Adding airbags question

I will be adding Firestone airbags for towing. Would it be of any benefit to add sway bar also ? Just got price from local dealer for SB $332 + tax.

 

Little benefit to the sway bar if you have bags.

 

The place a sway bar would help is when you are going through a turn (or heavy sway) with the bags under pressure with a load. Of course this is only if your bags have a T fitting rather than individually plumbed. In that condition, the sway causes the air to be forced from the low side and to the high side causing the sway to increase. Individually plumbed, it isn't an issue.
Without the bags being under load, they ride pretty much like normal. A way to avoid this is to use a properly setup anti sway hitch with your trailer.
Personally, I would use that money towards getting a self leveling kit installed instead.

 

I would disagree. Increasing the spring rate of the suspension resists suspension up travel (the wheels moving closer to the chassis) as additional weight from towing or hauling would do, but it does nothing to resist suspension down travel. In fact, a stiffer spring pushes down harder on the rear axle. As you go around a corner and the load shifts, the outside spring resists compression from the additional weight while the inside spring promotes extension being as there’s less weight pushing down on it. This is body roll, or sway.

Sway bars - or more properly, anti-sway bars, resist this change. It does so by translating this change in side to side suspension weighting to a twisting force on the sway bar. Much like a torsion bar suspension on many early iterations of IFS trucks, the spring resists change by twisting, effectively changing the spring rates of the existing springs - only, unlike the actual springs, they can induce a force opposite to the natural weight shift and increasing that force as the weight transfer increases. It is able to do this without increasing the spring rate of the load bearing springs like air bags do.

The air bags are able to resist the outside weight shift pressing down on that side of the suspension bringing the whee and chassis closer together, but also increases the force on the inside spring to push separate the wheel and chassis. And that’s only if you plumbed the bags separately. If you plumbed them on a single inflation valve, the air from the outside bag can transfer to the inside bag negating any anti-roll a stiffer outside spring would normally produce.

In short, air bags and sway bars work in very different ways. Bags can mitigate body roll in a very small way, but they do not serve the same function as a sway bar.

I have run bags for several years. I plumbed them separately in order to level the load side to side when needed, but also to better combat body roll. I had a TRD rear sway when I installed the bags. I then removed the rear sway bar thinking it wasn’t necessary. It didn’t take long before I re-installed the rear sway bar.