Prior Convictions: Back seat steering

Four-wheel steer seems like a win-win on paper, so why isn't it ubiquitous yet?

Four-wheel steering is back. Not that it ever, entirely, went away, but its history isn't unlike that of 3D movies: it has been dabbled with, trialled, sometimes even heralded, but never really accepted into the mainstream showbiz of new cars.

The theory behind active rear steering is straightforward enough. (M'colleague Lewis Kingston will, perhaps, be along at some point to outline it in one of his excellent 'PH Origins' columns.) When you're at low speeds, it makes the rear wheels turn against the fronts, to improve the turning circle and agility. At higher speeds (from 40mph and above, typically) it turns the rear wheels in the same direction as the fronts, to improve stability. A win-win, right?

Well, not always. Early systems were sometimes too sluggardly to turn, or made cornering too unpredictable, like a 3D movie shoving a freaking shark in your face one minute but being visually entirely bland the next. That was until five years ago, when ZF launched its AKC (Advanced Kinematics Control) system - the acronym for which kind of makes me think of fried chicken every time I read it. Mmmm...

There are two types of AKC system, one with actuators at each rear wheel (Porsche uses this on its 911s); one with a larger, central actuator (as Porsche uses on the Panamera), though with the same basic premise: they extend or shorten suspension arms to increase or decrease the rear wheels' toe angle.

Typically the range of adjustment can be up to five degrees or thereabouts, though most dynamics engineers will tell you that the vast majority of their work is done at considerably less than that. On full 'oppo', AKC keeps the new Rolls-Royce Phantom 8's turning circle down to just over 13m; but it's the little movements, on the way into a corner, of around one degree, possibly even less, where manufacturers seem to like the extra agility AKC can bring, or that they can fit a more direct steering rack and yet retain better higher speed stability.

But while AKC has been picked up and used by manufacturers including Audi, BMW, Porsche and Ferrari, to much greater success than any previous systems, it is still on the fringes. Since AKC's introduction, about 300 million cars have been produced worldwide, yet AKC (three piece meal with an extra piece, please) has only found its way onto around 125,000 of them.

Still, they are high profile cars: GT 911s, the (frankly quite terrifying) Ferrari F12 Tdf, and the 812 Superfast, for example.

With what success? A fair amount, mostly, although not entirely. There's that F12 Tdf, for example, where, branded as Virtual Short Wheelbase, it wanted to oversteer a lot, but seldom with any particular indication about how much it was going to. The Rolls Phantom has few dynamic vices for a car of its kind, but I wonder if some mid-corner, mid-speed steering stiction is down to its rear-steer's hesitance. And while the Porsche 911 GT3 is one of the most magnificent driver's cars on sale, and while AKC tends to be advised to just stay out of it if you're sliding, I do wonder if, in those transient cornering moments, it's making life slightly harder than no-rear-steer.

The advantages are there, then, but at the end of the day it's a by-wire system that will decide how much steering to apply how it wants, and when it wants, based on how it has been tuned during development. Perhaps little surprise, then, that Porsche does it best so far. And it'll also be interesting to see how Renaultsport - whose latest Megane gets active rear steering - will do when we say what it's like next week. If things keep progressing, perhaps active rear steer will start to become widespread on premium and sports cars, rather than a set of automotive red and blue lenses which we'll drop into the bin on our way out of the door.

OP Posts Only

Author

Discussion

Icehanger

Original Poster

394 posts

222 months