With the kids having perfected the simple balance-beam I'd set up for them, it was time to make something a little more difficult. At the same time, I wanted to make something that I would also enjoy rolling over (and hopefully one day jumping). The end result was a double-hump bike ramp that is fun for all.

To build this ramp I used reclaimed patio timber that I had lying around at home. The sides were originally one complete length that I then cut down to suit, while the 'slats' were again a long piece of timber I had to rip and cut to suit. I would have liked to make the ramp wider, but this is what was achievable with the materials I had. It works nicely though, and my 4 and 6 yr old can now both ride it with only the occasional crash.

Note that you could definitely get a better build using better materials and taking more time, but I was trying to stick to a budget and get it completed in time to stop the kids going completely stir crazy.

Also apologies in advance for the lack of 'in-progress' photos, as usual I was too keen to get on with the build to think to stop and take photos.

Supplies

Materials:

2 off Hardwood planks ~1200mm long x 230mm wide x ~25mm thick (ideally would used treated structural timber, I cut down 1 longer plank to make these two)
25 off hardwood boards ~230mm long x 65mm wide x 10mm thick (I cut these from 2 x hardwood lengths I had, I would use thicker wood and make these longer (to get a wider ramp) if I was doing it again)
60 off 8G x 45mm galvanised self drilling countersunk screws. (I'm not sure how essential the self drilling piece is as I pre-drilled all holes, but again I used what I had).
Wood glue (Optional)

Equipment / Tools:

Hand saw
Jigsaw
Drill
Drill bit (for pre-drilling screw holes)
Screwdriver bit (for driving screws)
Large drill bit or countersinking bit (to recess the screw heads)
Pencil
Long metal Ruler or flexible strip (optional)

Step 1: Marking Out

The Sides:

Both of the long planks need to be 'identical' in order for this ramp to work. The way I achieved this was by clamping both firmly together and making sure the overall dimensions were the same. I used my handsaw to get the length right, thankfully I didn't need to worry about the width as they were pretty consistent boards.
Keep the boards clamped together and start by marking the quarter points along the top edge, middle and bottom. (ie make a mark at 1/4, 1/2 and 3/4 of the length of the board). The 1/4 and 3/4 points are the tops of the 'humps', while the 1/2 point is the bottom of the middle 'dip'. I set this approximately 1/4 of the way down, so that the resulting bend could accommodate my wife's 29" wheel size without issue.
Now it's time to mark the 'inflection' points of each curve, or in other words, where the ramp stops curving one way and starts curving the next. Make a mark at 1/8 and 7/8 of the length at half the height of the board. You'll also need to mark at 3/8 and 5/8 of the length, at 1/8 down from the top of the plank.
To mark the curve I simply drew in the diagonal connecting lines, then just sketched them in by hand. Since then I've realised you could put a nail at the bottom, mid-point and top of each curve, then flex the ruler around these to get a more consistent curve. On this scale it doesn't matter too much, but importantly make sure the steepest angle of the ramps isn't too crazy, and that it's the same immediately before and immediately after the inflection points.
At the end of all of that, you should have a shape that vaguely resembles what I have above. If not, remark until you're happy with it (I took about 2 hours to get it somewhere near right, but that involved a lot of attempts at large-radius marking using various methods that ultimately didn't work).
To make it even easier you could use newspaper or something similar to mark the shapes on, then trace these onto the wood.

The Planks:

Each plank of wood needs to be the same length and ideally width, so mark and cut to suit.
If you've varied the approximate dimensions of the ramp (made it longer or shorter, or made the middle dip steeper) you may need more planks.
To calculate how many planks you need, use a piece of string or a measuring tape to measure the total length of the top of the ramp (my 1200 long ramp ended up being about 1800mm along the curve). Then measure the width of your slats, and add 10mm. Divide the curved length by this number, and that's how many slats you'll need (remember to round up).
Measure and mark out to suit, then cut to length. mine ended up about 230mm wide.

Step 2: Cutting and Shaping

With the general shape marked out, it's time to fire up the jigsaw and cut that curve out!

Either my wood was too thick, or the blade too short, so I couldn't cut both sides at once. What I did was screw the two pieces together temporarily, so that I'd be able to marry them up again afterwards. Then I unscrewed them and cut them individually, before screwing them back together.

Don't stress if the two halves don't match exactly!

By aligning the two, you can then use the jigsaw (or a hand-saw if you're really clever) to tidy up the two halves and make them match. Again, at this scale it doesn't need to be 'perfect', but within a couple of mm so that the resulting slats are as flat as possible.

Step 3: Assembly

Putting it all together

By now you should have two matching curved pieces of wood, and a pile of small slats.
What I should have done (and would do again) is mark, pre-drill and pre-countersink each slat at opposing corners. Make sure that your holes will be roughly in the centre of the side ramps.
Screw the two curved pieces together again so that we can mark where the slats need to go on each side, and hopefully keep the ramp straight.
Layout the slats on the ramp, starting at each end and working your way into the centre. You may need to vary the spacing slightly to get a good fit in the middle.
To get the spacing right, I placed a slat down, then put one on edge, then put the next slat down, then another on edge, etc. etc.
With the slats lined up, mark both the edges of each slat on each piece of curved wood.
Now split the two curved pieces apart again, and set out the slats.
Pre-drill the holes to try and prevent the wood from splitting, and install the slats with the screws. Take your time and do one at each end first, followed by one in the centre.
You will still be able to align the sides a touch at this stage, so use this opportunity to get them as straight and even as possible.
Once happy with the alignment, put 4 screws in the central slat to prevent further twisting.
Progressively install the remaining slats, and then that's it!

Step 4: Go Ride!

The ramp is now ok to be ridden.

As with anything to do with the kids, I always load test first by walking on, then jumping on, then riding over any construction. If it flexes at all, or worse yet breaks, go back and add more strengthening or stiffeneing, and re-test.

A lot of other ramps I've seen use internal struts to keep the ramp strong and straight. In this case I don't think it's necessary, as the slats provide bracing on the up, down and middle sections.

I may need to eventually add a block in the bottom middle to stop it from spreading over time, but at the moment it's been used daily for 2-3 weeks with no signs of movement.

Is this a perfect woodworking build? No.

Is this a simple project that taught me a few new skills, cost me $6 in screws, and has given the kids hours of fun? YES!