Another Drivetrain Idea - "True Octanum"

[SpeedFreed]

The steps I always take to make sure all wheels touch the ground are:
1) I don't make perfectly rigid drivetrains (on purpose!). You shouldn't be able to notice any flex, but it should be there in ever so slight amounts.
2) Mecanum wheels that flex, such as Vex Pro Mecanum wheels, actually have a small amount of give, so they could be ever so slightly off.
3) Diligence. I always use a CNC machine or mill with DRO to drill holes in my mecanum drives to make sure they are exactly in the right spot I do this for many other parts of the robot too).

But I can't imagine it would be all that different from a typical 4 wheel mecanum, just doubled.

[pimathbrainiac]

Interesting idea, taking off my/cjl2625's concept. How would the suspension work on this, since each corner of mecanum wheels are joined by bevel gears?

[SpeedFreed]

Well on first thought, you could have to pivot one mecanum wheel about axis of the other. While you wouldn't want to do this for anything extensive (I can't imagine mecanum wheels work well on significant angles) you should get a safe 1/8" of travel out of it, which would be plenty to account for any build tolerances. Then you just have to make sure the 4 solid mounted mecanum wheels are mounted such that they are coplanar (like any mecanum drive).

But, in my opinion, suspension is unnecessary complexity that should be avoided whenever possible. One of the goals in this idea was it's lack of articulation for simplicity and weight (and time and money...).

[Caleb Sykes]

Interesting, now I want someone to build it to test how it performs compared to a standard mecanum drive.

Unfortunately though, I don't think that the added weight/design complexity would be very worthwhile. If I am understanding this design correctly, any gains you get in the strafing direction will come at the expense of the forward backward direction.

For example, under a normal 4-wheel mecanum setup, top speeds might be something like 10fps in the forward direction and 5fps in the strafing direction. With the same reduction in your setup, my guess would be that you would see more along the lines of 7fps in both the forward and strafing directions (whichever directions you so choose those to be).

I suppose that if you wanted the robot to handle more like a swerve drive than standard mecanum, this setup could help. I know our driver did not like field-centric code with mecanum wheels since his max velocity was dependent on which way the robot was facing.

[Toa Circuit]

Correct me if I'm wrong, but wouldn't you be better off going to a 4 omni setup? You wouldn't have the losses from miter/bevel gears, it would be lighter, more robust (it seems omnis stand up better than mecanums), have the same strafe power as forwards-backwards, wouldn't require a suspension, and would be mechanically simpler? It's effectively what you're doing here already.

[SpeedFreed]

It is true that basically this only improves the strafing abilities and adds symmetry. While I can imagine it must have some effect on top forward speed, exactly how much is a question that can only be answered in testing. Also, speed can always we compensated for, like my teams new octanum--powered by 6.66 CIMs and does 22fps on mecanum (after mecanum loss). Likewise if you power each corner of this with 1 CIM and 1 Mini CIM, forward loss will probably be quickly forgotten.

As for 4 omni wheels, I'm not quite sure what you are getting at. If you mean replace one set of mecanum wheels with omni wheels, then you would need at least 6 independent gearboxes to drive all the wheels (4 for mecanum, 2 for omni wheels) (or differentials, but THAT truly gets in the realm of unnecessary complexity), and then that is some strange mix of H drive that really holds no advantage. If you mean just 4 omni wheels, then that would have to be holnomic drive, which only reaches top speeds on the diagonals (unless you do 4 omnis Killer Bees style, and then you have to be magic to drive it).

The biggest pro's I was getting at with this, is that it provides full motor power in the cardinal directions (unlike H drive), and is symmetrical like swerve. But it is also heavier, more expensive, and harder to make than mecanum, for probably not enough gain.

However if your team does mecanum every year without exception, and is tired of it's asymmetric performance, then this is the drivetrain for you!

[Certified Nerd]

Why not just use 8 sets of 2 omni wheels? Wouldn't this work fine and be much cheaper?

[jman4747]

Originally Posted by Certified Nerd

Why not just use 8 sets of 2 omni wheels? Wouldn't this work fine and be much cheaper?

Do you mean 2 sets of 4 = 8?

8 wheels*2=16

16*$18 a wheel (4" VEX Pro omni) = $288

2 sets of 4" VEX Pro Mecanum = 8*$30 = $240

8 total 4" VEX Pro omni wheels = $144

[JohnGilb]

I don't think this really helps you.

So, here's the thing (Ether or one of the mecanum experts can correct me here): in an ideal environment, there is no difference in power applied forward/backward vs strafe left/right. The reason you see bias in the real world is because of the internal friction in the roller bearings - this effectively means the wheels aren't providing power at 45 degrees, but rather some other number that is typically biased towards front/back.

All you're doing here is combining two imperfect drives orthogonally, so instead of max speed in one direction and 60% in another, you're probably going to have 80% in both, as the fast drive will be "dragging" the slow drive.

[jman4747]

Originally Posted by SpeedFreed

As for 4 omni wheels, I'm not quite sure what you are getting at.

Originally Posted by Toa Circuit

Correct me if I'm wrong, but wouldn't you be better off going to a 4 omni setup?

Did you mean 2 sets of 4 or 8 total? Like this: