Everyone,
I really do not know when the 74% came into effect for the
750s if it was right off the bat with the first ones or later.
It is just a matter of fact that the test riders liked this number
the best during tests of the new 750 engines. And of course
during the rotation cycle the bob weights on the crank are there
to cancel out the stopping and starting of the piston and rod
mass at both top and bottom dead centers. However, the
forward and aft shaking comes from just the bob weights as
there is no counter acting force from the piston and rods.
The "smooth" answer to this of course was the counter rotating
shafts used by other makers. However, Triumph did not have
the money or the time for new tooling to compete. So Triumph
went for the best feel. Work pretty good, but I have to say
even though I love my ride, the engine was really outdated
by the mid 1970s as compaired to other manufactures.
A good little article showed up in Classic Bike September 1990
about the balancing of cranks and stated;
"Writing of the new unit-construction Triumph Bonneville,
developed from the separate gearbox twin, he wrote(Bert
Hopwood): 'This was achieved with a minimum of unknowns
because the basic geometry and structural factors remained
unaltered... However, once the engine was fitted into the frame,
which was also of a new design, it proved to be a real shaker.
Time was not in our favour, yet our development team...
managed to compromise with a passable degree of roughness
which we knew the public would accept.'"
Remember this was about the 650 engine, and the 750 only
made matter worse.
Also covered in the article were other causes of ruff running
that seems like an out of blance crank. First and foremost was
out of blance carburators. Also alternators are a major cause
of vibrations, with there "bucking action" as the winding are
passed by the poles. This is even made worse by the use of
a Zener diode. Now three phase system with electronic
voltage control does not have the same problems that say a
two wire system would have with a Zener diode.
Good article worth a read if you have a copy.
Also remember that there is static and dynamic balancing.
Furthermore, a really matched set of pistion, rods including
all the small parts needed to weigh the same or one develops
a rocking pair offset. Remember, that there is already a
given distance between the two rods in this style of engine,
each power stroke causes a rocking motion from left to right.
A none matched set of parts will only increase this effect
of the rocking pair.
Pookybear