My advice is, when you are selecting a motor, search for the torque. Makers usually have pdf files with some usefull data like torque, proper voltage and rpm. They also have this info written on the product packages. So for your case you should buy (by my calculations) a 2.4 N.m motor. But here is the problem: the unity you are using is metric. Just look again in your original post and you will see that you wrote 20kg and 0.15m and so on ( your result gets, then, 2.31N.m) and those units belong to the metric system Unfortunately in U.S. and Britain they don't use metric system. From your post I could interpret that you misunderstood you result and interpreted it as being 2.31 g.cm, which is not enough. Converting the units 2.4N.m=240000g.cm and a motor with such a power is not affordable at all.
Since your description is lacking of lot of data I calculated the torque using the simple definition of force and torque Torque = force * radius = mass * accel * radius = 2.4 N.m.
If you want an advice, mine is: reduce the weight of your robot. He is too fat. 20kg is a monster(those are 4 packs of rice)(traumatized foreigner living in Japan who is sick of eating rive for breakfast, lunch and dinner
). I have no idea of the purpose of your robot but you said you are a beginner so I don't think it will be something complex full of heavystuff like batteries and lots of motors and so on. Simple robots can weight about 1kg or even less. Use plastic, correct batteries, an adequate microcontroller and plan well. I am sure you can reduce 80% of this weight.
Use not only 1 but 2 motors(one on each wheel)(in that case I recommend servos or stepper motor that are easier to control) or lots of motors connected to an axis. By doing this you will be able use weaker motors and/or get some extra torque.
Usually cheap DC motors and even some servos don't have enough torque but most of them have high RPM. By using gearboxes you can "convert" the excessive RPM in the necessary torque.
And if you really want to make a big and heavy robots, consider using bigger wheels as well. But be aware: avoid heavy wheels. Not only because of the mass but one should consider the rotational inertia as well, which will drain some extra power.
Anyway, I respect a beginner who recognizes the necessity of planning beforehand and calculating important aspects of a robot such as the torque of the wheel motor.Keep going and you might build something great
. Unfortunately, the calculations on the tutorials are very simple (no offence
). When building a robot there are a lot more variables to be considered like the number and efficiency of motors, friction, variable mass( in case the robot collect stuff), momentum, rotational inertia and momentum and so on. There are cases that one will have to solve diff. equations and/or look for data in tables. Of course, I am actually considering writing myself a more sophisticated, but relatively easy to understand, tutorial with some detailed explanations for the curious ones, both for mechanics and electronics.