we are building a 6WD and were trying to decide what wheels to use, the 8in rubber KoP wheels or 6in placation?
Our team usually goes with 8" wheels, not the KoP ones, though. Kind of depends on what gear ratio you are going with and how fast you want to go. It also depends a lot on the material. We always go with a fairly soft rubber for maximum traction.
It’s ultimately going to come down to what you’re wanting your drive system to do. 6" and 8" wheels are going to require slightly different gearing and are going to have somewhat different torque and speed outputs. In addition, the plaction wheels are most likely going to have a higher coefficient of friction than the KoP wheels, though I haven’t tested this so don’t quote me for truth on that. I would certainly believe that the softer rubber on a plaction is going to have more grip than the harder rubber of the kit wheels, though.
If you don’t mind doing a bit of reading, I would recommend this thread discussing the differences of different wheel diameters. They go a bit more in depth than I have here
Just to warn you, the KOP wheels are somewhat low traction
4 in wheels are the best for this game, because there are no obsticals, you need a low center of gravity and you need traction. Thats why the best wheels are 4in Plaction wheels.:yikes:
Thats what I was about to say, 4 inch wheels. The kit gearboxes are geared pretty fast, so smaller wheels make better use of that speed.
For us, the question is always, “what is the smallest wheel we can safely run?”.
This year, it’s 4".
Andymark has great 4" wheels for cheap that will give you traction with the best of teams.
I recommended the 4" plaction wheels for a dead axle setup (the bearing has wheels in it and rotates on a shaft, and the wheel is directly driven by an attached sprocket), and the 4" performance wheels for a live axle setup (wheel driven by hexed or keyed hub).
The lower wheels make it way easier to get a low CG, among many other benefits.
Thats exactly what we are doing! you read my mind! 
Our CG as of now is under 2 inches from the floor. Small wheels are the best!
ignore that we don’t have a manipulator yet, just for a second… 
We’re fans of 4" wheels as well.
As many have mentioned it all comes out in the wash depending on gearing.
The KoP wheels have a coefficient of friction of 0.9, compared to 1.2/1.3 for the plaction wheels with wedgetop/roughtop tread.
I am a fan of keeping the CG low, but for the purposes of high-speed impacts you want to keep your CG inside the bumper zone, which happens to be pretty darn low this year: 1" to 7" IIRC.
FWIW we are planning on using 6wd with 6" plaction in the middle where most of the grip is generated, and 2007 FIRST wheels on the corners (you can buy them from AM). They are a softer rubber than the 2011 KoP wheel so they have a CoF of 1.
We originally tried to design for 3" Colson wheels to eliminate a gearbox stage, but are going with 4" wheels to aid ground clearance.
For the 4in plaction wheels, would you go with roughtop or wedgetop tread?
AndyMark quotes a coefficient of friction of 1.2 for the wedge-top and 1.3 for the rough-top. I do not know if there are other advantages to either though.
So which of the two treads would you use, on your wheels?
Higher coefficient of friction is generally better. It means your more likely to be torque limited instead of traction limited. Though a difference of .1 probably won’t make a difference.
Our team went with the wedgetop tread. I have heard that the roughtop has a tendency to wear down faster than the wedgetop.
Our team went with the 6" plaction wheels. I really wanted to use the 4" plactions, but it was going to get a little tricky with the KOP frame and wanting to keep it as simple as possible. The more time we spent on our drivebase is the less time we have driving it!
We are using rough-top in the middle of our dropped-center 6wd for maximum grip.
We’re going with a 6WD 6" dropped pneumatic in the center and with omnis on the corners.