So I had a weekend project in which I attempted to replace a wheel hub on my POS Alero. The disassembly went smoothly until I ran into the wheel hub nut which I could not remove, which lead me to abandon the project and reassemble everything.
That’s not what this question is about though. In the process of taking apart and putting back together the brake-rotor-caliper components I noticed that the caliper seemed to fit very snugly on the rotor. Not so snugly that I couldn’t slide it on/off with a bare hand, but it was tight enough that I needed to force it into place. This led me to wonder if this is normal, it seems to me that it must be incredibly inefficient to have a rotor rubbing a pad pretty much constantly. It must create a lot of heat and wear on the pads.
That said, I have not noticed any issue with my steering/alignment/brakes (other than the sensor which spurred the initial repair attempt, natch) and a visual inspection of the pad and rotor seemed pretty normal to my untrained eye. So, what is a normal amount of friction one should experience when installing a caliper over a rotor?
The fact that you could remove the caliper without prying makes it clear that it was fine.
The caliper piston fits easily into the caliper bore (without the seal installed) – I don’t know what the clearance is, but I’d guess in the .020-.050" range. It’s a snug fit with the rubber O-ring seal installed, requiring more than hand pressure to press the piston in. The O-ring is square-cut, which is a critical design feature. When the piston is pressed out against the pad by brake fluid pressure (stepping on the brake pedal), the O-ring deforms from a square into a rhombus. When the pressure is let up, it returns to a square shape, which retracts the piston away from the pad a few thousandths of an inch. This leaves the pads just kissing the rotor, which can spin past them easily enough but doesn’t have any clearance to speak of. It’s this lack of clearance that makes removing and replacing the caliper a just-barely proposition. In some case it’s necessary to force the piston back into the caliper a bit, by prying on the pad or rocking the caliper body, especially if there’s a ridge at the rotor rim.
So in sum, the normal rest position of the caliper piston causes no appreciable wear or heat buildup on the pads, but does make for a snug fit when it comes to withdrawing and installing the caliper.
Thanks for the thorough explanation. My rotor did have a slight lip like you note due to wear/oxidation and did require a little more coaxing than the rest of the travel. It surprises me that such a narrow clearance would have no impact on heat and wear. In a precision machine part it’d be expected, but in something as exposed to the elements and under such varying temperature ranges it seems like there’s be quite a high risk of seizing. Guess the engineers know what they are doing.
There’s no conspiracy here; Gary’s explanation is spot on, and applies to everything from motorcycles to light aircraft. The fact that warped rotors happen to be noticeable is a by-product of a rather crude, but effective and simple design.
And this is what god made giant C-clamps for. Now if he just made the wheel wells bigger so there was some room to get the c-clamps in there. In fact, you’ll (the general you, not Gary you) notice that there’s often times a a flat spot on the caliper specifically for a clamp to have a spot to grab without slipping off.
As for a conspiracy. Pads are cheap, rotors are cheap and the whole system is mechanical with (basically) no help from the computer. The system, as designed a long time ago, works with the engine on or the engine off, it works with the one of the brakes lines cut, it works if you’re a bit low on fluid or vacuum. It’s self adjusting so you can ride the pads right down to the end. It’s a good system and it costs very little to maintain. Hell, if you get a new car and get rid of it in 4 years, chances are you’ll spend nothing on your brake system at all. As long as you don’t tailgate or drive two footed.
To the OP, I too was surprised to find out the first time I did a brake job the you are effectively always ‘riding’ your brakes. OTOH, it makes them very responsive since there’s virtually no play in the system.