[kstrauser]

Not significantly. Otherwise they’d get hot and work less well. Pads ride very closely to the disc so that road stuff can’t get wedged between the surfaces, and so that you don’t waste break pedal travel just getting the pads close to the surface. Well adjusted drum brakes should be arranged almost exactly the same way.

From https://mechanicswizard.com/how-close-should-brake-pads-be-t...:

> Typically, the optimal gap between brake pads and rotors is between 0.5 and 1.5 mm (0.020 to 0.060 inches).

[lukevp]

Are you familiar with how disk brakes on a car operate? They’re essentially a hydraulic piston clamping the brake pads against the rotor. When you let off the brake pedal, the pressure is removed, but there’s not a “retraction” of any kind. So there’s no pressure but nothing is actually making that gap. That article was articulating basically that the piston, when not compressing the brake pads, should not be too far out, because that’s just unnecessary travel of the pedal, but it does not mean that the brake pads are somehow gapped away from the rotors.

Drum brakes by contrast have springs that retract the pads from the braking surface, so the OP has a very valid point.

[kstrauser]

I'm very familiar, having changed many sets over the years. If there's no air in the system, pulling fluid away from a hydraulic piston must retract it. It can't just sit there unless there's an air bubble to expand or so much negative pressure that it forms a vacuum somewhere. Even then, pressure on the atmosphere-facing side would push the piston back toward wherever the vacuum or air bubble is.

[m463]

the seal/friction is what stops the disc from fully retracting.

There are clips you can buy to force the calipers apart. I wonder if this negatively affects safety though.

[datavirtue]

The rotors push the pads away from the surface if the slides and spring clip seats are maintained. I have seen many unbalanced brake systems with uneven pad wear of all kinds due to rust/dust/heat impacting the system.

[jackvalentine]

Something makes them retract slightly - how they’re seated in the caliper retaining clip maybe I’m not sure because trivially you can take the wheel off and press the brake and see it.

It’s even easier to see on a bicycle with hydraulic discs.

[rightbyte]

> When you let off the brake pedal, the pressure is removed, but there’s not a “retraction” of any kind. So there’s no pressure but nothing is actually making that gap.

The wobble pushes the pads back in, right?

[smitelli]

A healthy disc shouldn’t wobble enough to push much of anything. We’re talking like 1/20th of a millimeter.

[rightbyte]

Should be enough to relieve the pads I think? The less wobble the less the pistons need to retract too.

I mean you can hear scratching when the pistons get crusty and the don't retract like they are supposed to.

[AngryData]

Technically there is a slight amount of rubber spring back for a fraction of a millimeter and the air traveling with the rotor surface puts a small amount of pressure back on the pad and pushes it imperceptibly backwards. Of course that is only really effective if your rotors are perfectly flat and warp free, which most peoples are not because rotors never get resurfaced any more and 95% of them are super cheaply casted and turned which leads to warpage extremely quickly.

[BenjiWiebe]

Ya but they don't get hot when not applied, which means they're not wasting (much) energy.

[datavirtue]

The default for drum is less friction (gap increases, takes more pedal) over service life whereas pads default to more friction (slides gunk up and spring clip faces rust and swell) from failing to retract as well.