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There are a lot of things to consider before building a halfpipe. First is the investment: building a halfpipe will likely set you back at least $600, probably more. Second is storage: even pressure treated lumber won't stand up indefinitely in your yard, so if you have some place to stow your halfpipe, you will be better off. Another thing to think about is size. Bigger ones will be more expensive, as well as harder to build. For beginners, it would be best to start with a mini-halfpipe. This choice avoids verticals and, as such, is easier to ride as well as store.
Draw it Out
The first step is to draw out the curve. For a mini-ramp such as this, a radius of 7 ½' would be ideal. (NOTE: For those wanting a bigger ramp, a good rule of thumb is a 5' ramp has an 8' radius, and then 3 inches is added to the radius for every foot of height added to the ramp.) Lay a piece of ¾" plywood out flat and on the 8' side, draw a short line 3 ½" from the bottom in one corner. This is the starting point of your curve. The simplest way to ensure the curve is drawn properly is to butt two sheets of plywood together and then hammer in a nail 7 ½' directly up from the starting point. Using a piece of fishing line or other non-stretchable material, tie one end around the nail and the other around a pencil to mark the plywood – making sure the line will be drawn at 7 ½'. Beginning at your starting line, keep the line taut as you draw the curve on the plywood. To make the most of your materials, the mini-ramp has a height of 3'3". Therefore, on the opposite side from where you started your curve, measure that distance from the bottom of the plywood and draw a straight line across. This line will intersect with your curve, showing both the transition curve and the platform. After you have this first one drawn, cut it out and use it as a template to cut the rest. For an 8' wide halfpipe you will want four of them (you should be able to get them from 2 sheets of plywood).
After cutting the sides, you will connect them using 8' long 2x4's. You will need 15 of them per side. Starting at the bottom of your curve, screw the board between the two sides. Use a second board to screw the back together. (Screws are used as they will hold the boards together better than nails and will not pop up. Also, this process is easier if the holes are pre-drilled.) From this point, mark out where the remainder of your 2x4's will need to be placed along the curve (reserve three for the platform). Screw one at the top and one in the middle to keep the frame sturdy, and then screw in the rest.
Bending and Smoothing
Wet down two sheets of 3/8" plywood and apply one to the frame from the top of the curve to the bottom. Gently bend the plywood by standing on it – if it does not bend easily, wet it more. Screw the plywood into place. Next, apply the second sheet next to this one in the same manner. Cut off the overhang with a jigsaw. While plywood can be left as the surface, applying a ¼" massonite will make the ride smoother. Put it on in the same manner as the plywood, but start from the other side so that the seams do not overlap. Screw in a 1 ½" metal or PVC pipe across the top of the curve. Use the remaining 2x4's to stabilize the platform by screwing them at both ends and the middle of it. Add a section of plywood to the top of the platform and trim it to size with a jigsaw. Repeat this entire procedure for the other ramp.
Screw together four 2x4's into a square for the flatbottom. Screw another 2x4 across the square every 12". Apply 3/8" plywood on top as you did for the ramps, along with the optional ¼" massonite over that. There are a variety of ways in which the sections can be joined together - they can even be permanently joined during the construction of the flatbottom skeleton by screwing the 2x4's of the sections together. Choose the method that best fits your needs.
At this point, the halfpipe is ready to ride. This is a simple plan for beginners. Bigger and more complex halfpipes can be made in a similar manner. Be sure of what you want and draw out your plans before you start building to avoid costly mistakes.
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