Re: pic: Coax Crab, another variation.
 

Mmm, Craig you could make it so the robot could turn if you used two motors to control the rotation. You could have a warthog esk drive. Your cads make me want to start cadding again =]

Re: pic: Coax Crab, another variation.
 

This particular crab uses a "powerplant" style gearbox to drive it. As you can tell, there are no motors in the individual crab modules, but rather one cluster of them in an isolated location. If you look, you'll notice two sprockets on each of the 5 vertical shafts. Those 5 shafts are all connected with the chain. So each shaft is connected to the one before it and the one after it.

What you may be confused on is how it turns. Right now, it doesn't. But it looked like he was planning on adding some timing belts. The steering would come from a separate motor and not the powerplant. So the shafts are spinning on the same axis as the modules rotate. They call this a "coaxial" design.

Below are pics of another crab with very similar setup but different frame. It should give you a better idea as far as the gearbox works.

http://www.chiefdelphi.com/media/photos/28441
http://www.chiefdelphi.com/media/photos/28442
http://www.chiefdelphi.com/media/photos/28443
http://www.chiefdelphi.com/media/photos/28952

Re: pic: Coax Crab, another variation.
 

Joe, I think his intentions were to show 118's method of crab. Which doesn't really have a correct orientation. But I do like your suggestion a lot. Turning a crab drive tank style is very frusterating and really the best way I've seen is to have two sets of steering... or 6 wheel crab...

Re: pic: Coax Crab, another variation.
 

Exactly. Also, WOW this is old.... I re-did this one, making it about 2/3rds the weight, and MUCH shorter. The new one can be checked out here.

Also, 6 wheel crabs are a step in the wrong direction. You don't gain anything out of the extra two wheels, except for a more complex frame, more parts to machine, more weight, and more hassle. Crab is a LOT of work, and should only be chosen if the game demands it. (Note: in my opinion, 2006 was the most recent game that could be owned by a crab.)

Re: pic: Coax Crab, another variation.
 

Tell that to Nasa. In all honestly though, I was joking. It's still really cool...

Craig, I was hoping to get my hands on the cad files for your "Bullet Proof Drive". I tried shooting you an email but I must've sent it to the wrong address. If you get a chance, I'd appreciate it.

burchardta@uwplatt.edu

Re: pic: Coax Crab, another variation.
 

The only advantage that 6 crab modules provides is when mounting hills/obstacles is required. However, if climbing is required, a team is better off with a simpler skid steer system. It's lighter, MUCH simpler to build, and has a few orders of magnitude less failure points.

I also emailed out the BPD CAD today.

Re: pic: Coax Crab, another variation.
 

Thanks man, I think I finally understand it now.

Yeah, sorry about bumping an old thread, I just did a search on crab drive, and this came up. Those new pictures you posted make it a whole lot easier to see what is going on.

Re: pic: Coax Crab, another variation.
 

1075's 2009 iteration of crabdrive isnt a powerplant type, but it could be... since all 4 motors get the same output (except for our 'tank' steermode).

We use 4 steering modes. Car (rear steering locked at center, front steers with x axis of joystick), Monster(same as car, but rear steering goes exactly opposite to the front)(like a monster truck), Strafe (rear steering goes the same as the front for translational movement), and Tank (front and rear steer lock to center and the left and right sets of wheels operate as a 4wd setup)

These modes are all driver-selectable on the field during a match. Our driver frequently switches between them to maximize his mobility.