I can’t picture your driving bridle with the “upside down y” that prevents use of keepers. Bit Keepers might not fit the straps, but the bit has to get attached to the bridle somehow.
Per Kande04’s worry of the tire sliding up under horse, this is one reason to NOT start with little bitty tires unless your equine is also very small. Car tires have some weight to them, but not huge to have horse thimk tire is unmovable. You have teach horse to be brave, he will ALWAYS be able to move with his load, because YOU won’t ask too much from him. YOU build his confidence during training with small increases in weight pulled. Trust me, horse KNOWS there is more weight than last time! Ha ha Picking your ground to drive on, can make tire pulling less slippery in stopping. Our barnyard is gravel and crushed cement, flat hard surface so tire moves easily, offers friction without being slippery. Muddy ground will let tire fill with dirt, adding weight, moderate friction when horse stops, usully not sliding very far. Our Trainer has loose dirt in the covered arena, so tire fills with dirt but added weight is minimal.
Going immediately to fake shafts is OK, but they should not be tied together to start. We use very light string to hang our saplings from the rein terrets to start. NO baler twine because it won’t break easily. The object of the lesson is to have horse FEEL the fake shafts rubbing him, bumping body parts, swing out a bit on circles. That can’t happen if saplings are tied together with that board across the ends. Actually, that rigging is called a Travois (pronounced trav–oy). Used by the American Indians on horses to move goods during their seasonal migrations. With the light string attachment, the sapling can break loose if horse spooks, jumps, so it won’t chase him. You halt, he stands, you attach the string on harness and ask him to walk off again. This is a double lesson, standing well, sapling falling off is NO BIG DEAL. Didn’t hurt, and always popular, “I get a break!” happens. We start with one sapling, usually on the outside so ends swing wide on long line circles. I would NOT plan to ground drive in straight lines, open ground, when starting dragging poles. Sometimes they really jump, even after watching helper drag it alongside. You might not be able to hold him, so keeping things contained inside a fence/arena, is safer.
After some minutes of sapling on his outside, stop, remove sapling, lay it down, turn him around, put the sapling back on to learn dragging in the other direction for a bit. Again, if string breaks, slow him and stop, put the sapling back on. Act like it was planned! Nothing to get excited about, everyone stays calm, relaxed. Stuff happens while driving, shafts will jerk and lurch as cart goes over bumps, nothing to get excited about. So this preparation in dragging poles and tires that can “do unexpected things” is good as learning experiences. After some time dragging shafts on only one side, staying calm in all gaits, listening, you move sapling and attach the light string on his inside of the circle. Still only one sapling on him. Start slow, lots of walking because this time sapling will be staying close to him, rubbing more, because of centrifugal force pushing sapling outward, with horse in the way. Take your time, let him get very used to this new feel on him. Do circles in both directions to learn the feel on both his sides. Speeding up gaits will push the sapling against him harder, get him used to that. Your fake shafts ends are way out behind his longest stride, trying to prevent getting a leg over. When he is quietly going in both directions a while later, it is time to put both saplings on at the same time and have him walk off. Keep it slow so he can learn the feel of things, get used to them before asking for trot. You should be able to have him walk across the center of circle to reverse directions, drag things going the other way. Stop in the middle if you need rein adjustmenta. Everyone is calm.
Both pole dragging and tire dragging let animal experience odd touching and pulling of moving things, coming and going. Tires pulled will also swing outward on the circles, with stronger force as speed increases. Horse learns to accpt the pull, pressure of traces on haunches, occasional bounce of tire if it bumps something, just like happens unexpectedly in real life driving. I do think both pole dragging and tire dragging are really important teaching steps, to be done before you make fake shafts rigid with a crosspiece… They learn a lot more than just dragging stuff looks like, waching them move along. Repetition of these steps make them acceptable, not startling, before progressing to the next training step.
These basics “drilled” into horse are his source of life “experience” to figure reactions in new situations. A deep well of experience, never getting hurt or scared while learning, makes it more likely he will WAIT for you taking charge, giving directions, when a new scary thing occurs.
Your Trainer is coming, you will have tell us what she wants you to work on next!