4 Wheels vs 6 for games like 2014

[jkelleyrtp]

Originally Posted by Dunngeon

Alternatively, if you want 4wd, two omni's and two traction wheels works pretty good chained tank. It's a setup that worked well for 955 in 2014 under defense.

In this situation, your forward or direct traction would be similar to that of a standard four wheel drive, but laterally against the faces of the wheels, your friction force would be about half. It also changes your center of rotation, but making a 6wd with two front omnis would be a good compromise: wide wheel base for 4 traction wheels and the stability of a 6wd.

[KeeganP]

Originally Posted by jkelleyrtp

In this situation, your forward or direct traction would be similar to that of a standard four wheel drive, but laterally against the faces of the wheels, your friction force would be about half. It also changes your center of rotation, but making a 6wd with two front omnis would be a good compromise: wide wheel base for 4 traction wheels and the stability of a 6wd.

We used this setup in 2013 and loved it. In 2014 we switched to 8wd tank, with no drop but the far front and far back wheels being omnis. We hated it. Partially because our gear ratios were wrong and we had misaligned chains, but it didn't work for us -- we tripped breakers, popped chains, and still couldn't push very well. This year we went back to the 2013 setup (back traction, front omni) and again loved it. The problem is finding (non-custom) gearboxes to direct drive wheels that bolt on easily to 1x2" aluminum frames. Back in 2012 I was on a team that build a wide 4wd traction setup, which worked very well for bridge balancing, but "hopped" when it tried to turn and drive at the same time. Being wide made the drive better than 4wd long, but it still wasn't great.

We're getting ready to build a "Texas Tube" drivetrain -- 6wd drop center, chain in tube -- setup in the coming weeks, and may well run that in 2016, pending the game. The 6wd benefit over the 4wd traction/omni setup is traction, especially in turning situations with defense. We avoided most defense in 2013 (and our driver was insane -- he did spin-moves around other robots, it was crazy), so it wasn't a problem, but it can be.

I wouldn't recommend building a drivetrain for the first time ever during build season if you can help it. You never know that kind of problem might pop up, or how it will perform for you until you've tried, and it's far harder to rebuild a chassis and drivetrain after the robot it complete than during the offseason.

[Dunngeon]

Originally Posted by Munchskull

I had always thought that yoy had 6wheels with four traction and 2 omni guess i was wrong all this time. It was a very well built drive train either way and you drove it really well.

Thank you! We ran the 4wd for all PNW events, then swapped to 6wd at worlds. I can say with confidence that for 2014 (and 2013) we preferred the 4wd, 2omni, 2-tank. Friction pins were much less of an issue, and having the pivot point at the front (or rear) of the robot was nice. It does require a bit more driver skill when turning, but its easily learn-able in an hour or so of drive time.

Major downside was the limited traction when pushing, but you can adjust play-style to compensate.

[EricH]

Originally Posted by jkelleyrtp

In this situation, your forward or direct traction would be similar to that of a standard four wheel drive, but laterally against the faces of the wheels, your friction force would be about half. It also changes your center of rotation, but making a 6wd with two front omnis would be a good compromise: wide wheel base for 4 traction wheels and the stability of a 6wd.

Depends how you do it. You could do an "opposite corner omnis" 4WD, which has been done before--say you have an omni on your left front corner, you put another on your right rear corner. It has a similar effect in terms of friction reduction, but it keeps your center of rotation in the center of your robot. Now whether that's a good idea depends on your robot and the game--you might actually want to pivot elsewhere!

[PayneTrain]

Originally Posted by Andrew Schreiber

This.

The big change that happened in (2013?) was the new frame perimeter rules. Prior that we had max dimensions in each direction and it meant you were either building a wide bot that was unstable front to back or a long bot that had a longer wheel base[1]. With the change to a 112" perimeter we were able to optimize differently. Example - 125 ran a 6 wheel flat drive in 2014[3] and had no issues turning because we were effectively 28x28, and when looking at actual points of contact, much wider than we were long.

2015 allowed teams to do whatever they wanted.

Personally, I hope to see a return to frame perimeter rules as it is both easier to inspect than transporting a box[2] and allows teams more flexibility to optimize their chassis shape.

I think the 112" frame perimeter rule was an evolution to FRC design that I'm pretty sure was universally lauded. I really hope to see that return in 2016.

My biggest regret in 2014 was not running a flat 6-wheel because with a robot would have had a wheelbase of around 22" that year, I think. The reason I cited Daisy Thunder is because a Hall of Fame team took the opportunity with the 112" rule in 2013 to develop a 4 wheel drive and looking at results, they must have liked it well enough.

There is a lot of inertia when it comes to FRC strategic design. That's mostly a good thing. Now is a good of time as any to go out (or in... to your shop) and build and test something like this and see how it goes. In fact, I'm probably going to look into crashing something like this together!

This is a great thread, by the way.