Why Drum Brakes Make Sense on EVs
Drum brakes aren’t as old-fashioned as most people believe, but even so, these wouldn’t be the first choice for the rear wheels of 1600 kg car like the new Volkswagen ID.3. Why the German engineers opted for this solution, you ask?
Compared to drums, discs shine primarily when it comes to performance, meaning stopping power, especially in wet weather. These also dissipate heat noticeably better, which means they will do better job when the brake needs to be applied at short intervals, such as in an urban scenario or racetrack. But whenever engineers can employ drum brakes without significantly affecting the car’s intended function, they will.
Drum brakes are on the verge of extinction from medium to high end modern passenger cars. However, they have made an appearance on Volkswagen's new ID.3 & ID.4 electric crossover on rear axle. The low-cost antiquated technology is often seen in low end cars as a measure to keep the overall vehicle’s cost in check. So, what made VW choose the same for their latest technologically superior electric offerings. Is it to lower down the costs or there is some thought out strategy?
Let’s try to understand:
Rear brakes are used so rarely (remember, the motor on the rear axle does all the regenerative braking before the physical brake needs to kick in, first in front and if necessary in the back) that the disk would corrode. In practice, I think you’d experience “sticky” braking.
Rear drum brakes are used primarily to decrease rolling resistance. The disc brake pads have the tendency to slightly drag on the rotors as the vehicle is coasting. Drum brakes, just by nature of their design, do not have this tendency. Drag reduction leads to increased mileage.
EV's regenerative braking system handles most of the stopping. This leads to long inactive periods for brakes which further cause disc to corrode specially in the case of Rear Brakes. This means that the brakes no longer work evenly and may pull the car to one side. Drum brakes offer superior performance and reactivity after long periods of disuse as they form a closed system as opposed to Disc Brakes.
EVs are capable of getting all the braking they need in non-emergency situations from regenerative braking. Hence, the friction brakes are only needed to provide enough stopping power to make up the difference in an emergency situation. Due to dynamic load transfer during braking in majority of cars, all braking power is provided by the front brakes. Rear brakes are left with not much work to do.
What Does Regenerative Braking Do?
Regenerative braking uses the excess energy produced by a moving vehicle to charge the batteries, thus slowing down the car.
Normally, all that motion energy is wasted through the brake pads and discs in the form of heat. Brakes get red hot and wear out with use, so why not tap into all that excess energy? It has to go somewhere and regenerative braking simply puts as much as possible back into the batteries.
The operational principles of regenerative braking are simple, but they require special motors as IPM ones (SMAC Serie).
If you’ve ever driven an electric vehicle, you’ll know that its deceleration is a lot closer to a gas car with a manual transmission than a run-of-the-mill automatic. These cars decelerate quickly when you take your foot off the gas and usually don’t coast much.
This is both a design characteristic that improves safety and a result of regenerative braking in action. The load on the front motors, which generates power, also slows down the car—which makes coasting impossible.
Although it may be annoying at first, it’s actually a great way to reduce your chances of having an accident. If you’re inattentive and take your foot off the pedal, the car won’t keep blasting down the road without direct input.
Also, coasting is illegal in many areas due to the fact that a coasting car is technically out of control. This was once a major issue with manual transmissions, as drivers who chose to coast instead of using engine braking wore out brake pads faster and increased their stopping distance on hills.
Structure of Drum Brakes
Drum brakes are a brake system with brake drums (rotor) which rotate with the wheels. Inside each drum are brake shoes fitted with brake linings (friction material). Pistons (pressure mechanism) press against the drums from the inside to generate braking force, thus making is possible to decelerate and stop the vehicle.
How Drum Brakes Work
When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system) and changed into hydraulic pressure (oil-pressure) by the master cylinder. The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid). The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons press the brake linings, which are friction materials, against the inside surfaces of the brake drums which rotate with the wheels. The linings are pressed on the rotating drums, which in turn decelerate the wheels, thereby slowing down and stopping the vehicle.
Changing the public’s perception of drum brake technology as being antiquated and inferior to discs remains a challenge. But while we likely will never see a return of four-wheel drum systems, especially not in any high-performance and sports vehicles, “For city driving, we think it's the best technology that makes sense here for EVs.”
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