crab drive vs. mecanum drive system

[Chris is me]

Originally Posted by Passion

ohhhhhh, but if the wheels were placed at the corners, then the tradeoff would 100 % efficient going diagonally, correct?

Well, no.

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|_x frame of reference

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If you go in an X or Y direction, you're only getting one component of each motor's force. If you go diagonally (parallel to a wheel), you get only 2 of the 4 motors' force.

[Otaku]

Originally Posted by Andrew Schreiber

Won't work, Mecanum requires having 2 motors per side.

Personally I have always liked 6 wheel drive machines w/ a drop center wheel. It may not be as flashy as a 3 speed swerve drive but it is reliable. In my book reliability beats everything.

^ Words of sense.

Mecanum is simple to build (well, compared to Crab/Swerve), but the more complicated programming and driver training is one turn off, and the lack of traction is another. If somebody pushes your bot, it doesn't have the friction of normal traction wheels to prevent you getting pushed around everywhere. Add that in with the higher cost, and; though a cool system, there will be simpler, more reliable, and better ways to do it.

Crab, while you get a lot more traction, is even more complex and cost-prohibitive than Mecanum; and it can't be switched to a traditional skid-steer setup with a simple change of the wheels and a little hacking of the code.

I'm a fan of simple skid-steer 2, 4, or 6wd bots, especially "dropped center" bots since it helps them gain some agility while turning in place. 2wd drop-center 6-wheel bots are especially cool imo, as far as turning agility is concerned, since there's less drag to overcome from the "skid".

Originally Posted by Chris is me

I think the bigger issue with 2 CIM mecanum isn't torque but that you can't independently control 4 wheels with 2 motors. Well, with any amount of reasonability.

You would need a forward/reverse gearbox, with matching ratios, and a clutch so the gearbox could disengage and switch to forward or reverse. And to be able to do so quickly and effeciently would be a ridiculous engineering task considering the engineering experience (or lack thereof) of the Students, as well as financial and time limitations.

So yeah, it's unreasonable.

[AdamHeard]

I'm a massive hypocrite, if you asked me what kind of base you should run, I'd say 99% of the time run a 6wd, 1% a crab, and never use holomonic or mecanum.

That being said, let me contradict myself. There is no perfect drive for all games. Your team must analyze the game, determine what the requirements are for your drive, and pick based on that. They all have benefits which come at varying costs.

Even though I'm pretty darn sure we'll run a 6wd next season, my team will still stop and analyze to determine if it is really the best choice.

Originally Posted by Otaku

Crab, while you get a lot more traction, is even more complex and cost-prohibitive than Mecanum; and it can't be switched to a traditional skid-steer setup with a simple change of the wheels and a little hacking of the code.

A crab can be switched to a skid steer pretty easily in just about every application of it I've seen. Sure, it's obviously not the lightest and most efficient skid steer when in that mode, but it is equally as functional.

[Otaku]

Originally Posted by AdamHeard

A crab can be switched to a skid steer pretty easily in just about every application of it I've seen. Sure, it's obviously not the lightest and most efficient skid steer when in that mode, but it is equally as functional.

Alright, I'll give you that.

But this is FRC, where we need to keep weight, simplicity, and practicality in mind. Not to mention, the inevitable increase in flex (solid axle to frame members vs. mounting via several seperate links) makes it even less efficient.

I honestly can't see any practical reason why Crab would have huge advantages over a 6wd drop-center design. Especially not advantages that could be used as justification for the higher cost/longer build time/etc.