May 13, 2003
our PDF guide from our online library)
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.
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.
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.
state-of-the-art premium gearing systems, components, and service
for five-speed Big Twins and sport bikes focus on the relationship
between gears, clutch, bearings, and seals, especially during the
stages of acceleration.
trucks, for example, usually operate at a fairly constant 50–100%
of available engine torque. Motorcycles, on the other hand, typically
run under relatively light loads — except during hard acceleration,
or under race conditions. That’s when the shock factor can be
300-500% of steady state torque, and why we use transient load factors
in our transmission design.
Heart of the Helical System
heart of the TwistGear performance system is a unique, one-piece
countershaft that combines a shorter (lower tooth count) helical
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
With our helical design, there are always two teeth in
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
effort that means 20 percent more time and effort shifting instead
of riding. Performance? That's what a close ratio gear box is all
True Close Ratio Gearbox
the performance advantage of our helical construction is our close
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
thanks to shorter times through the gears.
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
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.
About 4th to 5th?
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
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
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
still trying to get there.
you're both heading for that optimum one to one in top gear, you win,
usually by a bunch. It's just that simple.
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
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.
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.
Shifting And Lower RPMs
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.
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
those six speeds
clattering around under your seat.
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.
overdriven transmission compensates for low torque output by adding
gears and multiplying the gear reduction in sixth, robbing horsepower
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
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.
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.
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
of constant shifting or the noise they generate.
Gears Are Designed
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
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
taken from handbooks written at the turn of the 20th century.
popularity and use of spur cut v-twin transmission gears is simply
economic manufacturing decision. They’re cheaper to produce,
but have no performance advantage, while inherent design inequities
optimization in today’s transmissions. Running spur gears is
about as effective as flogging a top fueler with a buggy whip.
gear’s tooth shape determines its characteristics. High contact
ratio gears generally have taller teeth and are quieter, while
tend to be stronger and nosier. Other clues include thin tips on one
or both gears, the slope of the flank, and height.
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.
Engineering’s computer designed cutting tools produce gears
that operate much more quietly, and with far greater strength and
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.
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.
Computer Design: Quiet and Strong
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.
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
computer enabled technology optimizes these two factors - contact ratio
and pressure angle - to produce higher strength and a quieter, more
Wide? Think WideGear!
demand for increasingly wider rear tires has manufacturers turning
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.
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.
in mind the fact that a Big Twins are seldom admired for their
handling characteristics, these approaches are short term fixes usually
in a bike that
is unbalanced and generates various degrees of handling difficulty,
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.
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.
the primary drive can introduce increased engine sprocket shaft
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
flex is eliminated.
designed a 30mm longer main shaft using our TwistGear
close ratio performance package. This means the transmission remains
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.
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.
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
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.
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
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.
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.
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.
More Messy Main Gear Oil Leaks
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.
Four-Point Bearing Solution
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.
need to point out that the axial force mentioned is minimal. While
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
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.
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 bearings
feature much larger shoulders, preventing any combination of radial
or axial ball edge loading.
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 bearings.
a radial strength double that of deep groove designs and a corresponding
increase in axial capacity, our
four-point ball bearings are used with both TwistGear and WideGear
and required when used with modified engines.
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
Main Gear Bearing Failure
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
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.
fifth gear countershafts and gears are two-piece designs, resulting
in a thin cross-section between the splines and teeth roots and
stress concentration factor of 1.7. TwistGear’s one-piece gear
and countershaft has an ideal stress concentration factor of 1.0,
in a significantly stronger piece with superior fatigue resistance.
TwistGear and WideGear fifth gear replacements are fully assembled
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.
The TwistGear Helical System
general reference, installing TwistGear is comparable to a fifth
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.
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
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.
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
of helical system performance products, and check out our new ratios
page for technical background on why and what our close ratio system
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.