Why do we drop wheels?

Why is it done? Why do we drop the center wheel on drivetrains? Why not just drive with all 6 wheels touching the ground? So many teams do a center drop of 1/8 or 1/16 and I can’t seem to fathom why.

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When you turn about your center, your wheels are dragging sideways across the carpet. Naturally, there is a friction force opposing this called turning scrub. With a long (front to back) wheel setup, there is more sideways movement, thus more turning scrub. Likewise, having a wide setup reduces turning scrub. Using a drop center makes it so only 4 wheels are touching the ground at any given point in most cases, thereby creating the desired wide setup to make turning scrub as small as possible.

Keep in mind the omni wheels, because they have side rollers, have effectively no turning scrub, which is why you will sometimes see teams opt for corner omni wheels instead of a drop center.


So assuming I use corner Omni’s there is no need really for a center drop?

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In terms of wheels skidding, yes.
Another attribute of center drop is if your weight shifts then the 4 out of 6 wheels you are are sitting on will change and this will change where your center of rotation is. You might want it in a specific place or not.
Choosing center drop or not, or Omni’s or not affects where your center of rotation is.

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This is part of the reason why 8 wheel drivetrains, while heavier and slightly more complex, are often preferred when possible. Rocking and changing your center of rotation is a nightmare with almost any mechanism, especially a tall elevator.

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No, there isn’t, but keep in mind using omnis means you have less side to side friction, so it’ll be easier to get pushed from the side.

I wouldn’t go as far as to say it’s a nightmare, or even an inconvenience. I believe with most games the rock/drop is negligible to performance provided it isn’t a huge drop.

Fair enough, but there are specific situations where things get a bit too hairy to mess with turning center and rock, like carefully placing a 3rd level cube in 2018 without knocking more off, or being defended/bodyslammed while placing on level 3 of the rocket. These are certainly edge cases, but finding out about them mid or late season is never fun.

That’s why you build a turret, so you never have to worry about this stuff :stuck_out_tongue:


Dont need to worry about CoG if the turret never moves


That is an option, but it makes getting turned by defense very easy

“Nightmare” is a huge overstatement. In most cases, it is at worst a minor annoyance. More configurable turning scrub and more precisely repeatable autonomy is the main argument I see for 8WDs - but it’s really preference.

Andrew’s right, it is simple geometry. A 1/8" drop is essentially a max deflection of 1/4" at one end of the chassis. Very, very rare that 1/4" is the tolerance between success and failure in scoring. You need a long arm hanging out of the chassis AND a very precise scoring requirement - 2018 is the only game that really comes close in recent memory.

And that 1/4" deflection assumes a perfectly rigid floor and perfectly rigid robot frame - neither of which are the real world. Wheels sink into carpet, to the point where in many cases all six wheels end up touching anyway - just with the dropped middle wheel supporting more normal force.

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If/when you do a drop center, make sure your CoG for the entire bot ends up either forward or behind the center axle but pretty much never directly over it.

Having the rest position of the drivebase be predictable for your driver/programmers is critical.


I know. This is why I would much rather drive with Omni’s instead of 6 with a drop.

We did 8WD in 2018 and had a lot of turning scrub. We ended up just removing two wheels and adding omnis for our offseason events.
This year we ran 6WD with corner omnis, and a small center drop. We had slightly more rocking that I would have liked. Next year we might do 6WD with a 1/32" or 1/16" center drop to ensure the center wheel always gets traction.

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Personally I’d rather do 6wd. Or rn swerve or bust :yum:


Oof. Our 2018 bot was pretty evenly balanced, over middle dropped wheel… Was that why we sucked along with other mechanical fails?

Were there robots with 8WD in 2018?

Yes. i remember a couple from PNW DCMP/Worlds, 1595 and 2147 being ones i could find offhand.

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I don’t think we had 8 wheel in 2018, we had 8 wheel in 2019. (I think)

oh shoot, i totally missed the 2018 portion. updated post with correct teams.