Sub-frame

The zero-torsion sub-frame for this build. See the installation guide for the step-by-step field procedure and BOM.

This document defines the OpenOverland zero-torsion sub-frame as a standalone architectural object.

The sub-frame is the structural interface between the truck chassis and everything above it — the aluminum body, the camper, and all attached systems. It is designed first, in isolation, with clear boundaries and explicit intent.

Purpose of the Sub-frame

The sub-frame exists to:

• Isolate the body and camper from chassis torsion
• Provide a stable, level platform regardless of frame twist
• Mount to the truck chassis without compromising its structural integrity
• Define a clean mechanical interface between the truck and the body system

The sub-frame is not:

• A replacement for the truck frame
• A body or enclosure
• A camper tie-down system
• A decorative or aesthetic component

The sub-frame must remain valid as camper models, body configurations, and truck variants change. Only the mounting interface and dimensional variables need to adapt.

Design Posture

The sub-frame is conceived as a three-point zero-torsion kinematic system, where a single front lateral pivot and two rear fixed mounts form a statically determinate structure.

Key assumptions:

• Three-point mounting eliminates over-constraint
• The front pivot allows rotation on the lateral axis only
• The rear mounts are fixed — no rotation
• Together, the three points define a plane that remains stable regardless of chassis twist
• The body above sees no torsional input from the chassis below

The sub-frame does not fight the truck. It decouples from it.

Geometry

The sub-frame geometry prioritizes:

• Minimum deck height above the frame rails
• Maximum clearance from the tire swept volume at full suspension travel
• Clean load paths from the body to the chassis mounting points
• Compatibility with Ford factory body mounting provisions

Flat-top deck geometry is the default. The sub-frame presents a level platform to the body above regardless of chassis state.

Exact dimensions are intentionally parametric and adapt to the specific truck variant.

Structural Concept

The sub-frame is designed as a welded steel structure carried by the truck chassis.

Structural behavior is achieved through:

• Rectangular hollow section main rails
• Welded crossmembers at regular intervals
• Full-penetration welds at all primary load-bearing joints
• Outrigger provisions for body attachment extending beyond the main rails

The sub-frame:

• Carries the full body and camper load
• Transfers loads to the truck chassis through three defined mounting points
• Does not assume structural contribution from the body above it

Zero-Torsion Pivot System

The front pivot is the defining component of the system.

The pivot allows:

• Rotation on the lateral axis — left and right frame twist
• No rotation on the longitudinal axis — handled by the rear mounts

The pivot is constructed from:

• A ground steel shaft (the pin)
• An SAE 841 oil-impregnated bronze sleeve bushing
• A machined threaded retaining ring
• Steel laser-cut brackets welded to the sub-frame and the truck frame adapter plate

All pivot components are standard catalog parts. No proprietary hardware. No special tooling required for service or replacement.

Chassis Interface

The sub-frame mounts to the truck chassis exclusively through:

• Factory Ford body mounting holes — no new holes in the frame flanges
• U-bolt and shear plate assemblies per Ford Body Builder Guidelines
• Web drilling per LiquidSpring installation template where applicable — Ø21/32", web only

Drilling in the top or bottom frame flanges is explicitly prohibited.

The chassis interface is designed to be reversible. Removal of the sub-frame restores the truck to its original state.

Tire Clearance

The critical constraint in sub-frame geometry is the tire swept volume.

The swept volume is defined by:

• Tire outer diameter at minimum inflation pressure
• Full suspension jounce travel
• Any camber change through the travel arc

No sub-frame structure may enter the swept volume envelope. A minimum 25mm clearance margin is maintained beyond the calculated envelope at all points.

This constraint is modeled explicitly in CAD before any other geometry is committed.

Parametric Design

The sub-frame is fully parametric.

Key variables include:

• Truck wheelbase
• CA dimension (cab to rear axle)
• Frame rail width
• Frame rail height
• Suspension jounce and rebound travel
• Tire outer diameter at minimum inflation pressure
• Body width at the base interface

Changing these variables adapts the sub-frame to a new truck variant or suspension configuration without redesigning the system architecture.

Interfaces

The sub-frame defines interfaces, not implementations.

Interfaces include:

• Chassis interface — the three mounting points to the truck frame
• Body interface — the outrigger bolt pattern that receives the aluminum body
• Deck interface — the top surface that receives deck boards or a flatbed platform
• Air line interface — routing provisions for onboard compressed air distribution
• Electrical interface — wiring conduit provisions for tail lights and body electronics

Interfaces are explicit, documented, and conservative.

Manufacturing Neutrality

The sub-frame geometry is designed to be built by any competent structural steel fabricator.

Compatible fabrication paths include:

• CNC plasma or waterjet cutting from DXF files
• Manual cutting and fitting from dimensioned drawings
• Any combination of the above

No specialized tooling, proprietary processes, or specific equipment is required. The design is intentionally accessible.

Coating

The sub-frame is designed for a two-stage corrosion protection system:

• Blast clean to bare metal — Sa 2.5 standard
• Line-X polyurea applied directly to bare blasted steel — same day as blasting

No primer between steel and Line-X. Line-X bonds chemically and mechanically to the blast profile. Primers intermediate between steel and polyurea risk blistering under the exothermic application heat.

All bolt holes, threads, and mounting surfaces are masked before blasting and coating.

Open Source Intent

All sub-frame designs published under OpenOverland are released under an open license.

This includes:

• Complete engineering drawings in DXF and STEP format
• Bill of materials with supplier part numbers
• Fabrication notes and lessons learned
• Real cost data from verified builds

The goal is not a product. It is a standard — an engineering baseline that any builder anywhere can take to any competent fabricator and get a correctly built sub-frame at a fair price.

Closing Notes

The sub-frame is the foundation of the body system.

If the sub-frame is correct — level, isolated from torsion, properly mounted — everything above it can evolve. If the sub-frame is compromised — too high, too wide, incorrectly mounted — no amount of body refinement will fix it.

This document exists to ensure the sub-frame is treated as an engineered system, not a welded shelf.