Subframes & Engine Cradles

Engine cradles (or k-frames) for internal combustion engines are in the front of the vehicle. For electric vehicles, the cradle wraps around the motor housings to provide mount support and battery protection in frontal crashes. In either case, the subframe performs a critical role in the stability and the ride quality which are central to vehicle dynamics, safety, and in meeting customer expectations.

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Subframes are subject to considerable mechanical stresses, and environmental conditions from their location near the ground. They are sized according to the vehicle and the powertrain, and for a mid-size crossover or sedan, the weight is 14-35 kg, depending on the material and the size. There may be one in the front, one in the rear, or both. 

Subframe Design Considerations

The subframe is a critical element between the road loads and the passenger compartment. It acts as a mount structure for the suspension and it reacts to vehicle travel on corners, on bumps, and acceleration and braking. Critical features include high strength, torsional and flexural stiffness, fatigue, and robust performance for the life of the vehicle.

Extrusions can be produced as a hollow, which bodes well for weight reduction initiatives without compromising stiffness. Aluminum is naturally protected from environmental conditions, especially the liquid road salts commonly used today.

On the Porsche Taycan EV[1][2], it was noted that the front subframe protects the battery in a frontal crash, and in the rear, the components are designed to break away and intrude above and over the battery pack upon impact.

[1] Porsche Taycan
[2] Porsche Taycan

Design with Extrusions

Subframes are subject to twisting loads, so it is advantageous to use hollow structures because of the torsional stiffness. Aluminum extrusions have a clear advantage in weight-sensitive designs because it is a low-density material, that is readily available, and easy to fabricate.

The GMX211, Chevy Impala, program developed the front subframe using just extrusions. Most sections were 6061 alloys, yet a couple were 6063. Multiple tempers have been utilized for product performance in addition to alloy selection. The benefit of using 6000 series alloys throughout is that they can all be easily joined using traditional aluminum welding and other joining techniques.

Hybrid Lightweight Aluminum Subframe

Another means to produce a lightweight aluminum subframe is a hybrid, using extrusions as stiff connecting members, and aluminum castings on the corners. The castings provide overall durability, and the unique geometry to enable tight packaging and the attachment points.

Photo showing versatility of aluminum extrusions
Extruded aluminum rear mount
Why Extrusions are Well-Suited
for Subframe Design

Extrusions deliver an efficient geometry, offering multiple functions:

  • Lightweighting
  • Torsional stiffness
  • Many shapes available
  • Ease of assembly &and integration
  • Long life in corrosive environment

CONCEPT: Aluminum vs Steel

Aluminum offers an improvement and efficiency gain over the incumbent solution of steel in the form of: 

  • 35% weight reduction using 6xxx
  • Piece count reduction in weldment
  • Quicker and easier assembly 
  • Corrosion resistance
  • Up to 1000% savings on tooling compared to a stamped steel frame
  • Comparable piece price to steel

Corrosion Protection

Aluminum forms a natural aluminum oxide layer, and it protects well against environmental conditions. Not every material fares so well.

Materials & Performance Characteristics

The aluminum extrusion market is supported by a robust supply chain of material providers, extruders, and companies well-equipped to support post-processing including fabrication, application engineering, and full assemblies.

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Subframe Performance Characteristics

Compare the performance criteria and why extrusions are the best structural material:

Performance Metric: Why Extrusions?

  • Piece cost: Multi-void sections with no welding; piece count reduction
  • Tooling cost: Tooling is much lower than one-piece castings
  • Development time: Tooling in weeks, not months
  • Corrosion resistance: Aluminum is naturally corrosion resistant. No coatings required
  • Weight: Economical lightweighting
  • Maintenance: Threaded hole options, flow drill screws
  • Structural integrity: Strong, torsional stiffness, protect battery & powertrain
  • Crash management: Efficient crush structures around perimeter and in load path
  • Ease of assembly: Easy to shape & machine; weld & adhesive friendly
  • Supply Chain: Robust, global supply chain for materials & extrusions
  • Raw materials: Large variety of non-proprietary alloys

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