Transfer it to a horse and four wheel cart. You have the two stiff bars, one on each side of the horse ina fairly "stiff" (some give in the harness) connection. Piviot point in the middle of the front axel, under the front end of the cart. You can go quite fast with them as long as you go straight forward or take a big open turn, not a narrow sharp one. I guess the rest is about center of gravitym how wide and how high the cart is and the weight.

Maybe look into extending the lenght of the axels to make it more stable?