If you jack by the frame, you will need a much larger jack to lift half the weight of the load on the axles and have enough travel to get it off the ground, at minimum, a 8-10 ton jack. If you lift from the axle under the spring perch, you will only be lifting a quarter of the weight, 4-6 ton jack, and only need to lift a few inches off the ground leaving the other 3 tires firmly on the ground for stability and safety. Consider the conditions you are in if you are trying to change a flat on the side of a busy interstate or a narrow 2 lane road with little shoulder that is sloping off the highway. I don't want to be lifting half of by rig off the ground while trucks fly by rocking everything that is perched by a single jack. When we had our blow out, I-44 between Strafford and Lebenon Mo., the shoulder was not wide enough to keep the right side on the pavement. So it was off the pavement to give me what little room to get it out of the driving lane, I had to stand on the white line to take the lugs loose. I used the jack out of my truck to lift the one axle to change the tire. Trust me, when trucks a sailing by at 75 MPH, your trailer will rock. Not a good feeling or good situation to have half of a already leaning 16,000 pound load sitting on a single jack.