I have thought long and hard about the
arguments surrounding whether to rephase
the crankshaft in my XS650 to 83° or
90°. I have read every argument in
support of Phil Irvings original theory
and looked at the mathematical proof
presented with interest, but if I were
to believe these arguments I would have
no choice but to conclude that 74° was
the perfect rephase angle for the 447
engine in my XS650 and that is not what
I have come to believe. I do however
concede that 74° is the point where the
centrelines of both crank and conrod are
at right angles and as such is the point
where piston speed is at its maximum and
the piston has its maximum mechanical
advantage over the crank, but I do not
believe this is the optimum rephase
angle to achieve the greatest reduction
in vibration. I believe Brian Whoolley was correct when he asserted that Phil Irving got it wrong and that to achieve a reduction in vibration then balance is the key and that can only be when the piston and conrod assemblies are in the same relative positions at the rephase angle in both halves of the stroke cycle and the crank pins are in equal and opposite positions effectively cancelling each other out. This symmetry is only achieved when the rephase angle is 90°. By Brian Whoolleys' calculations this should give a theoretical reduction in vibration of 43.5%. This holds true for all vertical twins irregardless of bore, stroke or conrod length. As you see in the diagram, when the
rephase angle is 74°, the disecting
centrelines of the conrod and
crank/crankpin is 90° on the downstroke
and the piston is high in the bore, but
on the upstroke this is only 58° and
the piston is low in the bore, notice
also the relative positions of the
crankpins, they are some 32° apart when
the left cylinder is at TDC and BDC (Blue)
Although not absolutely correct, my assumption that symmetry was the key to better balance was on the right track, see Smoothness by Degrees for a better explanation of why 90° is better than the rest. |

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