms of how you want it to drive and how it interacts with other robots on the field when in contact with them.
For example, 2655’s robot this year was Bonus level. Four Coulsons in the the front and two omnis in the back. Not only was it quick and maneuverable it had quite a bit of pushing power. This also depends on gearbox ratios but that would be straying from the point. West Coast is very solid and customizable, and it’s a drivetrain that I would easily recommend.
148’s 2018 Uppercut drive wheels were not supposed to be like that. Constant iterations as JVN always says on his blog led to the ultimate 6WD with 3 different sets of wheels.
this link shows 148’s 2017 Rogue with 4 sets of wheels with the front 2 as omnis. https://youtu.be/2zu1EzyKRRE?t=91
whereas https://youtu.be/2zu1EzyKRRE?t=24 shows Rogue having 4 sets of the same wheels.