What gives a car body its rigidity.

[davehedgehog31]

Two eerily similar posts, seconds apart.

[RayMK]

For Nürburgring approved handling and roadholding modern cars generally have short travel, stiff suspension and stiff anti-roll bars (which take away some of the independence of the suspension).  This increases instantaneous loads on the structure and requires a stiff body to limit relative movement of panels and thus prevent doors from rattling and dashboards creaking.  Add to that the design requirements of energy absorbing crumple  zones, crush resistant passenger safety cells, the desire for refinement which often results in vibration isolating subframes and tight shut lines which reduce wind noise (and drag), then the accountants refusing to fund extra/better material or over engineering, and you end up with a very efficient structure which can be accurately modelled for stress analyis during the design phase, thanks to computers.  Fifteen years later when even some galvanised and aluminium bodied cars suffer localised corrosion, this 'efficient structure' can be severely compromised if the corrosion has occurred in a key load path which is neatly hidden from inspection by cosmetic plastic trim and covers. 

 

Older cars commonly lost so much structural strength through corrosion in key areas that jacking up one corner risked permanent distortion of the structure even if you could persuade the jacking point to stay in view rather than disappear through the sill or subframe.  This is why it is deemed sensible to allow owners to reinforce their crumbling cars with structural underseal and P38 without the need for official inspection.   As Junkman hinted, a sturdily repaired car will not necessarily behave as intended if you hit something.  Nor will one which has had corrosion hidden with plastic trim or metal cover sills. 

 

Non-articulated HGVs with a flatbead load area over a chassis will twist readily by design but still have huge reserves of strength for load carrying. The cab is usually isolated from the twisty bits and all is well.  The need for structural efficiency is even greater in commercial airliners yet they also flex.  This often leads to fatigue failures after umpteen thousand hours of pressure cycling but is all part of the predicted life of the structure i.e. Usage is accurately logged and areas of known stress concentration are regularly inspected so that approved repairs can be made in good time.  

 

Whatever you drive, avoid crashing or driving like a tit and all will be fine. 

[PhilA]

Re: That Cortina estate- you don't really see to find comments these days in car buying guides about finding hidden rot.

 

For Spridgets the buying guide states to watch for non-frogeye models with doors that catch at the top edge or don't close well as the body sags downward in the center, whereas the Frogeye you have to look for the bottoms of the doors closing up due to the different design of the rear suspension adding torque to the body in different ways (quarter- versus semi-elliptic rear springs).

 

Do buyers' guides have to warn of these or similar characteristics for modern ragtops?

 

Phil

[HMC]

With early monocoques it’s well known that a less than full understanding of loads and stresses lead to a belt and braces approach. Equally say with Colin chapman it’s interesting to see how structures can be made lighter by careful design, components being asked to do more than one role (eg the engine) and perhaps a bit of suck it and see. Looking at moderns it’s about satisfying specific safety parameters whilst using (usually) as little of the least expensive materials practicable.