 Techline, June 2007, part 3
Sprocket/Pulley Changes
The rudimentary external gearing rules to improve initial acceleration or, conversely, lower highway rpm listed earlier are not practical in every case, nor are changes made to every sprocket in everyday usage. Some sprockets are fixed on different years, and space limitations affect the amount of change possible on most years, although these are not hindrances to achieving gear ratio changes desired in recent years. Only the engine, transmission, and rear sprocket/pulleys can be changed on modern Harleys. The clutch sprocket remains fixed for most model years. They can be changed by one tooth from 1990-2006 (except the '06 Dyna) by swapping the clutch package and some starter parts, but this is hardly practical.
Primaries earlier than 2006 have no problem accommodating wild, off-the-line, 21- or 22-tooth sprockets and the more lethargic 25-tooth stock engine sprockets. Rear belt drive models can be brought either up or down a few teeth on the transmission pulley, while rear pulleys are common in the range of 61-70 teeth with correspondingly different-length H-D and aftermarket belts to accommodate them. However, there is no hindrance whatsoever created by making whatever changes we may have in mind to a particular size engine, type of riding, and rider and bike weight. Yesteryear models, before the advent of the rear disc brake in 1973, were restricted in ratio options by both the lack of aftermarket parts for the compensator engine sprockets and by the fixed clutch and rear 51-tooth chain sprockets that are riveted to the rear brake drum. However, 21- and 22-tooth engine sprockets were readily available, from the police FL models, to the 23-tooth ones available from later FLH Big Twins for civilians.
Different-length primary chains were also more difficult to get without modifying the endless chain by splitting it, cutting it, or adding to its length and rejoining it using master links. BAKER has resolved these issues for modern riders with its sprocket and primary chain kit combinations. Most changes made during the old days were to the transmission sprocket, which came in every size from 19 to 25 teeth, with the 24-tooth aftermarket sprocket being the most popular. We liked the 25-tooth sprockets just as much, but interference with the starter housing after 1965 became an issue with a loose or unevenly stretched rear chain. They were misadjusted due to inattention and usually had loose spots because of the uneven stretching characteristics of the drive chain. These restrictions, combined with a four-speed transmission with a 1:1-ratio fourth gear and the anemic Shovel and predecessor engines, limited our choices. I use this as a reference to the liberal changes allowed today with five- and six-speeds, and the wide array of options available from both Harley and the aftermarket for the changeable sprocket/pulleys, except for the clutch sprocket.
The Ring Gear
The clutch sprocket located at the rear of the clutch shell must not be confused with the ring gear located on the outer perimeter of the clutch shell. The ring gear is part of the starter mechanism. When the starter button on the handlebars is pushed in, the starter solenoid thrusts out the smaller jackshaft gear, so it engages the ring gear. This, of course, activates the starter motor, which turns the ring gear. Because both the ring gear and the clutch hub sprocket are a fixed part of the clutch hub, the starter motor turns the whole clutch assembly. The primary chain connects the clutch sprocket to the engine sprocket, so when they turn the crank also spins, moving the pistons up and down in their respective cylinders, drawing in fuel and air. When the spark plugs fire and the mixture is ignited, the rider’s thumb is removed from the button, the starter gear retracts, and the starter motor is deactivated.
Since the primary engine sprocket is always smaller than the clutch sprocket, we accomplish a gear reduction whereby the clutch turns slower than the crank and its attached engine sprocket. An increase in the turning force (torque) is also achieved. The big change, which bears repeating, with the new primary ratios combined with the H-D six-speed is the size of the engine sprocket at 34 teeth and the clutch sprocket at 46 teeth. All predecessor Big Twins from 1994-2006 have a 25-tooth engine sprocket with a 36-tooth clutch sprocket. Earlier Big Twins utilized a 37-tooth clutch sprocket with a 24-tooth engine sprocket. So, the five-speed models have drastically different sprocket sizes and conservative primary sprockets, creating much different gear ratios than on the six-speed Big Twins. We now have a whole new ballgame.
Secondary Drive Ratios
Secondary drive ratios are determined by the relative sizes of the transmission pulley/sprocket and the rear wheel belt pulley or chain sprocket. The formula here is:
Rear Wheel Pulley Teeth/Transmission Pulley Teeth x High-Gear Ratio = Secondary Gear Ratio
High-gear ratio refers to the final gear ratio inside the transmission, which, for all H-D stock four-, five-, and six-speed transmissions, is 1:1. Therefore, the FLH is 2.22 (51/23 x 1.001 = 2.22), which means that the transmission sprocket and transmission mainshaft turn 2.22 times for one rotation of the rear wheel sprocket. To state the obvious, the rear wheel turns one time for each revolution of the wheel sprocket. The secondary gear ratio for all rubber-mount five-speeds from 1985-2006, except the '06 Dyna, is the same: 2.19:1, since they all use a 70-tooth rear wheel pulley and a 32-tooth front transmission pulley. Therefore, on these models, the transmission pulley and transmission mainshaft turn 2.19 times for one rotation of the rear wheel sprocket. The secondary gear ratio for the 2006 Dyna still uses a 32-tooth transmission pulley, but use a smaller 68-tooth rear wheel pulley (68/32 x 1.001 = 2.19). This results in the transmission pulley and transmission mainshaft turning 2.19 times for one rotation of the rear wheel sprocket. Finally, the secondary gear ratio for all 2007 Big Twins is calculated by using the 32-tooth transmission pulley and the smaller 66-tooth rear wheel pulley (66/32 x 1.001 = 2.06), resulting in the transmission pulley and transmission mainshaft turning 2.06 times for one rotation of the rear wheel sprocket. We can see from the chart that both primary and secondary drive ratios have steadily decreased, producing smaller final drive gear ratios over the years to the lowest levels ever with the 2007 Big Twin.
Final Drive Gear Ratios
It’s time to explain the final drive gear ratios and how to calculate them. Final drive ratios, which are the ones that concern most riders, are completely influenced and determined by the primary and secondary ratios. The formula is:
Primary Gear Ratio x Secondary Gear Ratio = Final Drive Ratio
Therefore, the 1973 Ironhead XL 1000cc final drive ratio is 1.74 x 2.43 = 4.23, which means that the engine turns 4.23 times for each rotation of the rear wheel. The 19?? FLH final drive ratio is 1.61 x 2.32 = 3.74, which means that the engine turns 3.74 times. The final drive ratio for all rubber-mount five-speeds from 1985-93 is 1.54 x 2.19 = 3.37, which means the engine turns 3.37 times. The final ratio for 1994-2006 (except the '06 Dyna) is 1.44 x 2.19 = 3.15, so the engine turns 3.15 times for one rotation of the rear wheel. The ratio for the 2006 Dyna is 1.35 x 2.13 = 2.88, which means that the engine turns 2.88 times. Lastly, the final drive ratio for all 2007 Big Twins is 1.35 x 2.06 = 2.78, which means that the engine turns 2.78 times. Changing the rear pulley size easily modifies the BAKER and S&S six-speed final drive ratios.
Breaking In My Glide
Personal riding examples will demonstrate the effects that this change has on engine speeds because of rpm reduction. I have owned many Shovels over the years. The FLH models had higher gearing for adequate city riding and allowed normal highway speeds for the day, but not really high ones. My Shovels really wouldn’t allow me to attain main highway speeds during break-in periods for a new engine. I had to stick to secondary two-lane highways and suburban thoroughfares in order to keep the engine rpm at an acceptable level. Even when broken in, these bikes simply cannot keep up with the sustained highway speeds that the Evos handle so effortlessly.
Stock-geared Evos were much easier to break in than the Shovels in today’s fast-moving world. I could easily stay in highway slow lanes while putting the first all-important miles on them, gradually increasing and decreasing engine rpm to let mating parts like cylinder walls and piston rings wear into each other. I have ridden many Evos, after they were broken in, on cross-country trips at a steady 80 mph. A Shovel can only reliably do this at 60 mph. I owned a 2006 FLHXI that I took onto a major highway with less than 20 miles on it. My friends promised to stay at 60 mph or less so I could gradually break in the engine. Soon, they forgot their promise, and we were going 65 mph and then 70 mph. It was a cool day, and the engine suffered no ill effects. The sustained speeds of 90-100 mph did not burden the bike at all once it was broken in. My new 2007 FLHXI and I suffered the same fate but in subzero temperatures. I knew that the cold weather would allow me leeway at increased speeds. Once more, my friends began to leave me behind. I had to stay at close to 80 mph to catch up, but the rpm on the tachometer assured me that I was within safe limits. It’s not totally broken in yet, but I can tell that this will be the fastest top end Big Twin ever produced.
Soon after this ride, I took my engine apart to hop it up. I was anxious to look at the cylinder walls and piston thrust surfaces to see if I was correct when I decided it would be safe to go so fast while breaking in the bike. There was no damage whatsoever, and the FLHXI was breaking in just as Father Harley would like. This is quite remarkable, and it's all due to the new gear ratios being married to the Cruise Drive six-speed transmission and engine in a match made in heaven.
The examples above show how much the primary and secondary ratios, with the resultant final drive gear ratios, have improved by being lowered over time. The modern Harley can now hold its own on any highway, including the European ones, except the autobahn fast lanes.
Conclusion
Performance is all about increasing the engine’s horsepower and torque. The third component, drive ratios, delivers this power in a way specific to the rider's needs and wants. Those who forget about or ignore primary, secondary, internal, and final drive ratios do so at their own peril. There is no quicker way to lose a race or have an uncomfortable touring motorcycle. |
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