A Tale of Two Tunnels

The original rings of the tunnel were made from 3/4 inch plywood. I made a circle cutter out of some metal stock and gerry-rigged it to my router. It is very important the the outside cut forms a perfect circle, since that is the edge that rests in the wheels. In this picture I made the rings 12 inches wide. This was unnecessary. I later cut them down to 6 inches.		This drawing shows the best way to maximize the plywood. This gives you a 12 foot diameter circle.

The picture above shows the first two rings completed and the third half way assembled. You can see the Unistrut frame and the BMX wheels in place. You can see the rings are attached together with 9 1/2-foot sections of 1/2 inch galvanized pipe with floor flanges on each edge. Note that I added and additional floor flange at the outside edges for stability.

The BMX wheels are held in place with some L-shaped pre-drilled steel stock. The stock is attached to the Unistrut with an angle plate made by Unistrut.

The unit worked great. Then it rained. Wood warps in the rain. The rings were ruined. So I started over. This time I would use 1/2 galvanized pipe. I also decided that neither rain, nor snow, nor dead of night would stop be. So I bought a building to house the tunnel in. If you are indoors, the plywood would work fine.

	I started by trying to bend my own pipe with a pipe bender. That didn't work. I couldn't get the perfect roundness required.
	So I had the pipe bent at a fabricators. This cost about $240 including the pipe. They would only bend 20 foot sections, so I had to recall my High School geometry and figure out the circumference of a 10 foot circle. 31.4 linear feet of pipe were needed. I wanted to be able to store this easily, so I cut the arcs into 15.7 foot sections by measuring a piece of string and using it to measure the outside of the pipe This gave me 2 halves of the circle.

The quarter sections were attached together with a smaller diameter pipe bent at the same angle inserted inside the pipe.

The cross pieces were attached with L-shaped and T-shaped hardware from the hardware store, but when I disassembled the unit I found that some had sheared.

My neighbor Steve created the following system.

Here you can see the rings. Each forms 1/2 of the circle.	This is an end ring. This is the male end. The base of the "T" is where the cross brace is attached.	Here is the male end of the center ring. Notice that the cross brace can be affixed on both sides of the ring.
Here is the female side. When it slides into the male side, the two flanges slide side-by-side and are bolted together. Use grade 8 bolts everywhere. Standard bolts will sheer!	This is the midpoint of an end ring.

Here they are assembled.

The bridge is 2 20-foot sections of 4x4 I beams (note: do yourself a favor and paint these when you receive them. If you wait like I did you will spend hours scraping off rust before you can paint them. Here you can see the two platforms that the I-beams rest on. Stairs were later built to access the platforms. 3/4 inch plywood spanned the I-beams. 2x4 cross pieces were added between the beams at 2 foot intervals to add support to the platform. These were notched out on the top corners to fit inside the I-beams and a small strip of ply was attached to the top edge to bring it up the the top of the I-beam. I used 3 4x8 sheets of 3/4" ply to make the bridge. You can see the beams are not 4 feet apart. I brought them in about 4 inches on each side to lessen the span across from side to side and to give space to mount the UV fixtures underneath (more on that later).

Note that if your I-beams are 20 feet long, your tunnel will end up being about 19 feet long, since 6 inches or so of the ends of the I-beams need to rest upon platforms at each end. Make sure you take this into account when making your overall design.

If I had it to do over again I'd use three engineered wood beams. They are less expensive, much lighter, and don't rust. They are available from building supply centers. Make sure you understand the weight limitations in order to determine the number of beams required. I've heard anywhere from 3 to 5.

Here you can see the motor unit. The belt simply runs inside one of the BMX wheels. The pulley on the motor has 4 different sleeves to adjust the speed until the best is found. The motor is a 288RPM gearmotor from Grainger. Model # 2Z850.

You can see the handrails. These are made by Speedrail and accept standard pipe. They were bolted through the plywood and into the I-beam and were very sturdy. That is an important factor, as people will lean on the rails. Hard. You can also see the UV fixtures in the above picture. There are 3 18-inch fixtures on each side, mounted right underneath the platform. The platform sticks out about 4 inches from the edge of the I-beam for mounting the UV fixtures.

The cover was made from commando cloth, which is a velour-like flame retardant material. It is very important that any material is flame retardant. Under no circumstances should you use visqueen or other plastic sheeting. In case of a fire, this will drip molten plastic on your guests. You will also never pass a fire inspection.
The commando cloth I purchased came in 72 inch width, so some major sewing was required. I sewed it lengthwise, so the seams went along the length of the tunnel. It was attached to the frame with wire ties, which went through the wheels easily. I had planned to attach snaps to the pipes and cloth, but there never was enough time.

I built ramps up to the tunnel, because I wanted to make sure it was wheelchair accessible. This meant 24 feet of up ramp and 24 feet of down ramp in order to comply with the ADA. If you want to use stairs, I would still recommend a ramp at the exit, as people will be dizzy upon exiting. At the least, you should have an employee at the exit to assist guests,

Another good idea is to have a panic button or switch at the entrance so an employee can stop the motor in an emergency (like when someone freezes in the middle).

That's it. Even with the mistakes, I spent about $1800 on the project. I'm happy to answer any questions you might have. You can email me directly at jbrown@timepasses.com.