Bogey Wheel---- One of a number of modern tank-tread double-wheels used as an Idler/Tension Wheel.

Idler (Tension Wheel)----Component that transmits no power but guides or tensions a track, belt, or rope.

Tank Tread ---Individual links combined together to form a continuous section. On the robot, the tank tread distributes the vehicle's weight more evenly, enabling the robot to maneuver in loose sand, or on spongy surfaces that would otherwise cause the robot to sink.

Tank Tread Drive--- A drivetrain which utilizes a continuous track as a drive member; this track is driven by one or more drive sprockets.

Track---- A series of connected treads, drive sprocket(s), idler(s)/tension wheels(s), and sometimes support rollers and suspension components when necessary.

Sometimes people incorrectly refer to tracks as treads Sometimes people incorrectly refer to tracks as treads. Treads are actually components of the tracks, not the tracks themselves. Many treads are used to make up a track. Tracks are made up of treads, the drive sprocket, idlers/tension wheels, and if required, support rollers and suspension components. Track drive systems have been around since the early 1900s, but their major usage did not happen until World War I when a variety of different vehicles were developed to traverse the diverse terrains of Europe. Their diverse evolution continued in World War II and continues today.

Track Drive System Advantages One of the major advantages of a track drive system is the large contact area with the surface being traversed, effectively spreading the weight of the vehicle out over a large area so it does not sink into the surface. In fact, military tanks put less force on the ground per square inch than a person’s foot. Because of this, tracks provide substantial advantages in soft, muddy, sandy, rocky, and loose terrain such as snow.

They can also carry heavy loads which might sink on a wheeled vehicle They can also carry heavy loads which might sink on a wheeled vehicle. A famous example of this is the crawler-transporter used by NASA to carry space vehicles to the launch pad. The crawler-transporter can be seen below.

Another advantage tank tread drives have is their continuous contact patch from front to back. This enables them to cross over rough terrain without getting hung up or high-centered. Wheeled vehicles may end up riding down into ruts or holes that a tank tread drive will go right over.

Wheeled vehicles have a small contact patch with the ground, whereas tank tread drive vehicles have a large contact patch. Wheeled vehicles have great traction on hard surfaces like pavement, but on loose surfaces the force of the wheel is applied to a small area. If the force is great enough, it will cause the ground to break away and the vehicle will lose traction. The tank tread drive applies the force over a larger area, and as such the ground does not break away and the vehicle does not lose traction. For this reason, some vehicles rely on tank tread drives for higher traction.

Tank tread drives, unlike wheels, can also have varying angles of approach and departure. Different track profiles are good for different applications. When designing a track profile, it is important to account for surface and terrain conditions (slope of terrain, size of possible obstructions, surface material, and so on).

A basic track profile with just two sprockets is shown below A basic track profile with just two sprockets is shown below. This is extremely low profile but has a very low angle or approach and angle of departure. It is not good for climbing objects, but it has all the other advantages described above.

Adding a bogey wheel assembly turns the front of the vehicle into a ramp, making it a better climber. This is called a single ramp configuration.

An additional bogey wheel assembly creates a ramp on the back of the vehicle as well. This enables the machine to go over large obstacles either forward or backward. This is called a double ramp configuration, and is commonly found on military and construction vehicles.

The simplest and most recognizable single-track vehicle is the snowmobile. It provides a wide contact surface that enables it to move over snow with cleats to provide traction. This design must provide another mechanism for steering because the track can only provide forward or backward force.

Tank tread drives have a number of disadvantages as well Tank tread drives have a number of disadvantages as well. Tracks are ideal for a variety of situations, but they do have limitations. Because they contain so many moving parts, these drives are full of friction and require a lot of lubrication and maintenance. They are also difficult to change when compared to a tire. To change out one bad tread, the entire track must be removed. This is a difficult and time-consuming job.

Tracks do not have the cushioning effects of air-filled tires, so the ride can be really rough. Often tank tread drives require some additional form of suspension

Although the tank treads in the VEX Robotics Design System differ from treads in real life in many ways, VEX Treads are not subject to the lubrication and maintenance needs of real tracks. However, they do require proper tensioning. VEX Tank Treads come with four primary components: the tank tread links, drive wheels, single bogey wheel assemblies, and double bogey wheel assemblies. These components can be used to create a variety of tread profiles and configurations designed to overcome any obstacle a robot encounters.

Challenge In this challenge, you modify your tank tread drivetrain in an effort to climb a 3 to 4” high step. Modification Instructions: Take an exact measurement of your obstacle. A good example of an obstacle is a stack of textbooks. The surface of the obstacle should be wider than the robot. You will need to know this number to help design your modifications. Brainstorm ideas of how you can possibly climb the obstacle. Record these ideas in your engineering notebook. Sketch out some of your ideas. Look back to the Think Phase from this unit to gather some possible ideas. Perhaps do some research and look at pictures of real life tanks. Evaluate your ideas, and choose the one you think is best. Implement your idea by modifying your tank tread drivetrain.