Re: pic: Octocanum Module
 

This is a really bad idea from a durability perspective. If you make the traction wheel the pivoting wheel, you will have to deal with a lot more sideways loading on the module than if a wheel with rollers is the pivoting wheel. This has been a documented problem in other drop drives (e.g. 148's 2010 drivetrain) and isn't recommended if you don't have a really robust way to deal with the side loading.

Re: pic: Octocanum Module
 

With omnis, I would agree with you. However, mecanums generate as much side force as forward/reverse force by design even when the robot is driving directly forward. With octanum, I'd consider the traction wheels as having less sideways loading under most conditions, probably only exceeding the peak mecanum side load when being T-boned. And in that case, the solution is to switch to mecanum and strafe your way out of the t-bone, or at worst into a legal pin that (under most years' rules) is time limited.

Re: pic: Octocanum Module
 

First things first, if your solution to a design problem is to drive around the failure mode, that's just plain bad design. You should never have a robot designed in a mechanically weak manner just because the driver "shouldn't" have the robot in that mode when you expect damage. You can't bet your drivetrain on perfect play.

The strafing force a mecanum wheel puts on the robot is very different than the force of being pushed to the side in traction mode. When being pushed in traction mode, the full weight of the robot resists motion until static friction is broken. When strafing in omni / mecanum mode, there is no such resistance - the wheels are rotating and the rollers are allowing the motion to happen.

Finally, while you are technically right that mecanum wheels exert as much force in the sideways direction as they do in the forward / back direction, it is worth noting that this is ~70% of the peak force a traction wheel of the same gearing would put in the forward direction, assuming the same CoF for both wheels (which is also not true, all commercially available mecanum wheels have a lower effective CoF than most high traction wheels).

It is simply more robust to put the omni wheel on the pivot and the traciton wheel at the axle.

Re: pic: Octocanum Module
 

Again, no argument as stated, but Mecanum ain't Omni.

I wasn't so much betting on perfect play, as planning on the play I've seen (which has not included t-boning, for the record). Once T-boning was under consideration based on evidence of others, the defense is straightforward. Training for this situation would be included in driver practice, not left as something that we hope the driver would invent on the spot. In any case, we would design to handle the (externally applied) strafe forces of the worst case we expect to see. If 1/8", there would likely be no pocketing. If 1/4", there would probably still be minimal or no pocketing. As I've stated before: Due to our limited machining capability, our team's general strategy is to select material for our robot, not engineer it.

Our team is not planning for an octanum drive train, but if we did, we'd be at least as likely to have the mecanums as the "fast default" and the traction wheels as the "shifted shoving" state as any of the other three possibilities.

Also, as a final fallback, we'd design any "quad-drop" drive train to have predictable behavior should the modules be pushed by extreme strafe forces. It might involve a drive shaft pushing against a plate with unreasonable friction, but the outcome would still result in a fairly reasonable, predictable outcome.

Re: pic: Octocanum Module
 

So what I'm hearing is either configuration (mecanum or traction wheels pivoting) will have sideways forces that need to be dealt with. For traction, the big problem is T-boning. For mecanum, the problem is the constant force being applied at a 45* angle. Can anyone comment on whether they think both the 1/2" Hex shaft and the 1/8" plates (without pocketing) would be strong enough to stand up to either of these forces?

Re: pic: Octocanum Module
 

It's not the constant force of the mecanum or of a T-Bone. I'd be worried about that initial shock load when someone slams into your(2014) and you have pivoted the traction wheels down. You can only react so fast and you're not going to see every robot coming.

Re: pic: Octocanum Module
 

Honestly, we never had an issue with it. Heck, only time we had down time was when fasteners backed out into the frame (and yet we still won matches at an offseason just using it as a brake). In my experience, it is better to have the suspension on the traction wheels since then you don't loose contact with the ground when pushing, while having suspension doesn't fix a cg out of place. In addition to that, it seems that having the mecanum wheel on the end of the pivot is worse for survivability than a traction wheel that is only in use rarely. Plus, you should make the traction wheel as small as possible to cut down on weight by making the overall module smaller.

Re: pic: Octocanum Module
 

Smaller wheels would also have the side effect of making that moment arm shorter. Reducing the torque is probably even more important than weight savings.

Re: pic: Octocanum Module
 

Is there a frame element in the foreground, running parallel to the 2 x 1 tube that you have hidden?

It looks like you have only two thin standoffs tying the two plates of the octanum module together. They may "twist" relative to one another as forces are applied to the module. The plates look pretty thin which may make this worse. If they are really thin, the pocketing will not save you a worthwhile amount of weight. If you are going to pocket the module plates, radius the corners (with larger radii near the standoffs). In 2014, we used 1/8" plate. I seem to recall that they did bend a bit after a number of hard hits from the side so you may want to make sure they don't bind on the frame if this happens.

You may also want to consider how the modules will be assembled and how you would service them, say to change a wheel. Do they have to be assembled in the frame? Our 2014 modules took over an hour to assemble, each. It may be best to investigate how 148 designed theirs. They could assemble theirs outside the robot and swap one complete module for another in just a few minutes by removing one or two bolts. You and your team mates would be much happier working on a design more like 148's than ours.