TwistGear Helical
Transmission Systems

 

 

 

 

Johnson Engineering
100 Blake Road
Denver, IA 50622
(319)984-9298
(319)984-9299 fax
info@twistgear.net

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May 13, 2003

Quick Guide to TwistGear Helical Transmission System

What Is Helical Technology?

Acceleration Load Characteristics

The Heart of the Helical System

A True Close Ratio Gearbox

What About 4th to 5th?

Faster Shifting And Lower RPMs

Direct? Or Overdrive?

How Gears Are Designed

Determining Gear Characteristics

Optimized Computer Design: Quiet and Strong

Thinking Wide? Think WideGear!

WideGear Drivetrain Technology

Sport Bike Blastoff

Better Buell Cruising

No More Oiling

The Four-Point Bearing Solution

Stop Main Gear Bearing Failure

One Piece Countershaft

Installing The TwistGear Helical System

What About Ratios?


quoteWhat Is Helical Technology?

(download our PDF guide from our online library)

catalogBeginning 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.

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.

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.

Acceleration Load Characteristics

Our state-of-the-art premium gearing systems, components, and service items for five-speed Big Twins and sport bikes focus on the relationship between gears, clutch, bearings, and seals, especially during the transient stages of acceleration.

Commercial 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.

The Heart of the Helical System

fifth gearThe 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.

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.

A True Close Ratio Gearbox

power performanceSharing 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.

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.

What About 4th to 5th?

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

TwistGear’s computer enabled technology optimizes these two factors - contact ratio and pressure angle - to produce higher strength and a quieter, more efficient transmission.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 bearings feature much larger shoulders, preventing any combination of radial or axial ball edge loading.

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 bearings.

With 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

 

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.