Discs & Drums

If you know me, you’ll know that I tend to obsess over a topic until I know everything about it. Witness my entries from a couple of months ago about cars, AWD, and so on. Recently, I’ve been wondering why in the world we still use drum brakes on our cars. I think it’s obvious to everyone that disc brakes are superior in their stopping power; most 4×4 trucks come with 4-wheel disc brakes standard, and so do many performance cars. Motorcycles also use disc brakes exclusively, but not necessarily for the same reason (where would you put a drum on a motorcycle anyway?). So, with that in mind (and with knowledge that even my car has drum brakes in the back), I set out to find the answer to my question.

It was surprisingly hard.

Nobody seems to say WHY drum brakes are used, specifically. There’s much talk of converting drums to discs on all types of cars, there’s discussions of the superiority of disc brakes over drums, and whole volumes of stories about older cars using 4-wheel DRUM brakes and how terrible they were braking under adverse conditions. So, why are we still using them? After an exhaustive evening of web searching, forum prowling, and just general Keith-style research, I’ve come to the conclusion that we still have drum brakes on our cars for two reasons:

1. Price

2. Parking brakes

Let me deal with these in turn.

Price: it’s an accepted fact that drum brakes are CHEAP. Cheap to manufacture, and cheap to install. This is a free-market economy & all that, so the cheapest effective solution wins. But, you might argue, they’re not THAT much cheaper these days, right? Well, you’re right on that point – the driving issue here isn’t necessarily just the pure price of drums vs. discs… which segways nicely into point 2.

Parking brakes: every car’s got one, right? It’s mandated by some highway safety regulation. A car must have a parking brake. So what does this have to do with the disc vs. drum issue? Everything. Turns out, a parking brake just has to be a manually operated brake system that can function when the engine is off. Now, since modern brakes are all hydraulically assisted, when the power goes out – and the brake pump with it – braking becomes VERY hard. Go ahead, try braking sometime when your engine is off. Not much fun, is it? And what if there’s an emergency and you need to stop the car, but the engine’s off? Enter the parking brake. (Sometimes referred to, logically enough, the “emergency” brake.) Since it needs to be manually operated (i.e. no power assist), and the simplest method of doing that is via a cable attached to a lever or foot pedal, you need a brake system that can be operated by a cable. And the simple fact is, you can’t work disc brakes with a cable (at least not easily – and certainly not CHEAPLY). So you see, the reason we still have drum brakes on the rear wheels of most cars & trucks is because it’s more expensive to put 4 discs on a car vs. 2, and because even if you DID put 4 discs on the car, you need to come up with some way to operate the parking brake manually – and although there are a few different ways of doing it, none of them is simple or cheap (relatively speaking). And since in a car with the engine up front (as in almost every car), under hard braking the front wheels do something like 60-70% of the braking, it seems like an almost logical decision to put the “good” disc brakes up front, and keep the inferior drum brakes in the rear, which not only saves you the cost of putting more discs on the car, but allows you to have a cheap, simple parking brake solution.

Who woulda thunk it?

And don’t even get me started on why big trucks (I’m talking BIG… semi-trailers & buses) use ONLY drum brakes… because I STILL don’t know for sure, but it seems to be price (again), combined with the logistics of operating brakes on air pressure vs. hydraulics. But that’s a topic for another day – or never, since I don’t drive a car with air brakes!

I could chew your ear off with more discussions of drums and their mechanical shortcomings; something about what happens to the brake under hard braking, due to inertial forces, heat, and a whole bunch of other technical mumbo-jumbo… but I won’t, because I hardly understand it myself. I’m just satisfied to know that there is a reason (albeit not a great one, but at least one I can understand) why drums are still on so many cars today.


By Keith Survell

Geek, professional programmer, amateur photographer, crazy rabbit guy, only slightly obsessed with cute things.