[Omkar]

I remember reading a couple of years back that drum brakes might make a comeback in EVs. This might, in fact, turn into a reality. Let’s look at why drum brakes make more sense in an EV.

How drum brakes work



The mechanism consists of a drum that is connected to the wheel and rotates with it. Inside, you have brake shoes with brake linings on top of them. The brake linings are the friction material and are responsible for braking. Then you have a wheel cylinder which has a couple of pistons that are connected to both shoes.

The idea is simple. When you depress the brake pedal, the power from your foot isn’t enough, so it’s amplified through a hydraulic mechanism. You have a brake booster that multiplies the force and hydraulic pressure is transferred to the wheels through brake lines that are filled with brake fluid. The pressure reaches the wheel cylinder that pushes the pistons outwards and ergo the brake shoes and brake lining. The brake lining makes contact with the drum’s inside walls which slows down the wheel’s motion and stops the car.

Sounds like a simple and effective mechanism. So, what’s wrong with it?

The mechanism has some inherent flaws in it. Since this is a closed mechanism that’s dependent on friction, the heat generated when using brakes is retained. The lack of heat dissipation from the system can lead to faster brake fade. Also, in wet conditions, water might accumulate in the drum which means that the braking won’t be as effective.

Okay, so disc brakes are better?

In some ways, yes. Brake pedal feel and initial bite are much sharper in disc brakes than drum brakes. Not to forget, they’re lighter than drum brakes and thanks to the open design of the disc brake setup, heat is dissipated quickly. They last longer under heavy usage as compared to drum brakes and hence, most IC-engine cars, nowadays, have an all-wheel disc brake setup as standard.

Tata Punch.EV gets all-wheel disc brakes:


However, disc brakes have a more complex design with multiple components and require more maintenance than drum brakes. They are more expensive than drum brake setups and costlier to maintain as well.

So, what’s best for EVs?

Since EVs have become mainstream, there’s a slight difference in the way people view cars. EVs in general are considered as point A to point B cars that have a low cost of running. But in the context of braking, there’s one major aspect of EVs that differentiates them from IC-engine cars – Regenerative braking. Thanks to regen, most of the braking is taken care of by the motor itself and that exposes some flaws of the all-wheel disc brake setup.



The rear brakes usually provide ~30% of the braking force and the majority of the braking is handled by the front brakes. Add to the fact that in EVs the mechanical brakes aren’t used as much, and the rear brakes are barely used.

The less usage of rear brakes means that heating and brake fade also won’t be an issue. Also having disc brakes at the front takes care of most of the mechanical braking. So, with a drum brake setup at the rear and regular maintenance, the brake lining won’t need replacement for a very long time.

Since EVs also bank heavily on the environmental aspect, there’s also the case of brake dust that weighs in on this topic.

Up to 55% of roadside traffic pollution is made of non-exhaust particles, with around 20% of that pollution coming from brake dust. These particles may be just as damaging to our lungs as exhaust fumes.

Source

Due to the open design of disc brakes, brake dust is released directly into the atmosphere. But since drum brakes have an enclosed setup, brake dust is collected inside the drum which can be cleaned during your regular service.

From theory to reality



It's not all theory, and some manufacturers have started adding drum brakes in their EVs. For starters, Volkswagen is using drum brakes at the rear for ID.3 and ID.4 EVs which are on sale internationally. Here’s an excerpt from the 2024 Volkswagen ID.4 media release for the United States:

A combination of brake types is used on ID.4. It features disc brakes on the front wheels and drums at the rear—the difference being specific to electric vehicle needs. As EVs rely on regenerative braking, the rear brakes get comparatively little use compared to a conventional ICE vehicle, plus there are friction advantages in using drums.

What do you think of the return of the drum brakes in EVs? Do share your thoughts.

[Omkar]

Thread moved out from the Assembly Line.

[dzayyy]

Originally Posted by Omkar

I remember reading a couple of years back that drum brakes might make a comeback in EVs. This might, in fact, turn into a reality. Let’s look at why drum brakes make more sense in an EV.
...

Interesting, very interesting. I like the concept as it makes a ton of sense and seems logical enough.

[StarrySky]

The braking distance on the MEB cars have come in for criticism in the winter tests done by a Finnish magazine. The MEB cars have had the largest stopping distances on snow and ice, with the ID.Buzz last year taking double the distance of the next car on the list. The braking distance of the ID.7 in the latest test has been much improved and is now in the same ballpark as the others, although still towards the bottom on the list. In the parking test, problems have also been reported in reversing the RWD ID.7 up a slippery incline for parking.

I am not quite sure how the tests are conducted. But having gone through a pretty snowy winter with my ID.4, I am happy to report that there have been no heart-stopping moments. I haven't been more circumspect with the ID.4 than with my earlier cars, and may have even been slightly more adventurous due to the (falsely?) assumed sense of security provided by the AWD system.

[Jeroen]

I don’t think drum brakes were ever phased out on ICE cars. Lots of small cars currently have drum brakes at the rear. Eg our Ford Fiesta.

Having drum brakes at the rear also makes for a very simple, ie cheap, parking brake system. Just a ratchet handle and a cable.

As far as I can see it is purely a case of cost cutting.

The notion that a drum brake is an enclosed system and thus doesn’t suffer from internal rust, is flawed. Ask any (DIY) mechanic who has worked on old drum brakes. Trust me, when not used they rust.

There is no sealing between the drum and the back plate. There is a small gap and water will get into it.

One of the big advantages of disc over drum brakes is the less weight. Less so called spring weight, improves handling. (For the anorak brigade: that is why some vintage race cars have drum brakes that are mounted internally, await from the suspension.

Jeroen

[sagarpadaki]

In my experience, disc brakes vs drum at rear does not make much difference to the braking performance in day to day driving. From a maintainence standpoint, I prefer disc brake anyday. They are far easier to maintain than drum brakes.

[NomadSK]

Originally Posted by Omkar

Regenerative braking. Thanks to regen, most of the braking is taken care of by the motor itself and that exposes some flaws of the all-wheel disc brake setup.

I agree, This is the main reason that the mechanical braking is not much needed in the EV's. But at the same time this also kills the momentum which one has already attained.

If a disc brake setup isn’t used frequently, you’ll notice that a layer of rust forms on top of the disc which hampers the braking considerably.

Thats a superfical layer of rust formed and is removed as soon as the car is driven and during application of the brakes.

Ideally, having drum brakes at the rear makes a lot of sense. Since it is an enclosed system, the rusting problem is avoided.

That's not true, yes, the drums are enclosed, but they aren't airtight, hence rusting happens, but its not seen unlike on the disc rotor.

What do you think of the return of the drum brakes in EVs? Do share your thoughts.

The articles nowhere mentions the weight of the EV's compared to the ICE's in order to have the reference with the momentum which needs to be killed during the motion. If EV's are 10-20% heavier than the ICE's, then it would require more braking enregy to be dissipiated (excluding regen braking). Specially with high torque available for EV's at the tap, I would definitely want an all disc setup.

Disc brakes design is far superior to drum brakes, seems this has been done purely for cost control practices under the garb of environment to justify the change.

[dhanushs]

Apart from the cost savings to the manufacturer, I dont think there is any functional advantage of drum brakes over discs as far as an end user is concerned.

Yes. Regen helps with braking and hence less brake fade, but, with the speeds the new gen EV's can generate, the drum brake will always be inferior. Not to mention the performance in varying weather conditions (eg: water..)

[OpenEXit]

Not so sure if still hold truth due to technological advancements, I read a white paper long back by Yamaha R&D that front disk and rear drum is best combination for braking.
Beyond that, everything is marketing gimmick. All 4 disk brakes are not as effective as drum/disk combination.

[Cresterk]

Lots of misconceptions in this thread. Here is a really good video about it, request everyone to give it a watch :

https://www.youtube.com/watch?v=lsp8sdGGvWg

[diablo29]

I have Nexon EV Prime model which has disc in front and drum at rear. In my 3 years of usage the rear drum breaks were found to be as good as new when opened during 2nd year service for cleaning. The brake pads inside the drums were also hardly worn out . I would attribute this to the regen braking as I drive mostly single pedal.

[sgmuser]

Very strange to know this that Drum brakes returning. I for one, have this habit of looking at the rear wheels all the time when I see a car and not sure why! I have this issue for some time already. Nexon update comes with dual disc brakes and I thought it is completely warranted for the extra power and torque it generates especially being an ev and it's instant torque delivery nature. This drum brake trend is unfathomable at least for me and I do not like it all. I for one, I badly want a disc brake in my Thar for it's poor brake and feel.

[pradkumar]

In case you missed it, there's been a lot of discourse surrounding electric vehicles and tires lately. Not only do EVs wear through their rubber and roads quicker because of their relatively extreme heft, but the instant power they put down also accelerates the process. Owners are shocked to learn this firsthand because, as J.D. Power reports, their daily drivers chew through tires like they're going out of style. And not only that, but many were supposedly never told this would happen.

https://www.thedrive.com/news/nobody...-through-tires

[oakpr]

On a related topic, do we see the Formula E braking technology seeping down to road EVs?

In current regulation of Formula E, rear motor does all the driving and front has a dedicated Regen motor (a very strong one at that). Idea is front Regen motor does most of the hard braking, allowing the friction brakes to be skinnier and thus further saving weight.

In this context of disk vs drum brake debate, do we foresee road cars getting a dedicated Regen motor for braking and go lean on friction brakes?

[dutta_d1]

The real reason behind it is the problem of implementing parking brakes. Disc brakes work using hydraulic. So even in ICE cars that have all 4 disc brakes, they need to have additional mechanism to keep the brake locked even during loss of hydraulic. Most such cars solve the problem of parking brake using a tiny drum brake in the middle of the rear disc. When the parking brake lever is pulled, it actually engages this small drum brake.

Since in EVs, the rear brakes aren't getting used much anyway, it makes no sense to increase costs by providing one small drum brake and one disc brake. It is sufficient to just have one drum brake.

In other words, the rear drum brake in EVs have just one practical purpose - Parking brake (not trying to imply that it doesn't do anything for friction brake at all).