Sensotronic Brake Control
Almost 20 years since its introduction, Chris Randall investigates the innovative brake-by-wire system.
Not so many years ago, stopping a car was an essentially straightforward business: discs up front, drums behind and perhaps ABS for an added dash of technology. Then, in 2000, Mercedesbenz announced it was taking a more ambitious approach. Developed in conjunction with Bosch – reportedly over a number of years and at a cost of tens of millions of pounds – the Sensotronic Brake Control (SBC) system was unveiled.
Hailed by its developers as a milestone in driving safety, the key difference with the new system was that it did away with the conventional connection between the brake pedal and the braking force applied at the wheels. Instead of the driver’s pressing of the brake pedal acting directly on a master cylinder, thereby generating hydraulic pressure that was delivered to the brakes at each wheel, SBC used sensors to determine the pressure the driver was applying and the speed of that application. A control unit then interpreted the signals and, via pressure modulators within the system, decided how much hydraulic pressure should be applied at each wheel. Essentially, this was brake-by-wire.
The other key elements of the SBC system were an electric pump controlled electronically, a reservoir operating at 140-160 Bar and a tandem master cylinder that could be employed in the event of complete electrical failure, restoring conventional hydraulic braking albeit with longer stopping distances. The system also incorporated a special simulator, the role of which was to provide an element of feedback at the pedal, allowing SBC to mimic the responses a driver would expect to feel underfoot as they braked. And there was an extra layer of complexity because the control unit also received input from the anti-lock and stability control systems, the result of which was impressively fine control of the braking at each individual wheel.
The first model to benefit from SBC was the R230-generation SL launched in October 2001. SBC would go on to be fitted to a wide range of Mercedes cars, from the E-class executive saloon to the SLR supercar developed with Mclaren.
The company was quick to extol the benefits delivered by the sophisticated arrangement, not least of which was a claimed 3% reduction in the stopping distance during an emergency stop from 75mph because the high-pressure reservoir and electronically-controlled modulators ensured maximum braking pressure was delivered much more quickly. A further benefit was that SBC could recognise any sudden move from throttle to brake; predicting an emergency, the system was able to ‘pre-fill’ by increasing brake line pressure, bringing the pads into light contact with the discs and minimising any delay in applying full stopping power.
That wasn’t all, as SBC also allowed Mercedes to introduce a host of new braking functions. One was Traffic Jam Assist, which allowed the car to be braked automatically in slow-moving traffic. Another was Soft-stop, which claimed that bringing the car to a halt in urban driving would be smoother, because brake pressure was ‘feathered’ in the last few moments. Meanwhile, Drive-away Assist prevented rolling back on hills. And, in wet weather, turning on the windscreen wipers meant the system gently applied the brakes at intervals to dry the discs.
There was more, too. Parent company Daimler had carried out research showing that two-thirds of drivers were alarmed by the pulsing of the brake pedal on Abs-equipped cars during an emergency stop, meaning they either didn’t push hard enough or were tempted to release the brakes altogether. With no connection between pedal and hydraulics, SBC removed this pulsing effect at a stroke.
It was clever stuff, but like all such advanced technology it wasn’t without problems. Complaints were voiced by owners, with the software and SBC pump singled out as common causes of trouble, leading Mercedes to recall more than a million vehicles for checks to be carried out. While the company said problems mainly affected highmileage cars subjected to frequent stopping – city taxis were cited as an example and all cabbies in Europe were offered a free check – buyer confidence had been dented.
So much so, that the decision was taken to abandon SBC for the facelifted W211 E-class introduced in 2006, although it would continue on SLR and Maybach models until their production lifecycles ended. Its introduction had been a brave move for Mercedes, but one that ultimately failed to pay off.
But it wasn’t the end for the concept of brake-by-wire, because a number of manufacturers persevered with the technology, most notably Toyota and Lexus, both of which employ it on vehicles to this day.