. For the first time, the model will come with the ability to turn all four wheels, so we thought this would be a perfect time to take a look at just how and why Audi chose to add a new dimension to Quattro.
Although this is a first for the A7, Audi has been toying around with four-wheel steering and it recently featured on the A8. The idea is pretty simple: more wheels steering means more turning. Applying the technology is a little more complicated than that, though, because at different driving speeds you want different things.
To control the system, Audi hooked up the rear axle steering to its Electronic Chassis System (ECP) to intelligently pick between turning the back wheels in the same direction or in the opposite direction of the front wheels.
At low speeds (about 5 to 10 mph) the rear wheels turn by up to five degrees in the opposite direction of the front wheels. That has the effect of reducing the A7’s turning circle by about 1 meter (3 or so feet), making it easier to drive around parking garages and tight corners.
On the highway, though, you aren’t really concerned about the car’s turning circle. What you really want is stability. Traditionally, a long wheelbase has been the only way to achieve that (this is one reason why old luxury cars are huge), but you give up on maneuverability when you do that.
By making the rear wheels turn by up to 2 degrees in the same direction as the fronts, the A7 becomes more stable at high speeds. That means when you’re changing lanes, the car feels steadier and smoother.
Along with the rear wheels, the A7’s steering ratio varies from 9.5:1 (for low speed maneuvers) to 16.5:1 (for high speed driving). Switching from one ratio to the other requires something called strain wave gearing. Put simply (like, very simply), an elliptical gear inside a round one allows massive amounts of torque to be transmitted, at varying rates, with no play.
Audi has been using this type of “Dynamic Steering” for a while now and they’ve gotten pretty good at it, now it just has the added wrinkle of including the rear wheels.
Based on the , Audi is doing a pretty good job of wedding its dynamic steering with rear-wheel steering. They haven’t stopped at simple highway and city driving, though. The new system helps when conditions get slippery, too.
The A7 makes rear-wheel steering decisions via the ECP, which can exchange information with the car’s brain every 0.005 seconds. Along with the active roll bars and air suspension, the A7 can also control the front and the rear wheels. That means that the steering angle can actively be adjusted for understeer or oversteer.
So far, Audi hasn’t used the four-wheel steering to improve turn-in on track, like , but the RS7 could easily benefit from such a system. That system allows the rear wheels to turn in the opposite direction of the fronts at high speeds, too. It’s ideal for track driving because it helps you round corners, but so far Audi has only introduced the system on the SQ7, the A8, and now the A7, which aren’t exactly track cars.
For now, the system allows Audi to virtually change the size of the A7, based on what’s preferable. Around Saint-Tropez, it’s a tiny city car. On the Autobahn it’s a massive luxo-barge. And when the driving gets slippery, it can help catch you.