I am trying to build an autonomous robot with six wheel drive. I am still working on the drive train, because I am trying to find a way to get a differential to keep the wheels from fighting or causing each other problems. So I was writing to see if there was a way to program a differential into VEX or if there was a differential I could buy.
Are we to assume that the robot is set up more like a car than a tank? The latter tends to be more popular 'round these parts.
(By the way, welcome to the forums–it’s best to only post a question in one forum. 
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welcome again. A little more info as to how the vex bot is setup will help us out greatly. I will star with this.
If you drive each side (3 wheels) together, you will drive like a tank and no not need a differential. If you are going for like a 6 wheel drive off road vehical (I am not positive) then you would need a diferential. Let us know what you are thinking and I will think about how to do this.
If you’re using tank style drive, three wheels on a side, then you can consider using the Vex omni-wheels in the fore and aft wheel positions. This will give you nearly the same tractive force while improving turning substantially.
In this configuration you can run with 1, 2, or 3 motors per side. The only problem with this solution is you’re limited to using the small wheels that match the omni-wheel diameter. We haven’t found a reasonable vex gear ratio to use the omnis with the larger Vex wheels.
The robot is set up in the tank mode.
I drive all three wheels on one side together. I plan on driving the robot on rough terrain; which is why I am using the five inch tires. The reason I think I need a differential is the fact that when the robot turns, it is causing the other wheels to jerk or have problems turning.
I do have three motors on each side, one for each wheel, but I am using the five inch wheels. In my first build with the smaller wheels I did consider this, but when I decided I wanted the robot to be able to handle rough terrain I switch to the five inch wheels.
The classic FRC solution: have the middle wheel slightly lower than the others. I think you could pull it off with VEX, just move the middle wheel down a hole or two and you should be fine.
or the other clasic First fix, zipties if you wrap zip ties around the front and back sets of wheels, the wheels become a cheep omni wheel 