This page tells you what wheels can be used, and what prep the wheels need. Most of the wheels you get will be castoffs; fortunately by being un-picky we can use most of them.
A Gray-B-Gon evapotron uses three bike wheels, 24" and up: two for the drum and one for the propeller. Your mission is to select usable wheels, refurbish and adjust them, and group them in threes.
What’s usable? Wheels have to have axles and working bearings, and that’s about it. Usable wheels may:
• be front or rear wheels, hollow or solid axle,
• have one axle extending less than OR more than 1/2"
(two different supports),
• have bent rims, up to 2” or so out of true,
• have loose spokes or (a few) missing spokes,
• have cracked rims,
• have less than perfect bearings,
And for rear wheels only,
• have a freehub or a threaded hub
• have a gear cluster, which you'll have to remove.
Use only alloy wheels. Set a chromed steel wheel aside, to be used as a last resort.
Drum wheels are supported (on the outer face) by angle brackets drilled to receive an axle end. Axles that extend 1/2" or more will rest in an angle bracket with one hole drilled out to 26/64". Some hollow axles are very short or nonexistent; to support one of these, drill the angle bracket to 13/64", then slightly bend a short or long skewer and force it gently through the angle bracket and the axle.
Rear wheels are particularly useful. A freehub will support a propeller pulley; a freehub or a threaded hub just fits a 1-1/4" PVC pipe, and so can be used to stabilize a PVC drum axle. If there's any chance of running short of freehubs for propellers, avoid using them on drum wheels.
Classify wheels as potentially propeller or potentially drum. The two drum wheels must have the same diameter. Each can be either a front or rear wheel.
A propeller needs a rear wheel with a freehub, and need not match the drum wheel size. If the axle is hollow, you may be able to use a quick-release skewer. If you don't have one that fits, you'll need an extra nut (see Extra Axle Nuts below.)
Refurbishing Dispose of tires, rim tape, spoke reflectors, and gear clusters. Wipe the wheels clean -- not sanitary, just well enough that they don't soil everything they touch. Alcohol and a paper towel work well. Adjust the bearings by loosening one cone far enough to squeeze some grease in on both sides, then adjust the cone to a generous amount of play; perhaps a half turn or a full turn out from perfection.
Missing spokes Install soft iron wire to substitute for missing spokes. Cut 15" of soft iron wire and make a small coil at one end that will anchor at the rim hole. Guide the wire through the rim hole and in toward the hub with needlenose pliers, weaving it past the crossing spokes to the spoke hole in the hub flange. Tighten, bend over, push through, bend over, and clip to 1" or so. It won't carry the load of a real spoke, but it will support a propeller blade or maintain twine spacing.
Giving front wheels a center We use 1-1/4" PVC pipe for the drum axle. It fits freehubs and threaded hubs perfectly, but on some front wheels the pipe end skates around on the spokes and won't stay centered. On such wheels, build a cage looking like a three-petaled flower of soft iron or copper wire. Pick three spokes, more or less symmetric, close to the hub. Wrap wire once around one spoke, then guide it to the next spoke, leaving about an inch of "slack", then repeat on the third spoke, and around to the first again. Bend as needed to make a roughly triangular cage that fits the PVC pipe.
Preparing axles On each drum wheel, one end of the axle must show 1/2" or more of exposed thread To lengthen the exposed axle, you can either remove a spacer, or “shift” the cones and locknuts toward the other end. On the finished drum, these axle ends will be caged by angle brackets; they won't need axle nuts to clamp them.
On a propeller wheel, one face carries a freehub which will eventually support a pulley. Turn the wheel over. Over this axle end place a GA2 angle bracket drilled out to 26/64", and either a nut tightened on the bracket, or a skewer with its lever facing you. Adjust the skewer (by turning the skewer nut, or by adding or removing spacers) so the lever tightens the axle against the angle bracket.
Extra axle nuts A solid-axle propeller wheel, or a hollow axle missing its skewer, needs one extra axle nut (or cone nut, or cone used as a nut), for clamping the axle to a metal angle bracket. There are a half-dozen or more different thread standards, so finding the nut you need can take some work.
Salvage the axle nuts, cone locknuts, and cones from wheels you've rejected.
Ask at bike shops; they probably have a collection of old nuts, and they may be willing to give/sell you some, or to order new ones to sell to you.
Some threads may be available from other sources. Some Home Depot hardware departments carry 10/1 nuts (diameter 10 mm, pitch 1 mm) which fit most modern large-diameter hollow axles.
I have used epoxy putty, wrapped around an axle, to cast a nut.
Final grouping Make up groups of three wheels. On the propeller, mount the extra axle nut or skewer and wrap a few inches of “blue tape” (blue or white masking tape) around it so the propeller is identified (and so it won't unscrew by accident). On every wheel, write the shop or co-op's name on a piece of white tape and attach it to a spoke. This will tell the workshop organizers which shops to reimburse with $4/wheel donations.
Tie the three wheels together with a piece of string. Leave about a foot of slack, to make the wheels easier to stack for transport.