Vehicle dynamics & chassis: Current affairs
Power steering was first, now it’s the brakes, but maintaining safety and pedal feel is critical
- Published in Focus.
The first of many: The technology launches on the R8 e-tron but other models will follow
One of the main benefits the electric motor has to offer is instant torque.
This attribute is frequently cited in OEMs’ marketing material for electric vehicles because it helps to get across the message that they are still fun to drive.
Using its R8 sportscar as the basis gives it something of a headstart here, but Audi’s e-tron has four motors, and the resulting 4,500Nm certainly brings a smile to your face when you press the accelerator. I drove a prototype model for only five minutes outside the firm’s Ingolstadt plant in Germany but this was enough to tell me that people wealthy enough to buy one when the R8 e-tron goes on sale in 2012 are not likely to be disappointed.
But just as electric motors provide the vehicle’s traction, they also help it to stop. Unlike those clamping the carbon discs at the front, the rear brake calipers are actuated not by high-pressure hydraulics but by electromechanics – 12V motors turning a ball screw via a set of gears.
Audi has been working on the system for several years, and it will enter series production on the R8 e-tron. Powerful, fade-free brakes and excellent pedal feel are prerequisites for any sportscar. But an electric sportscar also needs to get good range from its battery, meaning maximum regenerative braking.
This system helps balance all these requirements. “We can control the brakes independently of the driver’s foot, which means that we can do perfect blending between recuperation and normal braking,” says Karl-Heinz Meitinger, responsible for electromobility chassis development at Audi. “That’s a big challenge and that electromechanically actuated brake caliper is one very good solution for that.
“It has some other advantages – one of them is that for emergency braking you have quicker reaction times because hydraulics take some milliseconds to build pressure. This system gives you a few extra centimetres less braking distance.”
Getting the brake feel just right is difficult. Meitinger says that meeting this challenge means a lot of up-front simulation work but that this only gets you so far – after that it’s down to lots of application work and prototype testing.
Particular attention is paid to the ESC system. To maximise range you use the highest possible level of regenerative braking. But this level is influenced, among other things, by the battery’s state of charge and also by the vehicle’s dynamic behaviour.
