What is the best set up for a tank drive 6 wheel drive? Will the robot turn correctly if we have all of the wheels be high traction wheels? Will it turn better if we use a high traction wheel in the middle and omnis on the sides? What do you think if the best set up for a Vex 6 wheel drive?
I think that the best 6 wheel setup (in most situations) is omnis on the outside and high traction wheels on the inside. Like this : (omni) (high traction) (omni). If you have all high traction wheels, the robot does stutter when doing point turns. But when using the omnis on the outside and high traction in the middle, you have to be careful, because the omnis are just a little bit bigger, and the high traction wheels don’t get much traction on the ground. Our team solved this problem by stretching a piece of non-slip around the wheel and zip-tying it in place. And if your chassis is too short to fit 3 wheels side by side, you can stagger the wheels, by placing the high traction wheels closer to the center of the robot.
I would definitely suggest omnis in the front and back. When we tested traction for all 6 wheels we had problems with the motors overheating and turning at low speed. When we switched to only traction in the center most of the issues went away. We did not test using all omnis since keeping center traction was acceptable for what the team wanted.
The answer, like so many things in robotics, is “it depends on your robot.” There are countless factors that come into play when selecting a drivetrain, such as weight, weight distribution, size, ratio of length/width, gear ratio, motor count, and so on. A drivetrain that works great for one robot may struggle to move another, and the only real way to learn to predict this is to learn the math behind it, and/or get lots of experience through trial and error.
Using 6 traction wheels in an inline arrangement often produces too much turning scrub. This can be overcome by building a drivebase that’s wider than it is long, or overpowering the friction with lots of drive motors, but generally, this isn’t ideal.
Another popular option is drop-center 6 wheel drive, where the center wheel is a bit lower than the others. This reduces the effective length of the robot’s wheel base, while still giving it a fair bit of stability. Additionally, drop center allows you to keep 6 high-traction wheels. Some manipulators can be negatively impacted by the natural “rock” of a drop-center, but it’s definitely something to look into.
I do agree with this, the 6wd drop center is one of the most common and successful drive-trains in FRC. In VEX it is a little harder to get the drop needed without affecting everything else on the robot. So this year I’m trying something new, a Simbotics style, raised back wheel drive-train. It’s a 6wd, with all 2.75 traction wheels, but the back wheel is just raised a whole. It should be finished next robot meeting, so I’ll take some pictures and video of how well it works.