Just came across this image from week 2 of the 2008 OverDrive season.
We fitted mecanums on our chassis, but got a really really jumpy ride…
We hypothesized that it might be due to the robot being so light… so we added about 20 bricks to the top it still jumped around like it was trying to tap dance and we eventually wrote them off for the season.
we will be solving the problem during the off season…
It seemed that the problem was the durability of the wheels. Andy said that the future iteration of these wheels would have several upgrades (listed in the thread), so you might want to wait until they come out before purchasing the mecanums in that size.
The problems 1675 experienced with the 6 in wheels where that there was not enough distance between then roller edge and plate edge causing the plates to grab into the carpet making any turning or strafing impossible. That was mostly remedied by taking each wheel and grinding each and every edge down. I took Ryan and myself a total of 3 hours to grind the 4 wheels. that doesn’t count removing the stripping the rollers, reattaching the rollers, and mounting the wheel at the following regional. Also we had problems with the plates them selfs concaving under the strain of strafing.
AndyMark is redesigning the wheels with the corners removed and thicker plates for the 2009 FRC season. Granted they will get a tad heavier but, they are so much smoother and smaller then the 8 in.
As for the X vs O thing.
Mecanum can be used in 2 different “orientations” we played with this for the 2007 season. X gives you more turning but less strafing, O gives you more strafing less turning, so thats how that works, your not “wrong” but you may not be using the orientation you need for the specific game.
I’m pretty sure both orientations would give you the same quality strafing, but that you absolutely need an X on top.
The reason is that with O, your rollers on the bottom look like an X. Since the rollers roll perpendicular to their axle, X on the bottom allows the robot to spin around freely. This doesn’t sound like much of a problem (until you’re getting spun around on the field), except for this:
You can’t turn under power with X on the bottom. All the force that would be turning the robot is redirected by the rollers into an X pattern rather than an O pattern. So the robot is really pushing inwards on itself when it tries to turn, rather than pushing itself around.
Sorry if that’s confusing. The bottom line is that O on top doesn’t allow turning at all, and rather causes the robot to skid and jump around. You need X on top if you want to turn.
If your mecanum wheels are placed in a rectangle rather than a perfect square, you actually can manage to turn if they’re installed with the wrong orientation. The vectors won’t all cancel completely unless all the roller axes intersect at a single point.
They still cancel to a large extent, and it still doesn’t turn very well.
The proper configuration has the rollers angled so that each of the wheels could move freely in a direction toward or away from the center of the robot without the wheel itself turning.