Actually Foxglove, tire jerking, a bounce off something like a post while pulling is expected, part of the training. Heavier tires of a pickup truck size are a bit weighty, a lot more than a tire from a small car, so the larger tires are less likely to bounce like a ball when moving along behind a horse.
We show our young horses the tire laying on the ground, let horse follow a dragged-by-person tire, to see them in action. The horse may be asked to walk over the tire, get a snack off the tire laying there. This is all before we think of attaching the tire to the singletree for dragging behind the horse. We like the tire to start dragging with because of it’s shape of round, and the “giving” makeup of tires being rubberized. Logs or poles will catch on dirt lumps, then JUMP as horse continues forward. The logs and poles roll on turns, can swing out wide when doing long-lining things. Just not dependable in how they react when pulled. Tires also will swing out on long line circles, but don’t leap and jump badly even over slightly rough ground. An advantage is how nice the work area looks after being smoothed by the tire’s countless circles behind the horse! Ha Ha. Tire swing-out in circles will help get horse used to some rubbing, tugging of the traces on his body. Horse is expected to ignore that stuff to go where reins tell him. He is not jumping when surprised by harness jerking on him when hitched.
Not sure what kind of hay bales you have, but just trying to pull a hay bale here would result in broken (rubbed thru or pulled off) strings, wasted hay on the gravel. I don’t think a hay bale would last one circle behind the horse, so I have never considered using a small hay bale for training a horse to drive.
We start our horses at an older age than Foxglove. We have come to believe that our horses for driving, don’t have the mental ability to accept a great deal of training for anything, at very young ages she mentioned. They can’t pay attention, are quite silly before age 4yrs. With the bit of age on them, previous daily handling, they are ready to accept training much easier and not get started playing games or avoiding requests. They just “get it” so much easier, respond better, so we can move right along thru the training steps they MUST understand to be a good Driving horse.
Ours are not Drafts, which I know a lot of folks start at very young ages. Not the same kind of horse accepting work mindset. We do a lot to prepare them to be Driving horses. There is no safe place to drive around here with a bolting horse, totally unacceptable for any reason because someone is going to get badly hurt.
There are lots of ways to train Driving horses, tools to use pulling. Horse has to learn and be dependable to voice commands, situations, before he ever gets hitched. Horse has to stop and stand EVERY time you say Whoa, from any gait. Not ONE extra step allowed on a loose rein. That is your emergency brake should some terrible thing happen like a broken rein or broken harness, vehicle problems. We have NEEDED that brake a couple times over the years, no one got hurt. Vehicle tipped over once, had a broken hame allowing harness to come mostly off, but horse stopped and stood with voice commands, while we sorted it out! Horses praised to the skies for OBEDIENCE. Driving is not riding, horse has to be trained to a higher degree because of the things we hitch onto him.