I'm putting together a swing set in my backyard mostly following this Youtube video. I'd like to make sure that the legs of the A-frames don't sink into the ground over time when it rains, etc. What the best way to do that without making concrete bases for the legs? Will the swing set anchors with spiral discs be enough for that? The anchors I see on Amazon etc. look small (only ~2" disc) so want to confirm. If such anchors are not enough, what if I connect the (bottom) tips of each A-frame with another 4x4 or 2x4?
I Assume your set is like the one in the video and has wood 4x4 legs and that you have cut the correct angle on the bottom of the leg so it is flat to the ground.
You could cut a square piece from a 2x6 ( or 2x8 if you like, treated or outdoor wood ) and attach it the bottom of each leg. This would spread the weight out over a broader area. IF you leave them on top of the grass You could use a router or a saw set at an angle to round over the all the corners and edges to eliminate all sharp right angles both top and bottom ( top for safety and bottom so you can drag it to a new location without the edges digging in the grass ) With this method the base/foot will travel with the set if it get moved. But i would recess them into the ground to avoid a tripping hazard.
Another possibility is buy a some pre-made concrete pavers but these are very hard obviously and i would recess them into the ground and glue foam padding all over them or cover with a layer of sand. You do not want a kid to fall and hit their head on them ( or the wood for that matter.) This option would not allow for moving the swingset with out re-setting the pavers.
Or go full child safety and do like they do on school playgrounds and anchor the set deep and put down sand or shredded rubber to lesson the IMPACT when child falls of the swing.
Bury the posts (1) The posts will resist moving vertically and laterally if they are buried in 6" or more of dirt. (2) The depth of the holes can compensate for a modest slope between the legs.
Use pressure-treated pine lumber rated for ground contact. These are easy to find and routinely used as fence posts. They will last in the ground long enough for your kids to swing your grandkids on it.
(1) Build the basic A-frame (around 1:00 in the video)
(2) Set the frame directly on the surface where it will be installed and use pavers or scrap wood as shims until it is level. Any posts that required a shim will need more gravel to make a shallower hole. Jot this down on a piece of paper and mark the location of each post in the ground with a small stick or spray paint.
(3) Move the frame out of the way and dig holes for each post. Plan for 3-6" of gravel under each post (I prefer #57 crushed stone). This will help rain water to drain away from each post and deter rot.
(4) Pack a couple inches of gravel around each post and then cover with dirt/sod. Proceed with the rest of the swing construction.
I wouldn't bother with holes deep enough to avoid frost heave. With such long posts I would think that any heaving will be compensated for across the structure and won't cause problems.
Probably you can stick to 8 ft posts and just plan to make the swing a few inches shorter. But if not start with 10 ft posts and cut then down to 8 ft + however much you plan to bury.