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F22 / F23 2 Series (2014 - Current)
The 2 Series coupe is the replacement for the E82/E88 1 series coupe. Production starts in November 2013 on the 228i (N20) and M235i (N55) coupes. Look for them in dealerships in February 2014. The convertible F23 2 series will follow in the fall of 2014.

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Old 06-20-2019, 08:38 PM
DrivingPassion DrivingPassion is offline
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What retracts the brake pistons?

When your foot is off the brakes, what retracts the pistons in disc brake system?
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Old 06-21-2019, 02:32 AM
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Doug Huffman Doug Huffman is offline
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Nothing.
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Old 06-21-2019, 06:09 AM
marcozandrini marcozandrini is offline
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To add to Doug’s elegant reply, the brake pads are in constant, but very light, contact with the brake rotors whenever the brake pedal is not depressed.
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Old 06-21-2019, 07:05 AM
DrivingPassion DrivingPassion is offline
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Originally Posted by marcozandrini View Post
To add to Doug’s elegant reply, the brake pads are in constant, but very light, contact with the brake rotors whenever the brake pedal is not depressed.
Wouldn't that wear out the pads?
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Old 06-21-2019, 07:43 AM
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Originally Posted by DrivingPassion View Post
Wouldn't that wear out the pads?
Yep, and the discs.

My BMW even has a feature that automatically tightens the brakes if I come off the throttle quickly and also periodically tightens the brakes in the wet. BMW Dynamic Stability Control uses the brakes to prevent wheel spin like a limited slip differential.
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Scepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.
(The Works of George Santayana p. 65)

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Old 06-21-2019, 07:49 AM
jaye944 jaye944 is offline
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Douglas,

That is the shortest reply, post I have ever seen you make.
made me laugh dude... I was expecting at least a 3 line reply,
citing various sources and some anecdote

LOL
yr da best !



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Nothing.
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Old 06-29-2019, 09:36 AM
DrivingPassion DrivingPassion is offline
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Quote:
Originally Posted by Doug Huffman View Post
Yep, and the discs.
[...]

Why such a design?
Are all disc brakes designed this way?
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Old 06-29-2019, 10:03 AM
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The pads need to float on the discs for minimum delay when applying the brakes, to keep the discs clean (of rust, e.g.) and dry. All designed this way, without active retraction, yes. But remember that they don't touch but float, much like a hard disc RAM read-write head floats above its 10,000 RPM disc.
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Scepticism and Animal Faith (1923)
Scepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.
(The Works of George Santayana p. 65)

Eschew eristical argumentation. I am responsible for what I write, not for your understanding of it.
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Old 06-29-2019, 10:18 AM
DrivingPassion DrivingPassion is offline
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Got it. It made sense now. Thanks!
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Old 06-29-2019, 10:31 AM
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Just don't drive with one foot resting on the brake pedal like I've seen some people do!
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Old 06-30-2019, 04:51 PM
MikeNola MikeNola is offline
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The caliper piston is sealed to the caliper bore by a square seal set into a groove machined into the bore just below the top.

When the brakes are applied, hydraulic pressure pushes the piston out, which twists the square seal. When the brakes are released and the hydraulic pressure dissappates, the square seal returns to its "untwisted " normal shape and retracts the piston as it does.

Mike
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Old 06-30-2019, 05:54 PM
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^^^^

Barbara Streisand.
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Scepticism and Animal Faith (1923)
Scepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.
(The Works of George Santayana p. 65)

Eschew eristical argumentation. I am responsible for what I write, not for your understanding of it.
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Old 07-30-2019, 02:15 PM
lmacmil lmacmil is offline
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Quote:
Originally Posted by DrivingPassion View Post
When your foot is off the brakes, what retracts the pistons in disc brake system?
When you apply the brakes the caliper expands slightly and when you release the caliper returns to its unpressured shape and sends the fluid back to the master cylinder. The piston seal design and the shape of the groove it sits in determine the distance between the pads and the rotors, which, as was pointed out, is pretty much nothing.
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Old 07-30-2019, 04:11 PM
DrivingPassion DrivingPassion is offline
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Thanks for all the clear description.
It's nothing like seeing the inside.

https://www.aa1car.com/library/brake_calipers.htm

I still cannot fully understand how the seal can retract the piston.
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Last edited by DrivingPassion; 07-30-2019 at 04:18 PM.
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Old 07-30-2019, 06:03 PM
lmacmil lmacmil is offline
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Quote:
Originally Posted by DrivingPassion View Post
Thanks for all the clear description.
It's nothing like seeing the inside.

https://www.aa1car.com/library/brake_calipers.htm

I still cannot fully understand how the seal can retract the piston.
The seal is pretty stiff rubber and almost square. As the piston moves, it stretches the seal and when the pressure is released, the seal returns to its original shape.
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Old 08-08-2019, 07:20 AM
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Also note that the distances involved here (the amount the pads "retract"; movement of the piston seals) are on the order of a thousandth of an inch, if that. As Doug explained, the pads essentially float on the air film created by the spinning rotor and this is a Good Thing™.

Jack up a wheel and spin it by hand and you will hear the pads dragging ever-so-lightly on the rotor in the absence of that air film. The additional wear from this contact is technically non-zero but in practice, is completely negligible: with each brake application, a few additional microns of pad/rotor material are lost to this "dragging" effect until all contact is eliminated. After that there is zero wear until the brakes are next applied. So, think of it as a normal "tailing off" effect each time you touch the brakes--it's not additional wear, it's just a part of normal wear that is imperceptible.
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