How TwistGear Works

 

 

 

 

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

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April 17, 2003

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. (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.)

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.

schematic

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.