Walter,
Throughout our prototyping process we tried numerous different sidewall materials including:
- wood
- aluminum
- lexan
- delrin
- teflon
- gum rubber
- various types of grip tape
- hockey tape (for our friends to the north)
- wedge top
- rough top
From this testing we determined that rough top was the best option available. The goal here is to make sure you impart the maximum amount of energy from your shooter wheel to the disc.
Basically it all boils down to Newtonian physics, "For every action there is an equal and opposite reaction." The equal and opposite action we want is maximum rotational inertia of the disc as it exits the shooter, and minimal slip between the shooter wheel, the disc, and the opposite wall. By using a material with a higher coefficient of friction on the wall opposite your shooter wheel, you are minimizing the energy lost to slip when the disc is in contact with the wheel.
The reason we chose rough top tread over other high friction materials is the compliance it added to our system. This allowed us to maximize the disc contact patch on the wall, as well as giving some play in our wall to wheel distance.