Almost every project uses wheels. they are one of the oldest, most used, and most important mechanical device ever. but sometimes it seems hard to make your own wheels. even though it seems as though you cannot get anymore strait forward. all you need technically is a round object (more specifically a cylindrical object). but it seems that simply using rollers will not do for everything and every project uses different axles, different motors, different sized bodies and so on. for this reason the same type of wheel will not work for every thing. the first thing that we should do is look at why and how the wheel works so well. Like most things there is physics behind it, and when you look at the physics in things it can surprise you how much is really going on in such a simple thing.
Step 1: The Physics of the Wheel
It seems that there should not be much going on in an object as simple as a wheel, but there is actually a lot happening. To begin imagine trying to move a heavy object. Let's say it is too heavy to heavy to lift so you have to drag it. lets also say you are on a completely flat landscape so gravity is not an issue. Your main problem in moving this object is going to be the friction it has with the ground. to overcome this a lot of ancient civilizations used wooded sleds to reduce the amount of contact with the ground the object had which would in turn reduce friction. But even if you were to place the object on the sled there would still be a lot of resistance. to overcome this it would probably be best to use a wheel because it is very good at reducing the friction. But how does it do this? If you are to look closely at a moving wheel, you'll notice that the point of contact on the wheel is not being dragged across the ground like with a sled. the point of contact is constantly changing so that there is no dragging. No dragging means that theoretically there should be no friction. However we know this not to be true. there is still friction in the wheel. if you were to place our heavy object on a cart and roll it across the flat road, it would slow down and stop (though not as fast as the sled by any stretch of the imagination). the main source of friction on wheels would be in the axles. where there is dragging going on between a rod and a hole the rod is in, though this friction is limited and very small compared to the sled. there is another source of friction in the wheels, which comes from the fact that there are going to be small deformations in the wheels which will cause friction with the ground, but again very little.
Step 2: Primitive Wheels: Rollers
rollers are the most basic form of wheel. take a bunch of cylinders that are all about the same size, line them up and when you push a heavy object on top of them it will move with very little friction. there are clear disatvantages to using the rollers of course. Mostly in that you must constantly move the ones the object just passed over back in front of the object if you want to keep it moving. this is probably the most primitive form of wheel. and was used by many ancient civilizations to move large objects.
Step 3: Wheels With Axles
most wheels that we see have axles. They are definitely better than the rollers, because you need less of them, you do not need to0 move them, and there can be less contact with the ground. for pretty much all projects using wheels you will use this kind. there are very many ways to make these from trash, and most of the ways are very effective. All you need is something of the right size for your project, that is cylindrical, and is thin enough to make into a wheel. many things can be sawed to the right size, and you'll probably find wheel potential in very many objects.
Step 4: Giving It Traction
Once you make your wheels, (take your object, drill a hole in the center for axle, mount to body using axle) you may notice they are not really doing their job. they are not rolling when you push your object. they may be sliding. this is probably due to a lack of traction. Traction is how the wheel is able to grip the ground in order to role rather than slide. This is more useful for wheels that are supplying power (i. e. connected to a motor) but is a good idea for every wheel. Adding rubber bands is a cheap, fast, and easy way to give your wheels traction without having to give them tires.
Step 5: Mounting to Servos
When I say supply power, I of course mean gives kinetic energy to the rest of the system. Mounting your wheel to a motor it typically the best way to do this. One motor that is particularly good for this is a servo motor. Horns that are on them typically have small holes to mounting wheels with small screws is very easy. the one thing to watch out for is the fact that the shaft of the servos typically have a very shallow reach, so you need a thinner wheel. also you need very small screws. I used screws that screws I took from a small toy a while ago. to get continuous rotation, you are either going to have to buy a special servo, or modify a regular one. You are also going to need to set up a 555 circuit, or use a micro controller to actually run the servo.
Step 6: Mounting to Regular Motors
If you are like most people, you do not want to have to use a servo, so you are likely to use a regular electric motor. unfortunately most hobby motors do not have a lot of torque, and some have shafts that are difficult to mount things to. I suggest you use a gear box to increase torque. beyond that most gearboxes have hexagonal shafts so it is easier to securely mount wheels on them. Tamiya has very good gear boxes. to make your wheels able to be mounted on the gearbox, take a piece of scrap wood, drill a hole in the center of it, and mount it in the center of your wheel the same way you would mount the wheels on the servo (see previous step). Once this is done all you need to do is slide the shaft from the gearbox into the hole (be sure the hole is about the size of the shaft, slightly smaller would be better though.