Ducati Gearing Sprockets and Primary Drive Ratios - Updated Feb ’05: 749R
Often people ask what the best gearing for their bike is, and then they compare it to what others are running. What they don’t realise is that there’s a couple of reasons why the comparison may not be valid. I’ll try to explain them in this article and give some gearing guidelines.
The gearing system goes like this. On the end of the crank is a gear. This drives another larger gear that is attached to the clutch basket (the outer clutch bit). This gear spins on bearings mounted on the gearbox input shaft, allowing the gear (and clutch basket) to rotate independently of the input shaft. This pair of gears is called the primary drive gears, and the number of teeth on them gives the primary drive ratio. This is the first rotational speed reduction.
The gearbox input shaft is driven by the clutch drum (the centre bit) which is splined to the input shaft often incorporating a rubber cush drive arrangement of sorts. The input shaft has gears on it that run in mesh with gears on the output shaft. Which gears actually do the transferring of power depends on which one per shaft is locked to the shaft at any given time. These are the gearbox gears, and the number of teeth on the matching pairs of gears is the gear ratio.
The drive is taken from the output shaft by the countershaft sprocket (the front sprocket’s fancy name) by the drive chain to the rear sprocket (no fancy name afaik) which is linked to the rear wheel - usually through another cush drive arrangement.
So there are 3 rotating speed altering devices between the crank and the wheel. The first is fairly hard to change unless you’re a manufacturer or own a gear cutting company, the second similar (and you have to split the engine to do it). The third is the easy one.
Primary drive ratio
The primary drive ratio is one of those things that is usually ignored because there’s no real need to pay any attention to it. But it does need to be considered when comparing gearing on different Ducati models. Since they were introduced all the big engines (900/944 2V and all 4V) ran a 2:1 primary drive ratio, where the crank gear had 31 teeth and the clutch basket gear 62. 62/31 = 2. So the crank speed is halved before the drive enters the gearbox. This was changed with the introduction of the 916SPS in ’97, the first 996cc production model. I was told that when they started racing 996cc bikes in SBK that they ran into rear sprocket problems due to the size of the rear hub limiting how small they could go. If you can’t change the external gearing and don’t want to change the gearbox ratios (we’ll get to that later) the easiest thing to do is to change the primary drive ratio. Which they did.
Although, there’s a couple of other benefits this can bring. The first is clutch capacity. Power is defined as Torque multiplied by Rotational Velocity. The actual formula for rotational velocity is a bit wacky (it’s measured in radians, of which there are 2 pi per revolution), but the implication is, for constant power transfer, if you lower the speed of transfer you increase the torque. However, in this instance, they raised the speed of the clutch by reducing the ratio from 2:1 to 1.84:1. This was done by replacing the 31/62 teeth gears with 32/59 teeth gears.
As the clutch is now spinning faster with the same power being transferred the torque at the clutch is lower. This means the clutch on a 996 sees 8% less torque than it would have with the old primary drive ratio. Given the capacity difference between a 916 and 996 is about 8% this means the torque the clutch sees is about the same on both. On the race bikes it means the torque the 996cc race clutches had to transfer would have been a bit lower than the (916) 955cc race clutches.
There is another benefit of equal or maybe more importance to the race bikes too. The ’96 model bikes were breaking primary drive gears as in teeth coming off. When you have a 2:1 ratio the same teeth work against each other every second revolution. This encourages wear to happen much faster than otherwise. When they went to the 32/59 gearing they achieved what is called a “hunting tooth pattern”. This means that the same teeth see each other much less often - in this case, every 1,888 revolutions. This is a massive reduction in wear promotion on the gear teeth. This irregular relationship is also used in the gearbox ratios, timing belt and pulley teeth combinations and in sprocket sizing and chain length combinations as well.
The other benefit of the 32/59 is that the primary drive gear on the clutch basket is a touch smaller in diameter, which helps reduce the torque the teeth see a little more.
The other race bike derived primary drive changes include the change from crank drive taper location to spline on the Testastretta bikes. Taper drive means the crank and gear have a matching taper ground onto them. When the gear is fitted on the crank and torqued up the matching tapers form a bond on each other that is, for an otherwise free joint, truly amazing. This is how a lot of flywheels are attached, and I’m sure many have been frustrated trying to remove the odd flywheel or two over the years. Removing the primary drive gears after they have been torqued just the taper joint remember involves very heavy duty pullers (usually hydraulic) and heat and a very big bang when it lets go. At a training course a few years ago the big bang was the factory tool breaking the primary drive gear was still firmly stuck.
However the taper joint didn’t handle the 955cc race bikes too well. With lightweight internals the rapid acceleration and de-acceleration rates of the engine coupled with the torque output were enough to loosen the nuts (with around 200Nm or torque) and break the taper “set”. Then the helical gear tooth design would force the crank gear sideways, machining its way through the primary drive cover, coming out just in front of the clutch. The splines fixed both problems.
So that’s primary drives in theory. Now onto the useful info the actual ratios. I sure all these are correct, but the 600 and 750 changeover points can be a little hard to pinpoint.
748, 851, 888, 900SS and M Carb models, 916 31/62, 2:1
ST2, ST4, ST4S, S4, S4R, 996, 900SS ie, 900M ie, 749, 999 32/59, 1.84:1
The 916SPS and ST2 were the only models to have the 1.84 ratio with the old 2 phase Ducati alternator.
750SS up to engine number 007223 36/71, 1.97:1
750M 31/62, 2:1
750SS/M ie 33/61, 1.85:1
600SS up to VIN 001852, 600M up to VIN 002961 32/73, 2.28:1
The next part of the gearing story is the various ratios available. There are 5 late model gearboxes 3 big engine and 2 small. The ratios are as follows
Applications (think I got them all):
The spread is the range between 1st and top. You can see that both the big engine boxes share the same first and second gears, and that 5th in the standard is 6th in the close ratio. Also the spread of the 600/750 five speed is the same as the spread of the standard box for 1st to 5th and the same spread as the 6 ratios in the close ratio box. And the small 6 speed spread is not much greater than the 5 speed’s, and certainly nothing like the standard box spread.
You can also get a feel why the Corsa box isn’t the best street box the ratio spread is only 2:1. It’ll go half as fast in 1st as it will in 6th. Cool!
It’s also the only box with a direct top ratio. All the others are overdrive (ratio numerically smaller than 1) to some extent. The 800 has 1:1 as 5th, while the standard box has 5th and 6th as overdrives.
The next table is for interest only, and gives percentage gaps between gear ratios. No real benefit to knowing, but it’s another table full of numbers and we like them don’t we.
Final Drive Ratios
Aka the sprockets. The range available (as I know it) is shown below, although many of these are application specific or available only as Ducati Performance parts or aftermarket. Styles available include the normal 6 bolt SS, M in 520 pitch chain, 6 bolt ST series in 525 pitch, 5 bolt 749/999 series and the 748/916/996/998/S4R/Multistrada/MHe ranges in both 520 and 525. See a dealer for specifics for your application it’s too confusing for me to explain.
The smaller the ratio the “taller” or “longer” or “higher” the gearing is said to be, giving less RPM for any road speed. The bigger the number the “shorter” or “lower” the gearing is said to be, with more RPM for any given road speed. How this effects top speed depends on many things RPM at which max power occurs and how much power is available can mean a bike re-geared shorter may reach a higher top speed, or that you will hit the rev limiter (or reach the red line) before it stops accelerating. Depends very much on the body type, engine type, state of tune, intended use and practicality thereof. As in, there’s no point gearing your S4R for 300km/h unless you’re going to strap yourself on.
There is a better way to explain all this though, and that is to relate it to an application. The next table shows approximate engine RPM for a road speed of 120 km/h (70 mph approx) in top gear for the various final drive, gearboxes and primary drive ratios. What I was talking about at the start, but have only now got around to showing. I must say I have crapped on in this report for much longer than I expected. The assumed tyre circumference here is 1.95m, about right for most of the 160 to 190 series tyres on the market.
I’ve shown in red my preferred gearing options for the bikes I’ve owned or ridden the most standard gearbox big engine bikes. You can see how over-geared a 996 is it has 15/36 as std fitment with the standard gearbox and 1.84 primary drive. Much like the ST4S or 1000SS with 15/38. Or the ‘91-’97 900SS that had 15/37 or 916 with 15/36 and the 2:1 primary drive. Changing the gearing at the final drive can make a huge difference to the way a bike feels in most situations, and is about the easiest performance change you can make.