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Re: Weight come from?(long and technical) (archive)

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Posted by Fred Kern on September 28, 1998 at 22:26:25:

In Reply to: Where does weight come from? posted by Praveen on September 28, 1998 at 15:28:33:

My physics prof would like me to say mass x gravitational acceleration.

Open top cars like the Z3 typically weigh more than equivalent-sized sedans due to the bracing that it required to prevent cowl shake and chassis twist. The roof panel is relatively light and located far from the twist axis -- sort of like the flanges on an I-beam. If I remember my strengths of materials course, the moment of inertia of a beam is proportional to the fourth power of the beam height. Assume that the car wants to twist around the center of its floorpan. A roof has the height of the interior compartment to resist this (50.7" for the Z3). In an open car lile the Z3, this resistance must be borne by the rocker panels, which have half the body width (33.3") to work with. If I am right about the X^4
moment of intertia, then the roof only needs 1/5th the strength to resist the same torque as do the rocker panels.

Take a good look at the Z3. The built-up sills in the door openings help to increase the effective height of the rocker panel to resist axial twist. These curve into the sheet metal in the top well to tranfer some of this load into the box structure formed by the relativley high sheet metal under the top. The strength of this area is evidenced by the fact that the seatbelts and shocks mount to this structure, as well as the roll hoops in newer Z3's.

If you looked at the cutaway of the Z3 shell in the Zentrum, there are two formed thicknesses of sheet metal in the windshield frame over a hardened roll hoop which ties into the frame rails (I am immensely glad for these as they prevented me fom having a lapful of Bambi). If you have had the sdashboard apart, then you have seen the trusswork behind it. This working in conjunction with the windhield bracing prevents cowl shake.
The Z3 may be heavier than the tintops, but there is a reason. I am impressed when I look at its structure how everything contributes to the structural integrity of the design.

You would have had to own an older sprots car to really appreciate the Z3. I used to have an ALFA Romeo Spider. Everthing flapped in the breeze when you drove it. You could actually watch the hood move in relation to the fenders, see the dash shake and feel the steering wheel shift in your hands. If you jacked up a wheel, and a door was open, you couldn't latch it. Don't get me wrong, I loved every minute of owning it, but it was basically a 1966 design (the chassis was from the Giulietta, and dated from 1953). A lot of improvements have been made since then, still a well-designed roadster will have to weigh more than a coupe or sedan.

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