I don’t see a specific place to comment on Ray’s columns, but I want to add just one thing to this week’s column about brakes. Everything that Ray says is exactly correct, but he left out a couple of important mechanisms. While it is true that the seal around the brake piston pulls the piston back a bit as it returns to its original shape, how much it pulls depends upon the size and shape of the seal. We call them “square seals” although not all of them are actually square. On some cars, like my old Toyota, the square seals pull the piston back so far that the wheels spin freely when you jack up the car. On other cars, like my BMWs, the pads drag noticeably when the car is parked, though the brakes are in perfect condition. There are two reasons why the pads don’t continue to drag at highway speeds. 1) The rotor assembly is not rigid. Even if the wheel bearing is good and adjusted perfectly, there is a little give in the spindle and bearing such that the rotor moves slightly, relative to the caliper when you corner or hit a bump. If you wheel bearings are loose, this becomes very pronounced and results in excessive brake pedal travel. 2) More importantly, the surface of a spinning rotor drags a very thin “boundary layer” of air along with with it. The brake pads literally “fly” on that boundary layer. This is what keeps them from touching the rotors at highway speeds.
I was surprised he didn’t mention the dust boot that’s on most caliper pistons. That would have some additional effect, I don’t think O rings would do much, square seals might do a bit.
I agree but consider the audience he’s responding too… Your comments are spot-on but maybe too advanced for the casual CarTalk reader.
A lot of work was done in the 90s to bring brake drag down to zero to help fuel economy. Bigger cross-section, stiffer rubber, more compression on the seal, carefully engineered seal grooves with a trapezoidal shape with nice radii to pull the pistons back. I’ve seen calipers (from my '71 Saab) with return springs behind the caliper pistons - Ate brand calipers.
And then the master cylinder needs a bigger bore to get more volume to shorten the extra pedal travel, the bigger bore reduces the pressure so the booster needs to be bigger…
All a big circular feedback calculation!