I've been curious why Toyota chose to put drum brakes on the rear of the Prius instead of discs. Is it simply to reduce cost or is there some other perfromance related reason?
Not required. I believe 4-wheel disc brakes are required in Europe. Besides, most of the time we'll be regenerating so the brakes won't be used. Heck, my front discs are starting to show some rust. Gotta get those cleaned!
I am really starting to wonder if 4 wheel disk brakes are all that important. At one time I automatically assumed you *needed* 4 wheel disk and cars/trucks without them were cheap or somehow inferior. My 1990 Toyota 4Runner had rear drums and they worked fine. I replaced the shoes - myself - at around 210,000km. My 1992 Honda Prelude had rear disk, with a lever actuator on the caliper to make the parking brake work. The Prelude had weak parking brakes and if I didn't disassemble the rear caliper slide pins twice a year, they would seize up. It became a PITA to always worry about the slide pins. In 2000 I realized I didn't need so many vehicles. I sold the Prelude and 4Runner and looked for a pickup truck. I really liked the Tundra, but discovered it had rear drums. Since Ford, GM, and Dodge already had rear disk, I thought the Toyota Tundra was somehow inferior and passed on it. Big, big mistake. I ended up ordering a 2000 GMC Sierra K1500 extended cab 4x4, with rear wheel disk. I thought it was neat how they used a "drum in hat" arrangement to make the parking brake work. It was a brake rotor with a small brake drum inside it. If I didn't get that "drum in hat" thing adjusted every 4-6 months, the parking brake refused to work. I was also surprised when I checked the rear caliper slide pins the next spring, one already had severe corrosion. So twice I year I had to take the rear calipers off to lube the slide pins. Overall, that truck had - IMHO - dangerously anemic brakes. Despite all the brake flushing and bleeding I did, I never had truly strong brakes. I drove other 2000 GMC Sierra and Chevy Silverado and their brakes were anemic too. I got used to using both feet to brake in city traffic, and had plenty of scares and some close calls. Dealer - of course - claimed it was "normal." Glad I found a buyer in mid 2004 and got the Prius. The brakes on my Prius are many times better. Oh, and for 2005, GM went back to drum brakes on the rear. Hmmm, something about "pedal feel" and "towing performance." Wonder if they'll offer to replace all those 1999-2004 trucks they made?
Back in the days of 4-wheel drum brakes even on full-size cars (1960s)many European countries used to require front disc brakes on any car with a 1.5 liter or larger engine. My brother's 1967 imported bug (1500cc engine) had front discs (which USA-spec cars lacked) drove parts guys nuts. He finally resigned to telling parts guys he had a '66 Karmann Ghia, which used the same brake assemblies and spare parts. Do they now require 4-wheel discs? The ICE is 1500cc. Or maybe it's just because they work better. On this car, front-heavy, and generator-assisted most of the time, the rear brakes just don'e have much to do. Perhaps it was a cost and performance issue-- not worth the marginal benefit.
Don't forget that rear drum brakes on pickups and suvs are probably better than rear discs since they partially shield the pad from the mud and crud of off-road travel.
Drums will give slightly better milage. Drum brakes also have lower friction since the shoes don't contact the drum when the brake pedal isn't being depressed. Disk brakes always have contact between the pad and the rotor for to keep the disk dry and to warm the pads which work better at optimal tempratures.
