So I picked up a cute metal easy entry cart and harness just for fun thinking I would use it on our 34" Mini Donk. It was previously used with a 38" mini. The harness is nice black biothane and the cart has 48" shafts and 20" wheels. The harness should work on the mini, yes? Or so I really need to go out and find a 38" mini now?
I've found a 40" mini who is a real deal, but was told I would need longer shafts. Is this correct? Thanks.
I would start with the harness, put it on the donkey and see if it fits. In a perfect setting, the holes should be snug at the center hole of straps. In real life, being in the biggesr or smallest hole setting, means you can't do any adjusting to an animal who may change sizes with weight loss in getting fit, or growth or muscling. Other big thing is that harness saddle has a tree in it. Like a riding saddle, the tree supports the saddle ABOVE his spine, no pressure on the bones. A treeless harness saddle can put a huge load on his spine, is meant for a 4-wheel vehicle that will have almost no shaft weight on him. Treeless is not the best choice in harness saddles, but common in less expensive harness. Just should NOT be used for 2-wheel vehicles. To see the load on his back, hold the cart shafts FIRMLY and have someone get in the cart. Have them sit down, lean forward and back, so you can really FEEL the weight they exert on the harness and animal. Always a surprise when you hold the shafts and learn about cart loads on him! In ideal conditions, good design carts, the consistant shaft load should be quite light, not more than about 8-10 pounds. Really! Large equines might go up to 15 pound shaft weight, but more is hard work on them. Wide saddles are recommended for pulling heavy vehicles, even with light shaft weights. Know that adding pads under the harness saddle does NOT remove or lighten spine pressure, just compresses the pad under shaft weight if you have it.
You don't know any of this, until you put the harness on him and see if you can adjust it to fit comfortably. Donkey will have a different build than a pony, so some parts might not work like they would in a pony. Less rump is usually mentioned, maybe shorter backs, narrow neck, wide heads. It is not real expensive to buy spare parts to fit "that" animal, but then you need to decide if you require a lot of parts, might be easier, cheaper, to just get a correctly sized harness for him. Sell the other harness.
It is going to be the same thing with the cart. Height is a help, but some animals are longer bodied, longer strided, so how a cart fits them is not what a shorter bodied animal could use.
In a good fit, cart shafts NEED to be level on his sides when hitched, height is along his mid-barrel. The shaft tips should end at the tip of his shoulder, not longer or shorter. Then you need to extend his hind leg out as far as he reaches backwards, to see if he might clip a wheel or the cart when moving out. He should have about 15 inches between his rump and singletree where the traces attach. With harness correctly hitched, the cart won't bump him from behind before his breeching stops it moving. Plenty of room behind to move around, extend his hind legs out behind.
These are the BASICS on harness and cart fitting. There are a lot more details, but it will give you a rough fit size, to know if what you have will work for him or not. Donkey can be fun to drive, just has to TOTALLY understand what is being asked of him. They don't think like ponies or horses, so getting good communications with him is your starting point.