ZF 9-speed automatic transmission for transverse engines
The 9HP transmission is indeed a breakthrough product in many ways. One of the most important, perhaps, is that it’s extremely compact; it can fit in essentially the same packaging space as ZF’s six-speed automatic transmission that preceded it, thanks to a number of fresh engineering approaches, such as the incorporation of two special dog clutches.
That new design may have played a part in making the transmission more challenging to tune. So is a design that aims not for gears that are very closely spaced but for a very wide ratio spread—enabling an incredible 9.8 ratio spread for this transmission; that’s about triple that of old three- or four-speed automatics and about 50 percent more than most six-speed automatics or continuously variable automatic transmissions (CVTs). That means not only quicker takeoffs from a lower first gear, but better mileage on the highway, potentially, from a taller top gear.
Frieder Mohr, an application engineer who’s worked with software and the internal controls for the nine-speed, said that the transmission goes through two levels of ‘adaptation’ to fine-tune its shift quality. One of those is as the transmission properly adjusts its shifts to internal tolerances, both when it’s new and as it wears. The second is adaptation to driving style, which takes just a few minutes and a few shifts through each gear.
Tips that the dealership probably won’t tell you
Mohr gave the following useful tips on how the nine-speed’s software works:
- If you’re the gentler driver in the household, a few lumpy shifts are to be expected each time you get in after the more aggressive one. If one driver drives rapidly most or all the time, then it’s possible that the low-speed, low-load shifts won’t work so well, because the transmission will still be assuming the style of the more aggressive driver. It is possible that each time the gentler driver gets into the vehicle, the first few shifts may be firmer, but the software softens shift quality after just a few minutes of driving.
- It’s actually important to mix driving styles when the vehicle is new. This is so that the transmission can ‘learn’ each shift over a range of conditions. The transmission could take longer to adapt if driven in a steady continuous way, but shifts should even then become smoother and more predictable over the long run.
That said, ZF isn’t willing to concede that all that we observe as odd drivability traits are related to their choices. When I asked about rough downshifts during a common American maneuver—a gentle rolling ‘stop’ followed by a right turn and moderate acceleration (one that’s a non-event in the Discovery Sport), Mohr deflected to FCA’s choices. “It sounds like what you’re describing there is related not to adaptation but to shift strategy, which is determined by our customer,” he said.
Land Rover Range Rover Evoque fitted with ZF 9HP nine-speed automatic transmission
Another engineer said that the supplier has found that Europeans expect partial-throttle downshifts on the highway in some cases—the nine-speed's apparent restlessness between eighth and ninth gear, in the four-cylinder Cherokee, for example—where Americans might judge it to be a flaw.
And that’s really the third component to this—the shift strategy itself. It’s the one aspect that’s not entirely under the ZF’s control, yet the supplier has likely learned a lesson that it should be more proactive.
You can be sure that other automakers are taking note. It might be time for automakers to wrap driving style into the keyfob profiles that are kept for a number of vehicles; that could be an easy remedy for this complexity. With more gears comes an even heavier burden on software and controls, and real-world tweaks.
Yet that hasn’t stopped several automakers—Ford and GM, for example—from working on ten-speeds. The more, the merrier.