It's now common knowledge that the Allegro's bodyshell was so floppy that when you jacked it up, the back window popped out. Well, that's what they'd like you to think.
JH Gillson reckons they missed the point...
The Allegro and its Allegedly Weak Bodyshell
rom the updated development
history of the Austin Allegro: "The Allegro's torsional stiffness is
about 6000 lb ft degree - one of the highest in the industry." Interesting
that, laying to rest the old canard of the Allegro's floppy body-shell. Perhaps
more interesting is that the figure is less than half that for the A30/A35 –
according to the development story for that car - but greater than that of the
Morris Minor by a healthy margin...
All of which follows on from the MG Rover: The Road to Perdition article by Ian Elliott: "A graph of beam stiffnesses measured by PSF for various bodyshells is interesting. It shows the Allegro shell having a figure of around 5.5 MN/m, much lower than the surprisingly high stiffness of the ADO 73 (Mk.2) Marina at 9.7 MN/m. However, it was still higher than the 5.2 MN/m of the contemporary VW Golf, the 4.7 MN/m of the Ford Fiesta or the 3.7 MN/m mustered by the Renault 5. These things are all relative… "
Is there a difference between beam and torsional stiffness? Somewhere, I’d read some engineer or other saying something along the lines that any idiot can design in beam stiffness but the real skill is in trying to achieve acceptable torsional stiffness.
Having read Ian Elliott’s article some time ago I’d wondered if the Allegro might have had acceptable beam stiffness but was badly lacking in torsional stiffness. Hence the old stories about windscreens or rear screens popping out when, after jacking the car up, the bodyshell would twist, but Harry Webster’s quote suggests the car was more than acceptable in this regard.
Digging around on the internet I found this quote: “18,000 [lb ft per degree of deflection] also sticks in my head because I seem to remember that was the quoted stiffness for the old Austin 1800 "Landcrab" monocoque.” And the following figures relating to the 75/ZT: Torsional Stiffness: 24,000Nm/degree or 17701.49 lb ft/degree.
I converted the Newton metres figures to lb ft using a conversion table found on t’internet. This is interesting if only to demonstrate how incredibly stiff the Landcrab was back in 1964 - the Crab being as about as stiff as the 75, if not more so. Incidentally, the Allegro figure converted to Nm/degree is 8134.91, and that for the A30 is 17625.63. Perhaps the structure of the tiny 1951 Austin is even more an impressive achievement than the Landcrab’s...
Then again, the figure for the A30 is probably for the two-door saloon whereas the bigger Crab had four doors and I would imagine that achieving that level of stiffness is more difficult when designing a larger saloon. If, as Rover stated at the car’s launch, the 75’s structure was two-and-a-half times stiffer than that of the Alfa Romeo 156 then the "Riley Tipo" has a figure of 9600 NM/degree or thereabouts.
Of the Maxi: “The much-vaunted fifth door…has resulted in a torsional stiffness less than half that of the 1800.” So less than 9000 lb ft per degree but still more than competitive back in 1969 if in Harry Webster’s words the Allegro’s figure of 6000 lb ft/degree was one of the highest in the industry in 1973.
What does all of this mean?
Well, the Allegro’s bodyshell was much less stiff than those of its Issigonis-designed forebears – and the A30 - but by the standards of 1973 it was probably no worse than the products of its competitors. However, the car was a booted saloon with no opening rear door, and judged in that context the car's structural integrity was a disappointment - being probably much less strong than even the Maxi - but not a disaster compared to the first generation of small hatchbacks that were appearing at the time.
It's perhaps just as well that BL didn't develop the car as a hatchback, and I wonder how stiff the Allegro estate's structure was, or how the front subframe that was originally mooted for the car - but then deleted - affected the car's stiffness? In other words, it seems that the Allegro's alleged structural weakness could only be considered a defect in relation to the truly exceptional strength of the cars that preceded it. Compared to other cars in its class, and of its era, it may well have been better than the norm.
As Ian Elliott said, "These things are all relative… "
The other conclusion to draw is that the Landcrab's structure was almost supernaturally strong and confirms for me that the poor car is the most underrated device conceived by the artist formerly known as BMC, or Rover or whatever. Had the V6 that BMC were developing in the late Fifties appeared in the car, and had Pininfarina been given a free hand in its styling we'd all be looking fondly back on a car that was conceptually similar to, as strong as (if not stronger), and as comfortable as 1998's Rover 75.
But with more room. And smaller.
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