Carli Suspension Full Progressive Leaf Spring Kit | 9-Leaf Main + 3-Leaf Overload | Block Eliminating | 15" Rear Wheel Travel | 37" Tire Fitment | 2011–2016 Ford F-250 / F-350 4WD Diesel — 3.5" Axle Tube | SKU: CS-FFSP-LVL-11

Verified Fitment

Does NOT fit:

• 2017 and newer Ford F-250/F-350 — different rear axle tube diameter (4.0") and different leaf spring mounting geometry; separate Carli leaf spring kit required
• 2010 and earlier Ford F-250/F-350 — different platform
• Ford F-250/F-350 with 4.0" axle tube — the 3.5" axle tube is specific to the 2011–2016 diesel Super Duty; the 2017+ uses the larger 4.0" tube
• Ford F-250/F-350 Gas — this spring is designed and load-tested with diesel-equipped trucks; gas applications may produce different results
• Ford F-250/F-350 2WD — leaf spring geometry and axle tube sizing are different on 2WD applications

Axle tube identification: The 3.5" axle tube refers to the outer diameter of the Dana 60 front axle housing. The 2011–2016 diesel F-250/F-350 4WD uses the 3.5" tube. The 2017+ Super Duty uses a 4.0" tube. To confirm your axle tube diameter, measure the outer diameter of the front axle housing at the center section. If you're uncertain, text (469) 407-7538 with your VIN before ordering.

Component Specifications

Factory vs. Carli Full Progressive leaf spring — the design difference: The factory 2011–2016 Super Duty rear leaf spring pack uses 2–4 heavy main springs plus an immobile lower overload — a configuration Carli describes as "making for a jarring ride." The heavy main springs carry load aggressively from the first inch of compression; the immobile overload engages suddenly when the main springs bottom against it, creating the abrupt transition that Super Duty owners experience as a harsh, stiff ride when unloaded. The Carli/Deaver 9-leaf main + 3-leaf overload pack replaces this with a progressive rate design: the rate builds gradually as successive leaves engage, producing a softer initial compression under light load and a smooth, progressive stiffening as load increases. The result is meaningfully more compliant unloaded ride, more rear suspension travel, and retained capacity as the pack compresses to full engagement.

Why block eliminating matters: A standard rear lift block is installed between the axle perch and the leaf spring pack to create ride height. Lift blocks introduce axle wrap — particularly under hard acceleration or braking — because the block's height creates leverage against the U-bolt clamp, allowing the axle to rotate slightly in the spring perch. The Carli Full Progressive leaf spring pack achieves the correct rear ride height without a block, eliminating that axle wrap mechanism and allowing the suspension to cycle correctly through its full 15-inch travel.

1/2" rear high stance — calibrated intentionally: Carli designs this spring to sit 1/2" higher at the rear relative to the Carli 2.5" and 4.5" front leveling systems. This slight rear-high stance produces a level truck under normal load conditions — the rear compresses slightly under payload and towing weight, bringing the truck to level. Running the spring under constant zero-load produces a slightly tail-high appearance by approximately 1/2".

What Is and Is NOT Included

Load testing data is publicly available. Carli provides comprehensive load testing documentation for the CS-FFSP-LVL-11 on their website. Before ordering — especially if your application involves regular towing or payload — review the load test results to confirm the spring is appropriate for your specific load profile. Text (469) 407-7538 for a direct link.

Critical Compatibility

3.5" axle tube is the primary technical gate. The leaf spring center pin, U-bolt spacing, and spring eye geometry are all dimensioned for the 3.5" axle tube on the 2011–2016 diesel F-250/F-350 4WD. The 2017+ Super Duty's 4.0" axle tube uses different dimensions throughout. Ordering this spring for a 2017+ truck produces hardware that will not fit correctly.

Carli Pathfinder system compatibility — shock tuning is calibrated for this spring. The CS-FFSP-LVL-11 is the spring that Carli's Pathfinder (CS-FLVL-PF-11) and Backcountry (CS-FLVL-BC-CS-11) shock valving is calibrated for. Carli explicitly states their shock tuning is optimized for use with this spring pack. Running Carli shocks on factory leaf springs, or running this spring under non-Carli shocks, produces a mismatched front-rear damping balance.

F-350 rear spring note. The F-350 has stiffer factory main springs than the F-250 due to higher payload ratings. The Add-a-Pack (CS-FPAP-11) adds progressive leaves to whatever is already in the truck — on an F-350 with a 3-spring main pack, the Add-a-Pack provides minimal benefit when the truck is unloaded. The Full Progressive pack (CS-FFSP-LVL-11) replaces the factory pack entirely with a purpose-built progressive design — this is why Carli specifically recommends the Full Progressive pack for F-350 applications running a Carli leveling system.

Installation Notes

Skill level: Intermediate to advanced — full rear leaf spring replacement on a 3/4-ton solid rear axle truck. Heavy axle, significant disassembly. Professional installation strongly recommended.

Tools required: Vehicle lift or floor jack with rated stands (must safely support F-250/F-350 rear axle weight), spring pry bars, torque wrench, basic socket set.

Installation sequence:

1. Safely support the vehicle and place rated jack stands under the frame — the rear axle must be at full droop.
2. Disconnect rear shocks from the lower mount (both sides).
3. Remove factory U-bolts and factory rear blocks.
4. Loosen spring eye front and rear mounts.
5. Lower the axle and remove the factory leaf spring pack.
6. Install the Carli/Deaver Full Progressive spring pack — align the relocated center pin with the axle perch.
7. Install the new Carli U-bolts and rear bump stop drops.
8. Torque U-bolts to Carli specification with the axle at ride height — not at full droop.
9. Reconnect rear shocks.
10. Lower the vehicle to ride height and confirm stance.

No factory block — do not reinstall the OEM block. The Carli Full Progressive spring pack achieves the correct ride height without the factory block. Do not reinstall the factory block alongside this spring — doing so would over-lift the rear.

LTL freight notice. At 318 lbs, this ships LTL freight. Residential delivery included — the freight carrier will contact you to schedule a delivery window. Inspect for damage before signing the delivery receipt. Note any damage on the bill of lading before accepting.

Common Questions

Why is a full replacement spring better than the Add-a-Pack?
The Add-a-Pack (CS-FPAP-11) supplements the factory spring pack by adding progressive leaves on top of what's already there. The factory main springs and immobile overload remain in place. The Full Progressive pack (CS-FFSP-LVL-11) removes the factory pack entirely and replaces it with a purpose-built 9-leaf main + 3-leaf overload design co-developed with Deaver Leaf Spring. The result is a fundamentally different spring rate curve — progressive from the first inch of compression, supporting up to 15 inches of rear wheel travel, without the abrupt transition of the factory immobile overload engaging.

Will this affect my payload capacity?
Slightly — Carli specifies "a slight reduction from a standard 3/4-ton load capacity." Carli provides comprehensive load testing data on their website that quantifies this. Review the load test before ordering if your application involves regular near-capacity payloads. For typical towing and hauling use, the difference is not operationally significant for most owners.

Why does this spring sit 1/2" rear-high?
Carli calibrated the spring to produce a 1/2" rear-high stance when the truck is unloaded. Under normal payload and towing loads, the rear compresses to a level stance. This slight rear-high provides a buffer against the tail-down appearance that a precisely level spring produces under even light loads.

Does this work with the 4.5" Carli lift, or only the 2.5" leveling system?
Both. Carli explicitly states this spring is designed and calibrated for their 2.5" and 4.5" suspension lifts for the 2011–2016 Super Duty.

What is the recommended tire size with this spring?
Carli recommends 37" tires as the optimal fitment. The center pin is relocated to accommodate up to 38-inch tires. The rear bump stop drops included in this kit help maintain correct bump stop engagement at the new ride height with larger tires.

The Obsidian Take

The CS-FFSP-LVL-11 is the single most impactful rear suspension upgrade available for the 2011–2016 diesel Super Duty running a Carli leveling system — and the reason Carli marks it as the default recommended rear spring selection on every Pathfinder, Backcountry, and Pintop system page.

The factory rear leaf spring pack on these trucks is calibrated for maximum GVWR — heavy main springs and a hard-engaging immobile overload that delivers the harshest possible unloaded ride in exchange for excellent maximum payload control. For the majority of owners who are not running near GVWR daily, that is the wrong spring for their actual use.

Carli and Deaver built the 9-leaf main + 3-leaf overload replacement specifically to address this. The progressive rate means the truck rides on the lighter main leaves when unloaded, with each additional leaf progressively contributing as load increases. The 15 inches of supported rear wheel travel is more than twice what a block-lift rear configuration delivers — and without the block, the axle wrap mechanism that blocks introduce under hard acceleration and braking is eliminated.

The shock calibration point matters: Carli's rear shock valving on the Pathfinder, Backcountry, and Pintop is tuned for the CS-FFSP-LVL-11. Running factory leaf springs with Carli shocks, or running this spring with non-Carli dampers, produces a front-rear damping mismatch that defeats the purpose of a system-level suspension build. This spring and Carli's shocks are a matched pair — order them together.

This SKU is for: 2011–2016 Ford F-250 and F-350 4WD diesel owners (3.5" axle tube) running a Carli 2.5" or 4.5" leveling system who want the complete rear suspension transformation — full progressive leaf spring replacement, block-eliminated 15" rear travel, 37-inch tire fitment, and shock valving that's calibrated for this specific spring pack.

This SKU is NOT for: 2017 and newer Super Duty owners (4.0" axle tube — different kit required), gas-engine Super Duty owners, 2WD owners, anyone expecting to pair this with factory or non-Carli shocks without understanding the damping trade-off, or anyone running constant maximum payload loads without reviewing Carli's load testing data first.

Manufacturer Data Disclaimer
Product specifications, fitment data, and feature details are sourced directly from Carli Suspension's product page and manufacturer documentation current at the time of publication. Carli Suspension reserves the right to modify product specifications without notice. Confirm 2011–2016 model year, diesel engine, 4WD drivetrain, and 3.5" axle tube before ordering. OFAS is not responsible for incompatibility resulting from 2017+ axle tube applications, gas engine applications, 2WD configurations, or non-Carli shock pairings.

Prop 65 Warning
⚠ WARNING: This product can expose you to chemicals which are known to the State of California to cause cancer and reproductive harm. For more information, go to www.p65warnings.ca.gov.

Non-Returnable Notice
This item is non-returnable once installed. Verify 2011–2016 model year, diesel engine, 4WD drivetrain, and 3.5" axle tube before ordering. Questions before ordering? Text (469) 407-7538.