Note: My son weighs 44 pounds. If you're making one for someone significantly heavier than about 60 pounds, or for a kid who will apply significant impact to it, you'll need to use a different species and diameter of dowel. More on that later. It is your responsibility to engineer this with sufficient strength for your purposes. That is why I am not going to give exact dimensions.
The project was a bit less straightforward than I expected, because after making the first version, it turned out that it was quite uncomfortable for the kids that the rod turned as they pulled up on it. So we needed to do something to keep it from turning.
1. Hardwood dowel, preferably oak, of diameter and species sufficient for the weight of the user (see next step) and width sufficient for your door frame.
2. Plywood, 3/4" thick. You don't need much. About 2" x 8" will suffice if you don't need the anti-rotation attachment, and if you need that, you'll need another another 2" x 3" or so piece. I used Baltic Birch plywood, as that's what I had lying around, but cheaper stuff may be fine.
3. Wood screws of sufficient length, strength and quantity to anchor the plywood to the door frame.
4. Wood glue (I use Titebond II) for the optional anti-rotation attachment.
1. A drill with a bit (Forstner, paddle or hole saw) that can cut a hole in the plywood with diameter equal to the diameter of the dowel.
2. Hand-saw, jigsaw or hacksaw.
3. Screwdriver for the wood screws.
4. Clamps for optional anti-rotation attachment.
Step 1: Choosing a Dowel
- Thick enough to safely support both the weight and the additional force of swinging, pulling up, etc.
- Fits in the doorway, with room for U-shaped supports (one could cut off the inner side of the U and use the central protrusion in the door frame, maybe.
I am not an engineer. It is your responsibility to do this correctly.
With that said, here's what I did. I'm going to do this in Imperial units. I looked up the "bending strength" and "modulus of elasticity" of the species of dowel (e.g., here ; n.b. "Dry: 1,580 @ 1,000 psi" seems to mean a modulus of elasticity of 1580000 psi). For instance, red oak's bending strength is 14,050 psi. (If you get the numbers in MPa, you may need to convert .) Then go here and fill out the calculator in imperial units for "beam supported at both ends, load at center". F is the weight of the user, L is the length of the bar, E is the modulus of elasticity you looked up, and y is, as far as I can tell, half of the diameter of the dowel. You also need to calculate the moment of inertia I. This is pi d^4/64, where d is the diameter of the dowel. Then read off the maximum deflection--that's how far the beam can be expected to bend in the middle, and the maximum stress. You need to compare the maximum stress to the breaking strength you looked up, but you need a large margin of error. Not being an engineer, I don't know how much--I would probably feel good if the maximum stress was about a third or quarter of the breaking strength.
For example, for a weight of 200 lbs, red oak modulus of elasticity 2280000, a 32 inch length, and a 1.5 inch diameter, y = 0.75 and I = 0.2485, you get a deflection of 0.24" and a maximum stress of 4829 psi, which is 34% of the breaking strength of 14050 psi. Whether that's acceptable depends on how vigorous the bar's use will be, I expect, but again I am not an engineer. We used a dowel where the maximum stress was about a quarter or less of the breaking strength.
Once you choose the dowel, cut it to fit in the doorway with as little play as possible.
Step 2: Cutting Supports
Then, we drilled and countersunk wood screws into the door frame. Make sure you choose wood screws that can hold the user's weight. You might want one or two to be long enough to hit a beam in the wall--in our case, with a 44 pound user, I didn't.
Make sure both hang at the same height.
At this point, you can try out the rod carefully, to make sure it doesn't seem to creak, flex worryingly, etc. You might even be satisfied at this point and done with the project.
But my kids complained about the rod turning in their hands in ways that made it hard to hang on. Hence the last step.
Step 3: Anti-rotation Attachment
So, I went back to my 3/4" plywood, and near the edge drew a rectangle the same width as the diameter of the rod and a little less than twice the diameter of the rod in length. Getting the length exactly isn't an issue. Getting the width right is good. I drilled a hole with the same diameter as the rod in the center. Ideally, the hole would have been tangent to two sides of the rectangle, but I am not good at getting things just right.
I then cut out the rectangle, which will split in half in the middle of the big hole if you got the hole in the right place (mine needed another cut) and got two two pieces, each with a semi-circular cutout. The idea is that the pieces will be glued to the ends of the rod, thereby adding enough rectangularity to the ends to make the rod not rotate. Make sure the pieces nicely fit in the U-shaped supports on the doorway, on top of the rod. If not, sand or trim until they do.
Then glue the pieces onto the ends of the rod with wood glue (I used Titebond II), making sure they are aligned well enough. (The extra white clip in the photo is because the plies split a little and so I was also gluing them together.) Wait for the glue to cure, and you should be done. If the result doesn't fit in the U-shaped supports, sand as needed.
And then you're done. Insert with the anti-rotation stops pointing upward, of course. A little play should be harmless. (Might even be a good thing because if someone comes through the door and doesn't notice the beam and it's at forehead level, the beam may lift up a little, moderating the force of the blow.)
Again, you're responsible for safety, supervision of users, etc.