Leaf Spring Technical Documentation

[Fenix Motorsports]

The amount of information I've consumed over the years, and all the forum threads on leaf springs read like Egyptian hieroglyphs that I have to spend hours trying to decipher, or spend a couple hundred on design books and still not get quite enough info for what I'm doing. So heres a technical document I wrote up using an amalgamation of my own experience/knowledge as well as other stuff I've found over time from other forums and books like Adams' chassis engineering and Millikens' race car vehicle dynamics.

Feel free to add, criticize, or correct anything, I'm sure there's nuance, niche, and otherwise forgotten (or at least unknown to me) topics, tricks and tips. I wrote this as an internal reference document since I design leaf spring stuff so infrequently that I always forget everything and have to go redo all my research, but I figured I'd throw it up here for people to add and reference back to if needed.

Some of the points here aren't as fully polished as I'd like, but a bulk of the info and big rocks are covered that I can fill in the gaps easily enough. There's also repeated info in a couple areas that I need to clean like going over bump stop setup twice. I also would like to get some good reference photos, especially since I'm currently designing leaf spring setups for both the 2G-3G and GMT800 right now.

 

[Fenix Motorsports]

Leaf Spring Technical Design Points and Considerations

1. Axle Positioning for Mockup and Design Process​

Purpose:
To accurately establish design constraints for high performance leaf spring suspension systems

---

General Procedure​

Similar to mock-up procedures used in linked-style suspension systems, the axle should be set at full bump when establishing geometry for a leaf-spring suspension.
This position defines the suspension’s maximum compression and serves as the baseline for locating bump stops, shock mounts, and checking tire and frame clearances.

---

Critical Setup Notes​

A. Hydraulic Bumpstops

• Remove the Delrin pad and fully collapse the bumpstop body to simulate its true compressed height.
  
  
• Set the bump pad position on the axle housing accordingly.
  
  
• This ensures accurate final placement once the bumpstop is reassembled, accounting for long-term wear and nitrogen pressure variation.
  

B. Polyurethane or Rubber Bumpstops

• Assume these materials compress completely flat under load, regardless of their nominal durometer or spring rate.
  
  
• Mount the bump pad on the axle so that the bumpstop contacts the pad squarely at full bump.
  
  
• Verify clearance between bumpstop brackets and nearby suspension hardware.
  

C. Shock Absorbers

• With the axle at full bump, the shocks should measure ⅛" from full compression at least to prevent internal piston or seal damage under impact.
  

---

Leaf Spring Setup​

• At full bump, the main leaf should be flat relative to its eye-to-eye datum line.
  
  
• Avoid compressing the spring into a negative arch; excessive reverse bending can cause premature fatigue, loss of rate, or cracking.
  
  
• For mock-up, disassemble the leaf pack down to the main leaf only. This allows free cycling of the suspension without internal friction and full spring rate between leafs and makes accurate shackle/hanger positioning possible.
  

---

Key Takeaways​

• Always locate shocks and bump stops with the axle set at full bump.
  
  
• Confirm at least ⅛" shock shaft remaining to prevent bottoming damage.
  
  
• Avoid negative-arch loading of the main leaf.
  
  
• Use only the main leaf during mock-up to freely cycle suspension geometry.

2. Fixed Hanger Location​

Definition:
The fixed hanger is the immovable mounting point on the vehicle’s frame—typically a welded or bolted bracket—that captures one eye of the leaf spring. It serves as the primary pivot point for the entire suspension system.

---

Function and Placement​

At full bump, the fixed hanger locates one end of the leaf spring, determining both axle position and wheelbase. The hanger’s pivot aligns with one of the spring eyes and sits at a distance from the centering pin equal to the arc length between the pin and that eye along the spring’s curve.

Which end of the spring carries the fixed hanger depends on the vehicle configuration and intended use:

• Front-mounted fixed hanger – Common for rear-suspension applications such as trucks, providing greater stability under acceleration and allowing the shackle to manage length change at the rear.
  
  
• Rear-mounted fixed hanger – Occasionally used in specialized setups to alter weight transfer or packaging but less common in high-speed off-road applications. Common in leaf sprung front axle applications such as the Jeep CJ or YJ

---

Application Context​

For this guide’s example—a rear leaf-spring suspension on a high-speed desert truck—the fixed hanger is located at the front spring eye.
This establishes a stable pivot point about which the entire suspension assembly (leaf spring, axle, wheels, and related hardware) rotates during bump and droop movement.

---

Key Takeaways​

• The fixed hanger defines axle fore/aft position and wheelbase.
  
  
• It provides the spring’s primary pivot point and structural anchor.
  
  
• Front-eye mounting (fixed hanger forward) is standard for high-speed desert or performance off-road applications due to improved directional stability.

3. Shackle Type and Orientation​

Definition:
A shackle connects one eye of the leaf spring to the chassis, forming a variable link that allows the spring to change eye-to-eye distance as it cycles through bump and droop. This motion enables smooth, bind-free suspension travel.

---

Orientation in Typical Rear-Leaf Applications​

For most rear-suspension systems—such as on a pickup or desert truck—the fixed hanger is mounted at the front of the spring, and the shackle is mounted at the rear.

• As the suspension compresses (bump), the spring flattens and the axle moves rearward.
  
  
• As the suspension extends (droop), the spring re-arches and the axle moves forward.
  

This arrangement maintains consistent wheelbase change and predictable handling characteristics.

---

Shackle Design Types​

A. Tension-Style Shackles

• The shackle operates below the spring eye and is held in tension as the suspension compresses.
  
  
• Requires a larger hanger assembly slung beneath and/or outboard of the frame to provide proper arc and clearance.
  
  
• Common on late-model trucks including Ford F-150/F-250/F-350, Chevrolet Silverado, GMC Sierra, Colorado/Canyon, and Toyota Tacoma (1st–3rd Gen) and Tundra (1st–2nd Gen).
  
  
  
• Favored in modern OEM applications and many aftermarket performance systems
  

B. Compression-Style Shackles

• The shackle operates above the spring eye and is loaded in compression as the suspension compresses.
  
  
• Usually mounted directly to the frame via a single pivot tube, resulting in a compact package with fewer brackets.
  
  
• Common in early Toyota pickups, 1st-gen 4Runners, rock-crawling builds, and certain prerunner conversions.
  
  
• Advantages include simpler fabrication and increased packaging clearance
  

---

Key Takeaways​

• The shackle provides the leaf spring’s variable length, enabling full travel without bind.
  
  
• Tension-style: larger hanger below frame, common in modern OEM trucks.
  
  
• Compression-style: compact packaging, common in early or custom off-road applications.
  
  
• Both styles function effectively when matched to the vehicle’s intended use and travel range.

4. Shackle Length — and Why It Matters​

Purpose:
To allow the leaf spring to cycle smoothly from full arch to flat (and occasionally into mild negative arch) without binding, distortion, or loss of effective travel.

---

Function and Importance​

While shackles are often viewed as a means of lifting or lowering a vehicle, their primary purpose is to provide the spring with the range of motion needed to change length as it compresses and extends.

• Too short: The shackle cannot swing far enough to accommodate spring flattening. This causes binding and limits downtravel, reducing usable suspension stroke.
  
  
• Too long: The shackle can over-rotate, forcing the spring into an unnatural “S” or “W” shape under load, which can permanently deform or fatigue the main leaf.
  
  
• Ideal length: Allows the spring eye to travel its full arc freely while keeping the shackle centered in its range of motion at ride height.
  

This becomes especially important in lifted or long-travel leaf springs, where greater free arch increases both the spring’s eye-to-eye distance and the total range of length change through its stroke.

---

Determining Shackle Length​

A practical method for estimating required shackle length is to measure the spring’s change in effective length through its arc:

Shackle Length ≈ (Curved Eye-to-Eye Length) − (Straight Eye-to-Eye/Datum Line Length)

This difference represents the amount of travel the shackle must accommodate as the spring transitions from fully arched to flat.

Example:
For an OEM application rear leaf spring:

• Measured along the arch: 55.375″
  
  
• Straight-line eye-to-eye: 52.375″
  
  
• Required shackle swing: ≈ 3.0″ (center of frame pivot to center of spring eye)
  

Note: This example is based on a stock spring. Most aftermarket or performance off-road leaf packs—with increased arch and longer travel—will require a longer shackle than this baseline.

---

Key Takeaways​

• Shackle length governs the freedom of motion in the suspension, not just vehicle height.
  
  
• Too short = binding and restricted droop.
  
  
• Too long = over-flexing and main leaf distortion.
  

Always validate shackle length during mock-up at full bump and full droop to ensure smooth travel.

 

[Fenix Motorsports]

5. Shackle Hanger Location​

Overview:
Shackle hanger location is largely application-specific and depends on both spring geometry and suspension travel goals. The correct location ensures full bump and droop travel without binding, interference, or inversion while maintaining consistent rate behavior through the stroke.

---

Compression-Style Systems​

For compression-style shackles, the primary goal is to prevent shackle inversion and maintain predictable ride quality.

A widely used baseline formula for locating the hanger is:

Hanger Spacing = (Spring Flat Length) − (Shackle Length) + ½″
(½″ accounts for bushing deflection under load.)

This provides a starting point, not a final dimension. Each vehicle’s frame geometry, spring arch, and shackle length will require small adjustments to achieve proper clearance and geometry.

---

Important Note — Tension-Style Shackles​

The above formula is derived from compression-style geometry, where the shackle hangs down from the frame and pushes on the spring eye, often with the upper mount forward of the rear spring eye on the frame.
That math assumes:

• The shackle pivot is above the spring eye.
  
  
• The shackle swings rearward as the spring flattens in bump.
  

In a tension-style system, the opposite is true:

• The shackle is located below and behind the spring eye.
  
  
• It pulls on the spring eye as the spring compresses.
  

If you apply the compression-style formula to a tension setup, the calculated hanger position will land too far forward on the chassis, placing it under the leaf pack instead of behind it.
This creates several problems:

• Immediate interference at full droop, where the spring eye and shackle swing into the hanger.
  
  
• Severely limited downtravel and premature shackle binding.
  
  
• Unintended rate change due to incorrect geometry (a secondary concern compared to clearance loss).
  

---

Correct Method — Mock-Up and Scribe-Arc Placement​

For tension-style shackles, the hanger location must be determined by physically cycling the suspension and plotting the path of the spring eye.

Procedure:

1. Set the vehicle at ride height
   
   • With the suspension set for mockup with the stripped main leaf, set the suspension at ride height
     
     
   • Install the shackle to the spring eye and hold it at the desired angle.
     
2. Establish the neutral shackle angle
   
   • The shackle should usually sit perpendicular (90°) to the spring’s eye-to-eye datum line at ride height.
     
     
   • Avoid any rearward lean at ride height — this stiffens the effective rate too early and can cause rough ride and poor handling..
     
3. Check full bump
   
   • Cycle the axle to full bump (spring near flat).
     
     
   • Ensure the shackle is not forced into a flat or locked position that binds the spring or over-loads the bushings.
     
     
   • If binding occurs, verify shackle length — a shackle that’s too short can cause premature lock-up.
     
4. Check full droop
   
   • Let the axle droop completely.
     
     
   • Confirm that the shackle and spring eye clear the hanger, frame rail, and spring pack with no hard contact.
     
     
   • Droop clearance, not bump clearance, is usually the limiting factor in a tension setup when finding the hanger location
     
5. Mark and verify the final hanger position
   
   • Tack-weld or secure the hanger in the verified location after both bump and droop clearances have been confirmed.
     
     
   • Re-cycle the suspension to double-check all clearances before final welding, bolting, etc.
     

---

Weight Consideration​

Vehicle weight has a direct influence on ride-height geometry. Heavier vehicles compress the spring’s free arch more at static ride height, changing the effective shackle angle.
Always confirm the final hanger position with the full vehicle weight installed, not on an unloaded chassis.

---

Key Takeaways​

• Compression-style: use the spacing formula as a baseline, then fine-tune.
  
  
• Tension-style: determine location through mock-up and scribed travel arcs.
  
  
• Droop clearance, not bump clearance, usually defines the usable hanger position.
  
  
• Verify geometry under real vehicle weight before finalizing.

6. Bumpstops and Limit Straps​

Purpose:
To protect suspension components and maintain predictable performance by controlling the limits of compression (bump) and extension (droop). Proper bumpstop and limit-strap setup prevents over-extension, harsh bottoming, and premature wear of shocks and springs.

---

Bumpstops​

Function:
Bumpstops act as the suspension’s final line of defense during compression, absorbing and decelerating the last portion of travel to prevent hard contact between the axle, frame, and shock internals.

Types and Behavior:

• Polyurethane or Rubber Bumpstops:
  Common in OEM applications. These compress almost completely flat under load, offering limited tunability but low cost and simplicity.
  
  
• Hydraulic Bumpstops:
  Use a nitrogen-charged chamber and internal valving to slow the axle progressively near the end of its stroke. Highly tunable and rebuildable for performance use.
  

Setup Guidelines:

1. Polyurethane / Rubber:
   
   • Assume full compression to a flat state.
     
     
   • Mount the bump pad on the axle so the stop contacts flush at full bump.
     
2. Hydraulic:
   
   • Remove the Delrin pad from the bumpstop before setting the pad height.
     
     
   • Collapse the bump completely against the axle pad to establish its true compressed height.
     
     
   • This accounts for pad wear and possible nitrogen pressure loss over time.
     
3. Shock Clearance:
   
   • At full bump, ensure at least ⅛″–½″ of shock shaft remains visible to prevent internal piston or seal damage.
     
     
   • For bypass or coilover shocks, confirm that secondary zones or stops are not engaged prematurely.
     

---

Limit Straps​

Function:
Limit straps prevent the suspension from over-extending at full droop, protecting shocks, springs, and driveline joints from damage.

Setup Guidelines:

1. Measurement:
   
   • Measure the required strap length with the axle at full droop and the shocks 1″ short of full extension at least.
     
     
   • This ensures the strap, not the shock, controls maximum droop.
     
2. Stretch Allowance:
   
   • Fabric straps typically stretch up to 1″ over their service life.
     
     
   • Compensate for this by using adjustable clevis mounts and setting initial strap length accordingly.
     
3. Installation Tip:
   
   • Recheck strap length after the first few cycles; early stretch is normal.
     
     
   • Maintain at least 1″ of shaft inside the shock body at full droop.
     

---

Key Takeaways​

• Proper bumpstop and limit-strap setup ensures reliable, repeatable suspension performance.
  
  
• Always leave at least ⅛″–½″ of shock shaft at bump and ≈1″ at droop.
  
  
• Cycle the suspension through full travel after setup to confirm no interference or premature engagement.

8. Shock Absorber Setup​

Purpose:
To correctly locate and set up shock absorbers for maximum control, reliability, and suspension efficiency within a leaf-spring system.

---

Shock Selection​

To achieve consistent damping and tuning capability, it’s recommended to use a universal-style performance shock—typically rebuildable and externally tuneable (bypass, coilover, or smooth-body).
These shocks allow precise adjustment of compression and rebound characteristics to match vehicle weight, spring rate, and terrain.

---

Mounting Guidelines​

1. Set at Full Bump:
   
   • Position the axle at full bump (with bumpstops engaged or mocked).
     
     
   • Mount the shock so that ⅛″–½″ of shaft remains visible at full compression.
     
     
   • This ensures the shock reaches full stroke without bottoming internally and prevents seal or piston damage.
     
2. Shock Angle:
   
   • For optimum damping efficiency, aim to position the shock as close to 90° to the axle as packaging allows.
     
     
   • Angles up to 14° off vertical are acceptable with minimal performance loss.
     
     
   • Excessive lean reduces effective damping and increases lateral loading on bushings and shafts.
     
3. Mount Clearance:
   
   • Verify that both upper and lower mounts provide sufficient clearance from the frame, tires, leaf pack, and any crossmembers.
     
     
   • At full articulation, the shock body and reservoir should not contact the spring, hanger, or shackle.
     
4. Cycle and Verify:
   
   • After initial mock-up, cycle the entire suspension from full bump to full droop.
     
     
   • Check for any binding, interference, or shock body contact.
     
     
   • Confirm that bumpstops engage before the shock bottoms and limit straps engage before it tops out.
     

---

Key Takeaways​

• Always mount shocks based on full bump position, not ride height.
  
  
• Maintain ⅛″–½″ of shaft at bump and ≈1″ inside the body at droop.
  
  
• Keep the shock near 90° to the axle for maximum damping efficiency.
  
  
• Cycle the suspension to verify clearances and stop sequencing before final welding.
  

​

 

[Fenix Motorsports]

9. Shackle Angle - Progressive vs. Digressive​

The shackle angle plays a critical role in determining the effective rate of the suspension — that is, how the spring’s actual force curve translates into wheel rate as the geometry changes through travel. As the shackle angle shifts relative to the spring’s eye-to-eye datum line, it either increases or reduces the effective spring rate.

How Shackle Angle Influences Rate​

Many aftermarket long-travel systems are intentionally designed to exhibit a falling rate (softening) early in the stroke, transitioning into a progressive rate (stiffening) near bottom-out.

• The transition point occurs when the shackle is at 90° to the spring’s datum line.
  
  
• Below 90° (leaning back / toward the rear): The geometry produces a progressive effective rate — the wheel rate increases as the suspension compresses.
  
  
• Above 90° (leaning forward): The geometry produces a digressive effective rate — the wheel rate decreases as the suspension compresses, resulting in a softer feel through mid-stroke.
  

Simplified Measurement Method​

For lower-travel or near-OEM systems, you can evaluate rate tendency from basic hanger spacing:

• If the distance between the fixed hanger and shackle hanger is less than the spring’s flat (eye-to-eye) length → Progressive geometry.
  
  
• If that distance is greater than the flat eye-to-eye length → Digressive geometry (falling-rate).
  

Design Note​

Nearly every modern multi-leaf pack is inherently progressive by design — as the spring flattens, additional leaves come under load, increasing the actual spring rate. Shackle angle modifies the effective rate seen at the wheel, but it doesn’t change the intrinsic stiffness curve of the pack itself.

 

[UR_Mawm69]

No, but seriously, ill dive into actually reading all this once i start fucking with my rear end bullshit. seems like it could all be really helpful.
IF i don't just say fuck it and buy a whole new 64" spring pack kit. hah

 

[Fenix Motorsports]

No, but seriously, ill dive into actually reading all this once i start fucking with my rear end bullshit. seems like it could all be really helpful.
IF i don't just say fuck it and buy a whole new 64" spring pack kit. hah

Yeah, it took me about 5 hours to write this document and get it even to this level of refinement so I know its long and pretty in depth in sections. I also know that a lot of people just use the method of "fuck with it until it works" for leaf springs.

I mainly wrote this to reference back to when doing CAD work and don't have that option, especially for product design when I need repeatable and consistent results. Luckily it should still be applicable even if you swap to a 64in pack, assuming the geometry isn't all preconfigured, which I imagine it probably is.