This article helps you to know about the different techniques used to build the chassis and drive train of relatively small robots.

"Here you must know what I mean by small robots, is the category of light weight robots where aspects like aerodynamics and vibration damping Are not taken in account"

Most of the fundamental theory remains the same for all kinds of robots, such as robot stability, traction and steering techniques. However, this article is dedicated to the construction of relatively small robots, which explain how to the implement your designs using material and equipment at the reach of any average level hobbyist.
There are many types of mechanisms that have been developed to move a robot from a place to another, more than you could imagine, however, we will only study some of those techniques which are the most adequate to relatively small robots:

1-Traction and steering mechanisms

1.1- Differential Drive system:
This is the most common locomotion scheme in the world of robotics. It's easy to build, easy to control and can permit the robot to move in all required directions. In that system, two motors are connected, one to each of the two drive wheels at the right and at the left of the robot's base. Those two motors are responsible of driving the robot back and forth as well as steering in any required direction. This system can even allow a robot to pivot in its place.
Figure shows in a simplified way the principle of operation of differential drive. When the left and right motors are turning at the same speed, the robot moves forward or backward in a straight line. In order to turn right for example, the right motor is slowed down and the whole robot steers to the right. The bigger the difference of speed between the two wheels, the tighter will be the steering curve.
As you can also notice in the upper figure, this system needs one or more caster wheels (also called free-wheels) to support the rest of the chassis
while freely following the movement of the robot engaged by the two main drive wheels. Some robots use 2 caster wheels, adjacent to the drive wheels for better stability, but again, in the category of small robots, one caster wheel can be enough.
Figure

shows the chassis of one of our line follower robots where skids are used along with the drive wheels to support it in a horizontal position. This technique becomes interesting when used for lightweight robots, where caster wheels can cause even more friction than the skids, and add additional and unnecessary weight to the robot.
You can also notice in the figure that the rear skid is not touching the ground, and thus is not a primary skid, but rather a secondary one just in case the robot pivots around the drive wheels to the back during a sudden acceleration.

1.2- Four wheel differential drive system:
Another variation of the differential drive system is the 4 wheel differential drive system, which resembles the drive trains of tanks and bulldozers.
As figure shows, it consists of only two motors and 4 wheels. Each pair of wheels are mechanically connected together, usually by the mean of a belt forcing them to move together, driven by a single motor. The behavior of this drive train is similar to the previous differential drive system; if the two motors turn at the same speed, the robot moves forward or backward, if there is a difference between the speeds of the two motors, the robot will steer in the direction of the slower motor. The advantage of this system is that it is very easy to build and is most suitable for running on low friction and dusty surfaces where other drive systems would suffer.

1.3- Car-type drives system:
This drive system has many names, but calling it "car-type" make it clear to any reader, that it consists of 2 wheels coupled on the same axe at the back and one or two wheels capable of steering to control the displacement of the robot. While this technique is very difficult to implement mechanically, it provides two major advantages over other traction systems: Ease of control and accuracy. The best way to build your first 'car-type' drive train is to look at any old RC toy car. You can also use some parts of it's drive train to build your own, Following Figure shows a simplified layout showing the principle of operation this drive system. Connecting the front (steering) wheels to the motor can be very complicated, but a basic rack and pinion arrangement can be enough for very low steering angles, otherwise, for relatively high steering angles, the rack and pinion arrangement will have to be enhanced with a sliding mechanism allowing the green axe linking the two front wheels together to move freely along the axis of the chassis.

1.4- Divided chassis drive train:
This is another locomotion scheme that we experienced, which has the advantage of being very powerful and highly configurable. Indeed, this system relies on 4 independent motors, each one coupled to one of the four wheels, allowing the robot to move forward or backward with the accumulated power of four motors or to turn right and left with approximately equivalent power, but with minimum frictional losses.
Figure shows how to implement such a configuration, and shows how the chassis is divided into two parts, hinged together, allowing it to change its shape to turn in tight curves. One last special feature of this system is that it has neither head nor tail, in other words, there is no difference between the front part of the chassis and the rear one, making it possible for the robot to change it's direction, to adapt to any situation, without
having to execute a 180 degrees turn.

2-Mounting the Motors

There are many items that you can buy at your nearest hardware store, to use it to mount the motor on the chassis, I present some ideas that I find to be the most adequate to this category of robots.

2.1- Pipe Clamps:
Figure shows how a pipe clamp can be used to mount the motor on the chassis. As you can notice, one of the main advantages of using a pipe clamp, is that you can easily adjust the distance between the motor and the base of the robot to which the motor is fixed, which is not the case with other solutions, like the U-Bolt idea.

2.2- U-bolts:
The U-bolt is designed to hold a circular object by pushing it against a fixed surface. As figure shows, in our application, the circular object is the motor and the fixed surface is the chassis. so the height of the motor from the ground depends on the height of the part of the chassis where the motor is fixed. In contrast, with Pipe clamps, you can easily adjust the distance between the chassis and the motor.

2.3- Building your own bracket:
You have to use your imagination, and cope with the components, tools and materials that are at your reach. At this point i'll only give you hints of materials you could use to easily attach your motors.

A rubber band:
can hold your motor in place exactly like a U-Bolt, while giving it some elasticity to absorb shocks and vibrations. You can obtain really strong rubber belts from old cassette players.

Thick copper wire

3-Transmitting power to the wheels

3.1. Direct transmission from a DC Motor,
Shows an example of how a small plastic wheel can be mounted to the tiny shaft of small motors. As you can see this is the same idea used in most toy cars, where parts are simply forced in their place. This is more interesting than more advanced shaft coupling techniques when you're working with tiny motors, because it would be very difficult to drill it's output shaft to install setscrews or pins.

3.1 Direct transmission from a gear head DC motor
This is the most common method used by robotics, specially for the category of robot we are talking about. If the wheel you are using doesn't have an extended shaft as shown in figure, you will have to find another mean of fastening the motor's shaft to the wheel, but virtually any wheel you will find is designed to be mounted on a shaft, so using your imagination, you'll end up finding a solution to that problem, depending on the shape and size of the wheel.

3.2- Undirected transmission
Figure shows a classic undirected transmission system; where the rotation of the motor's shaft is transmitted through a reduction pulley arrangement, lowering the RPM of the wheel, while increasing the torque. Notice how the full weight of the robots is distributed only on the two rolling bearings, making this drive train much more robust than other where the wheels are directly mounted on the motors.
Both Figures shows a practical implementation of this technique on one of our early robots, where each wheel is mounted to a 6mm steel shaft, supported at both ends with rolling bearings. There is no obligation to put the rolling bearing at both sides of the wheel but you have to put two of them, each one as distant from the other as possible, to reduce shear stresses on the rolling bearings. While you're looking at the pictures, you can notice how the motors are mounted with pipe clamps as explained before.

6-Finding motors and building material

The easiest way to acquire all the material and equipment you need to build a robot, is to buy it! But that's not the cheapest way, and sometimes You won't find all what you need in the stores. That's why - if you plan to dig your way in the field of robotics - you should start collecting all kind of devices from which you can scavenge all kind and sizes of DC motor, mechanical parts, gears, pulleys, etc.

From my experience, I tried to classify the kind of devices from which you could extract useful items; at least, this is what I do with those devices before throwing them away.

Old cassette players and walkmans: Small Motors, pulleys, belts, gears.

Old Floppy disk and CD-ROM drives: Very small motors, worm gear, sliding mechanism, gears, limit switches

Old Photocopier machines, printers and scanners: quality Gear head DC motors, sliding mechanisms, rubber wheels, gears.

As for the chassis, it can be built with a multitude of materials. My preferred material is Copper boards (made for PCBs) as you can solder many metals to it like screws and nuts, you can also design the main board of the robot on the same board to reduce size, weight and cost.

My second best choice is the Fiberglass. It can be easily formed with a cutter, can be easily drilled, can sustain high stresses, and looks very neat an professional.

Another good choice for building the chassis of the robot is the 3mm Medium Density fibers (MDF wood) which has the advantage of being very cheap, and can be very easily cut, drilled and grinded to any shape you want. The only disadvantage of this material is that it can easily deteriorate with water or even too high humidity.

Those were the material I found to be more suitable for this category of robots.