A Triumph of Damping

by Wheelnut

We don’t hear the expression "there’s nothing new under the sun" so much these days, as car technology becomes ever more like science fiction. But back in 1980, when Triumph launched the Drophead version of theTR7, many old car buffs were highly amused to hear that it resurrected an interesting 1930s engineering concept – a harmonic front bumper. In those pre-WW2 days, most cars had a separate, channel-frame chassis that could suffer torsional vibrations of the front end under certain conditions.

Pioneer car designer Fred Lanchester had already shown how torsional vibrations in a crankshaft could be damped out. He did this by connecting a sort of mini-flywheel to the crankshaft nose through a spring medium, such as rubber, and then tuning this mass/spring system to absorb the energy of vibration at critical resonance speeds.

A harmonic bumper is intended to do a similar thing for a chassis if it ever starts to ‘quiver’. Birmingham car parts maker Wilmot Breeden supplied harmonic front bumpers to several, mainly up-market car-makers, and in particular to Rover, who used them until 1947. These had substantial lead bob-weights mounted in the rolled ends of the bumper blade, with the whole bumper assembly able to move relative to the chassis on a full width steel spring leaf.
Why did the TR7 Drophead need such a device? Because it had been designed purely as a fixed-head, two seater, in response to draft American Legislation that was intended to outlaw new open-top cars on safety grounds. By the time that the USA High Court had rejected this, it was too late – the TR7 design and tooling was fixed for its 1975 USA launch.

Converting a tin-top to a roadster retrospectively isn’t easy. Pressed Steel Fisher body engineers used contemporary finite-element computer techniques to work out ways of getting sufficient stiffness and fatigue-resistance into a soft top TR7 without adding too much weight, (which compromises performance, economy and emissions). The answer lay in a moderate amount of extra reinforcement in the body structure plus the harmonic front bumper. This made use of the substantial steel bumper armature that was already required to meet the American 5 mph impact regulations, but specially mounted so that it could also function as a damper. Did it work? Well, the prototype survived, intact, the full 1000 miles of the notorious MIRA Pavé test, which certainly isn’t for wimps.
So - anyone restoring a TR7 Drophead should keep the front bumper system intact – changing it for a standard bumper off a fixed-head TR7 is not advised.

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