Wheels



		Your robot has to move, and the easiest way to do that is with wheels. Many builders have worked with walking systems and track drives. For the novice builder, it's best to keep it sweet and simple.

There are two common types of wheels using on bots, pneumatic and solid. Pneumatic or air filled tires can be a good choice for you robot. Pneumatic tires come in a variety of sizes but smaller sizes can be very difficult to come by. Pneumatic tires frequently come with holes in the rim, this can make it easier for mounting your wheels on a hub. Pneumatic wheels also offer good traction and by changing the air pressure in the tire you have a means by which to make small adjustments in traction and speed. When you reduce the air pressure in the tire you allow the tire to have a larger contact patch with the floor to give you more traction. At the same time you will reduce the diameter of the wheel, this will reduce your speed but also provide you with more torque or pushing power.

The disadvantage of pneumatic tires is the risk of puncture. To overcome this, some builders will fill the tires with foam, however this will increase the weight and eliminate the ability for you to make adjustments to tire pressure.

Solid wheels are more durable than pneumatic tires and are typically made from solid rubber or plastic materials. The best ones have a grippy rubber bonded to a solid hub. These are fairly light and come in a wide variety of sizes. The disadvantages are that they can be difficult to mount to a live axle. They also tend to suffer from traction loss sooner than the pneumatic tires. An additional consideration is that if a solid wheel is damaged or worn you may need to replace the entire wheel. With a pneumatic wheel you may be able to replace the tire or rim. A simple puncture may only mean a patch using an inexpensive bicycle tire tube repair kit.

If you are thinking of using hard plastic wheels such as those found on lawn mowers, you will find that these are a very poor choice. The hard plastic varieties of wheels lack traction and lack the rugged construction for the rigors of battle.

The two wheeled drive, tank style steering robots are definitely easier to build however they do require more practice to learn how to drive effectively. The two wheel drive system is simple to build because one motor drives one wheel and another drives the second wheel. When both motors are going forward the bot moves forward. When you reverse one of the motors, the robot turns. This is the same method by which tanks and bulldozers steer and drive. Another advantage to the two wheel drive system is this allows the robot to turn on itself thus making the robot agile and maneuverable. Besides being more difficult to drive, traction is somewhat limited due to the limited amount of contact of wheel to floor. You may also need to use castors or ball rollers to allow your robot to roll easily.

The four wheel drive skid steer robots offer more traction to provide greater pushing power. Designing and steering a four wheel drive robot is similar to the 2 wheel system. The wheels on one side are driven by either one or two motors and the wheels on the other side are set up in the same fashion. The disadvantage with 2 drive wheels on each side, now the wheels must skid or slide when the robot is turning. This means using more battery power and additional stresses on the drive system and motors. A well designed 4 wheel drive robot can offer strong pushing power and the ability to turn on itself. The disadvantage is the more complex drive system requiring either more sophisticated gearing, pulleys or sprockets or will require more motors.

A rare version of the 4 wheel system is a two wheel drive with two wheel steering. This is a similar system to what is seen on automobiles. This type of drive and steering has the disadvantages of being much more complex to design and build, and has a large turning radius when compared with combat robots that use tank or skid steering. The only real advantage is that this type of steering and drive is relatively easy to drive.

Usually found on combat robots that have been designed to push their opponents around the arena, 6, 8 and even 10 wheel drive systems offer even greater traction. Once again the idea of skid steering comes into play. To reduce the amount of power required to turn a bot with this type of drive system some builders will mount the wheels at either end of the robot slightly higher than the wheels near the middle. This will tend to give the bot a slight front to back rocking motion but at the cost of sacrificing some traction. This type of drive system requires an even more complex drive system of chains and gears or even more drive motors. If you choose to build this type of a drive system using multiple drive motors in your bot you will need to carefully evaluate the power requirements of your batteries and the capacities of your speed controllers.

Wheel size depends on how fast and how heavy. Most combat robots move at approximately 10 feet per second. Of course rams and wedges tend to move faster where as vicious shell spinners can move slower. To calculate how fast your robot will move with a specified wheel is simple.

The speed here is reasonable if you are building a smaller bot, such as a feather weight or light weight or even if your building a heavy weight with a strong weapon. Its a bit on the slow side for a heavy weight wedge or ram.

How heavy the robot is an important factor in wheel size. You should try to have about 2 pounds of torque for every pound of robot. Again, a simple bit of math is required.