Road and Track, July 1st, 2006
It used to be that enthusiasts eschewed two-pedal driving, and for good reason. The traditional torque-converter/3-speed planetary/PRNDL was for Mom and Dad. We wanted to stir gears, by artfully wielded hand and clutch foot.
How times change. Today, some of the world’s most desirable cars don’t even have a clutch pedal.
But not all of these sequential transmissions are equally worthy of our attention, even among the smart ones. Here’s a survey of the many options to two-pedal sequential motoring, together with a chart collecting information from automakers.
HOW DO YOU STIR THEM?
Each of these transmissions has at least two basic modes: automatic, in which the soft- and hardware do the shifting for you, and manual, in which you’re responsible for working your way sequentially through the gears. By “sequentially,” we mean a linear progression with no gear skipping.
Sequential selection may involve nudging the console-mounted shift lever into a dedicated slot, then tapping it one direction or another. Many automakers have chosen a shifting movement that’s longitudinal, forward for upshifts and backward for downshifts.
BMW and Mazda are the opposite, and not simply capriciously. As you accelerate or decelerate, the physics of the situation forces you respectively backward or forward. Thus, BMWs, Mazdas (and race cars with sticksequentials) upshift with a rearward tug and downshift with a forward push.
Others, Mercedes-Benz and its Chrysler siblings among them, slap the lever from side to side, upshifts to the right/downshifts to the left. Others augment the lever with paddles, either steering-wheel- or steering-column mounted. There are tradeoffs with each, at least in part depending on how you use your hands on the steering wheel.
Those who “shuffle-steer” (i.e., keeping their hands more or less at 9 and 3) tend to prefer the paddles on the steering column. Those of us who “cross over” (I learned my technique at Bob Bondurant’s School of High Performance Driving) tend to like wheel-mounted paddles better.
The most common scheme is right paddle upshift/left paddle downshift. A variation uses buttons or toggles on the wheel in lieu of paddles, again typically righthand upshift/left downshift.
Some automakers allow single-handed actuation with upshift controls—buttons, levers or paddles—on the steering wheel’s facing surface and downshift controls on the opposite surface. Either right- or left-handed shifting is feasible.
Porsche’s latest Tiptronic allows singlethumb actuation, up or down, with wheelmounted toggles.
Last, combinations are common: Tap the steering-wheel paddles or nudge the console lever—your choice.
TWO (AND A FRACTION) TYPES
Which is the best actuation? In truth, once learned the motion is less critical than what’s invoked in the underlying gearbox. And here we make two (and a fraction) distinctions:
One type of sequential layout employs a traditional planetary-gearset automatic transmission. Left in Drive, a sequential automatic’s shifting is similar to that of a conventional automatic, its sun, ring and planetary gears variously clutched and disengaged to yield the different ratios of input and output shaft. Tipped into sequential mode, it’s your actuation— not purely internal hydraulics—that determines the gear. In its simplest form, think electrohydraulic PRNDL—optionally under your own control.
By contrast, the second type employs what is nominally a traditional layshaft manual gearbox, the sort we’re used to stirring with a lever. Only here, the controller does the stirring as well as the clutch actuation. I say “nominally” because the latest of these are anything but conventional. For example, BMW’s 7-speed SMG III (as in Sequential Manual Gearbox, third iteration) has internal shift geometry that isn’t amenable to any external gating of a single lever. That is, this particular gearbox exists only as an SMG.
Left in automatic mode, the controller does its best to simulate a traditional planetary automatic, with smooth shifting up or down dictated pretty much by your right foot. Used manually, your actuation of paddle, button or lever signals the controller of your intent. It handles the rest, often even to over-rev protection and rev-matching downshifts.
And then there’s the VW/Audi Direct Shift Gearbox, the one keeping this analysis from being a straight dichotomy. For full details, see “Technology Insight: A Gearhead’s Delight,” June 2004. Briefly, DSG is akin to a traditional manual, only its even and odd gears are on concentric shafts, one hollow and the other within it. Dual clutches at the input side dictate which of these shafts carries the torque. During this engagement time, the controller preselects the next gear and locks it to its free-spinning shaft. Appropriate action of the dual clutches brings about the actual change of ratio.
Left in Drive, the DSG controller handles it all automatically. In manual mode, steering- wheel paddles bring about your intent. DSG is mechanically complex, but innovative, elegant and, for me personally, the best sequential on the road at the moment.
PLAYING THEM OPTIMALLY
If your motoring is anything like mine, some of the time you’re really into it, having fun, treating gasoline as what I’ve come to call “liquid entertainment.” Other times, though, you’re just driving, getting from one place to another, often in more traffic than you’d like. This is the perfect time for nudging the sequential into automatic mode.
Set, forget and be confident that the layout will do the right thing. Even here, though, there’s lots of transmission smarts taking place. Most systems have some sort of hill finesse, recognizing up- or downhills and all but eliminating an annoying hunting from gear to gear. Many have a multiplicity of driver-selectable modes, Normal or Sport, for instance, or even a Winter calibration that’s less prone to spin wheels on icy startups.
At their best, sequentials in automatic mode can be every bit as well behaved as a good conventional automatic. At their worst, they can be clunky, with prolonged shifting particularly under light load. We’ll invariably comment about this in our First Drives, Road Tests or Comparison Tests.
More interesting to enthusiasts, though, is what happens when you tip out of automatic mode and into whatever manual operation the sequential has to offer. Again, there might well be driver-selective calibrations, Sport/Normal, Power/Economy. Indeed, in its M fitments BMW’s SMG III Drivelogic has a total of 11 different calibrations, six in manual mode as well as five in automatic.
One feature I appreciate in manual mode is speed-matched downshifts. These little blips of the throttle do a lot more than simply sound cool: If it’s an underlying manual box, there’s less wear on the gearbox synchronizers. Whatever its underlying nature, there’s reduced strain on the entire drivetrain—and potentially less discord at the tire/road interface. On a slippery road, for instance, an unmatched downshift can be quite enough to break traction and initiate a slide.
Another telling feature is how over-revs are handled. Typically, a downshift request is delayed (or even ignored) if its execution would exceed the engine’s redline. At the other extreme, sequentials handle accelerating to redline in one of two ways: By design, some upshift automatically at designated rpm, perhaps extending this engine speed in Sport mode. Others stay in the selected gear, even to bouncing off the rev limiter. Generally I prefer the latter, though in truth I don’t go there that often, so it’s no big deal.
Sequential specs sometimes boast of shifting speed, 100 milliseconds, 250 milliseconds, and the like. This requires some analysis: Does it encompass the complete progression of clutch disengagement/gear change/clutch reengagement? Or is it just timing the gear change? Data don’t always make clear which is being cited. Also, to put these in perspective, a quick shift of a really good traditional manual takes perhaps 300 milliseconds, i.e., 0.3 sec., clutch-in to clutch-out.
What’s more, to me the duration of shifting is less important than having the shift occur when I expect it, whether it’s manually invoked or prompted by my accelerator foot. The best of sequentials do this with uncanny accuracy and predictability.
Two-pedal driving? It’s not just for Mom and Dad.
