One of the most common types of drivetrain is known as a skid steer drivetrain, which may also be referred to as a tank drive.

A skid steer drivetrain consists of two independent sets of powered wheels (or treads), one on each side of its chassis.

By running the sides of the drivetrain at different speeds, it is possible to steer the robot in arcs.

Drivetrain Turning

One of the major attributes that defines a drivetrain’s performance is how well it turns. There are two main properties that affect drivetrain turning: turning torque and turning scrub.

This drivetrain is also capable of a zero-radius turn (it will spin in place) if the sides are run at the same speed in opposite directions.

In the previous robot, the wheels on the left and right of the robot are drive wheels, while the wheels on the front and back of the robot are wheels only for stability.

When the right wheel moves forward and the left wheel moves in reverse, the robot spins left.

This occurs because the left and right drive wheels are each providing drive force on the ground. This force provides a torque about the center of the robot.

Each wheel adds to this torque, causing the robot to turn. This is called the turning torque. This torque is the force applied by each wheel, multiplied by the distance the wheels from the center of rotation.

For the robot to turn, the front and rear wheels need to slide sideways across the floor. There is a frictional force between the wheels and the ground.

This force of friction causes a torque which opposes the turn; this is known as scrub torque.

Scrub torque is the result of the frictional force of the wheels multiplied by their distance from the center of rotation.

For a robot to turn, the turning torque needs to exceed the scrub torque.

For the purposes of simplification, the above robot design makes it easy to see the difference between scrub torque and turning torque because of the configuration of its wheels;

most robot configurations are not this straight forward. A typical 4WD robot configuration is more like the one shown in the following illustration:

In this case, all four wheels contribute to the turning torque, and all four wheels contribute to the scrub torque.

Each wheel applies some force that contributes to turning, and each wheel needs to slide sideways and contributes some friction to scrub.

Turning torque and turning scrub are both torques about the robot’s center of rotation.

As you know from Unit 5: Speed, Power, Torque and DC Motors, a torque is defined by a force at some distance from a center of rotation. 

Now that you understand the components of the torques that affect robot turning, you can begin to understand how to alter them to make a robot turn more effectively. 

The turning scrub needs to be less than the turning torque for the robot to be effective.

How do you reduce the turning scrub? 

You can do this by reducing the force of friction between the wheel and the ground; you can reduce this friction by either reducing the normal force (robot weight) resting on the wheel, or reducing the coefficient of friction of the wheel itself.

Similarly, you can increase the turning torque.

You do this by increasing the force of friction between the wheel and the ground; 

you can increase this friction by either increasing the normal force (robot weight) resting on the wheel, or increasing the coefficient of friction of the wheel itself.

Notice that to make the robot turn by reducing turning scrub, you need to decrease wheel friction.

But to increase turning torque, you need to increase wheel friction; this means you may not be affecting the overall turning ability of the robot.

Often, the most effective way to affect robot turning is to modify the chassis configuration.

This long and narrow drivetrain configuration will likely have poor turning characteristics because of its low turning torque and high turning scrub.

This drivetrain configuration is short and wide and will likely have good turning characteristics because of its high turning torque and low turning scrub.