How to determine the required torque while choosing a DC motor?
Here is a simple scenario. Think of a 4WD or 2WD vehicle. The max load the vehicle will have is 100kg.
Q1. How do I determine the required torque for the motor?
Q2. Should the Rated Torque(Kg-cm) more than 100 or the Stall Torque more than 100
Q3. And if use 2 Motor rated at 50kg-cm torque can it carry a load of 100kg?
Need to specify how much you will be accelerating the vehicle. Let's call this $a$. For example, you could decide that you eventually want the vehicle to travel 1 m/s and you want to get to that velocity from a complete stop in 10 s. That means you will need to support an acceleration of 0.1 m/s$^2$.
Once we have $a$, we know that the force required to accelerate the vehicle will be $F = ma$, where $m$ is the vehicle weight.
The torque applied to each wheel will be $T = Fr$, where $r$ is the wheel radius.
You also need to specify the final speed you wish the vehicle to travel at. Let's call it $v_f$.
If $r$ is the wheel radius, then $C = 2\pi r$ is the circumference of the wheel. The vehicle will travel a distance of $C$ per each wheel revolution. If $w$ is the wheel rotation rate, then $v_f = C\omega = 2\pi r\omega$.
Supposing that each wheel uses a gear with ratio $N$, the final rotation rate of the motor at each wheel is $\omega_m = Nv_f/2\pi r$.
Start off assuming $N=1$ (i.e. no gear - direct drive by motor) and look for a motor that can support a repeated peak torque of $T$ and rated output speed of $w_m$.
If $N=1$ doesn't work, try $N=2$, $N=4$, ... up to $N=160$ or so. Look for a motor that can support a repeated peak torque of $T/N$ and rated output speed of $w_m$ and make sure that the gear can support the repeated output torque of $T$. Above that gear ratio (if you really need to go that high) you aren't likely to find many solutions.