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As far as I know brake pads are supposed to maintain slight contact with the rotor at all times. As usually the only thing pushing the piston back after braking is flex of the square cut seal around the piston (which has quite a short travel path), eliminating drag completely is difficult (and probably pointless). However:

  • When is it safe to say that brakes are dragging too much? For example, should the wheel be spinning freely when you turn it by hand (jacked up, of course) even if there is a slight dragging noise? Should the discs be cool after a calm drive? Should only the wheels be cool?
  • Is a seized piston the only reason for brakes dragging (if properly sized pads and rotors are installed)?
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  • It seems to me that when cars had drum brakes on the rear, the self-adjustment kept the front brakes from dragging? Now with so many 4 wheel disc cars, I see that the front wheels (only) on nearly all are covered in brake dust. Is this a real issue? Why did we switch to 4 wheel disc if it is creating such a problem?
    – user15009
    Apr 12, 2016 at 15:55
  • @nocomprende Drum brakes have a shoe return spring which prevents drag, so dragging brakes probably wasn't as common in the old days. Drag only an issue when the calipers are in a poor condition, as I understand, normal drag should barely have any effect. At the same time disc brakes are much more effective, less susceptible to fade and self adjusting, so no wonder we are leaving drums behind. The question is: how much drag is too much to be considered normal?
    – I have no idea what I'm doing
    Apr 12, 2016 at 16:20
  • Well, what I am saying is that as far as I can tell "none" used to be normal, but now "some" is normal, based on the ubiquitous presence of dust - only on the front wheels of the cars I see, including my own. Why only the front if there are discs all around? If they are self-adjusting, why are the front not adjusting themselves in the same way as the rear?
    – user15009
    Apr 12, 2016 at 16:23
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    @nocomprende as far as the car cares "none" is pretty close to "some", if it's just barely touching. There is dust on the wheels because the brake pads are wearing and leaving it's friction material around. The design of disc brakes is "open", while the brake pads of drum brakes are inside the drum, so you can't see any dust on the drum braked wheels. And you see it mostly on the fronts because the fronts are doing most of the braking (at least 70%, I believe). The fronts are adjusting themselves the same way rears are.
    – I have no idea what I'm doing
    Apr 13, 2016 at 7:04

2 Answers 2

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What is a freely spinning wheel?
Of course, you should be able to spin the wheel by hand, but beneath the brake, bearing and transmission can apply lots of drag. My rear wheels do about 1.5 to 2 turns, my front wheels not more than half a turn when I put them in rotation by hand. While it's the same for the front wheel of my mother's car, its rear wheels seem to spin forever.

It's best to compare the left and right wheel to get an idea about the expected drag, furthermore, one can compare the drag before and after removing the caliper as well as after removing the pads. (Pads alone can be seized due to dirt / corrosion, too)

Temperature also gives a good hint on whether a brake is seized or not. Highly seized brakes produce lots of heat, slightly seized ones still a little. But again, it is not easy to judge if a wheel is slightly seized, as the temperature depends on drive behavior, weather and the car itself. And the bearings and tires also produce some heat.
It is again best to compare right and left side.

In my experience, I once had a highly seized brake in the rear which still wasn't noticeable while driving. But after, I could feel the heat on my skin when standing away one meter from the wheel...

I now have a infrared thermometer and measure temperature of wheels and discs from time to time. The second time I had a highly seized brake (front), the wheel had a temperature of 100°C (210°F), and the disk of nearly 300°C (570°F) after driving 120-140km/h (75-85mph) for 30min plus driving home at low speed for 5min. The seizes brake was only noticeable while driving at that high speed, as it started to make noise and the car pulled to the side when braking. It was not noticeable when driving in the city.

However, don't be too paranoid. When my rear discs and pads had been replaced, the temperature difference between the discs was 20-30°C, but as the brakes broke in, the temperature difference decreased and now is in the order of 5-10°C.

My left/right discs now differ about 5-10°C in temperature, but when I had my rear discs and pads replaced, both sides differed by about 20-30°C.

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To my mind, a wheel should spin freely when spun on the jack. A little noise is acceptable but if the wheel is clearly being slowed down excessively or is particularly hot after use, something is not right.

As regards causes for brake drag, there do indeed include seized or sticking pistons but could also include sticky or corroded caliper slides (the pins, sleeved or shims that the yolk of the caliper moves on), corrosion of the pad backing plate of the pad itself or even anti-squeal shim can cause pads to stick in the caliper. Corrosion in the caliper itself along the aspect which the pads are moving. A pad that's worn down to the metal may cause a brake to drag or bind.

I've also seen debris such as stones entering the caliper causing problems as well as misshapen backing plates rubbing on discs.

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