Thursday, Apr 21, 2016 at 22:50
671 is right on the mark. Aeons ago, when Bedford trucks ruled the road, GMH went to great pains in the operator manual to explain the dangers of overloading your Bedford truck.
In essence, they pointed out that any overload translates to double the overload weight, when you shock-load the chassis and axle via hitting a pothole, a sunken culvert, an out-of-level
bridge approach, and the 100 other uneven road conditions that your
suspension and chassis copes with daily.
So if you overload an axle by 100kgs, that translates to a 200kg overload when any of the above are encountered. Speed exacerbates the impact.
I can recall a highly embarrassing episode for GMH when the HR Holden and the Vauxhall Viva were new, in 1967.
Lap-sash seatbelts had only recently been fitted as standard to the GMH range, and GMH were keen to get a lot of media exposure about the superb strength of their seatbelts.
To this end, they invited a media pack and all their photographers to a stunning demonstration, where they were going to lift cars by their seatbelts!
They rolled out a new HR holden and slipped the seatbelt tongues out the open front windows - the crane driver hooked onto the seatbelt tongues - and he steadily lifted the HR Holden
sedan 4 feet (1.2M) off the ground!!
The media pack oohed-and-ahhhed and the photographers clicked away merrily.
Then came the piece-de-resistance! The crane driver hoisted the HR Holden up about 6 feet (1.8M) and the GMH boys rolled a brand-new Vauxhall Viva under the HR, slipped its seatbelt tongues out the open windows, and hooked them to the underside of the HR!
The crane driver then hoisted both the HR Holden AND the Vauxhall Viva a good 8 feet (2.5M) off the ground - and the media went wild!
Photographers clicked away, while the GMH boys explained how the seatbelt tongues were designed to hold a load of 4480 lbs (2033kgs) for at least 30 seconds.
The HR weighed 1180 kgs and the Vauxhall Viva weighed 700 kgs (approx.), for a combined weight of 1880 kgs.
After about 10 mins of this, the crane driver got pretty bored, and he was moving around inside the crane cabin, gawking at something else.
In doing so, he accidentally bumped the lift/lower control lever with an elbow - just a fleeting bump.
The cars dropped about 2 inches (50mm) at most, with the lever bump - but almost instantly, the HR Holdens seatbelt tongues snapped! - and both cars crashed to the ground!!
Needless to say, the GMH blokes were appalled - and the media pack took a whole lot MORE pictures!!
There was a very hurried examination and discussion by all the GMH heads and engineering chiefs - and they promptly re-assembled the media pack to explain exactly what had gone wrong.
The chief engineer of GMH explained how the 2" (50mm) sudden load drop by the crane - which seemed like virtually nothing to the onlookers - had actually nearly DOUBLED the load on the seatbelt tongues!
This seemingly-slight impact from the drop, was enough to exceed the seatbelt tongues designed and engineered strength by a large margin! - thus resulting in the seatbelt tongues failure - and a very expensive media stunt!
Virtually the same effect occurs on axles when a heavily-loaded axle receives a heavy road impact - from something regarded as relatively minor - say, in the shape of a 50mm deep pothole.
The impact can send the axle load into the danger zone - and even though engineers design in shock-loading resistance into their axle designs - if you're running at maximum axle loading - or even just near it, it doesn't take much by way of repeated heavy road impacts, to result in axle damage.
Cheers, Ron.
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