Wheel combination for a tank drive design

[wue]

Our team decided to go for a classic tank drive/chassis design as our base. However, we are indecisive about what type/combinations of wheels we should use. Some good advice would be appreciated.
Thanks!

[IsANerd]

In general, the best option is front 2 traction and back two omnis, as it turns fast but can’t be shoved around so easily, but it depends on what you aim to do. All omnis has the best turning, but all traction protect from being shoved.

[Xenon27]

So you can do all omnis if you want to have the best mobility, but you’ll get pushed around a lot. You can do half omni half traction, which still has ok turning, and can’t be pushed as easily. Then theres all traction which is the worst option imo. I’m probably going with half omni half traction

[Doer]

First try half and half, and if you are dissatisfied with your robot’s turning, you can switch to all Omni.

[James6555]

Or you could have a six wheel drive with omi’s in front and back and a unpowered traction wheel in the middle.

[Gameoa]

Half traction half omni puts your center of turning far away from the center of mass. This can really hurt your motors and your turning if you have a heavy robot.

I doubt defense will be huge this season so I would go for all omnis.

[ranOOm]

There’s a few different things you need to consider:

1. how many wheels do you want?
   a. having a lot of wheels means that you can get more combinations - is this something that you’ll want/need?
   b. with those extra wheels, some might be unpowered - is this ok, or is there going to be an instance where having those extra wheels is going to cause you to high-center?
   c. Those extra wheels are going to introduce a little bit more friction if you decide to bring them together.
   d. those wheels are going to take up space - do you need this space?
2. how fast is your robot going to be moving?
   a. some combinations make more sense for certain gear ratios - for example, 5:3 gearing will work on 3.25" omnis, but not 4" omnis.
   b. as your robot starts to move faster, there’s going to be more “slip” when turning - the weight distribution of the robot is also going to change this. Do you think that this slip is something that will help you move around the field faster (it might), or do you think that you would prefer the control you get on wheels that don’t slip.
3. how heavy is your robot
   a. With a heavier robot, your robot will have more momentum, and want to continue in the direction it’s going already. This means that on omni wheels, you’re going to get a little bit more slip, but on tractions, you’ll have to have the motors fight this weight (especially if your turning center is not lined up).
   b. A heavier robot may or may not need a different gear ratio - keep that in mind.
   c. A heavier robot will do better defense than a light robot. Especially if it is engineered with that in mind. 8m Turbo will push 8M HS anyday if the 8M turbo robot knows how to drive.
4. how much defense you’ll get.
   a. This is similar to above, but the amount of anticipated defence you’ll have against you can drastically change the wheels that your robot uses. You need to consider whether you would prefer to be pushed and slid along the field, or if you would prefer to not be push-able and stand your ground.
   b. If you’re going to be playing a lot of defense, think about which wheels will have the most “grip”, or what speeds you’ll want to be going at, or what’s going to be easier to integrate.

At the end of the day, the best thing that you can do is buy a bunch of different wheels - take 4 omnis of any size (it really doesn’t matter this season), and 4 traction wheels of a similar size (again, it doesn’t matter, but I prefer 3.25" wheels), and put them onto your robot. Build a quick drive base, try 4 omnis, 4 tractions, half and half, 4 omnis + 2 tractions, 6 locked omnis, etc. Remember however, that before you drive your robot around the field and call it a day - load up the chassis. Put on 10-15 pounds of material and see if the robot still drives the same, weight can drastically change the way the robot handles and manuevers. Perhaps one drive will have characteristics that you prefer with all the weight shifted one way, and perhaps the other drive will have characteristics that are more consistent throughout, that could change the decision that you make, because an unbalanced robot is a pain to drive, and sometimes you can use your drive to make that job easier (think: consistent turning radius).

From my experience, here are the drives that I would consider

• All omnis (6 or 4).
  This combination gives you the most versatility, and is commonly used and seen for good reason. It allows for quick maneuverability around the field, without compromising your turning speed. The rollers allow the wheels to slip sideways, which makes getting out from defense on an open field or against a wall a lot easier. This drive puts the least strain on your motors, making it probably one of the quickest IF you know how to drive the robot. There will be a lot of slip sideways, especially if the robot isn’t balanced, and it might be weird, and you’ll overshoot a bunch at high speeds. If you can drive with this in mind, there’s no issue, but if you can’t, you’ll have to slow down and another drive might be better.
• Locked omnis // Bling drive (6 omnis)
  This worked super great in turning point because it was pretty resistant to defense, and I think there’s a few other advantages as well. This puts your turning point roughly in the center of your robot, usually where all the mass is. This means that your center of mass and turning center are inline (or very close), which means that there’s going to be less strain on the motors. With that same thought, it’s super easy to drive this robot because there’s virtually no side to side slip when you turn, and you’ll always know exactly how the robot will turn because of the locked middle wheels. This does also mean that getting out of a pin against a wall is pretty difficult, because you have to force some slip. But, with this drive, you rarely should be found pinned against a wall.

Edit: for this season, I would definitely gear up the drive, something faster like 7:5 on 4" wheels or 5:3 on 3.25" wheels. 200 direct for an open field is still good, it’s just that having a little extra speed, and having this same speed throughout the season will make driving easier.

That’s all from me, any questions/comments/disagreements, feel free to let me know.

[9457Y]

Imo Using 6 omni-wheels and locking the middle two omnis by pushing screws into the sideway rollers works best. Gives you great turns while still offering the benefits of traction wheels.

[invalidflaw]

Our team used 3.25" inch omnis on our little bot since we are in VexU. The traction wheels work very well in place of locked omnis at this size.

[ranOOm]

The only reason Locked omnis are better than Traction wheels on 4" wheels is because the OD of 4" omnis is slightly more than 4" (it’s closer to 4.125), so traction wheels don’t grip as much. On 3.25" wheels there doesn’t seem to be that issue, but 4" omnis are made of a grippier material than 3.25" omnis are.

[codynelson03]

Last year we used 3 omnis on each side with the middle locked, and it worked pretty well. Def. use locked omnis over traction wheels, they have more traction.