pic: High Traction Drivetrain Concept

[Steven Smith]

You might consider looking at this post, it's for VEX but concepts apply.

Vex Forums

Not to discourage you, it's great to see people looking and figuring things out for themselves, but most combinations of "what happens if I put X wheel in Y position" have been pretty thoroughly looked at for tank drivetrains. You are just left with a bunch of trade-offs, and the magic is in selecting the right drivetrain for what you need it to do, not necessarily trying to find a magic drivetrain that is far superior in all cases.

In this case, let's compare to a 4 wheel omni (basically take out your center wheels), an 8 wheel omni, and an 8 wheel tank.

Increasing the number of wheels is usually a factor of what you need to traverse. More wheels reduces the amount of space between the wheels, allowing it to claw over. Additionally, by staggering the drop of the wheels, such as in a 6 wheel drop center (or 8... 10... 12... whatever), you can trade off stability (rocking) for reducing turning scrub. Adding more wheels can also give you a little more "middle stability" in the neutral state.

An omni wheel already has pretty good traction in the forward direction, and can push fairly well. What it doesn't have is any resistance to lateral movement. If you look at a 4 wheel omni drive (like the 2014 JVN buildBlitz, or any number of other ones), it almost drives like a drifting car. With drive practice, you can do some interesting things to throw it around... but you are also at a higher risk of being spun.

So to your drivetrain, you have a risk if the omni wheels are preferentially weighted (design tolerances, weight shift under acceleration) of acting more like a 4 wheel omni drive. You probably don't have substantially more pushing power than an 8 wheel omni drive, but you have a little more resistance to being pushed sideways... while still more at risk of being spun. So basically, you have a set of tradeoffs that gives you some of the benefits of a 4 wheel omni and some of the benefits of an 8 wheel drop center tank, some of the disadvantages of either, etc. At the risk of beating this horse dead, I just want to emphasize that you don't get the advantages of both as well as dodging the disadvantages of both, it is just a blend.

That being said, depending on the year, it might all be a valid set of trade-offs and lead to a pretty solid drive train. Build it and see what you think

Edit: Also as a note, I'm not sure how significant the "lower bumper" comment is. Most drop drive trains (outside of pneumatic wheel years like last year) are on the order of 1/16" to 1/8". When you are talking about a several inch bumper contact patch, I think the drop center difference will be lost in the noise when compared to other variables (bumper construction, driver practice) when you talk about defensive play. (just imho)