8 wheel drives (drop center) share a lot of the same benefits as 6 wheel drives (drop center) along with a few key differences that may or may not be something that you can use to your advantage. The biggest difference, or at least the one most relevant here, is that an 8 wheel drive will have a tendency to rock much less often than a 6 wheel drive, assuming that the center of mass is reasonably close to the center of the robot. This creates a much more stable platform to build precision scoring mechanisms (think shooter) and can work to your advantage if 1*-2* of drive train rock will make a difference between scoring and not.
There's also the added benefit as mentioned before about 8 Wheel drives handling obstacles better than a similar 6WD, but there are ways around this. As a rule of thumb, if you're concerned with your robot potentially having a high center of mass, it's worth looking into an 8 wheel drive as it'll handle the crest/apex of most obstacles a lot more smoothly.
Small wheels have some pretty significant benefits, the most obvious of which being that they're lighter than a similarly constructed wheel that's larger in diameter. Some of the additional benefits come from how they can be integrated into your overall drive system, specifically that a smaller wheel will require less gear reduction for a target performance parameter when compared with a larger wheel which allows you to use smaller and/or fewer gears in your gear box, which obviously reduces system weight, but can also increase system efficiency - which is an added bonus.
The biggest downside to small wheels is probably increased wheel wear, since the wheel is going to have less tread (circumference) than a larger wheel. This may or may not be an issue considering the design and your intent, some teams like to be able to get an entire season out of wheel tread, while others have no problem replacing wheels per event, or even per match. Another notable downside is that a drive using smaller wheels (say 4" in the FRC context) can/will require a trick or two to handle significant obstacles, especially things like 2012's Barrier, but it's not all that hard.
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Originally Posted by nathannfm
What would you guys say the ideal ground clearance is for an 8 wheel bot with 4 inch wheels? (assuming "flat" field, like 2013)
And would the wheel type mater for your number ei. Colson, Versa, Performance with rough top?
Along the same lines what is the ideal center drop? I have heard 1/8 inch, is that pretty much the consensus?
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A lot of this is based on preference. In 2013, 341's Drive Train had ~.75" of clearance at the drive rails, and then ~1.875" at the belly pan. The lower clearance at the rail helped to minimize the chance discs being stuck in the drive, although it happened once or twice, and the higher clearance at the belly pan allowed us to drive over discs without noticing. More often than not, assuming the field is flat, you can get away with 1/8" to 1/4" more than the largest field 'bump'. (In 2013, the field bumps were ~1/2" at the pyramids, and in 2011 they were something like 3/8" - 1/2" at the towers)
Wheel type does effect the static (built in) drive clearance a bit, a wheel like a colson doesn't 'sink' into the carpet all that much at it's OD, but something like a versa wheel will sink in about 1/8". In my experience it's usually something best decided on a case by case basis, and if you're not sure it's easy enough to figure out by taking a wheel and pushing it into carpet by hand.
Drop is a bit of a preference number as well, but as a rule of thumb, somewhere between .090" and .125" is probably the safe minimum, although, you can usually get away with running 'more drop' than a comparable 6WD before you start having stability problems. On shorter wheel base 8WD's, I prefer to go with less drop, since it helps to make the drive track 'straight', but on long drives, I'll run a bit more drop to help with turning.