Half Omni, half tank, best of both worlds?

Team 81A of the Downingtown VEXMen have created this interesting base. It allows a pretty decent pivot about 1" in front of the center of the robot. It also has pretty good pushing power to play some 1-1 defense. The two front wheels allow that extra grip to move things along. I don’t think they have an offical name yet, but they did pretty well at yesterdays Technomage League event.

With the two traction wheels at the front, does this strafe in a straight line? It seems like the strafing would be either very slow or arc in some fashion.

Very nice configuration. Looks a bit like a half-track. Perhaps “omni half-track”?


  • Dean

Pictures? I’m a sucker for a new drive-system.

OHT Drive?


Exothermic teams often refer to this as a “bucket drive.” I’ve never seen a team implement it successfully because the angled rear wheels make it inherently slower than a tank, while the forward solid tires add friction when turning in place (since most robots have a center of mass towards the rear). So you kinda have the worst of both worlds, since you don’t drive as fast as a tank, you can’t strafe like a holonomic or h-drive, and you can’t turn as fast as any of the three. But hey, if it works, it works.

You won’t be able to strafe, and when driving forwards you back wheels will have a faster forwards speed than your front wheels, which will kill your back motors

The solution is kinda simple y’all…

If you split the wheel vectors into their resulting vectors, it’s a 45-degree angle. The cos(45) or sin(45) is sqrt(2)/2, which is roughly 0.707.

So, just scale the straight-facing wheels back 30 percent.

I think the goal for this drive system wasn’t so much to strafe. It’s a more effective way of turning, while still maintaining somewhat of a relatively fast forward motion.
As to adding friction in turning, I believe that’s the idea. It’s a compromise. Nothing says it’s gonna be perfect in either camp, but it provides traction, yet faster turning.

I must say, I really like the idea of OHT. But I’ll probably stick with holomonic, just cause that’s what I know, and those are so much fun. :slight_smile:

While it’s true that it increases turn speed for a TANK drive, it is still inferior in all manners to a full holonomic drive. The only benefits I see are more pushing power, slightly more forward speed and a little more space in the front of the chassis. However the pushing power and forward speed are between the capabilities of a full holonomic/full tank and the strafing provided by a full holonomic would outweigh the middling capabilities of the OHT.

Here is something to think about when talking about omni-tank systems


  • Andrew

Might I say that this conversation is now irrelevant now that we have Mecanum wheels? :slight_smile:

You may, but I would disagree. I’ve learnt a lot, and I’m sure many others have also learnt a lot from this. In terms of the competition it may be now irrelevant. In terms of increasing knowledge and understanding, it most definitely is not irrelevant.

I’m possibly not seeing the funny side as much as I ought to be.

It’s actually a lot more space (when looking a the 18" max) and that was the primary driver. But the better mobility and the “push power” were the other goals. The “push” isn’t really as much as getting some extra traction to move the game play mechanics around the field.

Not sure the new wheels would make a difference in the discussion.

It brings out the best thing about engineering “everything is a trade off, a good design makes good trades”.

The back wheels use 1/2 of Cody’s famous omni drive code, but basically the math works out to what UncleJoe said in his post.