Shazaam's Tech Library - Gearing

A useful technical article from guest contributor Larry Kelly of San Diego CA (aka Shazaam!).


Click the table for a bigger image

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.

The first recommendation (14/40) in the table applies to 748's.

You are correct to 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.

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.

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. The 748 and 749 series (with the exception of the 749R) all have torque outputs below 80Nm so they are supplied with 14-tooth front sprockets and 520 chains.

Starting with the higher torque 916-series (and 749R), and continuing with the 996, 998 and 999, the factory shifts 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 10 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.

I think that the above suggestions to put a 13-tooth sprocket on a 749S (because it’s cheaper) are ill-advised. A better solution is to change to a 41-tooth rear sprocket. This will require a new 96-link chain.

 

Extracted from this thread at Speedzilla

Tip for 748 -Change your 14/38 94-link chain to 14/40 using a new 96-link chain. Because of the eccentric chain tension adjuster used on superbikes, this will lower your rear ride height, so measure it first (see manual) and then after, raise it back using the rear ride height adjuster rod.

Effect on wheelbase 9XX

With a 96-link chain, a 15/39 combo will give you a longer wheelbase than a 15/40 (which BTW is stock length) by about 4mm (the 4 o’clock position), and also lower your rear ride height. You can recover the rear height by increasing the length of the ride height adjuster rod.

This is preferable to running a 94-link chain that places the adjuster at the 6:30 position because a longer wheelbase provides more high-speed stability.

Please note that Ducati-UpNorth.com cannot accept any liability for the accuracy or content of this section. Visitors who rely on this information do so at their own risk. If you are unsure it's worth contacting your local Ducati dealer who will be able to help. Do not attempt a repair or modification if you do not have the correct tools or knowledge to do so.