It's time the two of you get a lesson in equipment physics and how it compares to the type of equipment you purchase. Let's look at curve shaft vs. straight shaft. Curves shaft units loose up tp 20% of the power generated at the engine, depending on their design. Because the is curved, you must use a more flexible drive shaft, which transfers less power to the head. Second, with a curved shaft, you are putting uneven pressure on the head bearings and coupler, because the shaft is constantly trying to straighten itself. This pressure causes the bearings and coupler to wear out faster. The coupler on the head and clutch coupler must be looser, because of the torque on the curved drive shaft, causing them to wear faster. Curved shaft units can only support the string heads, while straight shafts can accept a variety of cutting heads, because you don't have the extra torque pressure on the clutch, coupler, shaft and bearings. Curved shaft units also run hotter at the head, because of the reasons listed. Curved shaft units are mainly for residential use where the home owner uses then for about a half hour at a time. Yes, they are lighter and easier to get around, but for professional/commercial use you are better off getting a good straight shaft unit, with a good backstrap system. They may be heavier and not as pretty, but if you are in the business, you care more about lasting quality than looks. If you don't believe me, take an old drive shaft, bend it a little and then try to twist it.
