I also wondered that. Our normal carts for road driving, starting, and just pounding the bajeesus out of have shafts that extend straight from the cart. Both our cultimulcher, (not much elegance there,) and our runabout have shafts that curve where they connect to the cart. They are jointed and move with the cart when we use them, generally on things that have more than two wheels. A two wheeled cart the horse carries the cart, more or less. (Properly balanced it should not weigh the horse down.) But other carts have more on the ground and are just pulled, so for give when the horse trots the shafts move up and down. Then again, this is how we make them, not particularly how it must be done. But there might be other reasons.
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