spider stiffness

["Hardwick, Robert C" <Robert.Hardwick@domain.elided>]

Since a car's structure is essentially a bridge between the wheel supports, at first glance the Ereminas stiffener seems rather dubious.  As Chris Prael notes, it has little vertical section (not much on the side, either since there's no crossmembers but there's no loading in that direction except in a crash).

But the spider is so compliant that just about any additional metal would be an improvement.  (I do have the software to calculate the stiffness, but don't have dimensions for the structure.)  My curiosity led me to ask Ereminas for some information, and they sent me the installation instructions (nice folks).  Typically, bolt vs hole tolerances significantly reduce stiffness of a joint.  Ereminas' installation solves this by having the car on jackstands so that the body droops in a concave down attitude.  When the car then rests on its wheels,  the stiffener is  preloaded, closing the gap between bolt and hole.  

Of COURSE a rollcage would be vastly better!  Now, can I bolt one in for $425 and have my top function?  This is really an unfair comparison.   A rollbar alone will contribute little to stiffness.  Among other things, it's adjacent to the luggage shelf box, just about the deepest section between the wheels.

Airplanes are like bridges, too, except with a wing holding it up in the middle.  They've got this really deep section keel beam made out of 80ksi 7150.  That's pretty strong for aluminum.

robert hardwick, seattle 

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