I want to install a 100 lbs heavy bag in my finished garage. I don't want to cut into my ceiling and also my daughter's room is directly above the garage so I'd rather not disturb her.
I was wondering if I could put a 4x4 between the two walls (which are just over 10 feet apart), supported on each side by 2x4s screwed into two studs. Then I could hang the bag from this 4x4 with an eye bolt.
Is this workable??
100 lbs. jostling about creates quite a bit of force. I'd consider a 4x4 adequate up to about 8 feet of span. After that it's going to quickly become a rubber bad. Or worse, it could fracture and come down (hard).
For spans of 8-12 feet, I'd put a 2x10 oriented vertically in a pair of joist hangers, and I'd put another flat on top of it, screwed every foot or so. This creates a rigid T configuration that should be stiff enough. On the wall, span a section of 2x10 across several studs to create a robust surface on which to mount each joist hanger.
For spans longer than that you'd need something even heavier, such as four 2x10s in a double I beam configuration ( like II, which would maybe be good up to 16 feet). Instead, consider spanning a shorter distance between two adjacent walls using 45 degree joist hangers.
Instead of all that wood, just bolt the 2x4's on each side and measure the exact distance then have a plumbing supply place cut you a 2" iron pipe and thread each end, screw on some flanges, then drill a hole through it for your eye bolt.
I think your plan is basically sound but I'd beef it up a bit.
Number one, I'd definitely use a spring when you hang your heavy bag, that's going to take a lot of the shock off the support.
Unlike a floor joist, where you want a big enough joist that it's stiff, with a heavy bag, some flex is a good thing. The more the timber flexes, the less the shock on your walls. I am concerned that the strain will mess up your walls - and it could mess up the drywall on the other side of the walls.
Your idea to use 2x4 ledgers is probably undersized, too hard to get a solid attachment to the 4x4 beam and to the studs. I'd use a 2x6 or 2x8 lag screwed into the studs. Try to hit three studs with a ledger 36" long.
My first choice to hang the beam would be to rest the beam in 3.5" x 3.5" notches cut in the top of the ledgers. My second choice would be a hanger type bracket like the Simpson U44:
If you want to be extra protection for your walls, I'd put 3/4" plywood under the ledger, all the way to the floor, screwed into the studs all the way down. That will stiffen the wall so the drywall doesn't get messed up on either side of the wall.
