Yeah I have bils on the rear as well... I have empty bed a lot of the time so thinking that upgraded coils would prob ride better than spacers. So no issues with 1.5" on the 5100 rears then I take it?
Think of this with intended usage in mind, from my Jeep or Toyota overlading experiences, which of course is different from the towing world, though i'll assume the same concept can be applied here, you want coil springs to handle the intended load, among certain things. Spacers are good to level, not exactly designed with the task of making a stock spring to handle more weight, the spring will be compressing the same given x amount of weight, you're adding Y amount more height with the spacer, so the when the spring compresses, the rear will drop the same amount minus the spacer, i won't go above 0.5" with a spacer expecting factory springs to handle weight abuse for example, but this is just my opinion, i only use spacers to correct a trim issue, push up a corner because say the gas tank pushes on the vehicle more from a side causing a lean) if i need more than 1/2" of lift, i'll use coil springs to handle additional load. it's typical for overlanding vehicles for example to be 300-600lbs heavier than factory at all times, so coil spring choices are needed when you want to distribute that weight, while trying to run 33's,35's 37's or bigger tires and keep your lift at a certain level.
If you add 1.5" of lift to the factory shocks, you shock is actually extending 1.5" more than normal, it's resting position is 1.5" higher in the stroke, so without a longer shock, you're losing suspension travel, and running more risk to over extend the shock if you're pressing hard on them, that's one of the reasons to run aftermarket shocks if you're lifting, whether coil springs or spacers, the other reason is valving, with more weight, your shock should be valved differently to handle the extra load. I'd think the factory did a good job of designing the shocks and springs to handle extra weight in our application, after all, this is a truck designed to tow and haul stuff, but you'll be affecting the design when you add 1.5" of space in the back or front for example.
To address it, you can choose air bags if you're always having an empty bed, but loaded once in a while, that way you air up when you have a load in the bed. Coil springs will be a better choice if you're always carrying load, optionally, the matter of a certain look.., do you like to see a full rake without weight? or do you want to appear level without weight etc.
If you have the Billies set at 2.1", then a 1.5" rake in the back will be a lot of *** up
, with the billies at 2.8 and the same 1.5" extra in the back, it seems the rake is back to about 1/2" more in the rear, again, all this is subjective, i wanted to absolutely kill the nose down approach from the factory, but still have around 1/2" in the back, so i went with 2" in the front only (Rancho QL), i'm not into towing or hauling heavy, and i bought the airlift 1000 air bags to install them if i needed to. i haven't decided yet to install them, because i want to make sure they won't prevent my suspension from compressing properly when offroading (i'm more into offroad/overlanding), so at the moment they're still sitting in my garage.
The other issue with coil springs, i've noticed we don't have a lot of options,(i come from the Jeep world, so the afterermarket is huge, and i mean huge and well known...) and for the ones available, i don't see specs listed, only height: example 1.5" coil springs from Icon, 3/4" from Carlie, etc. I've callled Icon to ask about specs (spring height, press rate in inch/lbs, etc), and they won't disclose that info. if you go with a heavy rate coil spring in the back or front, and no extra weight, you'll be making the ride harsher, and without a way to control your shocks valving, there won't be a lot you can do to cure it. If you go with heavy springs, you must push weight through them, and/or adjust your shocks to handle the load accordingly, assuming you can adjust them. to think it's so easy to choose from linear springs for example, typically you know the factory specs, say 200lbs/inch, that means it takes 200lbs to compress the spring one inch, knowing your spring specs, can help you choose how much weight you need to maintain, say 3" of lift, if you're adding steel bumpers,winch, rock sliders etc to your rig, then you can choose a shock that can make use of the total extension/compression in your springs.
It'll be useful to know the coil specs on our trucks, i want those Icon 1.5" coil springs, not because i want to hold extra weight, but because they're longer than stock, and are progressive meaning the coil can be open longer than a linear coil spring counter part because they have more coil packed in around the same lenght, so they can hold a longer shock, i want 27-28" extended rear shocks, and gain 2-3" more in droop, i hate to 3 wheel, especially when the truck is leaning out in a rut
. but if i want to run those in the back, i'll have to make the truck talller in the front, wish we had 1/2 spacers to add in the front haha, either that,or maybe i should add an RTT tent in the bed and leave it in there permanently... ummmmm, i think i've found the excuse to run those 1.5" springs, lol.
Hope it helps.
