Add a leaf may get you an extra 300-500lbs of payload capacity. however, A severely sagged stock leaf spring will in fact, be harder on any AAL, than a stock leaf spring pack in decent condition.
A bastard pack is in between an AAL and Full packs. It is just adding leaves to the current pack. People say they ride better than an AAL, but that is not necessarily true. Given that you are keeping the same main leaf, the lift is accomplished the same as an AAL, by increasing the spring rate.. You achive that arch by adding more leaves, which increase the spring rate, making it stiffer. Really, Its not much different than an AAL.
Full packs are the best option usually, because they are new, obviously. Cheap ones could not be much different than an AAL or bastard pack, accomplishing height by increasing rate. An unloaded lift pack could have the same arch as an unloaded stock pack.
Nicer ones are built with more arch, and because of that they can have the same rate as a stock pack. for example, take 150lbs/inch as an example. If you had a spring that had a 6 inch arch with 150lbs per inch, when loaded say with 600lbs, would sit at 2" of arch (600/150=4). With the same rate an 8" pack would sit at 4" (2" of lift), and ride the same. A cheaper one may have 7" arch, but a 200lbs/inch rate, which would sit at 4" also, but ride stiffer.
Heavy Duty Leaf Spring Swap From Stock 93/97 Rangers...
I had went this route until I could determine the Deaver leaf custom kit that will fit my needs as I always have gear in the truck for everything and anything. And when I say that, I mean the truck has at least the following in it at all times;
- His and Her totes each consisting of a Spare ground tent, 2 sleeping bags (which the one of the two zips to the other one in the other tote and there is a separate military sleep 3pc system good for up to -15* too), 2 additional fleece blankets ea., a wind breaker and a slightly heavier coat. The tote weighs in near 50lbs (x 2)
- MAC tool bag, capable of nearly everything to do anything on the side of the road or trail.., nearly. That's 86lbs
- Spare Alternator, starter, couple hoses & belt, 2 sets of spark plugs, misc.. 60+lbs
- His & Her G.H.B. (a) 34lbs (b) 29lbs
- 1/30' ARB snatch strap, 2/20' ARB snatch straps, 4 shackles, a tree saver strap, a full-size shovel, a full first responder trauma/first aid pack, 2 smaller first aid kits all of which weigh in near 60-70 lbs
- and a pack of water...
I havent included what I take along with additional recovery gear, the fridge, misc ammo and firearms the weight of the truck vault I am building along with on-board air system or the rear multicarrier I havent built yet. The Flip Pac weighs in at 329Lbs alone and adds to lots of things to consider weight wise as some of those things once added, will be a part of the vehicles new curb weight too.
I think you would be better off to go with a full leaf pack upgrade or build a leaf pack than using the AAL [Add A Leaf] kit. I've not heard of anyone who off roads or caries a consistent amount of weight have a good story about their AAL experiences across the truck platforms.. Not Ford, Not Jeeps, Not Toyota's, certainly not Nissans.
Not only will a full leaf pack ride better its capable of so much more between capacity of payload and flexing when asked. Over flexing a leaf spring will cut its lifespan dramatically, no matter what type of lift you install. Leaf springs simply cannot deal with the added stress of over-flexing.
Ride stiffness is as more dependent on your choice of shock absorbers and brand of lift parts than on the actual type of lift parts, AAL vs a brand new lift pack. All lifted vehicles will ride differently than stock and you should expect some compromises when driving a lifted Vehicle.
And although AAL does not contribute to axle wrap, Lift blocks do contribute to axle wrap and are the least desirable lift option. Lift blocks are also the most likely lift option to cause excessive leaf spring sagging due to the addition of axle wrap to the lift equation.
I seen this post was fresh so lots of people will weigh in on this I hope. I sadly can only parrot some things I've heard but my experiences are shared with what I do more often monthly than most do yearly.