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What Is Helical Technology?
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| Beginning in the mid '80s, the rise
in v-twin power output has been dramatic, coinciding
with the introduction of a new trapdoor based 5-speed
transmission. Today, aftermarket offerings exceed
130 cubic inches and dyno readings surge past the
200-horsepower mark.
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| But gearing for those transmissions
- the critical link between motor and pavement -
remains much as it was when the first four-speeds
were designed nearly 70 years ago. This means failures
are increasingly common as the design shortcomings
of critical components are exposed when subjected
to stresses they were never intended to sustain.
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| Johnson Engineering’s helical drivetrain
technology delivers superior performance to the
v-twin marketplace by employing sophisticated engineering
and exclusive design methods not found elsewhere
in the v-twin marketplace, like high contact ratio
gear teeth for less noise and much more strength.
At the same time we've developed an optimized close
ratio gear set that restores acceleration where
you need it most...in first through fourth...while
fifth remains an optimum 1:1 direct drive for comfortable
cruising or top end speed.
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Acceleration Load Characteristics
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| The heart of the TwistGear
performance system is a unique, one-piece countershaft
that combines a shorter (lower tooth count) helical
fifth gear flawlessly integrated with a much more
robust countershaft. Fifth gear now meshes seamlessly
with it's helical main gear counterpart, for an
incredibly smooth transfer of power from the input
shaft to the output shaft.
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| Strength? With our helical design, there are always
two teeth in total contact. And you can forget about
shattering the short splines that hook conventional
5th gear/counters together. Comfort? We think six
speeds are fine - if you're racing 125s in Grand
Prix competition. Otherwise, it's just one more
repetitive effort that means 20 percent more time
and effort shifting instead of riding. Performance?
That's what a close ratio gear box is all about. |
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A True Close Ratio Gearbox
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| Sharing the performance advantage
of our helical construction is our close ratio spread
that delivers between 10% and 13% reduction, depending
on the application. The results are twofold: a substational
torque increase that results from moving the ratios
closer together, and quicker acceleration through
the gears. We still end up in fifth at an optimum
1:1 direct ratio, but we get there much quicker,
thanks to shorter times through the gears. |
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| In practical terms, for the first
time it's possible to stay on the cam throughtout
the acceleration process, by eliminating the big
drops between gears and taking advantage of all
the power you've got at hand. And because power
equals torque times time, it's like getting free
horsepower from the torque you've already got. Result?
Instead of getting in line for the endless chase
for more power at the expense of reliability, you
maximize the power you've got by getting it to the
ground faster, more reliably, and less expensively. |
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What About 4th to 5th?
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| Let's get this off the table right up front.
Because TwistGear, WideGear, and SportGear all move
the first four gears closer together to achieve
a true close ratio spread, the lag between fourth
and fifth gets a little wider. Not a lot, just a
little. If that's the case, and you're still in
direct drive when you hit fifth, what's the big
deal? Consider what you're trying to accomplish
when you're hitting the on ramp at rush hour; you
want to get to cruising speed as quickly as possible.
With the spread in conventional five or six speeds,
that could take a while.
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| Johnson Engineering's close ratio
shifting is much quicker through the first four
(or five, if you're comparing a six-speed) gears
than the other guy, which means you're already in
fifth while the other guy's rowing around still
trying to get there. And since you're both heading
for that optimum one to one in top gear, you win,
usually by a bunch. It's just that simple. |
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| Our main caution is that whatever
you're riding, whether it's a Big Twin, Sporty,
Buell, or American other, care must be exercised
in not overgearing the bike. If you've got a big
incher or are generating a lot of horsepower, you'll
just sit there and spin the tire instead of moving
forward. The fix is easy, though, because you've
now got a lot a totally tunable transmission, using
either primary ratios, final ratios, or a combination
of the two. Remember, the goal is torque applied
to the pavement in as quick a fashion as possible,
and if you're spinning, you're not moving. |
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| One other item while we're in full
disclosure mode. If you use a speedometer, recalibration
is a must. Most shops have or can easily order a
calibration tool, which is required whenever the
drive ratio is affected, either through gearing,
pulley or sprocket changes, or taller or shorter
tires. |
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Faster Shifting And Lower RPMs |
| Quicker shifts result in both faster acceleration
and longer transmission life. Smoother shifting
and positive engagement are the results of our designed
in clearance on the back cut main drive gear dogs.
A side benefit is that porting the dogs is no longer
necessary for high speed shifts. |
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| Replacing a stock ratio 32/17 (early)
or 41/22 fifth gear set with TwistGear’s 41/20
ratio helical gears results in 10% shorter 1st through
4th ratios. Experience superb highway performance
without the fatigue of shifting, the expense of
a new transmission, or the racket of all those six
speeds clattering around under your seat. |
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| Want lower engine speed and better gas mileage?
A simple pulley or sprocket change lowers cruising
RPM’s while leaving original acceleration
characteristics unaffected. For even easier drivetrain
ratio options, swap the engine sprocket. Translated,
this simply means that if you install any of our
close ratio products to take advantage of a taller
(either primary or final) ratio, you'll turn lower
rpms in fifth and still keep near stock shift spacing
in first through fourth. |
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Direct? Or Overdrive? |
| An overdriven transmission compensates
for low torque output by adding gears and multiplying
the gear reduction in sixth, robbing horsepower
in the process. Though used primarily in trucks
when torque was inadequate for industry higher speed
highways and inner city deliveries, today, thanks
to advances in both engine output and gear design,
overdrives are an anachronism no longer needed in
most instances. |
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| With today’s impressive v-twin
power curves, overdrives, in addition to added expense
and complexity, simply aren’t necessary for
either comfort or performance. Compared to direct
(1:1) drives, which generate far less noise and
vibration, overdrives are generally less efficient,
less effective, less reliable, and in fact rob measurable
horsepower. |
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| Here's why. Overdrive requires two
sets of gears and bearing supports that run under
constant load. In direct (1:1) drive, the input
and output shafts are coupled together, unloading
the gears and bearings within the transmission.
This is a critical consideration, because today’s
riding often involves long distances ridden at steady
speeds. |
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| Our TwistGear, WideGear, and SportGear
helical fifth gear replacements work by increasing
the transmission reduction ratio by 10% or more,
resulting in more reduction ratio than most six-speed
overdrive gearboxes, and without the distraction
of constant shifting or the noise they generate. |
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How Gears Are Designed |
| The spur, or straight cut, gear design
methods used today were developed in the early 1900’s
as a necessary refinement of the transition to the
Industrial from the Agricultural Age. Tooth proportions
were standardized to produce designs that worked
well enough for the early machine technology of
the day, most of which was driven by steam engines
hooked to leather belts that in turn spun thrashing
machines or fabric looms. Today, those outdated
methods for cutting specific gear families still
depend on manual calculations taken from handbooks
written at the turn of the 20th century. |
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| The popularity and use of spur cut
v-twin transmission gears is simply an economic
manufacturing decision. They’re cheaper to
produce, but have no performance advantage, while
inherent design inequities absolutely prevent optimization
in today’s transmissions. Running spur gears
is about as effective as flogging a top fueler with
a buggy whip. |
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Determining Gear Characteristics |
| A gear’s tooth shape determines
its characteristics. High contact ratio gears generally
have taller teeth and are quieter, while short teeth
tend to be stronger and nosier. Other clues include
thin tips on one or both gears, the slope of the
flank, and height. |
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| Most motorcycle gears still incorporate
those antique designs, recognizable by relatively
thick tooth tips (i.e. stubby or chunky teeth) with
a tooth flank slope that’s usually around
20 degrees. The main advantage of this obsolete
technology is the interchangeability of parts. However,
these gears, and their associated cutting tools,
cannot be optimized for either strength or noise. |
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| Johnson Engineering’s computer
designed cutting tools produce gears that operate
much more quietly, and with far greater strength
and reliability, by constantly iterating through
thousands of complex “what-if” situations
to deliver the optimum design and deliver the maximum
combination of characteristics each and every time
a gear is ground. |
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| Because helical gear teeth are twisted
rather than straight, they allow more simultaneous
tooth contact which increases the contact ratio
and reduces the noise level. In fact, federal noise
requirements are a big reason the automotive industry
has turned entirely to helical technology. Not only
do helical gears mesh much more smoothly than spur
gears, they also possess superior tooth strength,
due to a high pressure angle and increased gear
width. |
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Optimized Computer Design: Quiet and
Strong |
| Our helical gear delivers a face
contact ratio of 1.1, and at least one tooth is
in contact at all times. The involute gear profile
provides a contact ratio of 1.4, resulting in at
least one tooth in contact at all times. Together,
there are at least two full teeth in constant contact. |
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| Gear performance is also influenced
by it’s operating pressure angle. The higher
the angle, the less stress, and the stronger the
gear. Higher pressure angles also reduce the contact
ratio, which tends to produce more noise. |
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| TwistGear’s computer enabled
technology optimizes these two factors - contact
ratio and pressure angle - to produce higher strength
and a quieter, more efficient transmission. |
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Thinking Wide? Think WideGear! |
| The demand for increasingly wider
rear tires has manufacturers turning out more and
more monster cross sections. WideGear was developed
to provide clearance for tires up to 230mm (1.18"),
using a coordinated system based upon proven TwistGear
performance technology. |
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| The usual routine of obtaining additional
clearance is to move the transmission to the left
and space out the primary at the engine with a jackshaft
and spacer, usually referred to as offsetting the
tranny; offset the rear tire, using various spacers
to achieve the clearance; a combination of both;
or offset the entire engine and drivetrain, the
least elegant but most expedient solution. |
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| Keeping in mind the fact that a Big
Twins are seldom admired for their handling characteristics,
these approaches are short term fixes usually resulting
in a bike that is unbalanced and generates various
degrees of handling difficulty, vibration, and rider
discomfort. In some situations, the bizarre handling
that results can strike onlookers as humorous and
the rider as terrifying at worst and/or embarassing
at best. |
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WideGear Drivetrain Technology |
| WideGear
solves tire clearance problems by extending only
the primary and it’s associated components.
This leaves handling generally unaffected, and the
engine and transmission remain aligned as they were
designed, with the weight centered in the frame. |
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| Extending the primary drive can introduce
increased engine sprocket shaft vibration, though
it’s usually not noticeable in low revving
stock engines. The WideGear crankshaft sprocket
nut is designed with a bearing race in the outboard
end, and when used with an outer primary capable
of accepting the bearing, flex is eliminated. |
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| We designed a 30mm longer main shaft
using our TwistGear close ratio performance package.
This means the transmission remains an integral
unit of like components for strength and reliability.
WideGear’s lightweight, one-piece heat-treated
cast aluminum spacer (STs only) easily compensates
for the additional stress generated by a wide tire
conversion while greatly reducing the potential
for failure. |
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| WideGear is a robust answer to a
complex problem that uses intelligent engineering
design principles to deliver a performance based
solution to the challenges of contemporary motorcycle
styling. |
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Sport Bike Blastoff |
| We haven't neglected Sportster or
Buell riders. If you want to steamroll the competition
and enjoy a more comfortable ride while you're doing
it, then take a look at the performance you’ll
enjoy after installing SportGear. |
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| SportGear’s the quickest way
to unleash the full potential of your Buell or Sportster,
without sacrificing around town performance. Incorporating
all the proven technology of TwistGear in an optimized
sport performance package, SportGear features our
exclusive four-point caged ball bearings and triple
lip oil seal improvements for long life and reliable
operation. |
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| In a nutshell, stock 883s benefit
most from SportGear and stock final drive ratios,
resulting in a much quicker close ratio gearbox.
This setup gives them the around town boost needed
for road performance from a small engine. |
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| Buells and 1200 XLs benefit from
SportGear coupled with the recommended final drive
pulley swap. This results in a lower overall ratio
that drops cruising rpms or extends your top end
before hitting the rev limiter, depending on your
riding style. |
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Better Buell Cruising |
| Running a 55/29 sprocket setup (recommended
for all around performance), SportGear delivers
13.6% more overall gear reduction than stock. Fifth
gear revs are reduced 16.1%, yet only 4.6% in 1st
- 4th gears. The result is a taller overall gear
ratio that still retains near stock acceleration
through the gears. It’s the same effect as
an overdrive, yet with all the benefits of a direct
drive and at a fraction of the cost. |
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| The recommended 55/29 sprocket setup
drops engine speed about 470 rpm at 60 mph in fifth
gear, while keeping the gear reduction in 1st-4th
gears very close to original. This provides four
narrow ratio speeds for street performance, with
a highway cruising gear that delivers about 30 mph
more top end before hitting the rev limiter. |
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No More Messy Main Gear Oil Leaks |
| Hidden improvements include a lengthened
main gear with an integral snap ring that keeps
the oil seal from cocking and falling out of the
bore (a common problem with OEM and other aftermarket
parts), and which also prevents the shell bearing
from walking. And to make sure the countershaft
stays put, we've added a longer Torx retainer screw
with 30% more thread diameter. Torque it down and
worry no more about the countershaft banging around
inside the cases because of thread stretch. |
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The Four-Point Bearing Solution |
| While spur gears exert only radial
force on the support bearings, helical gears generate
both radial and axial force. If a deep groove ball
bearing is next to a gear, the proportion of radial
force to axial force is normally large enough to
keep the balls contacting the groove. This is usually
the case for the output (main) gear, but not always
the situation for the countershaft trapdoor bearing. |
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| We need to point out that the axial
force mentioned is minimal. While hearsay on the
topic mentions loss of horsepower, the fact is horsepower
loss due to side loading is virtually non-existent,
and in fact can't be seen on a dyno run. On the
other hand, horsepower loss in six-speeds is significant,
and easily measured, often in the range of 3-5 horsepower. |
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| Under the loading condition encountered
when the driving gears are on the opposite side
of the trapdoor, the radial to axial force proportion
is small and the balls can contact the shoulder
edge, damaging both the balls and the race with
predictable results. Our four-point
ball bearing feature much larger shoulders,
preventing any combination of radial or axial ball
edge loading. |
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| Under heavy load, bearing races can move on the
shaft no matter how much press fit is allowed for
the bearing. The same is true for shell bearing
movement in the bore. Performance and reliability
are both enhanced by our use of retainer rings,
washers, and caged roller bearings, in conjunction
with our four-point ball
bearing. |
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| With a radial strength double that of deep groove
designs and a corresponding increase in axial capacity,
our four-point ball bearing
are used with both TwistGear and WideGear and required
when used with modified engines. |
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| Do you need to be running helical gears to benefit
from our four-point bearings? Absolutely not! In
fact, replacing the existing deep groove bearings
with our shouldered design results in a much more
robust assembly, capable of withstanding significantly
more horsepower and torque loads than OEM. |
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Stop Main Gear Bearing Failure |
| High and moderate horsepower applications
generate lateral movement of the inner drawn cup
bearing, which will eventually work its way out
of the main drive gear. TwistGear replaces the inexpensive
drawn cup bearing with a high
performance caged roller capable of much
higher load and torque than OE or other aftermarket
bearings. |
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| Snap rings over washers on both sides
eliminate lateral movement in either direction.
In addition, our bearing features 3.5 mm diameter
heavy duty rollers, three times the capacity of
standard 2.5 mm diameter needles. |
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One Piece Countershaft |
| Conventional fifth gear countershafts
and gears are two-piece designs, resulting in a
thin cross-section between the splines and teeth
roots and a high stress concentration factor of
1.7. TwistGear’s one-piece gear and countershaft
has an ideal stress concentration factor of 1.0,
resulting in a significantly stronger piece with
superior fatigue resistance. |
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| Both TwistGear and WideGear fifth
gear replacements are fully assembled for easy installation
in 1991-up Evolution® and Twin Cam 88 5-speeds,
usually without removing the transmission or modifying
the case. Earlier five speeds using tapered mainshafts
are easily adapated to a splined mainshaft clutch
and main gear design. |
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Installing The TwistGear
Helical System |
| For general reference, installing
TwistGear is comparable to a fifth gear R&R.
It'll take longer on some bikes, not as long on
others, depending on the difficulty encountered
on getting to and reinstalling the transmission
sprocket and trapdoor. Items replaced are trapdoor
bearings, main gear, fifth gear and countershaft.
Service items replaced are the transmission oil
seal and spacer. |
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What About Ratios? |
| Depending on your bike, engine, and
style of riding, you may want to swap final and/or
primary drive ratios in favor of taller gearing
that will lower high gear rpms or prevent an overgeared
situation. This is accomplished with a larger tooth
count on the transmission or engine sprocket or
pulley, and a lower tooth count on the rear wheel
or clutch sprocket or pulley. |
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| Chain final drives are the easiest
to work with, as they don't require dropping the
swing arm to service the belt. Sprockets and chain
are also more economical than pulleys and belts.
Finally, a chain final drive is highly recommended
for big inch engines that are capable of generating
the kind of torque that can snap a belt where you
least want to be stranded. |
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| We hope this gives you an idea of
the enormous possibilities for modifying your riding
when you install TwistGear. Please feel free to
e-mail any questions you may have concerning specifics,
and refer to our other single subject pages for
more specifics concerning our family of helical
system performance products, and check out our new
ratios page for
technical background on why and what our close ratio
system means. |
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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.
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