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I agree with what was said with pedders they are much firmer, however they cope with a fully loaded care pretty well, I want to change to king superlows as i have monroe gt sport shocks. How do kings cope with a fully loaded car and would i be better off with heavy dutie rear springs?
If you want to tow or haul heavy loads, maybe look into air bags. You know the type that go in the middle of your spring. I know alot of people that have extremely low utes and they still tow heavy work trailers just by inflating the air bag.
As for kings, I never had any problems with carrying loads, i used to tow my hillclimb car on car trailer from time to time. That was ultra lows and 19inch wheels.
The main problem I had with being this low was that when you've got a full car and you load it up like starting a burnout or drifting out of a corner, the tailshaft would hit the floor and make quite a racket. This is not ideal at all. ended up modifying the floor to prevent this from happening. Thats why I have bort an IRS calais this time.