Brooke here. The hinges are in.
Continuing the spirit of over-engineering the duck (quack) out of the fence, I found some black “heavy duty strap hinges” online. We purchased six; there are four 12″ hinges for the top and bottom, and two 5″ hinges for the center.
I was rather grumpy with the shipping costs, but they came in a box that was too heavy for me to lift and I had to drag it into the house. That previous sentence should be read as: I had no idea this hardware would be as massive as it is. Brown said that touching these hinges made you want to buy a cattle ranch and wear faded denim, growing gradually more rugged in the Western sun as Sam Elliott narrated how you casually moved a toothpick from one side of your mouth to the other.
Oh, you people always think I’m kidding.
Because the posts aren’t quite level (we aren’t sure why: they were level when we poured the concrete, they were level for a few months after that, and then they weren’t), we had to get adjustable hinges. You drill a hole 4″ into the post, then adjust the depth of the screw to get a level gate.
I’m not sure the gates will ever be scientifically level, but we’re aiming for cosmetically level, which is a totally different thing.
Once the plate and pin are attached to the post, you just take your gate and plop the eye hinge over the pin.
I’ve started building the gates. They will be 3″ thick and 5′ tall, and are already remarkably heavy. These hinges might be overkill, but not by too much.
Flying by night 
Our posts moved too–it took a couple of years, but enough weather and freezing and thawing will do it. The best thing we ever did was add an arbor above to stabilize the movement. Now the gate works perfectly in all seasons.
It sounds like a common(ish?) problem. Is there any possibility your company will be adjustable hinges? The ones we bought are not all that attractive, and we’re going to have to put up a baffle to hide the huge 4.5″ gap between the edge of the gate and the post. I’d love to see this idea executed by a company with strong design.
Best way to keep a new gate from moving is to plant a line of shrubs on at least one side of it. Of course, it defeats the purpose of a gate, but the roots hold the earth in place so you don’t get that settling issue with weather and freezes.
Are you sure that you need all five hinges at this point?
Probably not, but we’ve got huge dogs and gates are deceptively heavy over time, so better safe than sorry.
It’s interesting you bring it up because last week a good customer wrote asking if we were planning on designing some adjustable hinges. We’ve talked about it, and we’ve made them for driveway gates before, but those were always welded to steel posts. Most people don’t want the big gap between the gate and the post, except for those who need it to swing both ways. I think I have another source for adjustable strap hinges and I need to dig through my files and see if I can find it. If I do, I’ll post it. They had an extensive catalog of straps–I’m sure something was adjustable–but really nice quality and they definitely knew their stuff…
I am really adoring the overengineering posts. They’re giving me a better idea of how things are built properly, so someday I can aim at that.