The best on wheels

Seeing how the way in which the robot moves is evolving in FIRST, what would you say is currently the best alternative for robot traction? I am thinking of implementing the mk4i or mk4 transmissions next year, are these the better currently?

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TL/DR: We love our MK4. We have a LIGHT bot and are running L4; scary fast, no wheel slip. Set your current limits!!!

Now I’m going to give you the long answer :slight_smile:

In terms of traction, the rough belting treads have the highest coefficient of friction in FRC, at the cost of wearing out fast and needing maintenance. And occasionally throwing/splitting the belting. The only things out there that can beat it (assuming identical robots) are outlawed: file cards engaging the carpet, and the inverse hovercraft creation that vacuumed itself down to the carpet.

A swerve robot can deliver equal torque to all the wheels and avoid slipping any of them, delivering the highest potential tractive effort for a given robot weight and power/torque availability. A major plus on swerve is that the wheels are commanded to make specific rotations, meaning that a slipping wheel does not get an opportunity to spin unless all four spin. A tank bot can be easily set up with 50% more drive power/torque than a swerve, BUT a tank bot is always going to have wheel slipping while turning, reducing the available tractive effort. If you run tread on a tank, you MUST make sure you can turn… If you have too much traction you can’t actually get the bot to turn :wink: You will also find that a brushless drive train almost always has more torque than you can actually deliver to the carpet! And a 6 NEO drive train will brown your robot out in a heartbeat if you don’t set your current limits!

Any wheel that has low friction but still carries robot weight reduces your potential tractive effort. The classic 6 wheel tank with front omnis pays a penalty in pushing ability. Dropping the center on this configuration means you can get the four back wheels all driving when you are pushing -forward-, but reverse has maybe half the pushing ability.


In current FRC we are traction limited, unless you can get more traction (more contact patch on the ground with swerve (this is doable)) you are not going to extract much more out of the motors past matching your peak acceleration to the peak power with gear ratios and playing with current limiting - as @Weldingrod1 said.

Diff swerve is pointless until we throw more contact patch on the ground, simple as that.

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BTW, what @Skyehawk is getting at is that with the rough, spiky tread we aren’t precisely in the “coefficient of friction” realm. Its closer to bending strength… So, larger contact patch CAN equal= more traction. If someone dreamed up a flat bottom tank tread that fit into a swerve you could build something with nutty tractive effort. Assuming you could get your steering motor to -turn- the darn thing! Its very easy to make a tank tread machine that can’t turn…

For non-engaging tread materials (like Coulsons) the traction is NOT dependent on contact patch area. Just the force transferred between the wheel and the carpet and the coefficient of friction of the wheel.


how light is your bot? ours was around 90lb AUW and we wanted to know if we should upgrade (from L2) to L3 or L4

We were 86 pounds with bumpers n battery.
L4 is crazy fast. Drive circles around other bots fast.
ONLY suitable for a very low CG bot!!!
L2 is also darn fast with a light bot.

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Alright. I’m not sure how the rest of the team feels about having a light bot next year. (light=small size=hard to work with)

sadly the l4 gears don’t work with the mk4i modules.

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