My question is about a square shaped differential wheeled robot, I built, that has a hub-motor (BLDC motor) attached to each corner, i.e., the wheels on the same side, either left or right, have the same velocity and direction (skid-steer). Currently, I’m able to move it back and forth but I’m not able to steer it by using different wheel speeds, i.e., left side spinning faster than the right side and vice versa, or let alone to turn it in place, i.e., left side spinning at the same speed as the right side but with opposite direction. In the case of using different wheel speeds, it keeps going forward. When trying to make it turn in place, it just doesn’t move at all. I’ve tested the wheels spinning off the ground, and they did spin according to the differential drive system. The tests were performed on asphalt. The wheels are hub-motors for e-bikes (bike tires).
What could be the problem? Not enough power (torque) in the hub-motors? the geometry of the robot? What calculations I can do to calculate the power necessary of each motor to steer it with different wheel speeds or turn it in place?