Notes on building a PlayDome

• Two of three tire stores I approached were more than happy to let me rummage through their piles of scrap tires. Be a bit picky - don't use tires that seem overly worn out and flimsy. Aquatreds seem particulary difficult to drill through along their central seam.
• For large tires I used mostly P205/75/R15's and P215/75/R15's. For small tires I used P185/70/R13's. Their diameters are about 27 inches and 22.5 inches (respectively), which is about 10 percent less than what Curt suggested using back in the 1970's. What is critical is that the tires have diameters, radii or circumfrences (take your pick, they are all linear dimensions) in a 6:5 ratio. That way when you drill holes evenly around the circumfrences, the 6 holes on the big tires and the 5 holes on the small tires are all spaced the same distance apart (about 14 inches in my case). This makes for nice equilateral "triangles" inbetween the tires of a completed dome.
• As Curt says, the diameters can vary by plus or minus an inch. Or eqivalently, this two inch range on the diameter translates to about a 6 inch spread on the circumfrence.
• I used a 7/16 inch "wood boring bit" - a flat bit with a sharp point and "cutting wedges" on the sides. It bites into the tire real well and does a pretty good job of plowing through the steel belts. The exit hole is never very clean though. Reaming out the holes with a large screwdriver just prior to inserting bolts helps somewhat. The tangle of mashed-up steel belts is pretty nasty stuff - be careful of poking your fingers on it.
• Washing the tires with a stiff brush (I have one designed for cleaning cow-milking machinery) and a solution of grease-cutting soap (Palmolive!) did a good job of cleaning the tires prior to painting.
• Paint: A colorful paint job really makes the PlayDome, yet this is perhaps the toughest thing to get right. On my first dome, I used latex house paint ("Over All" from Parker Paint). It was difficult to cover the black tires with the yellow and red paint, though the blue and green worked well. However, with four coats of yellow it never cured too well, and a lot of it peeled away when the kids began climbing on it.

  For my second dome, I switched to enamel paint ("Marathon" marine enamel from Parker Paint). It covered well with one coat and the glossy surface looked real nice. However, very weird things happened - the yellow never hardened, and some of the green became transparent. On an individual tire, there might be a band along the inside that was fine, and a perfectly circular band around the outside that acted up - this makes me suspect there was some strange chemistry between the rubber and the paint. Since some of the paint never dried, I had to abandon the enamel.

  While waiting for my enamel to dry, we had a house painter working on our house. He suggested a super primer - Zinsser B-I-N Primer Sealer, a white shellac primer. I entombed my enamel paint jobs in it, flipped the tires over and started with the primer. Supposedly this stuff will stick to glass (and the interiors of walk-in refrigerators). Then I applied one coat of the latex, though two coats might have been called for (I was real tired of painting tires at that point). It seems to be holding up pretty well, though there is a lot of cracking as the tires get flexed. I'd go this route again, if I ever build another one.

• Even though the drill bit was designed to make 7/16 inch holes in wood, our 1/4 inch diameter bolts were still tough to push through the holes in the tires. We used 1 1/2 inch long bolts together with large fender washers. I wouldn't advise using any shorter bolts, you will go crazy trying to get the nuts on while trying to cajole the tires into just the right position during installation. You might consider galvanized or stainless steel hardware if you ever plan to dismantle it.
• You might also drill some strategically placed holes to allow rainwater to drain from the tires. I drilled four holes with the wood-boring bit at the corners of a 1.5 inch square. Then, by starting at the holes, I used tin snips to cut out the material inside the square.
• I tried to orient all of the bolts with their heads up, so that if a child's foot went *down* inside a tire, it would only hit a bolt head, and not a bolt end. This is harder (or impossible) to do near the top.

Created: June 9, 1996, Updated: September 6, 1996.