Understanding WCP Drive Train Calculator

[Chris is me]

Originally Posted by Oblarg

The thing is, in my experience, your 'case 2' is by far the most common.

Even if your robot can move, you can (and will) still slip the wheels if your motor torque is high enough to break static friction, unless you are being very careful with the throttle. .

If it's case 2, it doesn't matter, you're not pushing them even if your wheels didn't slip.

If your robot can move the load reasonably well (and low gear isn't unreasonably low), then no, the wheels aren't slipping - the load is moving instead. Except for the edge case where you are just barely able to move the load you are trying to push, the wheels apply force, and the robot+load start to move before the wheel slips. The motor isn't stalling anymore, it's moving, the torque is being "used up" to make the robot move.

[Oblarg]

Originally Posted by Chris is me

If it's case 2, it doesn't matter, you're not pushing them even if your wheels didn't slip.

If your robot can move the load reasonably well (and low gear isn't unreasonably low), then no, the wheels aren't slipping - the load is moving instead. Except for the edge case where you are just barely able to move the load you are trying to push, the wheels apply force, and the robot+load start to move before the wheel slips. The motor isn't stalling anymore, it's moving, the torque is being "used up" to make the robot move.

That's not how it works. If the torque output is high enough to break static friction, the robot moves *and* the wheels slip. Your effective acceleration will be limited by the dynamic COF of the wheel. This is the same reason we have antilock braking systems in cars.

You can get a robot and test this, if you like.

[Chris is me]

Originally Posted by Oblarg

That's not how it works. If the torque output is high enough to break static friction, the robot moves *and* the wheels slip.

You can get a robot and test this, if you like.

It depends on what current load is required to break traction. Almost every robot can break traction at some current below stall, but wheel slip is negligible when accelerating from a full stop across a range of gear ratios. (It's probably instantaneously slipping, but not really for long enough to matter) If you're traction limited at, e.g., 60% of stall or ~70 amps per CIM (lots of faster single speeds), you'll really never slip in a pushing match that you're winning. If you're traction limited at 15% of stall or ~20 amps per CIM, you're probably burning out every time you gun the throttle. Most low gears in FRC seem to aim for traction limited at anywhere from 40 to 55 amps per CIM, and I just haven't observed slipping in pushing matches the robot handily wins. The load on the wheels is relieved to some extent when the load starts moving / slipping, and momentum helps too.

228's robot this year was 4 cim / 2 mini, geared to like 7.5 FPS low gear, and tread-cut Colson wheels. As long as the drive didn't ride up on the pushing target (grumble grumble), it would push things like AM wheel robots without the Colsons slipping noticeably. And that's traction limited at like, 35 amps per CIM? maybe 40? I can't remember off the top of my head, but less than I expected.

[Oblarg]

Originally Posted by Chris is me

If you're traction limited at, e.g., 60% of stall or ~70 amps per CIM (lots of faster single speeds), you'll really never slip in a pushing match that you're winning.

This isn't what I've experienced. Now, I will admit that my teams rarely use super-high-traction wheels (AndyMark HiGrip and unmodded Colsons, generally), so that could be a contributing factor.

Mathematically, at full throttle you'll slip the wheels at any speed below where the motor's torque value exceeds the torque generated by the static friction of the wheel.

If you're traction-limited at 60% of stall, this means any speed below 40% of max (assuming a linear torque curve). This is probably not quite right, because of voltage drop and other issues, but I see a *lot* of pushing matches where the movement in either direction is clearly a lot slower than that.