Shifting Gears
|
| At 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,
and by that your total riding experience. In fact,
a large percentage of the questions we receive
are of the, "How does all this work?"
variety. This page will better explain the mechanical
side of the how and why of the TwistGear Helical
System. |
| |
|
Why hasn't this approach been tried
before? The answer's four-fold:
|
1) |
Factory goals are based on averages |
2) |
Shortcomings of spur gear design |
| 3) |
The
physical limits of a spur fifth-gear, and
|
| 4) |
cost to manufacture |
| |
| Mass 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. |
| |
| As 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 engine gets to your rear wheel. |
| |
|
Direct Drive Defined
|
| Direct 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 and
torque. |
| |
| Every time the 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 (or sprocket's) 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.
|
| |
| Engine 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. |
| |
| Ratios 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 system does its main performance
work. |
| |
| This 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. |
| |
|
Why is it a system?
|
| We 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 (main 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. The table below
shows the ratios for a typical FL five speed before
and after TwistGear's been installed. |
| |
Before and after ratio comparison
|
FL with OE compared to TwistGear
25/36 primary and 32/70 final |
Stock
ratios (1.86:1)
41 main/22 fifth
|
TwistGear
ratios (2.0:1)
41 main/20 fifth |
| 1st gear ratio |
10.11 |
11.12 |
| 2nd gear ratio |
6.96 |
7.65 |
| 3rd gear ratio |
4.95 |
5.45 |
| 4th gear ratio |
3.86 |
4.25 |
| 5th gear ratio
direct |
3.15 |
3.15 |
|
|
| |
| Every 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, even though it still turns. |
| |
| The 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 gearsets loaded
in high gear, resulting in lost horsepower, more
noise, and less durability. |
| |
|
| |
|
Additional ratios for reference
|
| The 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. |
| |
| Before 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. |
| |
| 5th
Gear |
Tooth
count (main/5th) |
Ratio |
| XL |
33/19 |
1.73 |
| Buell |
42/25 |
1.68 |
| SportGear |
42/22 |
1.91 |
| Big Twin |
41/22 (late
model) |
1.86 |
| TwistGear/WideGear |
41/20 |
2.05 |
|
|
| |
|
-Final
Ratio |
-Tooth
count (tranny/rear wheel) |
-Ratio |
| -Big
Twin |
- 32/70 |
- 2.19 |
| - Big
Twin modification (taller) |
- 33/65 |
- 1.96 |
| - Big
Twin modification (taller) |
- 34/65 |
- 1.91 |
| - Buell |
- 29/61 |
- 2.10 |
| - 883 |
-27/61 |
-2.26 |
| - XL/Buell
modification (taller) |
-29/55 |
-1.89 |
|
|
| |
|
SportGear™ 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.
|
| |
