#9-speedAutomatic

We moved past three on the tree. We’ve long since bid farewell to four on the floor. The ZF six-speed automatic transmission that helped to make the 2001 BMW 7 Series seem so forward-thinking at the dawn of the millennium was usurped by a seven-speed unit from Mercedes-Benz a couple of years later, and then by the Lexus LS’s eight-speed automatic in 2007.

Nine-speed automatics are all over the place: in the 2017 Acura MDX I’m driving this week, in numerous Fiat Chrysler Automobiles products, and in ten General Motors models by the end of 2017. Now the most popular line of vehicles in North America, the Ford F-Series, is arriving at dealers near you with ten-speed automatic transmissions.

But when is enough enough? How many gears is too many? Are there diminishing returns as the number of gears in an automatic transmission increases?

GM says nine is enough. Okay, ten is plenty if you insist. Ah, whatever, maybe more would be wonderful.

From the get-go, the nine-speed automatic designed by Germany’s ZF in the United States and built and tuned by Fiat Chrysler Automobiles was deserving of criticism. It was criticism that FCA could not righteously label as unfair, criticism the automaker could not deny.

“We have had to do an inordinate amount of intervention on that transmission, surely beyond what any of us had forecast,” FCA boss Sergio Marchionne said early last year.

The nine-speed, responsible for sending power from a variety of engines to the front wheels of a large number of vehicles, became a reliability nightmare for many buyers who either didn’t perceive its shortcomings on a test drive, or didn’t care. Unless drivers ventured well beyond posted speed limits, the nine-speed wasn’t even able to benefit from its ninth gear. Surely deserving of partial blame for the Chrysler 200’s demise, the ZF 9HP was clearly launched long before it was ready.

Nearly three years since my first exposure to the nine-speed in a 3.2-liter V6-powered Jeep Cherokee, I’m driving a 2017 Chrysler Pacifica Limited this week. It’s a stunning minivan, and at CAD $62,340, it’s a minivan with which one needs a whole week to get a full picture. Yet only a few minutes into our first drive in the new Pacifica, it was clear that FCA had finally sorted the previously dreadful nine-speed.

Almost. Mostly. Sort of.

A TTAC reader is an engineer with a major powertrain company, and offered his extremely detailed analysis of the ZF 9-speed. Consider this an AP level course in powertrain engineering.

Before we dive right in to the 9-speed gearbox, let’s take a quick refresher on the basics of gears. The simplest gear set consists of 2 parallel gears mounted on 2 parallel shafts. Shown in Fig. is a gear set with a 20 tooth drive gear on the right and a 30 tooth driven gear on the left. For this gear set the speed of the driven gear is 1.5 times lower than the drive gear, and assuming no frictional losses anywhere, the torque on the driven gear is 1.5 times higher. This gear set has a ratio of 1.5:1. This type of a gear set is usually not favorable for packaging since it requires 2 parallel shafts, and there are largest separating forces that push the 2 gears apart which means that the bearings supporting the shafts have significant radial loads on them, in addition to an axial load if the gears are helical.