Ok I€™ll go over the mathematics behind an effective disc as it may be helpful for others.
It all comes back to something I like to call cutter bite. A large cutter bite means that your disc, bar, drum etc will transfer more energy as a smack whereas a small cutter bite will result in cutting type action (think axe vs chainsaw. Axe takes big chunks whereas chainsaw takes many small bites to perform the same function)
To work out the cutter bite for any design, divide your rpm by 60 to get the number of rotations per second. For example we will use a disc with 2 teeth spinning at 4000rpm (fairly typical). So we would have a rotational speed of 4000/60 = 66.67rps. We then must divide 1 second by this number so 1/66.67 = 0.0150s. Then divide this number by the number of teeth in your disc, drum, bar etc. So in our example 0.0150/2 = 0.0075 (units are in seconds per tooth). What this essentially tells us is that if we have a disc with two teeth spinning at 4000rpm, it takes 0.0075 of a second for one tooth to leave a position in space before the tooth behind takes up this space again. If you also play with the numbers you will realize that the more teeth you have, the less this time is. So for 4 teeth it is 0.00375seconds/tooth and 8 teeth 0.001875seconds/tooth.
Now what we are interested in is a distance. The distance that the robot will travel forward in the time between teeth. Or in other words, how far into an opponent we will get to transfer energy.
To know this we have to know the forwards velocity of our robot. So lets take a conservative number of 1m/s (if you watch videos online not many spinner collisions are at high speed) or roughly 2mph. So we have a velocity and a time. To get the distance multiply the two. So for our example with two teeth moving at 1m/s we get 0.0075*1 = 0.0075m or 7.5mm. So what this means is that your tooth will only get a maximum of 7.5mm into an opponent before it hits at 1m/s forward velocity of your robot.
This also highlights why it is beneficial to have as few teeth as possible on a disc or drum with 1 being the optimum.
The other thing to remember though is that when the disc rpm drops, the cutter bite increases however with a lower rpm you have lower kinetic energy in the disc so it€™s all a balancing act.