If disk brakes are so much more effective than drums (they look simpler, too) why did it take so long for them to become standard equipment on production automobiles? Why, even today, are some “performance cars” still offered with rear drums? Why, indeed, are drum brakes not as hard to find as lap robes?
Are there any advantages to drum brakes over disks? Are they cheaper? Do they last longer? Provide better control? What?
Well, this is a good question alright. I’m a little surprised no one else has started a debate about it.
I was taught in my Senior Mechanical Design course that drum brakes had two advantages over disc brakes. First, their actuators were much cheaper than disc brake calipers, and overall the whole drum brake assembly was cheaper. Second, drum brakes have sort of an “auto-locking” effect with one shoe, so they work better if applied on the wheels where the hand brake attaches to.
Other than that, disc brakes are supposed to be superior in every way - stopping power, simplicity, they shed oil and water much, much better than drum brakes too. Brake dust does not build up and reduce stopping power. Disc brakes can run hotter, because they can stop the car more quickly, but they are also more exposed to cooling air.
Why do you still find them even on performance cars? I honestly do not know.
They are found in performance cars probably because the designer spends so much time trying to eek out that extra .5 horsepower that they forget about the rest of the car. Hell, my Trans Am has a solid rear axle for Krusty’s sake!
Here’s my take, FWIW. When cars have a mix, one usually finds the discs in the front and the rotors in the rear. I figured it’s because the disks have more stopping power thus are under the engine, where the weight is. With less weight under the ass end, disks would have more of a tendency to lock up. If the front end (heavy) locks up, you have much less skidding than if you lock up the back. Locking the back brakes makes the car want to swap ends.
As an experiment:
Drive on an empty road or parking lot when it’s snowy or slick (about 10 mph). Slam on brakes. Car will tend to skid in a straight line, even if you turn the wheel.
Do the same, but this time pull the emergency brake. Car will try to swap ends. Now turn the wheel and see what happens.
True, but the front end also has more stopping power because weight is also transfered forward, the car dives forward in braking, hench there is less traction to the rear wheels which will lock up easier. Most cars have tapered brakes, with most of the power going to the front brakes. You can test this if you really want to on a motocycle, 70+% of a bikes stopping power is done by the front brake.
Cost, probably. If you add disc brakes to a mid-'60s fullsize car, you have to add a vacuum booster as well. Drum brakes may stop the car safely, even though they will fade with repeated/hard use. Also, people didn’t expect that much from brakes back then.
As a side note, I remember reading that Jaguar dominated European racing in the Fifties because they were the first manufacturer to use disc brakes (can anyone back me up on this?) The disc brakes allowed them to drive deeper into corners before braking. Based on that anecdote, it took about fifteen years for disc brakes to make onto production cars, which doesn’t sound like an excessive amount of time compared to other innovations in that industry.
Because you usually don’t gain anything by adding rear discs. IIRC, front brakes do about 60-70% of the work on cars, same as on bikes. Since the rear brakes are lightly loaded, drums can handle the work without fading. Has anyone here had their rear brakes fade on the street while the front brakes still worked fine?
One reason might be the improvement in tyre construction methods.
Disc brakes can dissipate a huge amount of energy but only if the tyre is grippy enough and strong enough to withstand the forces acting on it.
Drum brakes probably suited the tyres of the time fine and even when those tyres improved, the existing machinery to manufacture drum brakes made them far cheaper, rather than invest in new machines and redesign car chassis to take the discs.
Of the vehicles I own, all are front disc/rear drum with the exception of my Passat, which is 4 wheel disc.
I’ve wrestled with this question for many years, and the only conclusion I can come to for using drum brakes at all, is that they’re cheaper to manufacture. From a mechanics’ viewpoint, I much prefer discs.
On the Passat, I haven’t noticed any lock up issues in the rear, and the handbrake works as well or better than it would on drums; it just uses a different mechanism. As for lock-up, this is handled by a proportioning valve in my case, which keeps the hydraulic pressure balanced front to rear in relation to brake pressure applied.
I will be the first to cheer if/when drum brakes fade from the automotive scene (pun intended).
If you build a “performance” car in the year 2001 and it does not have rear discs, you are NOT building a “performance” car. Both of my cars have 4 wheel discs. One is 14 years old(Mazda Rx-7 TII) and the other is a VW Golf. I believe all current VWs come with 4 wheel discs, but I could be wrong.
Uh…I think you may be mixing things up here. Most all cars with drum brakes are vacuum assisted too. Drum or disc has no (real) impact on the use of power/no power. In fact, the FIAT X 1/9 had non-power-assisted brakes up into the mid-1980’s, and never needed it (4-wheel discs).
And the 1968 Cougar with 4-wheel drums I looked at a few weeks ago had vacuum assist on it.
I also seem to remember it given as a general rule at my motorcycle vo-tech that disc brakes required more pressure. Then again, a lot of what they taught was either grossly oversimplified or simply wrong.
An X 1/9 weighs about a ton; most of the stuff coming out of Detroit in the late '60s weighed 3500-4500 pounds (I assume that the OP had the US market in mind.) To stop a midsized car with non-power brakes, the pedal would have to either have twice the travel, be operated with twice the pressure or a combination of the two. It’s been years since I drove an X 1/9, but IIRC (a big if), the brakes were fairly stiff, so even that estimate might be conservative. To wrap all that up, I assume that by the Sixties, no manufacturer would have used brakes that drivers would have to stomp or push the pedal to the floor to activate, hence the need for a booster.
It also seems telling that Detroit (Ford at least) had power drum brakes as an option for a decade and a half, then went to a power disc brakes option. While they used non-power four wheel drum brakes well into the '70s, they never had non-power disc brakes.
Holy nostalgia, Batman–you had to use a '68 Cougar as an example for power drums! My first car was a 1965 FJ-6A Jeep postal van with a '67 Cougar drivetrain (an ice cream truck that would go 90MPH! ) I don’t have notes on brake sizes (and I don’t think even NAPA could help me at this late of date), but the old rear axle was in my parent’s garage for years, and the drums seemed to be about the same diameter as the Cougar drums. In any case, the brakes worked well with the old (single piston) master cylinder. Of course, I didn’t corner much with this thing; it was as tall as it was long.
FWIW, on the rear disc/drum issue: I don’t know if anyone here wants to consider Neons as “performance cars”, but this site explains how disc brakes could hurt performance: