American Iron Magazine December 2017

TECHLINE Continued from page 24
tion should be restricted to keeping oil
in and dirt out instead of as a powertrain
brace. The 1970s were fraught
with primary chaincase breakage as
The Motor Company sought to save
money with thinner, die-cast primaries
and severely cracked the inner chaincase
around and behind the clutch hub.
Harley riders of the day somewhat
forgave these breakages because the
die cast alloy chromed perfectly, unlike
the stronger sand-cast aluminum.
Instead of resolving internal vibration
causes like the 45-degree cylinder
angularity and awkward engine igni-
rotation engine cycle (intake, com-
decided to mask and separate the shake
from the rider.
The primary chaincase system,
transmission, and rear fork were
isolated from the rest of the chassis by
1979 with the bagger FLT and again
in 1982 with the advent of the sporty
FXR. These early systems worked
well. However, the popular and great
handling FXR frame was labor intensive
to make. H-D began to supersede
this model with the Dyna Glide
in 1990, which used a simpler frame
with a new rubber-mounting system.
This did not go well. All too often, the
rubber-mounted Dyna engine would
rock so much that the bottom of the
primary casing smashed into the lower
frame member underneath. Physical
parts damage, premature parts wear,
back through the powertrain, and bad
vibration were all symptoms of the lack
of rigidity. It took many rubber-mount
redesigns over successive years before
the Dyna came into its own.
The aftermarket rode to the rescue
that Carlini Design was the originator
the Evolution Dyna Glide models. The
transmission on the opposite side of the
primary system. It connects the rear
of the engine’s timing gear cover with
the front of the transmission end cover.
mary
system in its thankless side job.
to the powertrain, and was instrumental
in reducing vibration caused by
In 1999, the bagger Twin Cam
design incorporated the ultimate
transmission housing bolt together and
become as one. The primary chaincase
added secondary support for the most
to date. Vibration was reduced further
while handling capability increased.
This, of course, carries over into the
Milwaukee-Eight chassis design.
The hot rod Sportster (1957 to
get-go by using what is called uniconstruction.
The progressive little
alloy cases to accommodate the engine
that also housed the primary/clutch
components and the transmission. The
rigidity produced is the primary (for-
the Sportster suffered mightily from
vibration, but the causes were found
elsewhere. Harley-Davidson has no
shortage of vibration centers.
transmission uni-construction with
engine chain-driven counterbalancers.
A rubber-mount frame will not work
properly over time with the triangular
rear fork. I know because I have
tried. The main center frame downtube
attaches to the top of the transmission
casing while the bottom of the
transmission attached to and supported
by the engine casing bolts securely to
the lower frame tube members. The
engine and transmission are now part
of the frame on the Twin Cam Softail
models. Conversely and cooperatively,
the frame is now part of the rigid unit
construction of engine and transmission.
However, the Softail was left with
one major rigidity problem with the allstyle
but poor functionality of the rear
chassis fork and shock system. Cool,
for sure, but a handling nightmare in
twisty turns at high speed.
Dual Softail engine-gear-driven
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