Mechanum Wheel Sizes

[MichaelBick]

Originally Posted by Nemo

Some people always make mecanum sound harder than it is. Let's assume that a team has already figured out how to build, program, and drive a basic 2WD/4WD/6WD FRC robot. For such a team, I would claim the following:

1) Mecanum drives are not difficult to build. You can get preassembled wheels and gearboxes from AndyMark, and hubs to connect them. All you have to do is find a way to fasten the gearbox to your frame. That is about as easy as a drive can get. No chains required. It is not very heavy, either.

2) Mecanum drives are not difficult to program. You don't even have to program it, because holonomic drive code is included in the libraries with our software. All you have to do to turn an arcade drive into a holonomic drive is add "+ x" or "- x" to the motors for the strafing axis joystick input.
Front left motor: y + z + x
Front right motor: y - z - x
Back left motor: y + z - x
Back right motor: y - z + x

Adding gyro feedback prevents the robot from driving crookedly or rotating when you don't want it to. That admittedly adds a bit of complexity, but only a bit. I am not convinced that encoder feedback is necessary if you have a gyro.

3) Mecanum drives are not difficult to drive. I'd like to know how many of the people claiming otherwise have actually driven one. I thought the whole point of a mecanum drive was to make it EASIER to get where you're trying to go, pointing the direction you want to point. A 6WD is less forgiving to the driver than a mecanum drive, because it's faster and easier to correct your slight driving mistakes when you can strafe.

Mecanum can be a reasonable choice for some teams. It is not going to work well for all games, but it was fine for 2011 (flat field, protected offensive zone, and peg goals that were easier to score on with strafing ability).

Mecanum is also not going to be as good as 6WD at the highest level of competition, because a 6WD robot (which has inherently better traction) with a really good driver does not need strafing to quickly get the robot exactly where it needs to go. However, it is okay to focus on getting more competitive at the regional level. The forgiving nature of a Mecanum drive (item 3 above) might, in fact, make some teams more competitive at that level than they would be with 6WD.

1) I agree with mechanum being easy to build(relatively). Our team from last year built one, and last year is a year that is not fun to remember. That being said, you do have reliability problems. If one wheel goes down, you aren't going to be able to strafe.

2) Mechanums are relatively easy to program. But, once you start putting gyros, encoders, etc. on(and you should, you will have problems driving straight otherwise), you then get a lot more complicated, vs the traction drive, where you really don't NEED any of those.

3) While mechanum may be easy to drive(it is basically Halo), you have to make a lot of compromises in strategy and build. You have to give up playing defense, and have to make sure you don't get into a pushing match while playing offense. Therefor, you must be a very good offensive robot, and it is STILL harder to drive, due to the fact that you have to avoid those situations. Furthermore, you must make some compromises in build. If you look at many of the successful swerve bots(just because better teams usually use swerve when they are looking for holonomic robots), all of them have made compromises in build. 148 in 2008 built a robot that was completely round. 118 built a robot in the same year that had a turret. 1717 from 2011 was very effective in the home zone, because they didn't have to turn much, but outside of the home zone, they were much less effective.