Quote:
Originally Posted by rosswond
Weighing in here as a mechanical engineer.
Oh, and my Honda Jazz has rear discs, and is the only car in it's class here that has them.
Some facts:
Firstly, in perfect dry conditions, drum brakes will give you more braking force than discs - the first time.
discs give you consistent results because they are linear (they don't self apply) and because if they do get wet, they shed the water quickly instead of trapping it inside.
Disc brakes work better under repeated hard usage because they are better at shedding heat.
Drum brakes are superior for a parking brake because they self energize and have a larger contact area.
If you have rear disc brakes, you have two choices for your parking/E-Brake:
1. Use a mechanically actuated caliper on the disc - adds weight and not very effective
2. Use a smaller drum inside the disc hub - less effective than a full size drum as an E-Brake and adds a lot of weight.
In the USA, I believe there are rules that govern the effectiveness of the secondary brake (which is why you call it an E-Brake and we call it a parking brake) This effectiveness is compromised when the size of the secondary brake is reduced to fit it inside the disc brake hub. Elsewhere in the world, the use of split hydraulic systems is deemed to cover the emergency eventuality.
In a light FWD car, most of the mass is at the front and most braking is done at the front. The rear brakes may in fact do nothing in extreme braking in an empty car.
In a light car, the weight saving from using drums rather than discs with an included E-Brake is attractive.
In an economy car, the cost saving of using a rear drum brake for service and parking brake is attractive.
Disc brakes give better pedal feel and work better with ABS.
Disc brakes are often fitted for "marketing" reasons to cars that don't really need them because there is a perception that they are "better".
I think the reason you get rear drums in the USA is for reasons of cost, fuel consumption, and meeting E-Brake effectiveness regulations.
The reason we get rear disc brakes everywhere else is for marketing.
You might fail your annual safety check (if you have them) with rear disc brakes if you can't achieve the mandated effectiveness of your E-Brake.
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As a fellow engineer who has designed and tested brake systems I disagree that drum brakes offer more braking force. The friction between disc and piston pad far exceeds that of shoes on drums. And disc brake systems actually weigh less than equivalent drums. That cast iron drum alone outweighs my entire disc and caliper. As a deciding argument note that nobody of consequence has drums up front. Guess why.
Its easier to put ebrakes on drums I agree but the difference is slight.
And there is no comparison when servicing is involved; the drum fails quickly. And yes, there are SAE standards for how much the e-brake will hold in terms on the incline.
Its not a perception; discs brakes are superior in performance, cost, and weight. As I said no one uses drums in front. And you can bet your last hamburger if they were cheaper they would.
Only the leftover suppliers who can provide drum brakes because their fabrication lines have long since been paid for and thus their cost can be less are the reason anyone uses drum brakes.
When you look at the active surface area between the drum and the brake shoe you quickly see that they are not as much in contact as you might assume. Therein the so-called self-energizinfg just isn't there. Its not quite the case that because the shoe travels into the drum rotation that self-energizing occurs. The secret is the force pushing the shoe into the drum and its much less than the force that a piston can apply perpendicularly to a pad against the disc compared to the force applied axially to the drum. And multiple pistons offer far more.
As in most things you can make a drum so wide and large diameter compared to a tiny pad on a small diameter disc that it will be the better performer but in reality that doesn't exist. Well, except at leMans decades ago. But it didn't take more than a year to change.
PS: be careful of the stuff on google. they're the same people who say synthetic oils aren't any better than hydrocarbon oils. Those of us with dynos know better.