What are your thoughts on using a drive which doesn’t only consist of omni-wheels? It wouldn’t be as easy to push the robot off the center platform if it were using a combination of omni-wheels and traction wheels.
I think that no omni wheels would be best, because, as you said, you cannot be pushed as easily.
If you’re doing a tank drive, two omniwheels and two grips aren’t so much different than four omniwheels for your own driving, while they provide a lot more resistance to being pushed laterally. Avoiding omniwheels entirely provides that much more resistance to being pushed laterally. But I could also see teams going for 2 and 2 expecting some resistance and with a plan for a robot pushing them laterally resulting in them being reoriented to fight back. And there are more options for a tank drive.
I expect we’ll only see all omniwheels on robots with holonomic drives, where the omniwheels will provide some resistance to being pushed laterally.
If your robot only has rolers (non omnis) when in a normal tank configuration, turning becomes a problem. the wheels jump and or slide sideways on the mat and cause a very large amount of friction.
2 rollers (say in the back) is not as bad but it changes your turning origin (the point in space that your robot rotates around) to the reer right between the 2 rollers. this can make maneuvering slightly more cumbersome and turning requires slightly more power.
In contrast, an all Omni drive has much less friction and rotates around the center of mass. This means that if you have tons of weight in the front as compared to the back, your origin will be closer to the front of the robot. In most robots it is close to center though.
Another option would be to put a 3rd set of wheels (rolers) cetered between a set of omnis. This would give you an origin at the center and a good amount of lateral resistance. the only problem i could see with this type of drive is that it would be easier to high center when attempting a park.
ooh, I didn’t think of that. =\ would mecanum wheels be viable?
mecanums also have some friction on turns just because they are so wide, but they would be way better than rollers. with mechanism, some power would have to be given to the wheels to resist lateral force but they would probably perform pretty well on that front as well.
Think we’ll see four grips with steering anywhere?
IIRC, this works fine on the smaller wheels, but on the 4" wheels, the center wheel in this configuration will have minimum contact on the floor due to some diameter tolerances between traction wheels and omnis.
There are ways to accommodate this difference though. One way would be to put rubber bands between the wheel and the tire to slightly increase the diameter of the wheel.
I was thinking something like Toss Up, where teams had 6 wheels with 4 motors, 4 omni , and 2 traction in between each pair of omnis on both sides. The benefit is that it allows for the friction you need to not get shoved, but still lets turning not be a pain.
Having the traction wheels in the center will also relieve stress on the motors on turning as the wheels are close to the center of mass. Ideally, this is the best setup for the park bonus as X-drives can have trouble getting over the bar and mechanum wheels are far too slow to be competitive.
The funny thing about this is with human-size vehicles there is actually a lot of added stress. The wheels won’t actually turn at the correct rates compared to each other when the robot is turning because they’ll be trying to rotate the same amount as the front and back wheels while going through a different radius turn. With H-drives (real ones, not the term used for some holonomic drives) the different radii result in driveline windup. The recommended solution is to do things like driving over curb instead of fully around them to give the axles a chance to relax when the wheel is lifted.
Even in this situation, I would think with enough turning it would still add stress on the motors because of the differences.
this is a good point that didn’t cross my mind. I wonder if it really makes that big of a difference though at this scale and weight of vex.
I think if you start spinning continuously in one direction you’ll start to notice it. Otherwise you’ll probably get enough little slips that it won’t be noticeable.
I would just use the standard Traction Wheels to reduce the chance of getting pushed off…
The Omni Wheels would increase the chance of getting pushed out of the Center Platform, if the opposing alliance pushes you by the side of your bot.
You could do a back wheel drive with only the front wheels being Omni, this would make the robot not slide but still turn easily
True, but the turning will be very different compared to all wheels as Omni. It will be a bit tricky to get the hang of it, but totally possible.
Try putting a cap on a pole with all omni without being pushed 
An all omni drive will be challenging especially with the amount of defense I expect this season. Many teams seem to being going with a configuration with different type of wheels in the front and back
8 inch Omni VEXU drive is gonna be fun