In 2002, Mercedes installed an electrohydraulic brake system in the E-Class. A sensor measured how hard the driver pressed the brake pedal, a computer calculated the braking force, and an electric pump built up the pressure. In case of a failure, Mercedes had installed a backup system: a conventional hydraulic system in which the driver’s foot once again built up the pressure. However, this system braked significantly less effectively. The driver had to press down much harder, the stopping distance increased, and only the front wheels were braked. Two years later, the weakness became apparent: with high mileage and frequent braking, the electronics could fail; in about two out of every thousand cars, the brakes switched to emergency mode. In this state, there were several crashes, and the company acknowledged multiple accidents resulting in property damage. Mercedes recalled approximately 680,000 E-Class and SL-Class cars to dealerships because the vehicles are not permitted to operate regularly in emergency mode. Without this backup, a failure of the electronic system would have meant a complete loss of braking. The new technology failed, while the old one held up.

In 2021, Shimano patented an electronic brake for bicycles. Pressing the lever merely sends a signal; an electric motor generates the braking force. The power for this comes from the battery or a hub dynamo. The published patent documents make no mention of a backup system like the one Mercedes had installed. The brake is not yet available for purchase. When asked, Shimano importer Fuchs-Movesa stated that there is currently no official information regarding future product developments for electronic brakes.

In 2023, the Italian company Raicam filed a patent application for a bicycle brake designed to function even without power. A spring ensures this: If the power fails, it pushes a valve back, and the brake lever once again acts directly on the brake caliper. It’s the same principle used by Mercedes, scaled down and adapted for bicycles. 

No one requires a backup system

In Europe, the Pedelec standard applies to brakes on e-mountain bikes, and the e-MTB standard also refers to it. Tests cover strength, braking performance, heat resistance, and consistent response. Neither standard specifies whether an electronic brake requires a mechanical backup. Marcus Schröder from the German testing laboratory EFBE confirms this. He writes that the standards are silent on this point. Nevertheless, he does not see this as a gap in safety: The notion that a testing laboratory can only test for what is covered by a standard misses the mark. Above every product standard is the obligation of all manufacturers to sell only safe products. If a suitable standard is lacking, the manufacturer must ensure on its own that its product is safe. Such a brake can be tested even without a standard, using a risk assessment and experts in functional safety. 

Schröder derives the obligation to provide a backup from machinery law, under which pedelecs fall in the EU. The new Machinery Regulation takes effect on January 20, 2027. It requires that neither a power outage nor an interrupted radio signal result in a hazard. This applies to machinery ranging from cranes to machine tools, without a single mention of bicycles. Schröder describes the requirement as abstract but clearly derivable. How the backup system is designed is left up to the manufacturer.

Lessons from Aviation

European certification rules for commercial aircraft explicitly require that a single failure must not lead to a catastrophe; the same principle applies in the U.S.: Every critical system therefore needs at least one safety backup. The Boeing 787 has been using an electric braking system for years, backed up by multiple control units. The Boeing 737 MAX demonstrated what a single sensor without a backup can do. In 2018 and 2019, two aircraft crashed, killing 346 people. The control software relied on a single sensor. If it provided incorrect readings, the software repeatedly forced the aircraft’s nose down. Since then, it has compared readings from two sensors and can be overridden at any time.
Mercedes placed the hydraulics beneath the electronics, while Boeing incorporated a second sensor into the software. Mountain bikes lack such a requirement.

Mercedes placed the hydraulics below the electronics, while Boeing integrated a second sensor into the software. On mountain bikes, the inclusion of a backup system is left to the manufacturer’s discretion. Even today’s hydraulic brakes fail if oil leaks or air enters the line. But there are warning signs: the pressure point shifts, the lever moves closer to the handle, and the rider is forewarned. An electronic failure often occurs without warning.