3S Single Axle Trailer Suspension

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Sponsors Matthew Swenson, George Tanner
  • University of Idaho [1]
Team Name 3S Trailer Team
Duration Summer 2020 - Summer 2020
Faculty Adviser Matthew Swenson
Mentor Max Johnson
Client Matthew Swenson, George Tanner
Team Members
  • Robert Carne
  • Caitlin Swenson
  • Kirk McKenzie
  • Anson Lunstrum

The goal of our project is to create an innovative trailer suspension system using a first-class lever to shift from dependent to independent suspension by simply pulling a pin.

Problem Definition[edit]

The objective of this project was to create a proof-of-concept prototype with production ready drawings and a BOM for 3 different weight capacities. (2000lb, 3500lb, and 6000lb) We will need to optimize the components in our design so that it is cost effective and has the highest possible functionality.


Proof-of-concept prototype of single axle trailer with 3S suspension system

Sponsors are currently working on a collaboration project with Ted Carton, who is the owner of a novel, patented invention for a game-changing concept in trailer suspension systems. The concept utilizes a first-class lever coupled with a pivoting axle to create a uniform suspension which balances each side of the trailer. This novel configuration negates the need for individual solid axles and independent leaf spring or TorflexTM suspension systems, offering several distinct advantages. The suspension springs on each side of the trailer work together, ensuring any response will not allow the trailer to lean on one side, resulting in far greater load stability. This effect is most notable when driving over rough terrain (e.g. agricultural applications) or in high crosswinds (e.g. over-the-road applications). By contrast, commercial solid-axle suspension systems available today allow the payload to dramatically lean during uneven or high crosswind events. Improving load stability will protect the payload from potential damage, increase overall safety, and enable increased capabilities. For example, a balanced suspension system can allow frame and loading deck height adjustment, facilitating easier and safer loading/unloading operations. To date, the sponsors have fabricated a proof-of-concept prototype of the 2000 lb. capacity single axle design to enable evaluation. This design was modified from an earlier prototype and functions as intended, although the design is not optimized for production.


The objective of this project is to leverage the concepts from the “proof-of-concept” prototype to create production ready Drawings and BOMs for three different capacities (2000 lb., 3500 lb., and 6000 lb.).

  1. Create “production ready” designs for single axle suspensions configurations for 2000 lb., 3500lb., and 6000 lb. capacities.
    1. Calculate loads and select appropriate sized components.
    2. Create full Drawings, BOM, and Manufacturing plan to build each configuration.
    3. Provide estimated cost roll-up for each configuration and compare to competition (SixRobblees, Inc.)
  2. Fabricated a small-scale prototype of a trailer frame and suspension system for demonstration purposes. This may be 3D printed or use fabrication techniques, or a combination of both.


1. Functional Requirements[edit]

User Interface Requirements
Product Requirements
User Installation < 8 hrs
Change from dependent to independent suspension < 1 minute

2. What the product should do[edit]

Attach to existing trailer as a modified Suspension
Product Requirements
Different styles of trailers Box/Flatbed
Different dimensions of trailers up to 8ft 6in wide

3. Mechanical Requirements[edit]

Strength Requirements
Product Requirements
:Static Rated load capacity 2000 lbs
3500 lbs
6000 lbs
Dynamic load Capacity (per tire) No less than F.S of 3
Dynamic load Capacity (Per spring) No less than F.S of 3
Product Requirements
Free Length 9 to 13 in
Outside Diameter 3 to 5 in

4. Product Characteristics[edit]

Total Space when Assembled
Product Requirements
Overall Length (Middle of Tire to center of spring) < 4 ft
Overall Width (Tire to Tire) 4 to 8.5 ft
Weight/Mass Requirements:
Product Requirements
Shipping Weight 10% of load capacity
Mounting / Interface Requirements:
Product Requirements
Weld-on to existing trailer
Weld mounting hardware to Trailer frame
Easy to use pin release to switch between independent and dependent suspension
Cut axel to correct lenght depending on trailer width
Slide cut to length axel through slip bearings
Select correct wheel hub and break combination for weight rating
Appearance Requirements:
Product Requirements
Surface Finish Painted Black
Durability Requirements
Operation without maintenance
Bearings 10,000 - 20,000 hrs
Bushings 10,000 - 20,000 hrs
Springs 10,000 - 20,000 hrs
Bolts 10,000 - 20,000 hrs
Welds 10,000 - 20,000 hrs

5. Product Requirements[edit]

Reliability Requirements
Reliability rating of components
Bearings 90 %
Springs 90 %
Bushings 90 %
Bolt 90 %

6. Environmental Requirements[edit]

Elemental Resistance
Debris in roadway

7. Regulatory Requirements:[edit]

Shipping / Delivery Requirements
Product Requirements
Small Compact box for Sales/Shipping < 4 x 2 x 2 (L x W x H) ft
In person purchases (No store or delivery methods)

8. Cost Requirements[edit]

Prototype Cost
Product Requirements
Maximum cost to build proof-of-concept prototype $ 500 2000lb model

9. Schedule Requirements[edit]

Elemental Resistance
Major Milestones Date
Approval of Requirements 6/25/2020
Ordering of Parts 6/25/2020
Calculations 7/7/2020
Detailed Design Review Plan 7/9/2020
SnapShot 1 7/13/2020
3D modeling 7/13/2020
Design review 7/14/2020
Engineering Release of Drawings 7/30/2020
Animations 7/30/2020
Complete Prototype Build 7/30/2020
SnapShot 2 8/6/2020
SnapShot 2 8/6/2020
Final Reports and Drawings to client 8/7/2020

Design Considerations[edit]

Concept Full View
Back of Lever Arm and Brackets
  • Universal Fit
  • Independent to dependent versions
  • Lever Arm Ratio
  • Leaver Arm Length
  • Welded vs Bent Sheet Steal
  • Spring Locking Pin vs Free pin
  • Where is the trailer to be used (environment)?
  • Selecting the correct hardware. (Zinc plated nuts and bolts for corrosion resistance)
  • Scaling for Prototype
  • Material Cost vs Strength for design
  • Rotating Spring Mount to keep compression linear
  • Pre-cut alloy steel pipe vs. Sch 40 pipe

Project Learning[edit]

  • Something always just needs to be decided you can’t make it perfect.
  • Parts can be bought or produced; we need to see which one is most cost effective.

Final Design[edit]

2000lbs Full Assembly
3500lbs Full Assembly
6000lbs Full Assembly
Full Assembly of 2000lbs with Sub Asseemblies
2000lbs Bottom Spring Mount Assembly
3500lbs Bottom Spring Mount Assembly
6000lbs Bottom Spring Mount Assembly



Mesh and Forces applied to Rocker arms
Von Mises Stress
Displacement of Lever Arm at Max load
Factor of safety of 3
2000lbs Rocker Stress
3500lbs Rocker Stress
6000lbs Rocker Stress
2500lbs Rocker FOS of 3
2500lbs Rocker FOS of 3
6000lbs Rocker FOS of 3
2000lbs Bottom Spring Mount Stress
3500lbs Bottom Spring Mount Stress
6000lbs Bottom Spring Mount Stress
2000lbs Bottom Spring Mount FOS of 3
3500lbs Bottom Spring Mount FOS of 3
6000lbs Bottom Spring Mount FOS of 3

Materials and Parts Chosen[edit]

STILL NEEDS FORMATTING Bolts: 3/8” Zinc Yellow-Chromate Plated Hex Head Screw, partially threaded bolts were chosen. These were chosen because they were the correct length and size in order to keep our suspension system locked down and they would be able to withstand the amount of stress that could potentially be applied to it. Washers: 3/8”X1” Flat washer, steel, low carbon, zinc plated washers were chosen so that the metal would not deform around where the bolts were tightened on the metal for our suspension system and the washers would not rust. Nuts: 3/8” nuts were chosen so that our spacer and our axle mount can hook together and lock on our suspension system. Springs: Spring mounts: coil spring top mounting plates were chosen to mount our spring to our suspension system. They can withstand potential loads of our system proved by the finite element analysis. Axle: The axle diameters chosen was for the 2000lb – 1.38” 3500lb – 2-¾" 6000lb – 3". These diameters were chosen based off internet research and deciding which axles would fit best with bearing sizes and our trailer mounting system. Pipe covering axle: sch 40 1-½" 3” and 3” nom steel pipes. We chose these because rather than buying expensive precut alloy steel, these pipes are much less costly and will do the job that needs to be done. These are also much easier to obtain than the specially cut alloy steel. Bearings: 1-5/8” high-load, oil embedded flanged sleeve bearings for the 2000lb axle, 3-¼" oil-embedded flanged sleeve bearings for the 3500lb axle, 3-½" oil-Embedded flanged sleeve bearings for the 6000lb axle. These were chosen because they can withstand the high dynamic load and can last at least 10K hours. They also fit our axles perfectly. Spring lock: A Spring lock was chosen so that a customer could easier switch from independent to dependent suspension after removing the first time. This makes it so that it won’t suddenly lock into place without you intentionally doing so and it can lock back into place if there a single person doing this.

Lever Arm Ratio[edit]

For calculating required lever ratio at extreme conditions (3x dynamic loading): F_spring = Maximum Load Limit (lb) Manufacturer Specification

  1. At less than max load; F_spring = (3/2)*F_load/Lever_Ratio
    1. K = Spring Stiffness (lb/in)  Manufacturer Specification
    2. Preload = 0.75*K (lb)  Chosen in order to have >100 lb loading at all times
    3. F_load = 2000 (or) 35000 (or) 6000 (lb)  Depending on configuration
  2. Lever_ratio = (3/2)*F_load/F_spring  Reworking of equation (1)
    1. Deflection_max = (F_spring-Preload)/K (in) Deflection at spring side under 3x dynamic load
    2. For verifying ~1 in deflection at wheel side; D_wheel = Deflection_max/Lever_ratio
  3. Spring_load_max = K*Deflection_max (lb)  Actual force at max load, must NOT exceed F_spring

Below is snippet of the calculation file we received at the start of the project. As can be seen in the cells that have been highlighted yellow, a spring could be easily found that met the mechanical requirements for 2x dynamic loading. However, this did not meet the 3x dynamic loading requirement (the red highlighted cell). This became the design goal of the project, the ensure that the spring would never plastically deform if a 3x dynamic loading event were to occur.

Excel for Calculating Forces

Before optimizing the lever ratio, we had to test the performance all springs that met our criteria. Spring selection criteria:

  • 9-14 (in) free length
  • 95 – 350 (lb/in) spring stiffness
  • 250-2000 (lb) maximum load limit
  • 3.0-6.0 (in) outside diameter

We were able to find 34 springs online that were within this range. Listed below are the specifications and calculated values for all springs that fit in our criteria. They have been sorted by calculated lever ratio in ascending order. The top choice for each trailer configuration has been bolded at the top of each list.

New Ratio for 3500 and 6000.png

Cost Analysis[edit]

Bill of Materials for all three axle ratings

2000lbs BOM
3500lbs BOM
6000lbs BOM

Physical Prototype[edit]

Prototype 3D Design and Manufacturing process[edit]

  1. A description of the manufacturing processes used to build the prototype, with particular focus on any unique challenges faced or processes used.
  2. Video of actual prototype
  3. Evidence supporting validation of prototype - A summary of the validation approach used to confirm the prototype meets the requirements.
  4. Recommendations for adoption or future development of the design

Manufacturing Process[edit]

  • Calibrate 3D printer and print test piece.
    • 3D printed components: 3x bed pieces, LHS & RHS rocker arm, 2x bottom spring mount, 4x spring retainer, 2x axle caps.
  • Levers printed axially outwards, to maximize laminar strength.
  • Warping effects significant; utilized unenclosed 3d printer by flattening benign layers.
  • Added supports to perimeter features of center piece, that could easily be snapped off.
  • Added pretensioned thru-bolts to wheel mounts, to reinforce most likely failure mode.
  • Added pairs of nylon bushings between surfaces of lever.
  • First print incorporated lego-like connections, but was changed to screws due small resolution.
    • Similarly, the axle mounts were enlarged and became a mating pair.
  • First bottom spring mount was small radius and was too brittle, so triangle gussets were added to both sides.
  • First spring retainer design only compressed one side of spring at mount.
  • All printed parts had to be sanded filed down considerably due to the raft adhering to the surface.

Validation of Prototype[edit]

NOTES: Use product requirements, look at scope from Swenson, add SW, talk about tolerance and print gaps

  • Scope requirements - Fabricate a small-scale prototype of a trailer frame and suspension system for demonstration purposes. This may be 3D printed or use fabrication techniques, or a combination of both.
  • Slowed down print, changed raft, skirt, brim,
  • Couldn’t scale exactly to full size, had to reinforce parts to ensure durability of prototype.
Top View of 3S Suspension and Trailer
1/8 Scale Working Prototype of 3S Single Axle Trailer Suspension
SolidWorks of Printed 3D Trailer Frame
Bottom View of Prototype
Side view of Prototype Rocker Arm

Team Members[edit]


Name: Caitlin Swenson
Major: Mechanical Engineering
Hometown: Enumclaw, WA
Responsibility: Team Leader, Documentation, Client Communication, Meeting Minuets
Email: swen6627@vandals.uidaho.edu


Name: Robert Carne
Major: Mechanical Engineering
Hometown: Sandpoint, ID
Responsibility: Vice Team Leader, Design, SolidWorks, Validation(FEA), Team Meetings, Wiki Page
Email: carn0700@vandals.uidaho.edu


Name: Kirk McKenzie
Major: Mechanical Engineering
Hometown: Hayden, ID
Responsibility: Design, SolidWorks, Validation(FEA), Project Management
Email: mcke9682@vandals.uidaho.edu


Name: Anson Lunstrum
Major: Mechanical Engineering
Hometown: Nampa, ID
Responsibility: Design Calculations, Prototype Manufacturing, Team Meetings
Email: luns6312@vandals.uidaho.edu

Additional Documentation[edit]

Project Schedule

3S Schedule

Meeting Minutes

File:3S Meeting 1.pdf
File:3S Meeting 2.pdf
File:3S Meeting 3.pdf
File:3S Meeting 4.pdf
File:3S Meeting 5.pdf
File:3S Meeting 6.pdf
File:3S Meeting 7.pdf
File:3S Meeting 8.pdf
File:3S Meeting 9.pdf
File:3S Meeting 10.pdf
File:3S Meeting 11.pdf
File:3S Meeting 12.pdf
File:3S Meeting 13.pdf
File:3S Meeting 14.pdf
File:3S Meeting 15.pdf
File:3S Meeting 16.pdf
File:3S Meeting 17.pdf
File:3S Meeting 18.pdf
File:3S Meeting 19.pdf
File:3S Meeting 20.pdf
File:3S Meeting 21.pdf



Client Interview

File:3S Meeting 4.pdf