Crazy Crab

OK so at the end of the chicago Regional my team and i decided that it will be a good idea in the future to have a crab drive system. We dont know much about how to make one so we are looking for help. If anyone has any suggestions that would be great. Even better would be some Inventor drawings if you ahve them handy. My email is [email protected] and i am grateful to anyone who contributes to the future of team 1525 the WARBOTS!

Crab drive can be quite beneficial. There are a lot of ways to design them, too. I’d be happy to shoot you an email and get some discussion going. I worked on crab steering in 2002 and 2003 with the ThunderChickens and I’m over in Chicago now, not too far from your team. Great job at Midwest, too!

I would say that if you are going to help out, help everyone out and post here with attachments instead of personal email :slight_smile:

My goal this summer is building a crab so during build season, we can make another one with build experience

I’ve got some cads, though you should take our design with a grain of salt, we were unable to get encoders, and found that it would be really overweight so it never got built. It’s actually nearing completion, but only as an off-season project. I’ll put up the cads later.

how extensive are your manufacturing facilities? I drew our warthog drive to be cut using a 20 year old cnc using 1/8 and 1/4 end mills. But it is very bulky. We managed to producing a functioning omni directional drive completely within the 6 weeks. No pre build or pre design.

I just put up some random jpegs of the ideas that were put together to build the drive train.

Just a suggestion: 111 and 118 are known for their crabs. 1625 likes them too. You might want to look up 118 in the 2007 “Behind the Design” book (or here), and talk to all three teams. Between them, they can tell you more than you ever wanted to know.

Remember, not every game requires crab. Don’t lock yourself into a design before the next game is even announced. However, it isn’t a bad idea to get educated so that you know what your capabilites would be and what it would take to get the functionality working. I would suggest stopping by our field some day/night when Raul is there. He has so many years of crab under his belt that he’ll be able to give you all the tips you can handle. Send him a PM to set up a time.

Some things to keep in mind-

A crab/swerve drive is not the only kind of omni-directional mobility base. The following is taken out of an email I sent to my team during the 2008 season.

(Note this email was sent in 2008: carpet was present and we could use any wheels we wanted. CAUTION: HEURISTIC REASONING BELOW. Do not flame me for inadvertently “attacking” a design of yours.)

"Omni-Directional chassis are really cool


  1. Easy to build, one motor on each wheel.
  2. Accelerates slowly, wheel slippage can be a problem
  3. Has trouble climbing ramps (have heard reports of slipping to weight-biased when climbing an incline)
  4. Straight strafing is a function of even weight distribution upon each wheel.
  5. Can be pushed around easy due to minimal traction

Verdict: I dislike them. Defensive robots will have trouble with the wheel slippage and offensive bots are easily defended against on these wheels.


  1. Easy to build, one motor on each omni-directional wheel
  2. Accelerates well, no wheel slippage (in the inline direction)
  3. Seems like it only works on flat surfaces. I doubt they could climb ramps with any substantial incline.
  4. Uneven payload distribution on robot seems like it would not affect strafing as much as it does on the mecanums.
  5. Can be pushed around (852 coded an anti-pushing response on their robot though… but there’s only so much that the motors can do to compensate.

Verdict: I like them in every way except the fact that they can be pushed around.


  1. Hard to build, must construct individual swerve modules, a motor power plant, and a mechanism to steer the pods two by two.
  2. Accelerates well and perfectly straight! (all four wheels are powered simultaneously from the same power plant)
  3. Can climb ramps just as well as any other four-wheel chassis (assuming ground clearance is high enough)
  4. Will always strafe straight assuming the steering motors are calibrated correctly.
  5. Will stand fast in the face of being pushed. No omni-wheels are needed, robot always has good traction.

Verdict: It’s the proper emulation of an omni-directional chassis. I like every aspect of it except it’s complexity.

Take a look at the link of the swerve chassis. The entire frame is made from stock. We could, theoretically, configure our own stock chassis and have meadows (our machine shop) only machine the powerplant’s gearbox and individual swerve modules. The machined parts will be considerably smaller, but more intricate. How does one of the swerve modules work?"

AGAIN: that’s soft logic above! I have never worked with a holonomic or a mecanum drive-train before, so do not quote me on anything. **These thoughts stem from the perceptions of a somewhat-astute observer. **

After you’re through digesting that, consider my team’s design of a crab/swerve drive this year:

The four wheels are all driven from a single toughbox (that’s not ideal because there’s no differential). Steering is done by two different motors. The front wheel modules are steered separately from the rear two. This allows the robot to make turning arcs, like a “warthog car” if the wheels twist in opposite directions. If you sync the twist rotation of the rear steering motor with that of the front steering motor, “crab-like” control is available. The robot will face a singular direction while “strafing” around. In this game, crab mode allows the robot to “orbit” the trailer.

Our experience with this told us that assembly was the easy part - control gets really complicated.

For instance, forget about TWO steering motors if you can’t close a position loop around them. Without a proper feedback system they will turn at different rates - which isn’t good when you want to sync the two motors for any sort of integrated control at all - warthog car or crab.

Also, decide whether or not you want the wheel modules to twist indefinitely in any one direction. If you do, then you disallow the use of a potentiometer to sense your rotation because they have a twist limit. You’re going to have to use a rotary encoder (or something) instead - which brings up even more problems, like alignment. The encoders that AM gives us are not absolute encoders (they don’t know what ‘forward’ is), so a separate sensor system may have to be implemented to position your wheels straight. You can simplify this a little by using absolute encoders which can give you a displacement from a zero point, but software may still get funky if you choose to change your sprocket/pulley/gear/whatever ratios between your steering motor and your wheel module.

What’s the point? Control gets difficult. Again, this is what I have observed on my team - I’m not saying that you will run into the same suite of problems we ran into, AND I’M CERTAINLY NOT SAYING THAT YOU MUST SOLVE THESE PROBLEMS EXACTLY LIKE WE DID.

Just note that fabricating the drive-train is only half of the challenge, controlling the drive-train is something that is often over looked like red-blooded mechies like myself.

A taste of software development:

I think it’s great if your team wants to try something new next year (getting locked into a routine design isn’t stimulating… and it’s not FIRST), but you would really manage your risks if you prototyped the thing - both the mechanical scheme and the control scheme. For design guidance: I suspect most users on Delphi would point over to CraigHickman and his posted CADs.

Best Regards,
Sam N.

Hey cool, one of my Crabs got mentioned! To be totally honest, that was a previous iteration. I hadn’t published this until now, but here’s the latest version (assuming we’re allowed to use our own wheels again).

If you’re going to do a crab, you’re really going to have to commit. Get your machining resources in line, and make sure you can do it before you try. A crab frame doesn’t convert to a 6 wheel/4wheel if you mess up, not very well anyway.

Good luck!

And it uses screws! I love it!

We attempted a very modular crab drive this year (actually our first time building one). Basically the system is broken up into two almost identical modules that can be removed and replaced in the robot with only 4 bolts per side.

Each side module fits inside a 2" x 6" aluminum tube which keeps it compact and also safe since there are no open running chains. Most of the pocketing seen in the pictures is for getting your fingers in there to assemble it and also to just look nice :yikes: That being said, they do take patience to assemble! Have some modifications in mind for next time to make assembly easier but overall we were happy with how it turned out.

They also have integrated, fairly simple chain tensioners which serve to also wrap the chain around the drive sprockets in addition to tensioning.

We have a spare module made up to swap in if we have a problem with one of them as well as for display. If you are in Finger Lakes this weekend, or Waterloo stop by to have a closer look. Can’t wait to try these babies out in competition!!

Antoine Trabulsi
Lead Mentor
Beaverworx - Team 2609

Craig, your stuff rocks. That would be a pretty sick frame to have.:ahh:

Ok, I apologize for the terrible lighting, and the lazy sketches, but here’s out swivel drive (concept). There are 3 modules, each independently powered (for perfect differential speed) and independently rotated (for perfect turning). The way the chassis is built arround the module requires exactly 6" on either side, leaving the maximum available space for ball collection. The modules are made from aluminum square tubing, all machined on a manual mill and bolted together. There is only one weld, the round tube at the top is welded down into the 1/4" square (milled to a u). The mess at the top of the module is (top to bottom) mounting block with bushing, thrust bearing, aluminum spacer (milled from 1/4" plate), 30 tooth #35 sprocket, another spacer. each drive shaft is powered by 1 chip, and one other motors (the front 2 are densos and the back is a globe). Anyway, I’m just rambling, hopefully this thing gets assembled and wired sometime soon, and I’ll have some more usefull information.

My team did crab this year. We have no machining resources available in our build space, and our primary machining sponsors work only in steel. We had three plates machined out of 16 gauge steel and used toughboxes for each wheel “pod”. The system is steered with a single globe motor distributed to each pod by timing belt.

Even with the heavy steel parts, with conservative building techniques for the rest of the robot we have come up almost a pound light. Crab systems don’t have to be only for the well endowed teams.

Note- We’d probably need more steering torque if the game this year wasn’t played on hard plastic… so that’s more weight if you’re thinking longterm.

I have only one other piece of advice that hasn’t already been mentioned here:

Make sure you have a programmer ready and willing to tame the beast once it is built. I have seen many beautiful crab drives fail because of a lack of proper programming. It’s not an easy task, so be nice to your programmers now.

Wow that is some great stuff! Very nice. I’ve never actually seen a modular crab drive but that is just awesome and looks very doable. Mind sending over some CAD? Very impressive work. I’m def really impressed with your overall robot and team. Amazing, can’t wait to see it in action!

We designed a swerve/crab drive last summer and built it for this year. Our main goals were simplicity and modularity as we do not have a factory/CnC to help, everything is hand built. I have several more efficient and complicated CAD’s and designs at home, I’ll see if I can dig it up later.

If you are interested, you can view our current system here:

How are you managing to keep the wheel’s vertical if their is only one support?

Sorry for the delay in getting the CAD up guys. Wanted to wait a bit anyways so I could give you the full scoop after its been battle tested through 2 regionals. Seems a little better than just posting about CAD files.

Overall the system has been amazing and we have loved answering all the questions and showing the drive to everyone who came by to ask about it at both Finger Lakes and Waterloo. Being just a 2nd year team with a brand new driver using crab for the first time, it took a full regional to get the programming tweaked and the driver used to using this drive system. We obviously had a better time driving around for most of the Waterloo regional :slight_smile:

We used 1/2" aluminum drive shafts which I am happy to say have held up great (was a little worried to tell the truth). #25 chain for everything. The only real failure we had was one of the tensioner sprockets coming off the bearing that it was press fit onto at FLR. Probably not the best press fit in the first place, but we solved it by drilling 3 small holes into the sprocket to hold the sprocket on with #6 bolts.

I’ve uploaded the iges file (24MB) for anyone interested here:
just enter the short code into the box, then hit the “Click to download” button, should be all good.

Any other questions just ask, or feel free to come by in Atlanta as we should have the programming tweaked even more by then. Trying to get some good traction control going as well so should be fun!

On top of all of the suggestions here, browse CD-Media. It will take you several hours, but you will find drive trains you’re looking for. Once you find a good pic, hit the little heart icon so it saves it to your favorites for future reference. More importantly though, you’ll have a better idea of what questions to ask.

Another alternative is linkage drive.. It has the benefits of strafing & diagonal movements while also being simpler to implement than most common crab drive trains. This year (2009) my team did that drive train with 4 independent pneumatic pistons for the linkage actuation, and I must say that it was the best decision we’ve ever made post-kickoff. It was easy to prototype in 2 weeks, and has been the most reliable drive train we’ve created to date (though I wonder if that has to do with the fact that we have no chains…). I does have its cons, such as the fact that we think our current specific design would explode with high traction wheels on carpet, but there are challenges to overcome in any design if you think about it.