Why "Rocker Panel"?

Where did the Rocker Panel on a car get its name? I seriously doubt that it was ever intended as a place to “rock” the car. A friend suggested that it may have been a corruption from some description that this is the part of the car that is hit by rocks flying up from the wheels, but that seems like a stretch to me.

Surely someone in the Teeming Millions is aware of the etymology of this??:confused:

Your friend is correct. On early cars a sacrificial panel was placed behind the front tires and ran to the rear of the car. It was covered with a rubber matting to keep things quiet. It was originally called a rock deflector. As body building techniques evolved and running boards and stand alone front fenders went away, the name rocker panel stuck and now it is the part of the lower body work between the wheel wells.

A Treatise on Carriages: Comprehending Coaches, Chariots, Phaetons. Published 1794. page 33. The Rockers, which are two strong boards firmly screwed or nailed to the inner part of the bottom side-piece, from which it descends farthest in the middle, and the descent gradually diminishes to both the extremities : on the bottom of those rockers the bottom boards are nailed; their use is to give depth from the seat, without affecting the external appearance of the body.

Well, the parts manual for my Triumph TR 3, ca. 1950s calls them “sills”.


I’m pretty sure it goes back to the original ‘coach builders’ who put the first bodies onto the chassis. These companies actually made carriages and coaches before the automobile, and the mid-section was set up with a bend/arch… which provided rigidity on early carriages and coaches. This was also a rocker section on cradles, coaches and chairs.

From “A Practical Treatise on Coach-building” by James W. Burgess published by Crosby Lockwood & Co London in 1881.

“Proportion in carriages applies to both form and colour; as regards form, it regulates the sizes of the various parts so that the whole may harmonise, and dictates the adoption of contrivances for lessening the apparent size of those parts which would otherwise be unseemly. Thus, the total height which is necessary in the body for the comfort of the passengers is too great for the length which it is convenient to give it ; therefore the total height is reduced, and to give sufficient leg room a false bottom is affixed by means of convex rockers, and which, being thrown back and painted black, cease to form a portion of the elevation ; they are, -like a foundation, out of sight, and thus the proportion of the front view (the side is called the front in coach-builder’s parlance) is preserved. In painting the body of a coach or chariot, it is customary to confine the ground colour to the lower panels and to paint the upper ones black, all except some stripes on the upper part of the doors…”


That must be an old style. I have looked at all the carriage drawings I can find and none mention rockers. Trying to figure out what it is saying it seems like those rockers support the floor (bottom boards). They are attached inside the bottom side-piece and do not show. I don’t think they are structural. Although it seems to have given rise to the present interpretation of the word.


Post a picture! I love vintage Triumphs. My dad had a TR 4 - not as nice.

That’s British English for the same thing.

Actually, we still have “side sills” in the body construction language in US English, too.

Merriam-Webster is having a hard time with it. None of their definitions of rocker are relevant. However, sill is defined as “a strong horizontal member at the base of any structure, e.g., in the frame of a motor or rail vehicle”. But a rocker panel is defined as, “the portion of the body paneling of a vehicle that is situated *below *the doorsills of the passenger compartment”.

So they are trying to divide up the sill and rocker panel when in fact it is the combination of the two that gives structure support to the body.

And to further confuse things, in all the cars I have done such work on, there is an inner sill panel that carries most of the load and an outer panel that is both load bearing and forms the outer skin that you see.

Here is a TR3 inner sill, the structural support that the A and B pillars, the inner fenders and the floor are welded to and bolts to the frame. : http://the-roadster-factory.com/TRFpartsProjects.php

Here is an outer sill that spot welds onto the inner sill along the top and bottom flange: http://www.kas-kilmartin.com.au/TR009L%20TR3A%20-%2060000%20on%20Outer%20sill%20-%20rocker.JPG
I note that over the course of time the term “rocker” is now the opposite of the carriage’s use of the term. Where once the rocker is purposefully hidden and was not really structural today it is in view but does carry bending load.