pic: Mars Game 2018 (looking for feedback!)

Water Game… who wants that when you can have a Mars Game!

This is 4904’s contingency plan for in case Dean springs some kind of (bumperless) all terrain crazy game on us next year. Here’s a bare drivetrain without any electronics, manipulators, or anything, really. It utilizes a rocker bogie suspension system similar to the ones found on MER (spirit/opportunity) and MSL (curiosity). Each side is able to rotate back and forth, and the two are connected by a differential. What this does is gear them together so that when the right side goes up, the left side goes down, and vice versa.

Now that I’ve finished the basic drivetrain, I’m planning to begin making tweaks to the swerve modules to make them more robust, CAD a robotic arm to mount on top, and add the rest of the normal FRC components (battery, pdp, etc…).

Like I said in my last post, this is kind of a just-for-fun thing, but I’d love to hear your feedback!

Looks hard to manufacture. I could build you one faster out of box tube that would work just as well.

Why would you use round tubing?

  • You’re going to rotate things (you’re not; in fact, now you have to come up with some locking mechanism)
  • You’re dealing with a complicated tubeframe that doesn’t obey clean planes (a la http://fsae.scripts.mit.edu/motorhead/images/c/c6/2011_frame.jpg )
  • Analysis becomes easier when dealing with a crossection without a direction (are you doing the analysis? If you are, it’s still just as easy to do that with box tube)

Circles that go perpindicular to each other are hard. Box tube would make this thing an ease to manufacture. As would making the bogies flat and maybe adding an offset if needed.

Depending on the game, why is it necessary to steer the wheels? Would be surprised if that’s needed. Do some searching into FRC #33’s 2010 drivetrain; do something akin to that where they force a drop-center for normal operation and turning. If you drop the steering you can also consolidate your power plants/ gearboxes into one per side.

If you must steer the wheels, do something less silly than running a bevel gearbox that still leaves the motor sticking up and preventing you from doing 360 deg of revolution! Might it also be worth to just slap a COTS swerve module on every end rather than design your own?

The ‘averaging’ or tying-together mechanism is all-together skimpy. I liked something like the linkages you had a while back on something; I’d trust that load path much more than some miter gears that could skip and then your orientation is all wack.

In all, you’re basically downsizing a mars rover- which has certain engineering tradeoffs that don’t actually matter for your use. You’re not being inspired by it and taking this solution and re-implementing it in a more useful way. Understand why Mars rovers are built the way they are (they don’t have predictable terrain, equipment is extremely sensitive, sinking may be a concern, power efficiency is crucial).

Thanks for your feedback! I totally agree with you- this drivetrain isn’t practical for any kind of game that we’ve seen before. As I mentioned before, it was mostly a fun project that I didn’t really design for any specific reason.

I actually designed a locking mechanism for the tubing. There’s just a little hole drilled in the end of each one, and a bolt goes through the joint and the tubing, holding them in the correct orientation. Additionally, (contrary to what you mentioned) the cylinders are not perpendicular to each other. The inside of each joint has a flat edge that the end of the tube rests against nice and flush.

My reasoning for sticking the motors up like that was

A)If they were rotated 90 degrees, the VP’s would have to deal with a lot of vertical load that they’re not good with,


B)Since the motors are mounted to the pivot module, rotating 360 degrees would just tangle the wires. I decided that if an all terrain game that required this kind of drivetrain were to arise, a true omnidirectional drivetrain wouldn’t really be necessary, so the motors wouldn’t need to be mounted in a way that let them rotate 360.

I’m honestly not too worried about the miter gears slipping… if it works on mars rovers and car differentials it’ll probably work here.

I think that you could make it a little simpler and more robust, without losing much functionality, if you got rid of the swerve modules and had a single speed gearbox to power each side. The torque would be transmitted through chain between the rotating points.

Also, I agree with the other poster - using square tubing and gussets with bearing holes in them would be simpler to manufacture than the circular tubing you’re using now. I think the point of what he was saying was that you should use circular tubing when the axes of the tubing aren’t planar, presumably since you couldn’t take advantage of the level faces of the square tubing.

I think your point about the wheels not needing to turn a full rotation is also true. And the miter gears are fine as long as you make sure they are rigidly held in place to prevent slipping.

Is there any chance you’d be willing to put the CAD model on GrabCAD? I’d love to get a more in depth view of the model.

I’m totally willing to! I’ve actually wanted to start posting stuff there for a while. The only issue is my team uses a “core components” folder where we source our basic parts (bearings, shaft collars, etc…) so I’d have to either copy to components out or upload the 17gb folder to grabcad. I’d be happy to do it though!

If you’re using SolidWorks (looks like it based on some of your prior posts) you can do whats called a pack and go. If you go to the file tab at the top right, about half way down you will see the option for it. This will take all associated files of an assembly (drawings, parts and sub-assemblies) and place them into one folder with all links made. It makes sharing files much easier and is great for scenarios such as this.

That’s awesome, thanks for sharing! I’ll go ahead and upload it then.

For all of you who want to see the CAD files, here they are!

I was kind in a rush while doing this, so I didn’t name features/use super great cad practices. It’s fine though.


Great job CAD’ing this concept.
Keep up the great work and don’t get discouraged by all the feedback.
The CD community sometimes gets hung up in the details and forgets that sometimes you just want to create something new and exciting.
I really enjoyed looking through your design and can’t wait to see what you do next!!!