Re: Question about wheel preferences
 

Team 957 used a modified 2wd kitbot frame. The "center" 2 wheels (standard kitbot wheels, not high-traction wheels) were driven directly, and the 4 corners were low-friction idlers (Lunacy wheels).

We were geared for a ridiculous speed -- I recall it being close to 20 fps theoretical, although I don't know what speed we actually reached. We had 4 CIMs on the drivetrain, using CIMple boxes. We had encoders mounted to the CIMple boxes.

Using feedback from the encoders, we electronically stabilized steering in our "high speed" mode (reducing the tendency to overshoot during corners), without limiting control authority.

We also had a "low speed" mode which directly controlled wheel speed (we used proportional controllers, not caring about steady-state error or derivative control). I believe we ended up scaling the joysticks to +- 2 fps -- this mode was intended to be for hanging the tubes, picking up tubes, and deploying our minibot. In practice, it was extremely precise and accurate (the closed-loop control more than replaced the resolution lost by gearing higher).

It's definitely possible to replace a shifting gearbox with closed-loop electronic control and high gearing in terms of control. However, I suspect that it would not have been traction-limited had we used "grippier" wheels, so this is not a suitable replacement if pushing power is important.

Re: Question about wheel preferences
 

We had a similar problem, but we needed more control at the minibot deployment stage. We implemented a "slow button", which reduced the joystick input by 1/2 before sending it to the motors when the driver clicked the trigger. This gave us full speed in open court and control at the tower. Just an idea, hope it helps!

Also, as far as the OP's question, we had mecanum wheels last season and did not have very much practice time for the drivers due to a variety of obstacles and our drive team did not have two many problems. We ended up using the joysticks, but considered using a ps3 controller. This might be a good idea for teams considering mecanum drive because of mecanum drive's similarity to some fps controls. We did not push other robots in the opposite direction, but we did bring some robots to a full stop when we attempted to. Not as good, but slowed our opposition down in addition to the tube herding that we implemented. Of course, results with any drivetrain vary based off of how the robot is built and the robot's purpose, so drivetrain should be a case-by-case decision.

Re: Question about wheel preferences
 

Our team has used mecanum for the last few years with increasing success. I will say that we got pushed around more than we would have liked when we got involved in a defensive battle with a tank drive bot.

However, we were pleased with our maneuverability and speed (15 fps). The big fix from previous years was avoiding chains and going direct drive to save weight and avoid complications. If it hadn't been such an offensive game in general, we may not have been as pleased though.

Re: Question about wheel preferences
 

HMMM! Situation-ally, No one pushes Truck!

Re: Question about wheel preferences
 

As rookies, Team 3489 used 4WD on nanotubes and 6 inch performance wheels with a custom overmolded urethane tread. The tread had a subtle tread pattern and was crowned to aid in turning. We produced molds on a 3D printer, and molded them with a 50 durometer low-viscosity urethane. The process entails a few secrets to improve adhesion to the wheel, but I'll share if anyone's interested.

As expected, the traction was good, turning fair. The trick to not getting pushed was the implementation of jack stands (think F1/Indy race cars). 4 air cylinders deployed conveyor belt pads to the carpet, lifting the 'bot about 1/8". Testing during build here was positive. Competition was a struggle as we had zero time to practice, and the defensive strategy didn't seem popular this year. For the fall SCRIW scrimmage, we changed out 2 urethane wheels for omnis, figuring that the manueverability would be better (much) and the jack stands would prevent our being pushed. As an [url="http://www.youtube.com/watch?v=Sd7vXl6h2JM"]earlier poster said, sometimes defense is just getting in the way. Video of us defending 1902 "Exploding Bacon" at SCRIW can be found below (hopefully). 1902 uses a 6WD drop center with conveyor tread. You can't see in the vid, but 1902 throws a tread, we have yet to lose one.

http://www.youtube.com/watch?v=Sd7vXl6h2JM