April 17, 2003
first glance, it doesn't seem possible that simply substituting a
couple of gears (main and fifth gear/countershaft combination)
could substantially transform the performance characteristics
of your entire transmission,
In fact, a large percentage of the questions we receive are of
all this work?" variety. This page will better
explain the mechanical side of the how and why of the TwistGear
Helical System. (You can also refer to our PDF TwistGear
Worksheet chart detailing the concepts discussed
below, in our PDF library, and learn more about ratios effect on
performance on our charts page.)
hasn't this approach been tried before? The answer's four-fold:
factory goals are based on averages
2) shortcomings of spur gear
the physical limits of a spur fifth-gear, and
4) cost to manufacture
marketers design for the broadest possible use - your transmission
is no exception. It's designed for
maximum utiliity, not optimum performance. The spur, or straight
cut, gears in stock and aftermarket trannys are a compromise between
strength and noise, limited to a 1.86:1 ratio, not the much more
aggressive 2.0:1 TwistGear optimized ratio.
we review what happens inside your transmission, you'll
encounter some semantic differences having to do
with parts nomenclature, i.e., the main gear or mainshaft,
input shaft or mainshaft, and straight cut or spur gears. Now lets
take a look at how the power from
your rear wheel.
drive transmissions are so named because when they're in high or
top gear, the output (main gear, output shaft,
or mainshaft) revolutions from the transmission to the rear wheel
are the same as the (mainshaft or input shaft) input revolutions
from the clutch, or 1:1 (direct). Overdrive transmissions use gear
reduction to reduce the final output, which means the transmission
sprocket is turning slower than the mainshaft coupled to the clutch.
Overdrive is achieved through negative gearing, which
means the countershaft remains loaded in high gear, creating stress
on the components, noise from the transmission, and draining horsepower
clutch sprocket (input) turns once, the transmission sprocket (output)
turns once. The rate of turn at the rear wheel, referred
to as the final drive ratio, is determined by the rear wheel pulley
relation to the transmission pulley (or sprocket). The rate of
spin for the clutch, referred to as the primary drive ratio, is determined
by the clutch sprocket's relation to the engine sprocket.
RPM in any gear is determined by all the ratios (1st through 5th
plus primary plus final) working together, but engine RPM in fifth
is determined only by the
primary and final drive ratios. That's because when the transmission
is in fifth (top or high) gear, the drivetrain bypasses 1st
through 4th and is directly coupled from the primary clutch input
to the final transmission sprocket output, and then to the rear wheel.
in 1st through 4th, however, are determined by the relationship between
the 5th gear
(input) from the countershaft and the main gear (output) to the rear
wheel working with the first four gears. This is where the concept
of close or wide ratios is established, and it's where the TwistGear
its main performance work.
is a critically important consideration, because a direct, 1:1 fifth-gear
ratio allows us to shorten the ratios in 1st-4th by about 10%, thus
achieving both a close ratio 4-speed gearbox and about 10% more torque
in the process (based on a Big Twin TwistGear system installation).
The net result is we do slightly lengthen the gap between 4th and
5th, but we get there (to 5th) much faster than a conventional straight
cut wide ratio gearbox.
Is It A System
refer to TwistGear as a system because there are a number of interelated
aspects to its unique advantages, all revolving around the percentage
reduction to the ratios of 1st through 4th. This is accomplished
by changing the ratio between the input (countershaft) and output
gear) by approximately 10% in the case of Big Twins. TwistGear uses
a 20-tooth input (5th gear on the countershaft, 2 less than stock)
and a 41-tooth main gear output.
shows the ratios for a typical FL five speed before and after TwistGear's
and After Ratio Comparison
FL with OE
compared to TwistGear
25/36 primary and 32/70 final
41 main/22 fifth
41 main/20 fifth
gear ratio direct
revolution of the input (clutch) side goes through the countershaft
first on it's way to the main gear's transmission sprocket in
every gear except 5th. When in 5th, the countershaft is unloaded,
schematic below, based on an FL application, illustrates the relationship
between input (clutch side) and output (transmission
sprocket side). The TwistGear system installation is noted by the
5th gear tooth count: 20t on the countershaft (one piece) and 41t
on the main gear. Keep in mind that in direct drive, the
gearsets are unloaded, whereas an overdriven application has
loaded in high gear, resulting in lost horsepower, more noise,
and less durability.
Ratios For Reference
table below lists popular ratios compared to stock. The issues
of drive ratios come into consideration when riders want to modify
or alter the performance characteristics for specific riding styles.
For instance, if the majority of your riding is done on the interstate
at a steady cruising speed, and the objective is to lower the engine's
rpms, then a taller (lower numerically) overall ratio is desireable.
This is most often accomplished by changing the transmission and/or
rear wheel sprockets or pulleys for a higher (transmission) or lower
(wheel) tooth count.
the introduction of the TwistGear family of helical close ratio transmission
modifications, more aggresive acceleration usually meant a lower
(higher numerically) overall ratio, which could only be accomplished
by dropping the tranny pulley's or raising the wheel pulley's tooth
count, which affected the entire ratio, including fifth, and severly
limited top end performance.
count (tranny/rear wheel)
Twin modification (taller)
Twin modification (taller)
and TwistGear® are trademarks of Johnson Engineering, Inc. Buell™,
Dyna™, and Sportster™ are trademarks of the Harley-Davidson
Motor Company. No affiliation with the Harley-Davidson Motor Company
is implied or inferred.