Prepare the skeleton for the base of your ramp. Lay the 2-by-4-by-70.5-inch boards parallel to each other standing on their 2-by-70.5-inch faces with 21 inches of space between them. Place two of your 2-by-4-by-21-inch boards between them so that each of the 2-by-4-inch faces of the boards contact the 4-by-70.5-inch face of the bigger board. Each of the smaller boards should be spaced about 3.5 inches from the nearest edge of the larger boards. Use wood screws and wood glue to fasten the entire structure together.
Add the vertical supports to the base of your ramp. Fasten the 4-by-23-inch face of a 2-by-23-inch board to the end of the inside 4-by-70.5-inch face of one of the 2-by-4-by-70.5-inch boards in the base. The board should be fastened so it is straight up. The two boards should make an "L" shape. Fasten the other 2-by-4-by-23-inch board to the inside face of the other 2-by-4-by-70.5-inch board the same way. Repeat this step with the 2-by-4-by-8-inch boards at the front of the ramp to make the supports for the low end.
Cut the 2-by-4-by-105.4-inch boards for the top of the ramp. Angle one end of each of the two boards that will support the top of our ramp so the ramp can run all the way to the ground. Lay one of the boards on its 4-by-105.4-inch face. Place the tip of a ruler in one of the corners of the face and rotate it until you find a line exactly 9.6 inches long from the corner to the opposite side. Mark and cut that line. Repeat for the other board.
Prepare the skeleton for the top of your ramp. Position the two boards that you cut in the last step parallel to each other, 21 inches apart, with their 2-by-96-inch faces on the ground. Place one 2-by-4-by-21-inch board between them, 24 inches away from the uncut 2-by-4-inch faces of the cut boards, with its 2-by-21-inch face on the ground. Place the next 2-by-4-by-21-inch board 48 inches away from the uncut 2-by-4-inch faces in the same orientation. Place the final 2-by-4-by-21-inch board 80 inches from the uncut ends. This board should have its 4-by-21-inch face toward the ground. The board's top 4-by-21-inch face should be level with the 2-by-105.4-inch face of the cut boards. Secure all of these boards in place with wood screws and wood glue.
Mount the long plywood board on the top of the ramp. Line up the 80-by-24-inch plywood with the uncut end of the top of the ramp. Use wood glue and wood screws to secure the ramp in place.
Install the gate portion of the ramp. Line up the 25.6-by-24-inch plywood with the cut end of the top of the ramp. Position the gate hinges over the place where the two plywood boards meet. Each hinge should be 6 inches away from the nearest outside edge of the ramp. Use wood screws to screw the one section of the hinges into the bottom of the 80-by-24-inch plywood and the flat 2-by-4-inch board below it. Then screw the other section of the hinges into the 25.6-by-24-inch plywood. Position the 2-by-4-by-20-inch board below the plywood so that the hinges are attached to the plywood and the board below it.
Mount the top of the ramp to the bottom of the ramp. Set the top of the ramp directly on top of the bottom of the ramp with the uncut end of the top of the ramp above the 23-inch vertical boards. Adjust the position of the top of the ramp so that the cut edges of the top rest evenly on the ground. Fasten the top of the ramp in place with screws and wood glue.
Drill a 3/8-inch hole in the the top 4-by-105.4-inch face of the left top support, 47 inches away from the hinges. Drill a 3/8-inch hole in one end of the metal bar, 1 inch away from the end. Notch the other end of the bar 1 inch away from the end. The notch should be 1/4 inch wide and 1/2 inch deep. Fit the metal bar to the ramp with a 3/8-inch lag bolt through the 3/8-inch holes in the bar and the ramp. The notched side of the bar should face up when the bar is next to the hinges. Tighten the nut on the bolt enough so the bar is snug but still moves easily.
Drill a 3/8-inch hole near the center of one of the edges of one of the 4-by-4-inch faces of the 2-by-4-by-4-inch block. Insert a lag bolt through the hole and secure it with a nut. Fasten the block to the bottom of the smaller piece of plywood. The block should be fastened on one of it's 2-by-4-inch faces 10 inches away from the hinges with the lag bolt pointing to the left side of the ramp. If fastened properly, the wood should not interfere with operation of the ramp, but the lag bolt sticking out of the side will. Cut a notch into the left support beam for the ramp to allow space for the bolt when the ramp closes.
Tie one end of the rope or wire to the ramp lag bolt near the bottom of the ramp and the other end of the wire or rope to the free end of the metal bar. When you pull the rope from the center the ramp and the metal bar should come up together. When you apply pressure with the front tire of your bike they will lock in place until you remove the pressure.