The Super Duty trucks are larger, heavier built commercial/industrial series pickup trucks with heavier-duty body-on-frame steel ladder frames, axles, springs, brakes, transmissions, more powerful engines, and all other heavier/bigger components (with much higher payload and towing capacities) than the older traditional equivalent F-250, F-250HD (Heavy Duty), and F-350 Ford truck lines. The two-wheel drive and four-wheel drive model frames are the same in the F-250 and F-350 series, making conversion from a two-wheel drive model to a four-wheel drive model less difficult than in other truck models. This is because the '99 model year of the F-series has all the bolt holes in the frame for both two-wheel and four-wheel drive precut by the manufacturer.