17-26 FORD F250/350 4X4 FULL PROGRESSIVE LEAF SPRING KIT – 1″ LIFT

Description

HIGHLIGHTS

1. Reduced Overall Spring Rate to “light 3/4-ton” for a Better Ride (more on capacity HERE)
2. Increased Progression (Spring Rate)
3. Progressive Overload (Factory overload is a hard-stop)
4. Dramatically Increased Suspension Travel
5. Elimination of Factory Vulcanized Bushing Bind – Added Ability to Service Cari Bushings
6. Maximized Spring Efficiency by Minimizing Friction Throughout the Pack
7. Custom Tuned Commuter, Backcountry, Pintop and E-Venture Shock Packages to the New Rate and Progression of the Carli Leaf Springs
8. Optimized Center Pin Location to Ensure Maximum Tire Fitment
9. Military Wrap on Front Eyelet for Safety
10. Sprayed, OEM Quality Coating Tested to 250 Hour Salt-Spray. The coating is applied to each leaf before assembly to ensure complete coverage.

FACTORY LEAF SPRINGS

Knowing whether to swap leaf springs starts by understanding what’s on the truck and the shortcomings of the design. The factory springs do one thing well – haul. They’re designed and built to handle the max capacity of the truck every day of the truck’s life and, when loaded with max capacity, they ride decent. Thing is, most of these trucks sell to people that will use the HD truck for its intended purpose, but far less frequently than the design intends.

Leaf springs are archaic in design – they either haul well or ride well. You can land anywhere in this spectrum you’d like but there will be sacrifices at the opposite end as you shift throughout. The factory springs slide the scale all the way to the “haul well” end of the spectrum. Let’s dig into their design…

The factory leaf spring are designed with 2-3 stages of spring rate. Pictured is a Stock F250 Leaf Spring Pack with 2 stages.

MAIN SPRING

First, there are 2-4 springs that make up the main support in the factory pack. These are high-gauge steel and ride, well, like high gauge steel. F250 High-Capacity/Tremor Leaf Springs or F350 Springs will have 3 main springs AND an upper overload (adding a 3rd stage of spring rate in the pack).

Pictured is an unloaded pack; under the weight of your truck bed, you’ll see less of a gap between the main springs and the overload.

OVERLOAD

The second stage of the pack is the below-highlighted lower overload spring. Providing support to the main springs, this immobile spring rate ceiling ensures the rear doesn’t sag below a predetermined point under load. As there is very little give to this spring, you’ll know when you bonce off the overload. Riding around town, you’ll likely note that small bumps are absorbed pretty well as you’re riding on the leaf spring bushings and main springs (F250, at least – F350 are pretty stiff everywhere thanks to the 3rd main leaf).

If you encounter a speed bump or culvert, especially at speed, the rear end hits much harder and often jumps up in the air as a result of the engagement of the lower overload spring. This design is cheap and proven on an HD truck. Unfortunately, it leaves a lot to be desired in ride quality, on and off-road, in the medium to large bumps, particularly when unloaded as there’s not enough weight in the bed, nor rebound valving in the shocks, to keep the rear planted.

CARLI LEAF SPRINGS

Carli aimed to design a leaf spring pack that would provide a more supple and planted ride with only a slight reduction from a standard 3/4-ton load capacity. Rather than built to haul, they’re designed with an emphasis on ride quality and consideration for capacity. In the “haul well” to “ride well” spectrum, we center the springs to ensure they’re still capable of hauling a capacity respective to the 3/4 to 1-ton platform (albeit, less than the factory springs), but ride much better.

We do this as there are load supplements available to ensure one can safely max the truck’s capacity when their use-case demands it, even with reduced rate leaf springs. Potentially improving ones hauling situation even relative to factory springs, utilizing a Carli Spring with a compatible load supplement affords users a better unloaded ride with adjustable rear height/load balancing of which a factory spring is not capable. More on this linked in our load capacity article…

As with the factory leaf springs, our complete replacement packs are 2-stage, but we implemented far more progression to the spring rate in a high-leaf-count pack design.

• The factory springs utilize 2-3 main springs –  Carli’s has 9
• The factory overload is a single, immobile rate ceiling – Carli’s overload is progressive as it is comprised of 3 leaf springs
• Carli leaf springs are optimized for the largest tire fitment possible. Larger tires means better small bump compliance
• Factory leaf springs are rated for the GVWR – Carli Springs are reduced to LIGHT 3/4 ton. Full capacity bed loads require an airbag system (More on this HERE)
• Lighter rate overall spread over more progression means softer ride that ramps up in spring rate the deeper you get into travel.
• FAR more suspension travel
• Factory Springs are designed to work with 2-4″ factory blocks – Carli’s Progressive packs bolt directly to the axle, NO blocks minimizing axle wrap and wheel hop

Optimizing a high-leaf-count pack requires many considerations… Most of all – efficiency of spring rate engagement. The easiest way to understand this concept is to consider an optimal result as you’d see in a coil spring. If you put 500lbs onto a single coil spring rated for 1,000lbs, you’ll want to see 50% sag. As there’s very little to interfere with the coil’s ability to do its job, the result would likely be very near ideal at 50% sag. Leaf springs, however, will vary based on shackle design, bushing bind, and coefficient of friction – all of which can impede the leaf spring’s efficiency.

BUSHING DESIGN

Factory leaf springs are designed with a vulcanized bushing. This means, the bushing material is high strength and bound to both the bolt-sleeve and an outer sleeve that’s pressed into the stock spring eyelet. The bushing position is set by the bolt when torqued. If the spring needs to roll the joint housing with the static bushing position, it applies a torsional load to the bushing itself. The leaf spring eyelet can only move as much as the bushing can deflect.

Carli utilizes a proven poly bushing and crush sleeve. This 3-piece design allows free movement of the bushing material around the bolted crush sleeve allowing unimpeded axial movement of the spring eyelet and subsequently, a much better ride. Combine the Carli Leaf Springs with the Carli Fabricated Shackle to eliminate ALL vulcanized bushings for the best result. The bushings are a high-durometer (hardness) poly material so one can expect a LONG service life.

The only downside to poly is that it can be noisy if not properly greased. More on that HERE. To ensure quiet operation, we include the proper grease and designed our bushings with grease flutes for extra grease capacity and proper grease migration.

Doubled Spring Eyelet

As a safety, all Carli Springs utilize a “Military Wrap”. This is a floating housing that engages to carry the spring in the event the main front pivot eyelet fails. We’ve never had a failure in which this safety is utilized but it’s nice to have, none-the-less.

FRICTION

Friction is the big one as there is potential for increased drag when increasing the leaf count given the increase in surface area. To mitigate the friction, we’ve implemented the following:

Diamond cut Ends with Sliders on every spring: Between each leaf spring, you’ll find a poly pad that serves two purposes. We’ve highlighted them in RED below. These sliders create space between each spring to ensure no friction along the length of the leaf while providing a much slicker surface (lowers the coefficient of friction) for the leaf springs to glide where friction is unavoidable.

Spacers between the springs at the Center Pin location. Doubling down on the slider’s purpose, these spacers were introduced to ensure no contact between each leaf spring for the entire length of the pack, front and rear of the center pin location.

Throwing some light behind the springs confirms they work; there’s clear separation between each leaf spring to ensure excessive, unintended friction won’t be an issue.

Final Notes

All of the above combine to make the leaf springs one of the best upgrades one can install to soften the on-road ride while adding the needed progression and suspension travel to take the Super Duty to the next level off-road. As we offer a 2.0″ IFP shock package that’s designed/valved to work with the stock leaf springs, we’ve optimized the valving spec of ALL the mid to high-tier Ford Systems (Commuter, Backcountry, Pintop, and E-Venture) to the full leaf springs.

Again, if capacity is a concern, we recommend looking into airbags. We get the query often – customers want to run our leveling system and stock leaf springs to maintain their capacity. What they do not consider is the new stance. Leveling the truck means that the stock leaf springs, albeit higher rate, will still sag to engage the needed spring rate leaving a rear-low stance. In turn, customers will install airbags.

If running factory springs AND airbags, the truck will ride on the massive, superfluous spring rate of the stock pack when unloaded, then, when hauling, you engage the airbags to level the truck which unloads the factory springs rending their capacity worthless. This is a ton of unnecessary, redundant spring rate. If you plan to run airbags, run a lighter spring on which you can ride when not under load and benefit from their compliance, THEN add the airbags to adjust the capacity in the rear for your varying loads.