Wheels vs Continuous Tracks (Tank Treads)

Question

I'm building a small robot using some cheap Vex Robotics tank treads. However, my choice of picking tank treads is almost purely based on the fact that they seem like more fun than wheels. I don't actually know if they really have much of an advantage or disadvantage when compared to wheels.

What are the pros and cons of both wheels and continuous tracks?

Answer 1

Well, you generally use wheels when:

• You want speed. Treads need a lot more torque to power, thus you generally use low-rpm/high torque motors.
• You want maneuverability: Treads are a big pain to turn with. Differential steering is very ineffective (the bot skids to steer, which may not work if you have really grippy treads)

But, treads are better when:

• You want to move on uneven terrain: Wheels get stuck in potholes/bumps, treads don't.
• You want traction: Treads in general are way grippier than wheels--climbing an incline is easier and you have less skids.
• You don't want the bot to be affected by pushing/gravity: A wheeled bot is easily pushed around (and, as mentioned, can skid on inclines). Bots with treads are hard to push and they don't skid.

Answer 2

The best option will depend on the type of terrain you expect to cover. You want the correct balance of several factors:

• ground pressure
• traction
• suspension
• steering

Ground pressure from tracks is less than ground pressure than wheels, so they're more suited to soft surfaces. Larger tires can help, but there are limits -- they may not work in something like snow (which is why snowmobiles use tracks).

(0.75 psi snowmobile tows a 40 psi ATV)

Traction from tracks is generally better than wheels, but still depends on the depth and firmness of the tread being adequately matched to the terrain for best results.

(Much like a gear, a tire or track engages with the ground underneath it. Tracks simply put more "teeth" into the ground.)

Suspension is related to traction; without the ability to conform the tracks or wheels to the terrain, the traction doesn't come into play, and your motors will just spin ineffectively.

Building good suspension is much more complicated for a tracked vehicle than for a wheeled vehicle, but by no means impossible -- as shown in this iRobot video:

.

But, compare the climbing ability of tracked vehicles to the rocker-bogie system used by the mars rovers.

Steering is necessary for maneuverability, and this is where wheels can have a significant advantage. For skid steering, good traction works against you... and in uneven terrain, you might find yourself blocked from lateral movement.

There are some hybrid approaches -- 4 tracks, non-skid steering -- that approach a "best of both worlds" design: Mattracks and hagglunds, for example.

One very important balance to to all of these factors is maintainability. The complexity of tracks means that there are a lot of ways to break them; you're adding a lot of moving parts that will wear out independently, or be prone to dirt and debris causing trouble. Troubleshooting tracks will be more difficult than wheels, and tracks will be more difficult to service. So from a durability standpoint, if you can get away with wheels then use wheels.