That packsaddle is a standard Chilean /Argentinian riding saddle frame (bars + cantle and pommel 'hoops') with modifications by us. Normally their saddles have more or less 7/8 rigging and a second surcingle-style cinch over the seat and overlapping the first.
We put new rings on so that we can use 'Y' rigging. Keeps the back of the saddle down on a horse who doesn't much like a belly cinch and who doesn't really have the shape for it either.
Second, because this saddle carries home-made pack slings, we added attachment points on the cantle and pommel arches so that we can clip the slings on directly. A second set of rings on the same arches allow us to put a light top load on the leather part of the saddle and secure it with two leather strings. The photo below doesn't show all of that in detail but does show the slings and the top load.
In terms of weight, that horse carries between forty and sixty kilos with a new load (he carries grain for the horses so his charge varies according to how much we find to buy) and it drops thereafter because the horses are eating between four and twelve kilos of it per day. As his load lightens, we transfer elements of the other horse's load to make things fairer. His top load rarely exceeds eight kilos.
The grain is carried in two waterproof cylindrical gear bags which have no protruding parts.
Although it's not as easy as with a 'proper' pack saddle, I've still loaded bales of hay using a barrel sling onto this saddle, and with minimum help (I'm 5'4" on a tall day).
The other horse (pony) uses an adjustable packsaddle. We can throw a barrel sling but apart from that we have no idea how to pack with ropes - our pack panniers 'hook' onto the Ts on the arches. The pony's load stays relatively constant, between 45 and 55 kilos, including a top load of maximum 12 kilos.
In terms of the swiveling bars, we have no problems with them. Bags are packed so as to have no hard objects on the horse side. No probs with sides either apart from one day when the pony had the trotting bug all day and had a pressure bump on one side of her ribs.
For both pack horses, we use a leather sweat-absorber directly on the back, a vetbed intermediate layer and a thick wool felt pad in a canvas cover. Given the weight we pack, we didn't see the need for side boards.
Ever four hours or so, the pack horses are unloaded for at least a half-hour.
Four horses, two people. 4000km in to the South American leg of our trip,and now in Brazil. Drop us a line on The Great Horse Trip