Ducati Gearing Changes
There's been a number of questions posted recently about changing the final drive gear ratio, chain and sprocket selection, and replacing these components with lightweight aftermarket parts. This discussion is geared towards superbikes, but has general application to all late-model Ducatis. It's probably more than you wanted to know now, but it may come in handy at some point.
Overall Gearing
The relationship between engine speed and road speed is a combination of the transmission gear that you select, the primary gear ratio, and the final drive gear ratio.
Transmission Gear Ratios
Six individual gears are provided to give you control over the overall gearing. Which gear you select depends on your desired engine speed, road speed and acceleration requirements.
Primary Drive Gear Ratio
The primary drive gear ratio is selected by Ducati based on the the engine’s torque and power characteristics. It establishes the number of revolutions that the engine makes for every revolution of the output shaft that drives the front sprocket of the final drive.
Final Drive Gear Ratio
The final drive gear ratio is determined by the relative sizes of the front and rear sprockets. In short, the higher the gear ratio the lower the engine speed in any gear, for a given bike speed. The lower the ratio, the higher the engine speed.
You can lower the ratio by replacing the front sprocket with a smaller diameter gear having a lower number of teeth and/or replace the rear sprocket with a larger diameter one having a larger number of teeth.
Any final drive ratio represents a trade-off between acceleration and gear range: the lower the ratio, the quicker the acceleration and the narrower the range of speed for any one gear. Consequently, a lower final-drive ratio means that while the bike scats aggressively in any gear, it requires a more shifting because the gear range is so narrow. Add to that a closely-spaced set of transmission gears and you have a bike that requires more fiddling with gear selection to stay on the torque curve.
You can expect to need different gearing for different tracks. The problem is that people here will suggest sprocket sizes to you that can only be used as a starting point. Gear selection is very dependent upon your style of riding.
In general, you would like to have a final drive gearing that allows you to hit the peak horsepower rpm at least one place on a given track. Otherwise, you’re not using all the gears in your transmission and aren’t taking advantage of the closer-spacing between the higher gears. (Daytona is an obvious exception because gearing for the high speed oval section will result in overgearing for the infield sections. This also illustrates the need to select a compromise gearing that doesn’t permit the maximum top speed but gives better drive out of the corners.)
So, you also need to find a final drive gear ratio that will minimize your number of gear changes and still place you at engine speeds that give you the best drive out of the corners. You need to build power quickly, sometimes at the slight expense of outright top speed in the straights. The fastest lap times are not so much controlled by top speed as they are by getting from one corner to the next as quickly as possible.
Lower gearing usually means more gear changes that lower your lap times. Sometimes you just can’t shift mid-corner, so you go in slower in a lower gear which allows you to come out harder. It is always a trade-off between gear selection and riding technique.
Sprockets
Ducati decides what front and rear sprocket sizes to install on each model depending upon a number of objectives and constraints. The standard gear ratio is a compromise that considers fuel economy, performance, and the need to meet environmental laws that limit exhaust gas emissions.
Notice from the table that 996's are geared unusually high, probably the highest of any Ducati model. The high gearing is the result of using the unique combination of the standard gearbox plus the 1:84 primary gear ratio. So sprockets that work well for 916's and 998's are still too high for 996's.
Tales of the Front Sprocket
One of the easiest performance changes that a new owner can make is to lower the final drive gear ratio by changing the sprockets. The stock gearing is selected to enable Ducati to reduce exhaust emmisions, but is simply too high for most road use.
Ducati bike models have a wide range of torque output and the size of the chain and Ducati’s selection of sprockets reflect this range of outputs. All Ducati current models, except the 748 and 749, come with 15-tooth front sprockets. The more powerfull 749R is 15T.
The 748 and 749 series (with the exception of the 749R) all have torque outputs below the 78Nm of the original 916, so they are supplied with 14-tooth front sprockets. The 748’s got 520 chains.
The 749’s get 525 chains, probably more for parts standardization than for strength. I remember from the product introduction that that was one of the major design objectives of the new models.
Starting with the higher torque 916-series (and 749R), and continuing with the 996, 998 and 999, the factory moved to a 15-tooth front sprocket and a 525 chain. Why? Because more torque means more chain tension and a 15-tooth front sprocket lowers the tension in the chain by seven percent. A 525 chain has a tensile strength that’s ten percent higher. So you get an overall 17 percent stronger setup.
Still-higher torque SP, SPS, R and Corsa models output over 100Nm so how do they get away with 14-tooth sprckets and light-weight 520 chains? That’s easy. Once you get over a certain torque level (for a given weight bike) the bike will wheelie before the chain tension exceeds it’s strength limits. At least for awhile, chains on these bikes don’t usually see 15,000 miles of service.
So, what does this tell us about changing our final drive components. Four things.
First, as a general rule, it’s better to increase the rear sprocket size to avoid the higher chain tension resulting from a smaller front sprocket. Changing from a 14-tooth front to a 13-tooth front, for example, increases chain tension even more (9 percent) than a shift from a shift from a 15 to a 14-tooth.
Second, the heavier the bike, the higher the chain tension needed to make it wheelie and the higher the maximum chain tension it will experience. So, a 680 pound 916-plus-rider will generate a higher chain tension than (say) a lightweight Corsa-plus-jockey or even a Suzuki GS-X. When a chain under tension elongates one percent, it needs replacement. Frequently for a Corsa bike.
Three, combining a change to a smaller front sprocket with a change from a 525 to a 520 chain on a higher-torque model Ducati will significantly weaken the final drive load capacity. Reports of chain failures are common enough, so it may not be wise to ignore this point for the sake of saving 275 grams of chain weight.
Four, there’s a practical limit on the size of the front sprocket. You end up carrying higher loads with fewer teeth. No manufacturer puts a sprocket smaller than 14-teeth on a bike with a torque output of a Ducati.
Note: Ducati uses a eccentric to adjust the chain tension on some models so remember to measure your rear ride height BEFORE you change your rear sprocket so you can later raise your ride height back to where it was after the change.
Linear Mode
