For carrying loads you can always fit very heavy duty springs but this compromises ride quality when empty. The solution is Polyair springs where you can adjust the air pressure in the spring to suit the many varied load weightsthat may be carried in a vehicle. If you are towing these springs can compensate for towball weight of the trailer. A vehicle that sits evenly on the road will handle and stop better plus will be easier to drive reducing driver fatigue and improving safety. Each spring has a separate valve ( the same as a tyre valve)so you can even compensate for different weights from side to side on the vehicle .