Winter 2004 Forward to a Friend

The Role of the Sub-Frame

The U-shaped sub-frame attaches to the front body structure, starting under the bulkhead between the engine compartment and the cabin and running underneath the body's longitudinal frame members to the front end. The bottom of the sub-frame's U shape crosses behind the front bumper. The sub-frame is located a few inches below the longitudinal members and is securely bolted to the unitized body (for example, 12 bolts used for the Impreza ensure a solid structure).

The front sub-frame serves a number of purposes. It adds to the rigidity of the front body structure and supports the vehicle's engine and some chassis components. Most importantly, the sub-frame helps to protect occupants in an offset frontal impact.

In a collision, the sub-frame first helps to absorb some of the impact forces on the side that is hit. It also channels some of the impact forces to the side of the vehicle not involved in the impact. This also helps to absorb some of the forces of the collision before they can reach the passenger cabin.

Hydroforming – The Power of Water

As a simple part of the body structure, the steel, U-shaped front sub-frame's hidden beauty lies in the technology of its formation and in its preparation for assembly. The steel piece is hydroformed, which means that it is shaped in a mold by extreme water pressure. Of course, there's a lot more to it than that. Here's a brief explanation:

1. The steel tubing that will become the sub-frame is cut to the desired size and bent in the approximate shape of the final sub-frame.
2. The steel tube is then placed in a machine die, which is closed and pumped full of liquid under high pressure.
3. The pressurized liquid fills the steel tube, which expands to the shape of the die, and then the liquid is released.

The process of hydroforming has been available for more than 100 years. It generally has not been applied to the manufacture of vehicle frames because equipment has been prohibitively expensive due to the extremely high pressures that must be produced for the process to work. With improvements in technology during the last few years, more vehicle manufacturers are using hydroforming for everything from sport car frame rails to motorcycle frames.

Contemporary body design often requires frames and sub-frames to make awkward bends and angles – shapes that ordinarily compromise strength and rigidity. Yet frames and sub-frames must remain rigid to serve effectively as mounting points for a number of chassis components. Hydroforming allows Subaru to shape steel frames with the required bends, bumps, turns, dimples, etc., while retaining structural integrity.

In addition, hydroformed steel components are generally lighter in weight than if manufactured by more traditional methods. The lighter weight contributes to improved handling and fuel economy.

Once a Subaru sub-frame is formed, lasers cut holes in the steel. Using lasers ensures consistency and precise measurements. This in turn means tight fit and tolerances that enhance the overall feel of quality and strength in the vehicle once completely assembled, and the vehicle holds up better through years of driving!

Inconspicuous, But Beautiful Nonetheless

Even though the front sub-frames in Subaru vehicles predominantly are hidden from view, you can appreciate the added safety that they afford. They improve the strength of the body structure without increasing weight, and enhance passive safety by helping to protect cabin occupants.

Subaru sub-frames are beautiful pieces of formed metalwork – all from harnessing the power of pressurized water!