[IKE]

My biggest piece of advice:

While asking for guidance and rules of thumb are a good way to start, learning how to test and tune a chassis quickly is where you will get a lot of benefit.

Center "drop" is there so that there is more normal force on the inner wheels than the outer. This reduce scrub (lateral friction which counteracts steering and turning). Too much drop will actually have the wheels off of the ground, and can cause tipping during starting and stopping depending on Center of gravity height and relative position.

Reduction in scrub can make a world of difference in smoothness and power draw when turning. Too little scrub can make a chassis feel "squirrely" or unstable. A good testing program will have you adjust things until you no longer like the performance (a different negative attribute shows up like pitching or difficulty driving straight), and then backing off that parameter to find a happy medium.

Make sure you run your tests at weight and on similar carpet. Testing on smooth concrete and only 50 lbs will lead to very different results than testing at 120-150 lbs on carpet. This is especially true for pneumatic tires. With hard wheels, the primarily "spring" element is the pile depth of the carpet, thus teams tend to run lower drops (1/16" to 3/16"). With pneumatic tires, the tires will also spring down a bit which varies a lot depending on tire pressure.

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Lastly, with regards to 6" wheels, you are at a disadvantage to impacting a 4" beam with the 6 inch wheels. In order to go over it, you will need some sort of "leading edge" or guide. Even with COTS chassis, this is important to consider. That is not to say a 6" tire will not go over, You just have to be a bit more clever. I saw a team this year using 4" wheels and it would go over the Rock wall just fine, but they were very clever with other chassis elements to help them maneuver (494/70).

Do some searching, and you will find good testing programs other teams have conducted. I know 234 has some nice white papers out, and there are several other threads discussion chassis and mobility.

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Lastly,
Buy extra sprockets to play around with gear ratios a bit. Use the JVN calculator, but then compare your results with the calculator, and try other ratios (say plus and minus 20%) to see what you think. Several years I have seen teams "go faster" by gearing slower... For many FRC games, robots are doing short sprints of about 12 to 16 feet. Sometimes sacrificing top speed will help improve time to distance.