TECH TORQ
hunting tooth gear, that is, the
occurrence of a chain running
over the same sprocket point
is multiplied by the number
of teeth on the sprockets, this
helps keep wear even over the
full chain length and sprocket
diameter.
“Robbo, 23/48 would work
well with your combo; the
100 cube engine will pull that
happily and sixth would then
be a reasonable overdrive for
cruising.”
HARLEY
VIBRATIONS
Doc, my old man had an
old Beemer, a R60 I think,
from the 1960s. I rode it a
few times as a teenager and
then again a couple of years
back just before he passed.
It rode smooth as silk and
it makes me wonder why
the Germans could sort out
engine smoothness years
ago and Harley are still
struggling with vibration,
getting around it with rubber
mounting in so many cases.
Does their engine design
need a thorough rethink?- Roger
Here’s the deal Roger. In a two-
cylinder, horizontally opposed
engine such as the BMW, the
pistons are timed so that onefires on one revolution of the
crankshaft and the other fires
on the next revolution, so one
of the two pistons fires on every
revolution of the crankshaft.
This seems logical and gives the
engine a balanced feeling. To
create this type of engine, the
crankshaft has two separate pins
for the connecting rods from the
pistons. The pins are 180 degrees
apart from one another.Conversely, a V-Twin Harley-
Davidson engine has two
pistons. But the difference – and
this is important – the Harley
engine crankshaft has only one
pin, and both pistons connect
to it. This design, combined
with the vee arrangement of the
cylinders, means that the pistons
cannot fire at even intervals.
Instead of one piston firing
every 360 degrees, the Harley
firing sequence is one piston
fires and the next piston fires
at 315 degrees, which equals a
405 degree gap. Then the cycle
is repeated. Which is why at idle,
especially in older bikes such
as Shovelheads, you have the
famous ‘potato potato’ sound. It
is not so obvious on later Harley-
Davidsons because their idling
revolution speed is set higher.
Now given all this, the Motor
Company has done a pretty
damn good job on the balanced
Twin Cam and M8 engines. HD