Dear Tom and Ray: All of the cheapest cars these days seem to have drum brakes on the rear instead of disc brakes. But drum brakes have more parts than disc brakes, and are much harder to assemble (I know from experience). You'd think that would add assembly-line costs and that it would be cheaper to put the better disc brakes on the cheapo econo-cars. What's up with that, guys? What am I not understanding? -- Barnaby

TOM: The missing element is the parking brake, Barnaby.

RAY: Right. A drum brake also can serve as a parking brake. So you're essentially getting two for one.

TOM: Disc brakes make lousy parking brakes. So if you put disc brakes on all four wheels, you then have to add two additional, separate drum brakes on the rear wheels to serve as parking brakes.

RAY: Or you have to add a very expensive, and failure-prone, component to the rear disc-brake calipers to make them work as parking brakes.

TOM: But drum brakes naturally make good parking brakes, because they have a binding, or self-wrapping, action (that's also what makes them lock up, and makes them less desirable than disc brakes for stopping the car).

RAY: So if you use drum brakes in the rear to stop the car, you already have your parking brake, and you save money that way.