First my experiences with T-posts then comments related to wood posts.
I find that my Kubota 4610HSTC with FEL will shove t-posts into the ground just fine with no ballast in the busket if the ground isn't too hard or rocky. If it is hard or rocky you can bend a post too easy and adding weight isn't the only answer. A pipe cut to the length of how much post you want left out of the ground will help keep the post from bending and gives you a gauge so the posts are uniform.
There are two ways to use this that beats starting them by hand: 1. weld a handle 3-4 ft or whatever length you like sticking out to the side so your helper doesn't get too close to the action or 2. weld on tripod legs (one adjustable) to hold the post up with no helper. You can attach a couple strings with weights (like a plumb bob) at right angles to each other to give you or the helper a "gauge" to indicate when post is vertical on the handle version or the tripod version. The bottom of the tripod legs need "feet" so they aren't shoved into the ground too easily. the pipe itself is suspended off the ground in the tripod version so there is clearance for the "barbed spade thingy."
A removable pocket that attaches to the FEL bucket (an inch or so of pipe in length and 3 in or so in diameter) will safely hold the top of the post from slipping from under the bucket. If you use too small a diameter of pocket it takes too much fussing to get it over the post. Previously I have raised the front of the tractor well off the ground many times in hard ground just before the post bends. This is why I like the pipe thingy. It helps prevent disasters.
Based on my experience with T-posts, I'd say for wood posts you need a larger tractor than my Kubota 4610HSTC and most likely a fair amount of ballast weight in the bucket. If you can get a 3PH auger a bit less in diameter than your wooden posts to drill pilot holes then you could likely drive the post with a FEL bucket a whole lot easier and still have a snug fit for the post. This I feel confident I cold do with my tractor or a considerably smaller one. I don't think I could drive a 6 inch round post even if sharpened with my FEL unless the ground was dead soft, nearly mud. LIkewise a 4x4 post would be a challenge. If there were no rocks and the ground wasn't too hard AND I had a holding fixture to ensure no helper in danger and so forth, I'd still prefer to predrill the holes. I have had the tractor front up in the air many times with T-posts and the force required for a decent wooden post (4x4 or larger) is way more than a T-post.
Just had an idea for a safer way to drive posts with FEL. A holding fixture that attaches to the forks with a chain or strap that runs up to the top of the post. This pulls the post towad the ground instead of pushing it, can't slip off, and you get an automatic depth gauge since you definitely will stop sinking the post when the forks hit the ground.
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