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From: jerome(at)supernet.ab.ca
>OK, maybe I'm missing something, but this doesn't make sense on
>reflection. Wouldn't a 5-main engine, given the same crank length,
>actually have *smaller* bearing areas and thus more wear pressure (and
>less oiled surface) per bearing? The webs for the two extra journals
>have to take space (length) from somewhere, and unless each web is also
>drastically narrowed (by 20% each?), then there is actually less shaft
>left over for journals than on a shaft with 3 journals, especially if
>"most of the internals being compatible if not equal" (i.e., the crank
>is not longer).
>My mentor (40 years in the BritCar business) pointed this out right
>away. Everyone/where else says 5 is better.
The bearings are slightly smaller "especially #2 and #4", but the total
bearing area is greater, and so is the main diameters.
The bearing area has thus gone up, AND the amount of crank
distortion goes down.
The real advantage is in that the crank is allowed to flex alot less,
and this allowed them to go to cheaper materials for the crank itself.
The vibration of the crank at high RPMs, and the subsequent stresses
is what propogates cracks, and causes fatigue failure.
I dont think that there is a big difference between the modulous of
elasticity of Cast Iron compared to Steel, but since the steel 3 main
is spun on three points instead of 5, identical forces will flex the
shaft at least double the displacement.
Sunbeams racing departments suffered terribly at the hands of
broken cranks and thrown rods on the 3 main engines.
The early engine was at the edge of what the engines capabilities
were. Building significantly more power meant sacrificing a lot of
engine life.
Any how, Rootes engineers were closely following inovations by Ford
on the 1500 and 1600cc cortina engine, and British Leyland on the
1800cc MGB unit.
Jarrid Gross
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