A-frame angle for swing

I'd like to figure out the smallest angle possible for the kid swing A-frame. I will be using 4x8 beam on top and 4x4 for the legs. I am reading two sources and each has different angles:

This site http://myoutdoorplans.com/playhouse/outdoor-playset-plans/ says 15 degree (each side)

This site has plan with higher swing beam but it uses 30 degree http://www.ana-white.com/2010/06/how-build-swing-set-playhouse

I think using the second option plan is OK for the high (10 feet) but the width is also 10 feet which is too much space. Is there like a requirement for the A-frame angle? Or can I use 15 degree just fine? Given proper support and everything else framing correctly.

• Why risk tipping the whole thing over? Go with wide stance and tack on anchors as well. Commented Jul 19, 2016 at 19:31

With the narrower angle (15), they add cross supports. The wider angle (30) is given with no supports.

Making a stable swing set is no joke. Unless it is part of a larger structure that weighs it down, the legs need to be anchored in concrete. When I was a kid I used flimsy backyard swing sets and could easily rock the whole swing set, frame and all, back and forth. Also, the swing set should be 10 feet from any fence, wall or hard surface (like pavement) if you want to avoid injuries. Some children (like myself) will swing out to the maximum height (about 8 feet) then leap out of the seat and go flying. When I was a kid this was a game we played, the object being to see who could catapult themself the furthest distance.

There is no hard and fast rule about the angle. It depends on the design of the swing set, especially the height, the material and the way it is joined. You can use a narrow angle, but the beams must be correspondingly stronger and better anchored.

The angle is not particularly important. With sufficient ground stability and fastening, you could do it with a zero angle: a beam supported by a pair of posts.

For each configuration with a greater angle, the need for strength in the supports and anchoring decreases. When the angle is 180 degrees, the system is completely stable without any anchors at all. Of course, it it useless as a swing, but the point is that it is plenty strong.

If I were you, I would just build it to be as wide as is practical if space is tight, or about 30° to optimize height vs. the amount of materials needed.

One belated consideration is that as soon as the swinger moves ahead of the front legs, their weight will have the effect of creating a rotational lift on the rear legs (with the front legs as the pivot point), and thate force increases with speed. This is how even a smaller child can "tip forward" a lightweight swing set that isn't anchored well. For my own application, I'm building a tall A frame for a large homemade wooden bench swing that can seat 4+ adults. I have to make sure my A frame is anchored well or wide enough to prevent the swing from going in front of the front legs (because you know the teens will try to swing high), because the force of the potential group of swingers, magnified with swinging motion, will far exceed the ability of the relatively smaller mass of the rear legs to counter the rotational force of the swingers.

• When I last made a swing, I incorporated a limiter that interferes with one side of the swing's tension lines when the user swings beyond my design threshold. The line breaks over an interference that effectively changes one side's swing radius but not the other side's. The two out of sync pendulums thwart abuse by twisting the seat all over the place. It sounds like you're already on top of the circular motion's acceleration and its additional load. Given θ as the swing's maximum angle away from vertical, the max downward force (at the bottom of the swing's motion) is Weight[1 + 2(1-cos θ)]. Commented Feb 3 at 7:36

I notice you have a tall frame, and the plans you showed apparently call for hanging the swing from the crossbar on long chains. That's fine for play and high swings past the horizontal, but for a relaxing swing bench you're better off keeping things low. With a beam under 7ft high, add cross-members on the A-frames about 5ft up and hang the swing bench on chains that hang from big horizontal bolts through these cross pieces. Along with the size of the swing bench, the short length of the chains will limit the swing angle. You'll still need to anchor the rear feet, though, as the mass projecting forward could tilt the A-frame forward. The Grand Hyatt resort in Kauai has gorgeous teak swing benches, with room for 3 large adults, where the top of the A-frame is just 6.5ft high and the only anchors are two steel angles keeping the rear feet on the ground.