What difference does the diameter and material of the rollers make? Why would a person choose 3" or 4" over 2", or aluminum over PVC? I'm into making my own stuff (so far made a bike stand and my own headlight), and have machine tools, so rollers would be easy.

Smaller diameters would be lighter, feel "slicker" and spin faster with little resistance. For the truly brave... Aluminium can be made lighter for the same strength as PVC or nylon, although they tend to be loud. yeah, rollers are easy to make. I actually made my first set out of wood.

I think Danno got that wrong. The smaller the diameter, the greater the resistance. I'm sure there's some physics major out there who can explain why.

Measure the weight of small-diameter aluminium rollers, then measure weight of larger-diameter plastic rollers. Google "polar moment of inertia" and plug into equations for acceleration.

Smaller is meant to be of greater resistance.
Aluminum is less likely to warp over time than PVC.

Easy answer: never use PVC because "plastic" rollers warp over time, especially in direct sunlight. I wasted almost $100 on a set of rollers that didn't get more than 200 miles on them...I now have a set of aluminum rollers that will last indefinitely, for only $20 more, and this set included a magnetic resistance unit and it folds for convenience! Asfar as rollers are concerned, PVC blows!

I agree that plastic rollers will warp. Aluminum is best and I guess wood is antiquated. While smaller diameter rollers are cheaper and easier to store, they will have more resistance because the tires will indent more. However, at 100 or 120 psi, it shouldn't be a problem.

Now I have a question: What is the recomended spacing for rollers? Should the front hub be directly over the front roller, a little ahead or a little behind, and by how much?

I have mine set up so that the front hub is just behind the front roller.

I like to set mine up with minimal distance between the rollers for the rear-tyre, perhaps 10" apart. Then the front-roller is directly beneath the front-hub. Gives me the most realistic feel as far as handling and how the bike responds to steering and weight-shift inputs.

+1

Larger diameter rollers will rotate slower and bearings will last longer in those rollers. Tires will also last longer too as the contact patch will be much greater. I highly recommend that a magnetic resistance unit be used due to the much lower resistance.

The front wheel has to be behind the front roller in order for proper handling.