The Close Ratio Effect
 

 

 

 

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

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April 3, 2003

Final Drive (Pulley/Sprocket) Combinations

We receive numerous requests for gear ratio combinations, especially since the introduction of a new Big Twin 21T engine sprocket (see the table at the bottom of the page for ratios) that's seen as a stopgap for restoring low end acceleration. (Use of this sprocket only increases RPM at any given speed in 5th gear.) With that addition, the number of variables becomes even more daunting for the rider who wants to individualize the riding experience.

The number one question we get concerns pulley combos and their effect on engine revs, so we gathered the data needed to publish the results in a format that's easy to understand. Check out these graphs for clues to the huge performance boost you can expect with TwistGear or WideGearin your Big Twin. The same type of performance applies to SportGear when used in either a Buell, Sportster, or Buell Blast. For a larger view that's easier to read, download our TwistGear Charts PDF version. You can also click on the individual charts shown for a larger screen view.


PERFORMANCE
COMFORT
pulley chart

Graphs are based on an FL 25t engine and 36t clutch primary ratio. Click on the graphs to see a larger image, or on TwistGear Charts (above) for a PDF version.

speed rpm

This graph plots four different final drive pulley ratios for an understanding of how performance curves are enhanced after TwistGear or WideGear have been installed. A fifth curve is shown for baseline stock transmission and final drive performance.

OEM is the red line. Fifth gear (1:1) is shown at a steady 3000 cruising rpm, with the mph shown at left. Shift points (1st - 4th) are made at 4500 rpm.

Gears 1-4 are read as MPH on the left, RPM on the right. Fifth gear is constant 3000 RPM and read as MPH on the left.

Here are the same pulley combos in fifth gear only plotted against a steady MPH or RPM. The graph uses only the final (1:1) ratio, also referred to as direct drive.

Curve shows MPH at 3000 RPM and RPM at 75 MPH. Select either the cruising RPM (left value) or MPH (right value) desired and install the pulley combination shown along bottom value.

Refer to graph at left for 1st-4th gear TwistGear or WideGear acceleration comparison to stock transmission at 4500 RPM shift points and final 3000 RPM cruise speed.

Figuring Final Drive Ratios

The table of sprocket (pulley) comparisons below demonstrates speed in gears at 4500 rpm and corresponds to the lefthand chart above. The first two columns compare TwistGear to OEM. Both use the stock pulley configuration of a 32t engine pulley and a 70t wheel pulley. TwistGear's close ratio allows the engine to wind up more quickly in gears 1-4, resulting in quicker acceleration before shifting into fifth gear direct drive.

As the pulley ratio lowers (left to right) the speed at 4500 RPM increases. What this means is that if the speed were lowered in the gears shown, so would the RPM, which usually translates into a more comfortable ride with less vibration and noise.

Table compares TG/WG
against OEM using 4500 rpm
shift points, speed in mph,
pulley ratio as shown

OEM
TG/WG
TG/WG
TG/WG
TG/WG
32/70
32/70
34/70
33/65
34/65
Engine rpm
mph
mph
mph
mph
mph
1st Gear 4500
33.7
30.7
32.6
34.0
35.1
2nd Gear 4500
49.0
44.5
47.3
49.5
51.0
3rd Gear 4500
68.8
62.6
66.5
69.5
71.6
4th Gear 4500
88.3
80.2
85.3
89.1
91.8
5th Gear 4500
108.2
108.2
115.0
120.2
123.8

Individual Gear Ratios

The table of primary, final, and transmission ratios shown below is intended to serve as a guide in determining the setup for your particular situation. Columns from left are primary, which is the relationship between the engine sprocket (smaller number) and clutch sprocket (larger number).

Next is the final drive ratio, which is the relationship between the transmission pulley (smaller number) and the rear wheel pulley (larger number). Multiplying the primary ratio by the final ratio gives you the overall ratio in fifth gear (direct). The higher the number, the more engine RPM in fifth gear.

We've recently concluded that tuning primary ratios is a much more cost effective and less labor intensive route to taller gearing and fifth gear performance than the pulley/belt swap route, although limited due to a lack of tooth count options. Of course, a chain final drive conversion makes matters a lot simpler, and also offers the potential of a lot more tuning opportunity because of the wide selection of engine and wheel sprockets.

The choice of final drive ratios in conjunction with our close ratio fifth gear system is what ultimately determines your performance profile. It's our belief that retaining the stock primary ratios and changing the final ratios according to individual riding goals yields the best results, regardless of whether the intent is improved acceleration or top end cruising comfort.

O
E
M
PRIMARY
FINAL
1st
2nd
3rd
4th
5th
25/36
32/70
10.11
6.96
4.95
3.86
3.15
24/37
32/70
10.82
7.45
5.30
4.13
3.37
21/37
32/70
12.37
8.51
6.06
4.73

3.85

T
G
25/36
32/70
11.12
7.65
5.45
4.25
3.15
24/37
32/70
11.91
8.19
5.83
4.55
3.37

21/37*NR

32/70

13.61

9.36

6.67

5.20

3.85

Are You Overgeared?

In a growing number of situations gearing up (taller, lower numerically) may be preferable as the best way to obtain both comfort and performance. With today's huge powerplants becoming more common, bikes can actually have too much gear, resulting in excessive tire spin through the lower gears at the expense of optimized acceleration and top end performance. What this means is that even if you don't list interstate touring RPMs as a main goal, your all around performance still could benefit from a taller gearing swap.

To view in a larger format, we've put a separate chart PDF in our online library. It's easy to download and even easier to read. Look for TwistGear Charts.pdf.

 

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.