Rear drum brakes...Why?
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VW says that the 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. So VW chose Continental’s new low maintenance drum brake which comes with a 150,000 km service interval. Smart move I’d say.
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Ok, this seems to make sense, but issues with panic braking comes to mind though. I guess we will see how it plays out in time.
VW also claims drum brakes are less draggy than disk brakes, especially over time. One of the video reviews of driving the ID.4 showed a panic stop and driver was surprised on how quick the car stopped. He didn't do formal stopping distance test, it was just a seat of the pants stand on the brake pedal test.
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As one-pedal-driving demonstrates, EVs are capable of getting all the braking they need in non-emergency situations from regenerative braking. That means that the friction brakes only need to provide enough stopping power to make up the difference in an emergency situation. Given that the vast majority of all braking power (in all cars) is provided by the front brakes, that doesn't leave rear brakes with much work to do, which means VW could optimize for things like efficiency and longevity rather than raw stopping power.
It's worth remembering that disc brakes aren't really a feature, they're just a specification (unless you want them for the aesthetics - fancy red calipers and all: that's different). If VW can provide the relevant features, such as stopping distance, lack of brake fade, ease of maintenance, etc., with drum brakes, what does it matter? People often see these kinds of decisions as the manufacturer "cheaping out," but in many ways, the very heart of product engineering is exactly that: finding places to "cheap out" in ways that don't significantly effect the end product. And if you're lucky, you'll even find ways to cheap out that make the product better in other ways (such as the reduced drag and very long service intervals mentioned above).
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The term "cheap out" scares me to death. These days that term manifests itself as "de-contenting" and as the bean counters continue to force their present concerns of the existing market condition on engineering we all should know who wins that contest. Here is a hint: I won't be the end consumer! I also would dispute your assertion that the quality of the end product will not or should not suffer as a result of the aforementioned "cheaping out". In my experience it will / does.
The disadvantage of drum brakes would be apparent if the car was tested with repeated heavy braking over a short period of time. A single panic application is a good enough test of the car for most drivers‘ needs, though. This car isn’t designed for the track.
"Value Engineering" is the official term. 
To be fair the beancounter-engineering compromise is needed to ultimately provide an affordable product that still meets the intended application.
Given as ChristophW well points out the motor's regenerative braking will handle the bulk of the effort, and then be augmented by the brakes themselves. These drums aren't your father's/grandfather's type, being far better materials technology and construction methodology. And you're not going to experience track-fade in an around town utility vehicle.
To be fair the beancounter-engineering compromise is needed to ultimately provide an affordable product that still meets the intended application.Given as ChristophW well points out the motor's regenerative braking will handle the bulk of the effort, and then be augmented by the brakes themselves. These drums aren't your father's/grandfather's type, being far better materials technology and construction methodology. And you're not going to experience track-fade in an around town utility vehicle.
I remember the transition from drums to discs. One of the advantages of discs is the superior cooling. Very important on the track, but also in real life. If one has to ride the brakes going down a steep incline, drum brakes will overheat and this was one of the reasons that in the olden days there would be signs on a hill to shift to a lower gear so that engine compression would assist in speed management. I can tell you that the regenerative braking on an EV, including PHEV's, makes braking totally unnecessary and is the EV equivalent of "compression braking". Weight and simplicity is also aided with drum brakes. Many cars have a small "hat" attached to the rear axle for a small drum brake for the parking brake. This adds complexity and un-sprung weight to the rear wheel assembly. While I was initially put off by the VW decision, I now applaud it for out of the box thinking. We'll probably see this having wider application for non-performance EV's.
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But that's my point: that what people dismissively call "cheaping out" is often just proper engineering. There's no such thing as a product without compromises. I would argue that the goal of any design engineer should be to make a product that leaves the buyer unaware of the compromises that were made behind the scenes, one that leaves them completely satisfied by its cost, performance, aesthetics, and longevity. I used to work with a statistician who always reminded us that "a difference is only a difference if it makes a difference" and I think that applies here. If there's no compromise in stopping distance, brake fade, longevity, or any other perceptible quality, why does it matter that a car has drum brakes rather than disc? Not to mention that (as others have mentioned) there are technical advantages to drum brakes in this application that have nothing to with cost.
One more thought; many vehicles with disc brakes have a small drum brake within as the emergency brake. If it's good enough for that condition ....
But for me the biggest EV-plus is that there's no drag in drums as there often is with disc brakes (whether by cleaning/cornering assist design or just natural occurrence). We want motor regeneration when slowing down but certainly not any other resistance to miles-per-charge.
But for me the biggest EV-plus is that there's no drag in drums as there often is with disc brakes (whether by cleaning/cornering assist design or just natural occurrence). We want motor regeneration when slowing down but certainly not any other resistance to miles-per-charge.
An article I read a few weeks (months?) back pointed out that disc brakes need to be used somewhat frequently or the discs begin to oxide & lose effectiveness, and the engineers felt that, with this design (regenerative braking, one pedal driving, whatever you want to call it), the rear brakes would rarely be used. So, in that author's mind, drum brakes were an upgrade as they would last longer with similar stopping power. Sorry I couldn't find a link to the article, but I did look thru my history - there's just so many ID.4 articles over the last few months.
Yup, I have read the same thing but it doesn't sound like it was written by the same author. I have been working on vehicles for 45 years, it is VERY rare that I find a disc brake that isn't rubbing causing some drag. In this application, a quality drum brake setup is superior to a disc brake for several reasons. Newer doesn't mean better, just look at the AC and volume controls on the ID.4. Most of the time they work the first time, but all too often they don't work. At least according to the reviews I have been watching/reading on the ID.3. I have seen the same complaints on the same setup on the latest Golf. I am not convinced a software change will fix this issue. Every review has noted/complained about the lack of a volume knob you can turn. Just because you can, doesn't mean you should.An article I read a few weeks (months?) back pointed out that disc brakes need to be used somewhat frequently or the discs begin to oxide & lose effectiveness, and the engineers felt that, with this design (regenerative braking, one pedal driving, whatever you want to call it), the rear brakes would rarely be used. So, in that author's mind, drum brakes were an upgrade as they would last longer with similar stopping power. Sorry I couldn't find a link to the article, but I did look thru my history - there's just so many ID.4 articles over the last few months.
In this application, drum brakes are a better choice than disc brakes.
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