Drum Brakes Vs Disc Brakes - Not What You Think汽车专题
It's commonly believed that disc brakes are the superior choice - but you would be surprised that it isn't always the case. Here's why.
As cars have evolved over the decades, they have become both quicker and heavier - which in turn means that braking demands have increased as well. There was once a time when having drum brakes at both the front and rear was a common thing - case in point, the classic Volkswagen Beetle - but nowadays consumers get upset if a car comes with drum brakes at the rear.
The complaints and comments are what led manufacturers to eventually transition to all-round disc brakes on the majority of their models, or at least in the higher-specification trim levels of their entry level models. The only issue is this: drum brakes are actually stronger than disc brakes for an equivalent size. But braking is a lot more than just force - so let's get into it.
Drum brakes, for the same package size, provide more surface area for brake force to be applied which in turn also helps to improve stopping power. That's just the reality of things - and if you want a good example, a rear drum brake (or drum in disc setup) is much easier to do handbrake turns with than a pure rear brake disc setup. Having that higher brake force allows you to lock the rear tyres easier, which in turns lets you slide the car more easily. While this isn't something you would normally do on the road, it is a good demonstration of the strength of these brakes.
One of the problems with drum brakes is that they are an enclosed system, unlike disc brakes which are fairly exposed to passing air. This means that it is easier to overheat drum brakes, and in some older performance cars, you would even find drum brakes with cooling fins on the housings to help cool them down. Once brakes overheat, they drastically reduce in performance - which you can imagine is a pretty bad problem to have.
What kind of disc brakes do you want? You can get them in solid discs, ventilated discs, cross-drilled ventilated discs, slotted ventilated discs, carbon-ceramic discs, and anywhere from one piston to eight pistons distributing brake force. Disc brake technology has evolved massively to provide better resilience to temperature, better braking performance, and better longevity - while drum brakes have remained pretty much stagnant.
Here's the real issue, and to understand it you need to have a rudimentary understanding of physics and weight transfer. For the majority of front-engine cars on the road, you will find that the rear brakes are significantly smaller than the front if they happen to be disc brakes. On top of this, you will also find that the brake bias (distribution of force between the front and rear) is towards the front as well.
The reason for all this is because under heavy braking conditions, inertia causes the weight of the car to transfer largely to the front wheels. Because all that weight is acting on the front tyres, you naturally expect there to be big, beefy brakes on the front - and that's definitely where a proper disc brake setup helps, especially with handling the heat generated in these conditions.
But what of the rear? The rear of the car loses as much weight as the front gains, which means the rear tyres don't have as much grip on the road and in turn means you can brake the rear wheels as hard. If you tried to, there's a high chance you may lock up the rear wheels and put the car into a spin. It's for this reason that we feel rear drum brakes aren't as big of an issue as people make them out to be, because the rear brakes are rarely doing the heavy lifting in the majority of cars on the road.
Ultimately, there's still an aesthetic reason for having all-round disc brakes - it just looks nicer. It's also a little more reassuring, but don't expect to gain any stopping power by converting to rear disc brakes, or picking a specific trim level with all-round disc brakes.