Air Bag Over-Load Conversion

After going through several rear suspension revisions on my '13, I finally came up with a setup that works well for me and what I do with my truck - an Air Bag Over-Load conversion.

I tow regularly and am virtually always hauling something in the bed, but still put a fair amount of highway miles on the truck, and even blast down some gravel roads and rutted two-track from time to time. I've found that virtually no matter what shock I swapped in to the rear suspension, I was always compromising somewhere - too stiff and bouncy for the ruff stuff but handled well on pavement; supple on road with light loads but wallowed like a greased pig with a trailer; etc. And due to the constantly varying load in the bed and on the hitch, I was continually compromising in spring force as well.

Here's the basic process and how it looks at the end, as I just did the same thing on my '21 that see's identical usage at the '13.

Existing suspension on my '21 - Heavy duty leaf pack from Rock Auto, Falcon adjustable rear shocks, TRD Rear Sway Bar, Perry Parts bumps. Look at how the overload is a hairs-breath from engaging.




The process is simple once the vehicle is secured on jack stands with the jack under the rear axle to move it up and down:
- remove the u-bolts,
- disconnect the rear shock and sway bar,
- clamp the main leaves together,
- unbolt the leaf spring center pin
- place a jack under the opposite side of the axle
- lower the axle down so that the side you are working on droops down at the leaf pack separates from the axle enough that you can get the centering pin out
- pull the overload spring down and remove it along with the centering pin
- insert and new/shorter centering pin (or the old one if it's threaded all the way) and recenter the main leaves
- tighten down the centering pin bolt

Should look like this




Now that the overload is gone and your leaf pack is back together, start installing your new parts:

- I prefer Firestones Ride Rite bags. I think the kit is simple and well thought out, initial quality is exceptionally high, and they stand behind their product as I found with my other truck.
- I also prefer to run cradles so as to allow more suspension travel and movement.
- Lift up the axle again to re-attach the leaf spring to the spring pad, taking care to get the centering pin seated. I always need to give the leaf pack and axle a little persuasion to get them to line up. For this step, my neighbor (who had meandered over to see what all the excitement was about ) ran the jack while I lined everything up.
- Reinstall the rear shock bolt
- Drop your u-bolts over the springs and reinstall the lower u-bolt bracket. I just thread the bolts on a few turns, then run them down a little tighter with the impact set to little kid mode, or my m12 wrench. Don't try to torque them down!
- Fit the airbag and upper bracket to the frame, taking care to line up the frame rivets with the holes in the bracket. You may need to flip the bracket over on the back or spin the bracket 180 degrees to get everything lined up.
- Snug up the washer and bolt holding the bag to the frame
- Run your airlines to the bag, install the airline fitting on the bag, then install the airline to the fitting.
- Fit up your cradles to the lower bag mount, and set the back mount on top of the axle, like so, taking care to align the bag and bracket. As a note - the axle doesn't move straight up and down, but articulates in a small arc, so you will need to adjust the lower bracket with the bags seated.




Now that all the parts are in place:

- Install the lower mount bolts, but leave them a little loose
- lift the rear axle to the point that the bags are seated and adjust the cradles/lower mount as needed. Mine lined up pretty darn well.
- Snug up the lower mount bolts so the mount doesn't move
- Reattach the sway bar link to the sway bar.
- Put your wheels back on and drop the vehicle to the ground.
- Torque lug nuts, rear shock bolt, sway bar bolt, u bolts, and lower back mount bolts to spec. I usually do them in that order, but make sure that you torque the u bolts, THEN torque the lower mount bolts or you may find the lower bag mount to be loose.





I did some before and after measurements:

- With the HD leaf pack and loaded bed, I was at 39.5" ground to fender.

- With the ABOL conversion and a loaded bed, minimum air pressure in the bags, I was at 38.75". The overload is ~.75" thick, so the main leaves are supporting the load properly without needing the overload. The problem is how close to the overload they were.

I had to go pick up my camp trailer about 30 minutes down the road and took the opportunity to go for a little drive with my normal load out, then hitch up the trailer, add some air, and drag it back home. I also took some measurements.

Trailer lined up, but no (real) weight on the ball




Tongue weight 100% on ball. Last time I scaled the trailer, it was just over 700 lbs tongue weight. Fender height 37.5".




~30 PSi in the bags brought it back up to ride height. 45 PSI brought it up to 39.25".




End Result: MUCH BETTER RIDE! With my standard load out and 7 psi, the truck road smoother than it ever has. With the trailer hitched up and 40 psi in the bags, the truck was firm and settled, but not uncomfortable. I could have dropped about 10 psi out of the bags, but I was making sure that the system would hold pressure, and seeing how much up-travel I still had.

So why go to all the trouble and cost to remove the overload and replace it with an airbag??

Simple: The overload spring, IMO, is responsible for the "truck-like" ride of the tundra. It is a compromise on ride quality in order to assure capability - which is a compromise I personally am willing to make. But in this case, I don't have to. At least not any more.

How did I arrive at this conclusion? Well, I'll tell you. Put your reading goggles back on your noses and pull up a mug...


The first thing I tried was some simple shackles for a little more suspension height and a mild decrease in spring rate until you hit the overload. It worked ok but had virtually no adjustability. With a Fox 2.0 shock, it was downright nauseating on concrete freeways.

Then I ran a stock leaf pack with RideRite airbags and Daystar cradles for several years. That setup was great - it allowed a lot of adjustability for heavy loads, still flexed out on the rutted two track, but limited up travel, which was a compromise I could deal with most of the time. But more and more I was finding that I needed 20 psi in the bags just for daily driving and decided to try and upgrade the springs.



Next up was an Icon 3 leaf add-a-leaf pack to a newer stock leaf pack that I bought off CL with 2k miles on it. I retained the overload and the airbags. It worked pretty well and allowed me to air down the bags a bit. It added some height to the back so I could maintain more rake with less pressure in the bags, and it seemed to should loads better.



A while later, one of the air bags started to fail - the wire retaining ring between the two convolutions started to rust out and separate. The bag DID NOT fail, but gave me warning enough that I could remedy the situation. Props to Firestone for warrantying airbags that had been in service for 8 years! After a little back and forth with them (mainly me inquiring what part number I needed and if I could order them direct form Firestone), they told me they were going to replace both bags - not just the one that was failing - and I'd have them in a week or so.



In the interim, the vehicle needed to remain in service, so I decided to cut down one of the factory springs removed when I installed the Icon kit, and simply slip it in bottom of the stack as the next progression of spring. It actually worked really well and rode great - at least up to about 5k lbs worth of trailer, plus my standard bed load out. So now I had a leaf pack with 5 leaves + 1 over load.

When my airbags showed up, I knew I needed to reinstall them as I had some heavy hauls coming up and really appreciate the stability they imbue. I stubbed them back in and found out that I was slightly oversprung unless towing. With my daily load-out, the ride was a bit harsh, but bearable.

Then it dawned on me that every setup with good or tolerable ride quality had one thing in common: they didn't engage the overload spring in normal driving. If I aired up the bags an extra 10 psi when towing, my ride IMPROVED instead of getting worse - because they disengaged the overload. So as a proof of concept, I simply removed the overload on my '13 and drover around. WHAT A DIFFERENCE! But being a truck, I needed to be able to load it up and drive safely down the road.

Firestone has an excellent reputation and have proven themselves to be reliable for years. Which is why I opted to remove the metal overload and replace it with variable air spring.

 

Nice work man. I never did like how the leaf packs are so close to resting on the overload. Genius solution really. Tempted to try it out when I put my lowering kit in...

 

Interesting setup! Thanks for sharing. You made me look at my overload leaf gap between the main leaves and the OL. I have the Toytec progressive 3 leaf AAL and Blue Sumos. I have a pretty good gap and a lot of travel before the overload engages. That said after looking closely I did see a contact mark at the front of my leaves where the overload touches the little bolt/rivet head on the AAL shackle at times. I suspect that is when loaded down towing the camper and hitting a massive pothole on occasion.

 

I like this. I also wish we could haul the same loads with this setup v mine. Very curious how they'd compare.

A quick overview of how I tackle this same scenario. I've got the U748s. It's a 9 leaf pack and is quite dynamic. Those are paired with 37s on 17s. It sounds strange for towing, but honestly, they ride about 30degF cooler and since that size tire is such a good spring, the comfort can always be dialed in. Full trailer load gets 50PSI, empty gets 23, and trail gets 15 (rear axle). Mixed bed weights I aim for three tread blocks of contact patch.

The downside (aside from the obvious when using these big of tires) is that I've got rake unless fully loaded. Like, a bunch of it. I'd like to try a rear sway bar for trailering but really prefer the feel of no sway bars, so I compromise with front only.

To make your setup unnecessarily complex, but cooler, add a tank and a self leveling arm with bleeder block like class 8s. Ha

 

Thanks. I don't know if I'd call it genius, but I think the simplicity of it helps it look that way. Haha.

If I was thinking about lowering my truck (and I have)... I'd do an under sprung rear leaf conversion, delete all the leaves expect for the main leaf, and add an airbag and cradle kit. Being under sprung, you are much less susceptible to axle wrap - which is one of the main reasons to use a progressive leaf stack on a leaf-over-axle suspension. Converting to a mono leaf (or just he main leaf and secondary leaf if you want to maintain that military wrap and give the leaf a little more strength) would give you al the benefits of leaf spring setup such as locating the axle, geometry, axle swing, and retaining stock frame mounts.

Since you deleted the springs from above the axle, you would retain a lot of space above the axle for the airbag so you wouldn't need to try and shoe-horn an airbag in there. You could probably even reuse a factory kit from Firestone with a modified bottom mount. If you needed a shorter bag, PacBrake uses a slightly shorter, single-convoluted bag (I have a set sitting in the shop, actually). If you only retained the main leaf or the tow main leaves, you would be relying on the airbag for daily driving ride height, giving you many of the suspension benefits or air: variable ride height, a progressive spring rate as the axle travels upward, and a smooth smooth ride.

I think you'd have a winner. Minimal work to improve the ride.

 

That's a WAY better setup than the stock pack! Funny thing - I looked at the same AAL and Sumo's while I was contemplating the changes I made above. I've had the AAL's and Sumo's in my cart on several occasions, and I may or may not have a set of Amazon Sumo's in the shop to test fit up... I also looked at Deaver's mini-pack and the overload replacement packs from Boise SpringWorks. If I didn't have to tow heavy from time to time, your setup is almost exactly what I was looking at doing. But I'm a weirdo and like stock-ish height suspension, so the airbags were my choice.

 

Haha. Unconventional, but if your setup works - it works. I think that's where I get tripped a lot on suspensions: what works for one person's needs may not work for another's. That's probably why it took me so long to arrive at this setup even though it is very simple. And I like a little rake on the truck. I can dial that in - or out - with the bags now.

I looked at U748's, but Deaver told me it was a no-go for me. Alcan, Atlas, Deaver, my local spring shop - they all told me I couldn't get a leaf pack that would be comfortable and handle my varied loads without retaining my airbags. Honestly, I was a bit miffed about it, reasoning that something like a nine-leaf pack plus two thinner, progressive overloads would do everything I needed. But I'm not the export, so I took them at their word and looked for a different solution.

Tire pressure, though, is a large part of ride tuning, but WOW, your pressures change quite a bit! But it makes sense.

Not sure I'll get a tank and bleeder block in there, but this will eventually get installed. Need to run power to the bed for a couple other items, but I plan on mounting in the bed, tucked away from the elements.

 

Haha yea true.

Yea you're totally right. Definitely onto something there. I have the sos performance full kit with the bags to install. Now you have me seriously contemplating removing that overload when I move the axle on top of the leafs... then just tuning the ride with the bags... the only issue I see with that is I would ideally need a way to adjust the bags on the fly. I was not planning on running any on board compressor or in cabin adjustment switch. Just a cordless Milwaukee air pump to air up on the seldom occasions where I tow. Hmmm.

 

I had airbags for a while installed with the Toytec (deaver) AAL, but had the airlift ones that had the jounce bumper inside which was a poor choice. If I did them again I would definitely opt for ones without the jounce bumper for a better ride. I was considering just swapping to regular bags, but figured I’d give the Sumo springs a try because I thought the bags may prevent the proper distribution of weight with my WDH unless I consistently used the same psi in them every time I used them as what I set the hitch up with. I also thought maybe the Sumos would ride better and be leak/maintenance free. I’m happy with how this setup performs with my 850# tongue weight and some extra weight in the bed. I basically have a level ride when loaded. If I was towing even heavier though I’d probably want bags or the black or yellow sumos. I like the idea of losing the OL leaf though for dropping the rear a little bit. I have about 2” of rake as it sits.

 

Im so glad this worked for you, how interesting i had the exact same idea a few weeks ago looking at my leaf pack…

 

Do you feel like the truck bottoms out more easily without the overload leaf?

I'm 100% stock plus RideRite bags and I feel the truck blows through the rear travel surprisingly easy. If I add more air to the bags the harsh bottom outs go away but then things start to get bouncy.

I'm figuring I'm probably at the limit of the valving from the stock TRD Bilsteins. I feel bottom outs more at low PSI because there are no bump stops given the airbags, but adding air pressure(spring rate) plus the extra weight in the bed overcomes the compression/rebound valving.

 

The short answer - no, the suspension doesn't bottom out. I actually have more up travel with this setup and I'll explain why.

So unless you are putting a couple thousand pounds in the bed, what you are feeling isn’t so much the suspension blowing through travel and bottoming out, but the suspension exhibiting an excessively high spring rate and restricting uptravel. One of the biggest issues with airbags is the limited uptravel compared to coils or leaves - but that’s actually a strength and part of why I got rid of my overload.

Your primary stage of your leaf spring consists of the first three leaves: one leaf would make for a linear spring rate while multiple leaves allows for a progressive spring rate (as well as combatting axle wrap when the leaf is placed atop the axle). As you add weight to the bed or hitch of the truck, the progressive pack allows you to push through the first inch or so of travel without a lot of weight, the second inch takes more weight, and the third inch even more. As made up numbers, let's say it takes 400 lbs to sag the truck 1 inch. It's going to take say, an additional 600 lbs to sag the second inch, and an additional 900 lbs to sag the third. Make sense?

Once you hit the overload, the amount of weight it takes to deflect the spring increases dramatically. Don't quote me on these numbers, but using a spring calculator based on spring length and thickness, the primary leaves are around 1700 lbs/in with all three leaves engaged, while the secondary or overload spring is 2100 or 2400 lbs. That's per side, not per axle. The first spring in the primary stage defects the most, with each spring below it deflecting less and less with each inch of overall suspension sag.

The large discrepancy between primary and secondary load capacity helps give the truck a better ride unloaded but still be able to shoulder heavy loads without bottoming out the suspension, but is IMO the reason why tundras have a "truck-like" ride as described by many. This is also the reason I wanted to ditch the overload and replace it with a variable-rate air spring. The primary stage sits so close to the secondary stage, especially with a few miles on the truck and springs started to sag, that the overload is engaged almost immediately, quickly ramping up spring force.

A neighbor kid picked up a clean '13 tundra with 100k miles on it last year. He was telling me how bad the chop was on a section of concrete freeway that I was very familiar with. Normally, the seams in the concrete excite the bed of an unloaded truck and causes some bed bounce that is very annoying. The stock suspension is quite bad in that section; his was downright terrible. I suggested changing out the stock shocks but once we took a peek at them, it was obvious that his stock, unloaded truck was resting on the overloads. Every time the suspension tried to move upwards, it ran in to excessive spring force instead of the bump stop, limiting up travel and creating a terrible ride. He needed new shocks and leaves.

In you case, you've added airbags per the MFG recommendations. A minimum 5 psi in the bags adds very little spring force at ride height - about 250 lbs if it were linear, but it's actually a little less than that. Compared to the 1700 lbs/in of the primary and 2100 lbs/in of the secondary, it's minimal. As you compress the suspension, the air inside the bag compresses and spring force increases. It has a progressively variable spring rate built in to the design. As long as you maintain the minimum amount of air required to level the load, you shouldn't be able to bottom out the bag on bumps. It should ramp up spring force to limit up travel.

BUT... the problem is you have progressive primary stage that ramps up spring force as the suspension compresses, and wildly high step in spring force once you hit the overload, and you are adding even more spring force with the airbags with compression. This all combines to add nearly 150% spring force in one inch of travel once the overloads and bags are engaged. You aren't bottoming the suspension, you are ramping up spring force too rapidly.

When towing with a stock pack and airbags, I found the truck to ride better when I added just a little more air than was required to get back to ride height. I realized this was simply unloading the overload, giving me a little more suspension travel before engaging it. This was further evidenced when I installed an add-a-leaf that raised the primaries off of the overload, giving me a better ride even though I had a higher spring force in the primary stage. @Silver17's setup also allows more travel before hitting the overloads, which I imagine makes for a better ride than stock even though he has an even more progressive stack. Having the jounce bumper in his previous airbags was only limiting his up travel, same as slamming in to the overload (I was lucky enough to pick bags without the jounce bumpers and am glad I did).

The way I have mine setup, I don't see any evidence of bottoming out the bags, and I don't feel it either. Neither do I feel like the spring force ramps up too quickly.

 

That's funny. Great minds think a... squirrel!

I tried this setup out on my '13 and have been running it this way for about 8k miles. I've towed heavy with it and found it to work to my liking, which is I why I felt confident doing the same thing on my '21. I plan on running it this way until I see a reason to change.

Already, though, I've thought about adding some 3/4" lift shackles from @Archive . Since I deleted a 3/4" thick overload, it would get my ride height back to "stock loaded up" and allow just a little more up travel in the bags. The downsides (and maybe and upside) would be a slightly softened spring rate on the leaf springs and maybe a little stink-bug stance on the rare occasion that I unload the bed. But that might just make for an even more plush ride.

The other thing I've been thinking about is a custom lower bag bracket to incorporate a bump stop. I technically shouldn't need one, but I'd bet curious to see if it ever engages. And since I'm using a cradle, it really shouldn't be difficult to fab up. Also, Firestone changed their brackets in this kit and no longer have the bracket turndown leg that rests on the axle; they are purely attached and supported to the top of the spring. I'd most likely place it forward of the u bolt on the leaf spring, out of the way of the back but directly between the spring and frame. Might give a little protection to the bag in case I don't air up enough.

 

I also run sumos for towing. They don’t compress fully so im sure they can handle the load without the overloads