My son started riding a 16" bike at 4 years old without training wheels. (He was first on a push bike without training wheels or pedals at about 3 1/2.) One day he decided to build a jump in the back yard. So he found a piece of 2 x 12 ramp (with random lumber laying around) and piled up some logs. He discovered it was a bit tricky to ride in the grass and hit his narrow ramp. So I said, 'hey, let's go in the street (dead end) and try this.' First one brick was used to provide vertical lift, and then a second brick. He loved it.

With two bricks, the ramp becomes a bit wobbly. Plus, a 2 x 12 is a bit narrow, and a few times he rode off the ramp's side. So I decided to build a jump with the following qualities:

1) Wider ramp
2) Adjustable height, so it will last for a few years as he grows
3) Portable, so I could drag it to the dead end or local schoolyard playground.
4) Safer (while still providing ample opportunity for skinned elbows and broken bones)

Step 1: Tools and Lumber and Hardware

Basic carpentry tools and simple lumber are required. I also had to re-teach myself basic trigonometry to determine the frame size to support adjustable heights.

You can view the trig calculations in this google spreadsheet, and save it as Excel or copy to your own Google accounts.

The jump design is modeled after adjustable outdoor lounge chairs where the height is controlled by moving the legs along notched rails.

Tools:

Drill + bits + drivers
manual screwdriver
T-square
Tape measure
Pencil
Saw. (I borrowed a compound mitre saw which was useful for making square 2x4 cuts, and angled 1x4 cuts. Could accomplish the same with a circular saw or even a hand saw, but with more effort.)
Router. I used this to notch the grooves. My initial design did not work, and the router is a better option.

Materials for the frame and adjustable ramp supports.

2x4 (2 x 8ft lengths). Cut into 2 x 50" lengths, and 2 x 16" lengths
1x4 (1 x 8ft length). Cut into 2 x 15" lengths, and 2 x 32" lengths
4 x hinges (smaller door hinges. The ones I bought included screws)
Screws (2 1/2", #10)
1 1/2" dowel (3 ft to be safe)
Pipe holding brackets (to attach dowel to ramp supports)
Handful of 3/4" screws to attach pipe brackets

Wood for the ramp.

I used 3/4" plywood cut to size at lumber yard. (Initially I used a piece of old shelving, but it was too narrow.)

Step 2: Final Ramp Assembly

This is what the finished ramp looks like. The 1 x 4 ramp supports move up and down the horizontal 2 x 4 framing to raise/lower the height of the dowel. The ramp rests on top of the dowel. This ramp will go to 11" from the ground. That's pretty high (too high for any 4 year old I've met). Lowest height is about 5".

Minimum jump height is 7", but you can lay the jump supports flat on the base to create a 4" jump.

Step 3: Assemble the Frame

Frame assembly is simple. Lay the pieces fat on the ground, measure, drill holes and screw.

The 15" cross pieces provide support. The rear piece is 10" from the ends of the 50" lengths. The front piece is 16" from the ends of the 50" lengths.

Drill holes and countersink, and then drive the screws in at an angle to make the connection. Do the top, and then flip the frame and screw from the bottom. This makes a very strong frame. Only screwing from one side makes a significantly weaker frame.

Step 4: Assemble the Adjustable Ramp Supports

Trigonometry helped me determine the length of the base 2x4s, and the length of the angled jump supports. Basically, the ramp supports form two edges of a triangle, and the 2x4 base forms the third edge (bottom) of the triangle.

The top sides of the triangle are adjustable using the hinges on the front edges, and the hinges connecting the each of the two sides.

The front (right of image) pieces of the 1x4 are 15", and the rear (left of image) are 32".

Step 5: Create the Cross Support to Support the Ramp

Attach the dowel with the pipe fittings to the top of the 32" 1x4 near the connection with the hinge. Ths provides stability to the two ramp legs, and the ramp will rest on top of the dowel.

Step 6: Final Adjustable Bike Jump!

Ride (or run & jump) over the ramp! When the kids get bigger and have more confidence, you can raise the jump even higher. So this ramp will last a few years while they get confidence and until they ask for a curved, BMX-style jump.

The ramp in this picture is the original, narrow shelf. I now use a 5ft x 2ft piece of 3x4 plywood, so the ramp completely covers the frame.