I just picked up a 2014 Kawasaki ZX-10R. In Kawasaki’s information for the bike it says a “race-style cassette transmission allows simple trackside ratio changes.” Can you explain how a cassette transmission is different from a regular transmission? And why would racers want to change individual ratios?
Mays Landing, NJ
In an engine with a conventional transmission, the two transmission shafts along with their gears are installed between the two case halves as they go together. This means that to service the transmission, or to replace an individual gear, the engine has to come out of the frame and the cases split to access the shafts. For a cassette transmission, the crankcase is designed with an empty cavity in the side; the transmission shafts, shift drum, and forks are assembled separately on a carrier, and the transmission slides into the cavity as a unit (much like the mechanism that drives a cassette tape does). This design allows the transmission to be removed and replaced with the engine still in the frame so it can be serviced quickly and easily.
For racers, changing an individual gear ratio rather than overall gearing can serve a number of purposes to help with setup or to improve lap times. Ideally, the individual gears in the transmission have ratios such that the engine is in the meat of its powerband at every corner exit. Obviously, if the same gear is used for more than one corner this could be an impossible task, but being able to change individual ratios allows the rider and crew to tailor the gearbox for a particular track’s layout, optimizing engine rpm at every corner exit.
Some other examples of where changing ratios can be beneficial: On a track with a very long straight that requires tall overall gearing, a short first gear could be fit to make launching the bike from the line easier. Or perhaps the rider is happy with the gearbox and overall gearing but the bike is slightly over-revving at top speed; here, a change in sixth gear alone could fix the issue without affecting how the transmission works elsewhere on the track. A similar situation could exist between two corners, where the bike is over-revving in a particular gear before the braking marker just enough that the rider must upshift and then make a downshift right away. Making only that gear ratio taller could prevent the bike from over-revving and save the extra shifts.
And a final example: With squat and anti-squat so important in handling and chassis setup on today’s powerful bikes, teams try to avoid changing final gearing whenever possible, as this changes the anti-squat geometry and affects handling. If an internal gearbox change can be made instead, that is one less variable affected by the setup change.
How important is all this? The racing kit manual for your ZX-10R lists six optional gearsets for sixth gear; four for first; three for second, third, and fifth; and two for fourth. Put them all together and include the stock ratios and you could build the gearbox 6,720 different ways, never mind your options for final gearing. Two years ago, World Superbike rules were changed such that teams can now only use one set of alternate gears in addition to the stock ratios, and these must be homologated at the start of the year.
While cassette gearboxes do offer a number of advantages to racers, there are fewer—if any—benefits for street riders as far as making use of those alternate gears, and the transmission is not something that requires regular servicing. Additionally, an engine with a cassette gearbox is likely heavier than one without, as there are more parts and fasteners, and areas of the crankcase must be made stronger. Given the recent change in World Superbike regulations, it’s likely we’ll see fewer bikes equipped this way in the future.