Another chapter to the 6WD vs. Mecanum debate

So here is my question, If a Mecanum robot and a 6WD robot come in contact and the mecanum strafes out of the way and heads forward can the 6WD catch up after it starts strafing?

It all depends on how fast your 6wd base is and how good your drive is. I would imagine that the average 6wd would have no problem catching up to the mecanum robot. I think it would be a cool project to get a 6wd robot and a mecanum robot in the same area and see who can out maneuver the other. I still have my money on the 6wd.

There are a lot of variables here that depend on both robots and their respective drivers…

But in general and in my opinion - yes. A mecanum can’t get past a 6WD just by going sideways for a second.

We are going to put them head to head and get a video up by sat. i will post it back here

From experience, a 6WD will push right through a robot with mecanum drive. A good driver could just clip someone with mecanum trying to slide to the side and throw that robot off target.

6WD isn’t that much more powerful than a mecanum drive. Of course 6WD is more powerful, but you are exaggerating quite a bit.

The argument pretty much boils down to who the drivers are and what they do. Certain 6WD teams will destroy certain mecanum teams, but certain mecanum teams will run circles around certain 6WD teams. It all depends on the team, the strategy, and the execution. The wheels themselves are unimportant.

Is he? Do you have anything to back this claim up?

Well, there is “a 6wd”, and then a 6wd with high traction wheels, 4 CIMs, and shifting.

One of them is anywhere from mediocre to good, the other is great. The great one will beat a mecanum any day of the week.

I would argue that shifting is not an absolute requirement to build a great 6WD. Other than that, completely agree.

A very solid base can happen without shifters, but if you take two 6wds equal in all but shifting, the shifting is superior. It will push harder (while drawing reasonable current) and drive faster.

True…but if the decision is 6 drive motors w/o shifting vs. 4 with shifting, then things get more interesting.

No, it’s not physics. It’s a myth. There is almost no loss of power at all during forward motion.

When a mecanum drivebase is traveling perpendicular to the axles of the wheels, as if it had traction wheels, the slipping of the rollers is almost nonexistent. There’s a teeny bit of roller motion that happens on a compliant surface like carpet, but 30% is a huge exaggeration.

What you do give up with mecanum wheels is maximum pushing force. Because the individual force vectors are in different directions, the sum of the magnitude of the forces on the carpet is greater than the magnitude of the sum of those forces on the robot. The wheels will slip at the same wheel/carpet force as any other wheel having the same surface, but when using mecanum wheels that happens when there is less total force on the robot than when using traditional traction wheels.

Emphasis mine.

Because a mecanum drive has less total usable force at any given speed than a 6wd, and power = force * speed, does this not mean that mecanum drive has less available power than a 6wd?

If my interpretation is correct (please correct me if I’m wrong), the power loss in mecanum comes from the force vector cancellation during straight-line driving, not from frictional losses in the rollers.

Yes. But that’s a power limit, not a power loss. This may seem like nitpicking, but it’s an important distinction.

If my interpretation is correct (please correct me if I’m wrong), the power loss in mecanum comes from the force vector cancellation during straight-line driving, not from frictional losses in the rollers.

The “force vector cancellation” does not cause any power loss unless you are operating on a very resilient surface at high vehicle force levels and speeds. And then it’s not a fixed 30% number but rather a function of the force and speed.

Once a mecanum’s wheels break traction (start slipping), then any additional power supplied to the motors is wasted in scrubbing the wheels on the floor.

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That makes sense, and I’m all about nitpicking distinctions, otherwise ambiguity would prevail.

The “force vector cancellation” does not cause any power loss unless you are operating on a very resilient surface at high vehicle force levels and speeds. And then it’s not a fixed 30% number but rather a function of the force and speed.

Once a mecanum’s wheels break traction (start slipping), then any additional power supplied to the motors is wasted in scrubbing the wheels on the floor.

This makes sense too.

Thanks for the input!

Depends what you mean by beat. A pushing match? Probably so. A test in maneuverability? I’d disagree with that. If your driver knows how to truly utilize the strengths of the mecanum drive, then you can juke 6 wheel drive robots all day long.

Picking one of them revolves entirely upon your team’s strategy for this year’s game.

I’ve literally never seen a mecanum “juke 6 wheel drives all day long”. Not saying it can’t be done… just saying I really, really want to see video of it. Partly for “proof” and partly because it would be really cool to watch.

You will see a difference in pushing performance due to the traction capabilities of the wheels used, not so much the power available.

The mecanums are usually a hard roller material, which will not provide much “grip” into the carpeted surfaces. Assuming the 6WD is using some sort of treaded wheel it will have more traction onthe carpet.

Think about walking across the playing field in your slick soled dress shoes. You can move, you can run, but you can also push your foot out to the side and it will slide across the carpet. In a pair of softer soled athletic shoes, you can also move and run, but if you try to push your foot out to the side it will take more force to do so.

The mecanum is like the dress shoe, the treaded wheel is like the athletic shoe.

As previous leader of the 449 Drive Team for two years, I’d like to put in a few good words for mecanum.

First off, there is almost no power loss in forward/backward movement in a mecanum drive - the rollers do not move unless you are strafing. The fact that mecanum is “less powerful” is a common misconception - mecanum is not omni holonomic, and the only difference in power is the difference in coefficient of friction with the floor between the roller material and the material of your six wheel drive wheels. Mecanum’s only real weakness with respect to being pushed is from the side, which in this game isn’t all that important as the other robots are not allowed to interfere with your goal scoring. Last year during the DC regional we had some issues with ball manipulation that forced us to change our strategy during the quarterfinals to “go into the front zone and push the defensive bot out of the way,” which we were able to do with no problems at all.

Secondly, strafing is *amazingly handy," especially in a game like this years in which staying lined up with a goal is crucial to success.

Thirdly, it is not particularly complex. One your team invests in writing/acquiring decent drive code, building a mecanum drive is as trivially easy as building a normal four wheel tank drive. Well, perhaps a wee bit more, as you need to have one gearbox per wheel.

Fourthly, mecanum is also fairly robust - we have lost rollers and drive belts and still been able to play the game fairly well, as even a mecanum missing the strafing capability can turn better than a standard 4wd. You really don’t even have to adjust that much for it on the controls. We were able to traverse the bump last year as well as any other robot at the competition without mucking up our drive in the slightest.

And finally, I am amazed at how AndyMark’s mecanum wheels have improved over the years. We had very few maintenance problems with our rollers last year, and the main issue we had (difficulty in adjusting the connection to each roller) seems to have been fixed on this years generation of 6’’ mecanum wheels.