Re: Another chapter to the 6WD vs. Mecanum debate
 

Of course, 469 in 2010, with their shifting, was no ordinary machine: both really quick and incredibly powerful. But if you check out the below video, you see them put a really nice move on our driver, then just shove us out of the way. It was really impressive to see, and I almost applauded..

youtube.com/watch?v=qAMlb2iugUs

Re: Another chapter to the 6WD vs. Mecanum debate
 

I think that's probably because the majority of teams in FIRST don't use mecanum. Walk around the pits at regionals if you don't believe me.

Mecanum is unequivocally more maneuverable than 6wd, and mechanically it's no more complex (and you can get working drive code online, I believe our team has published ours), and it's perfectly reliable - our team has never had a major drive failure (one which made our drive unusable or even less maneuverable than a 4wd) due to the mecanum wheels.

I encourage you to build a mecanum bot and push against a 6wd with it. I think you'll be surprised at how hard it is to actually get the mecanum to skid.

Re: Another chapter to the 6WD vs. Mecanum debate
 

Maybe that's because it's not as good - the VAST majority of the best teams in FIRST don't use mecanum, and it's not because they're stupid.

Is it? Honestly? I think 6wd maneuverability is quite underrated - there's a reason there's literally never been a mecanum on Einstein.

We've pushed plenty of mecanum bots no problem (and yes, the rollers spin)

Re: Another chapter to the 6WD vs. Mecanum debate
 

Yes 6WD can push harder than Mecanums. So? I did not catch Dean saying this year's main goal was to push around other robots. In fact I see there is a rule against pinning and contact during certain match times.

Next year, when the goal is simply to push each other I will be on my way to northern Minnesota for these:

http://www.mattracks.com/

Good luck to all, have a safe build season!

Re: Another chapter to the 6WD vs. Mecanum debate
 

When the Blue Alliance comes back up, watch matches with team 40 in 2006. They had a mecanum drive with a drop down "traction plate" to stay in place when they wanted to. When they were on the mecanum wheels, they got pushed all over the field, when the traction plate was down, teams rammed them until their angle was off and with the mecanum wheels they couldn't fight back for their initial position.

Mecanum wheels have strengths that traditional skid steer drives do not. Pushing is not one of them.

Re: Another chapter to the 6WD vs. Mecanum debate
 

I feel a little bad using this as an example because I think you did a great job on 469 and had a great robot last year, but I see too many mecanum drives like this one in this match. In theory mecanum can work welll, butt too often teams don't strafe or take advantage the 'ability to drive circles around slow 6 wheel drives' (which I'm still waiting of a video of, btw).

I think mecanum can be an effective drive system but 97 times out of 100 6 wheel is just better. The best utilization of a mecanum drive I've seen was 2171 in 2008, but their drive still wasn't as good as 1114 and the other 6 wheel drives that year. I think there is a reason none of the elite teams have used mecanum drives.

Re: Another chapter to the 6WD vs. Mecanum debate
 

I feel like mecanum bots will do pretty well this year. We also used mecanum in 2007. We resurrected our 2007 chassis and put the electronics from last years robot to do prototyping/let software have more then 6 hours with the robot. I just don't see a well designed mecanum bot having problems out-maneuvering a 6wd bot.
Here are our 2007 drive tests:
http://www.youtube.com/watch?v=v_vUYyJAkCc
http://www.youtube.com/watch?v=YbluSr2Mf5Q
http://www.youtube.com/watch?v=W0XN6iSvCXo
Here are a few more:
http://www.youtube.com/watch?v=L01Ok-6AMhM
http://www.youtube.com/watch?v=RXGbo3tQapE
http://www.youtube.com/watch?v=ZnoMhVEqx1c
http://www.youtube.com/watch?v=oamC4esbGfI
http://www.youtube.com/watch?v=KfOLERgWQrU
http://www.youtube.com/watch?v=fdS8BmQom58&NR=1

Re: Another chapter to the 6WD vs. Mecanum debate
 

Every fall I do the drive train presentation at FIRSTFare, our FRC technical conference in Portland. I go through all of the teams who were either division winners or finalists at the championships and try to determine what basic system they used for their drive train. Obviously the game has a lot to do with what works but here are the results from 2008 and 2010:

2008 Championship Division Winners and Finalists
14 Six Wheel with traction wheels all around
2 Six Wheel with omnis
2 Four wheel with omnis
2 Mecanum
2 Crab Drive
1 Four wheel rack and pinion.

2010 Championship Division Winners and Finalists
2 Four Wheel
5 Six Wheel
10 Eight Wheel
2 Nine Wheel (148, 217 partnership)
1 Mecanum
3 Crab Drive
1 Treads

I left out 2009, the Lunacy year, because there the drive train was more or less determined by FIRST. I might have gotten a few of these wrong because I'm just going by what I can observe from the team's website and Blue Alliance video.

What you can see is that very few Mecanum drives make it to the top levels of competition. How much of that is because many of these elite teams have a drive base they've developed over the years that they are happy with and how much of it is because Mecanum doesn't readily translate into a competitive advantage FOR THOSE GAMES is something we won't know. This might well be the year that Mecanum breaks out...or not.

Re: Another chapter to the 6WD vs. Mecanum debate
 

No, I think there are plenty of teams who have never tried it - I know that 449 had never used mecanum until '07, and since then we've used it in both games which allowed it. There's a lot of stigma around mecanum based on this myth that it gets pushed all over the place - anyone who has had experience driving a mecanum bot will tell you that this is nowhere near as big an issue as you think it is.

No, the rollers do not spin unless your wheels are already skidding - the physics of this has been explained multiple times earlier in the thread. As has been mentioned earlier in the thread, the only power difference mecanum suffers is the fact that the wheels skid sooner than traction wheels. You will never start pushing a mecanum bot and see the rollers immediately start spinning - it's the same reason that acceleration of mecanum drive is identical to that of a traction drive.

Yes, it is unequivocally more maneuverable. You turn just as well, you accelerate just as well, and you have an extra axis of movement. If that's not unequivocally more maneuverable, then nothing is.

Re: Another chapter to the 6WD vs. Mecanum debate
 

I'm going to throw a wrench into the works.

Back in 2005, 330 had this debate a bit ourselves. We decided to work a compromise: We'd build our frame to run both (quite easy to do) and rig it with 6WD and rig our kit frame with mecanum (6" fully nylon wheels, KOP trannies from that time frame--single speed, single CIM). At that time, our 2003 robot was still operational, so we decided to test against that (4WD, 2-speed, drill motor transmissions).

We could not beat the 4WD in speed. We could not get around it for shooting a gap between two goals. I distinctly remember the mecanum losing a push test, and after the first 5-10 seconds, it wasn't a contest! If we'd been hit trying to cap, we'd have lost position, every time we took a hit. We had trouble strafing with an off-balance load (tetra on a pole). If I had video, I'd post it. Just on motors, we should have been able to push the 2003 robot, 4 CIMs versus 2 drill motors, roughly equal weights.

The 6WD stayed on the competition robot. To this day, 330 has not done a competition mecanum. Not because we're scared to, but because the GDC has yet to give us a game where it's an advantage given our play style. The only competition omni-directional drive we've ever done was a 6WD rotational drivebase on our 2009 robot, because it fit a key part of our game plan (avoid getting pinned if possible).

Oh, and one other thing: At that point, we had two years of experience with small-scale mecanum, both programming and hardware.

Re: Another chapter to the 6WD vs. Mecanum debate
 

I think it should probably be noted that declarations about how rare mecanums were on Einstein don't really mean much till after 2007. At least.

2007 was the first year andymark offered mecanum wheels. So pre-2007, the only teams attempting this would be teams with the time and patience to make their own wheels and try them out.

Even then, I think it's a little arguable that they've had much of a chance at shining since 2007. Look at the run down:
2007: Debut of AM mecanums, with all the first-run, low volume problems.
2008: Mecanums arguably weren't much of an advantage.
2009: Lunacy, slicks only
2010: Bumps and plywood under the field reduced the effectiveness of mecanums.

So I don't think there have been all that many years where mecanums were both commonly available and obviously useful for that year's game. Now that may be because they just aren't ever going to be useful to the game, but I'd want a few more games to actually make that call.

Re: Another chapter to the 6WD vs. Mecanum debate
 

How much power-loss is there strafing sideways with a "normal" mecanum drivetrain?
Specifically -- how much current draw is there on a single motor?
How fast can you gear up the drivetrain before that "sideways" current draw becomes undesirable?
If you gear to a safe speed (without undesirable current draw), will you be making a tradeoff on forward/backward speed (i.e. would you have geared faster otherwise?)

In a game which emphasizes full field sprints, I think the "safe" gearing for a mecanum is just too slow -- but that is just my opinion... (one which is founded on calculations and engineering analysis, plus some anecdotal evidence on the side... yes, 148 build a mecanum drive this fall, and we hated it).

If you're a team debating between 6WD or mecanum my opinion is you should go 6WD. I promise you won't regret it.

However -- The most important thing is that whatever you do, you get it built quickly, get your mechanism on it, and start testing and improving as soon as possible! The more testing and driver practice you get the happier you'll be.

-John

Re: Another chapter to the 6WD vs. Mecanum debate
 

I compiled this information on a thread a while ago....It's not entirely complete, but if it means anything to this conversation I have the drivetrains of the winning alliance members of the championships.

2010
67 – 8 WD
177 – 8 WD (articulated is sets of two, front and back)
294 – 6WD

2009
67 - 6WD wide
111 - 4 wheel crab (non-coaxial), wide
971 - 6WD wide

2008
1114 - 6WD long
217 - 6WD long
148 - three-wheeled crab (coaxial), nonagon-shaped robot

2007
177 - 6WD long
987 - 6WD long
190 - 6WD long

2006
217 - 6WD long
522 - Treads, long
296 - 2WD long, Omnis in front

2005
67 - three-wheeled crab (non-coaxial). Flop bot.
330 - 6WD long
503 – 4WD long, omniwheels in rear


2004
71 - 4WD long
494 - 4WD long
435 - 2WD long, with casters in front

2003
111 - Four-wheeled non-coaxial crab (with dropdown skid for turning)
469 – 4WD Long
65 - 4WD Wide

2002
71 - 4WD flop bot with casters in front
173 - 4WD long
66 – 4WD long

2001
71 - ?
294 - ?
125 - ?
365 - ?
279 - ?

2000
255 - ?
232 - ?
25 - ?

1999
176 - 4WD long w/ Omnis in front
1 - tank treads, long
48 – 4WD, long

1998
45 – 4WD long with Omnis in front.

1997
71 - ?

1996
73 - ?

1995
100 - ?

1994
144 - ?

1993
148 - ?

1992
126 - ?

Important to note that mecanum wheels have not been available to first teams for this entire time, but the numbers are stacked in favor of skid steers. From 04 to 05 the dimensions changed from 30x36 to the current size, 28x38, so there are fewer 6 wheel bots as you go back before 2005.

I've had people argue that the reason skid steers win more is baecause they are more common, but I prefer to view this as "the best teams win, and by no coinsidence, they are skid steer bots." So basically they are winning because they are good, not because they are greater in numbers. They are simple, intuitive to drive, and comparatively pretty light, which leaves weight for an affective manipulator.

Re: Another chapter to the 6WD vs. Mecanum debate
 

We are building a mecanum drive. We feel it's best for our strategy.


BUT!!!!

We are also building into our drive the ability to quickly change to 6 wheel drop center. The holes are already cut into our frame. Add a nano box and a little chain and off we go.

Our first regional is week three, if we were week one I doubt we would attempt the mecanum setup. If we didn't have the ability or funds to build a second robot for practice and testing we wouldn't try mecanum.

Re: Another chapter to the 6WD vs. Mecanum debate
 

Our team decided on mecanum drive day 2 of build season, we had resurrected our 2007 chassis into a driving mecanum proto-chassis by the end of week one. We have 1/2 our chassis done now (waiting on more brackets from 80/20) and because we are making most of our robot from 80/20 we will probably make a practice chassis. I think that because 2007 was the introduction of andymark mecanum wheels that it seems they havnt had a decent chance to prove them selves since then, this seems to be that year.