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A closer look at the Frame

Most vehicles were originally a perimeter frame design but trending towards a lighter, stronger Unibody frame. Ongoing experiments in construction resulted in some Unibody Citreon models in the 1930’s; American made Nash 600 was produced in 1941 The American made compacts including the Ford Falcon, Mustang,  Corvair, Dodge Valiant and some Ramblers from the 1960’s are some further experiments with the Unibody design. There are cross members present which create torque boxes to help reduce chassis flex and vibration similar to the Chevy II and its semi-Unibody style with a bolt on front clip. The Hat style Unibody frame is now the most common version for the crossovers and other mass produced autos of this century. These latest versions actually use a combined variety of frame designs to achieve the most uncompromising support with the added benefit of being easy to manufacture.

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© Arkela | Dreamstime.com – Car Frame Photo

The automobile with a perimeter frame is made up of a pair of symmetrical rails running the length of the vehicle attached to each other by cross members. There are three most likely configurations of the rail shape to make the frames that support most automobiles. The C-shaped rail is the most common but the boxed and hat style variations are also used, The rails are rolled into shape from 1/8’ to 3/16’ flat stock composed of, in most cases, carbon steel or aluminium alloys, but could also be made of any other durable, light but strong alloy.

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© Rudi1976 | Dreamstime.com – Car Chassis With Engine. Photo

During the 60’s most conventional American cars have a boxed or four sided version of the “C” rail-spot welded where the edges meet. Stock cars competing on NASCAR circuits are often welded the complete length of the rail for additional strength. The original method for the box frame is to weld two C frame together but a more up to date way is to bend the flat bar into one four sided rail using only one weld to join it. The hat frame is similar to the “C” frame but instead of sideways the opening is either facing up or down to resemble a “U” or an “n” rather than sideways as in the “C” style. Many Studebaker models used the hat frame as does Chevy from the mid 1930’s until the mid-1950’s when the C type became the design of choice. The Unibody cars saw the Hat style rail revived in the last thirty or so years particularly although this design tends to rust quicker it is light with strength.

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© Joooooooooo | Dreamstime.com – Car Frame Photo

The frame is subject to large amounts of stress both from the road and high torque engines. The vertical length (height) of the “C” dictates how much flex the frame will have. The taller rail rather than a thicker one will resist the vertical flex or the force applied from the top with more pleasing results. A balance is engineered between thicker material and length to absorb the vertical flex when building the rail.

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© Suricoma | Dreamstime.com – Car Frame Photo

This is the “beam height” and can vary with every manufacturer for each vehicle model or platform they produce. The rails tend to be heavier in the rear, centrally and under the passenger compartment, then thinning towards the nose where the engine is mounted. The frame could be either arched over the axle or “Kicked up” to remain slightly higher ahead of the axle-streamlined accommodating the condensed weight of the engines compartment ending at the front bumper mountings.

The X-frame is the evolution of the perimeter frame and under every full sized G.M. product until the early 1960’s which gives the car a lower profile than previous perimeter frame does. The X offers good support beneath but without side rails is a design flaw and offers little side impact protection. From the mid-1960’s perimeter frames are used but now the crossmembers overlap the side rails both front and rear at the rockers. The last perimeter framed vehicle sold in the USA was the Ford Panther in 2011.

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