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Disc brake fun

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NutmegCT

NutmegCT

Great Pumpkin
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For starters, my 2010 Altima has a brake "anomaly".

But before I get into the ugly details ... How do the pads/calipers on a disc brake system retract after use?

On drum brakes, there are return springs which pull the shoes back when you release pedal pressure.

But on a disc brake, there are no return springs. Seems the pad would continue lightly touching the rotor after the pedal is released, which would cause friction (and heat) on the rotor. (right?)

Inquiring minds want to know!

Thanks.
Tom M.
 
OK Tom...since no one has yet to reply, I'll bite. I'll guess the anomaly is one pad on one side of the rotar has worn down faster than the pad on the other side. Could it be corrosion occurred when the brakes were exposed to road salt?
 
Thanks Jay.

Here's the "anomaly": Driving a short trip (for example over five miles), the right rear wheel brake drum and rotor get way too hot to touch. Not the left rear wheel, only the right rear wheel. All other wheels have hot rotors after driving, but not the drum.

To check if parking brake shoes were causing the problem, I removed the shoes. Did the test drive - same thing. Drum and rotor too hot to touch. Interesting thing is that the caliper and pads don't get hot.

Also pulled caliper and checked that the sliding pins are loose and piston is free. No problem there. And if the caliper and pads don't get hot, seems they're not causing the problem. But why does that RR drum get so hot? On a long drive (ca. 50 miles or so) I even smell a hot brake odor from that wheel.

But that got me wondering - when you apply the foot brake, the caliper piston pushes the pads onto the rotor to slow the wheel. But when you release the pedal, what pulls the pads *away* from the rotor?

Edit: I also replaced the hub/bearing unit, in case that was the problem.

Thanks.
Tom M.
 
Wow, that is a strange one. Sure sounded like a wheel bearing...obviously ruled out when you replaced it.
 
My understanding is there is nothing to retract the pads. Any slight imperfection in the 'staightness' of the discs is sufficient to lift the pads off the disc.
 
Nothing to retract the pads - that's what I've been finding. Doesn't seem "right", but that's the way it is. Would that cause the rotors to always stay hot?

Anyway, still need to figure out why the RR wheel drum gets so hot.

Thanks.
Tom M.
 
There isn't anything to retract the pads - but, remember that as you take your foot off the pedal the brake fluid flows back - so there is a vacuum of sorts.

Two thoughts.

1. the pad isn't retracting enough to clear but not sticking enough to drag. We had this on a trip to Florida - a front caliper ended up seizing. never knew what caused it but, it was literally within a week of getting new brakes - and the mechanic was trustworthy.

2. Likewise with the rotor. Typically a warped rotor will shudder. - on the front as it is transmitted to the sterring wheel. Not sure about the rear, but again it may be in that inbetween space - enough to rub but not enough to drag or shudder.

I would start with rotor.
 
Thanks JP. Regarding the fluid, there's no actual movement of the fluid itself at the slave. Just a change in pressure - so no "vacuum" is formed.

From many sources, it does sound like the pads are supposed to continue contacting the rotor even after the brake pedal is released. That sounds weird to me, as there would always be some friction - and thus, heat. But who am I to argue with the engineers???!!!

That's why there are vent holes in the caliper.

To me a very weird aspect, is that the rotor and brake drum get very hot - but the caliper and pads don't.
 
NutmegCT said:
Thanks JP. Regarding the fluid, there's no actual movement of the fluid itself at the slave. Just a change in pressure - so no "vacuum" is formed.
Click to expand...

Of course there is movement - think of your MG - you push the fluid with your foot at the MC - it pushes more fluid into the SC which pushes the piston which pushes the brake pads. You take your foot off the brake and the fluid flows back into the MC. Vacuum is not exactly the right word (no air) but, presumably there is some "drawback" You can't just increase pressure without movement.

NutmegCT said:
From many sources, it does sound like the pads are supposed to continue contacting the rotor even after the brake pedal is released. That sounds weird to me, as there would always be some friction - and thus, heat. But who am I to argue with the engineers???!!!

That's why there are vent holes in the caliper. To me a very weird aspect, is that the rotor and brake drum get very hot - but the caliper and pads don't.
Click to expand...

No - otherwise you would feel it when you spin the tire. Tire should spin freely. In terms of the calipers and pads, don't forget they are dissimilar materials - brake pads are designed to absorb heat (think asbestos) - but in turn wear out. Rotors are metal and therefore transmit heat - but don't wear out. The holes (and slots and shape) are there to disperse the heat (larger surface area) - and the holes, while mostly a "go fast" feature in a road car are there from racing.
 
Well, I don't think the fluid is moved at all. Pressurized yes, but the fluid is already in the slave cylinder before you press the brake pedal.

Edit: well, maybe a zillionth of an inch, to move the slave cylinder. Another way of looking at it - why do drum brakes have return springs on the shoes?

Anyway, if the rotor isn't true, then wouldn't the pads heat up? After I drive the car, the rotor drum is too hot to touch, but the caliper and pads aren't nearly that hot.
 
The fluid is already there but there is movement. As you depress the brake pedal you force fluid from the master cylinder through the lines in the slave cylinders. When you let off the brake pedal the master cylinder draws that fluid back into the reservoir. There is definitely movement of the fluid.

Just for clarity's sake are your rear brakes drum our discussion? You keep saying drum but then talk about the rotor, pads, and caliper.
 
Thanks Walt. On many cars (like my Nissan Altima), the rear disc brakes have a parking brake drum. There are small shoes inside the drum which activate when you pull the parking brake handle. The drum is like a "top hat" at the center of the rotor disc. Similar to this:

trw-disc-rotor-nissan-sentra-n16-front-nfautopart-1701-18-nfautopart@3.jpg

Hope that clears things up.
 
NutmegCT said:
Well, I don't think the fluid is moved at all. Pressurized yes, but the fluid is already in the slave cylinder before you press the brake pedal.

Edit: well, maybe a zillionth of an inch, to move the slave cylinder. Another way of looking at it - why do drum brakes have return springs on the shoes?

Anyway, if the rotor isn't true, then wouldn't the pads heat up? After I drive the car, the rotor drum is too hot to touch, but the caliper and pads aren't nearly that hot.
Click to expand...

More than a zillionth of an inch - 1/4 in at least X 2 and possibly X4 depending on whether you have two or 4 pot brakes - the "extra" fluid pushes the brake pot towards the rotor and "fills" the void behind it.

Again, the pads disperse heat quicker - think of the silicone bricks on the space shuttle - it isn't that they don't get hot, it is that they get rid of the heat faster.
 
I'm going nuts. JP did you just post the same thing twice?

Also, help me out here ... what is a brake pot?

:confoozled:
 
Tom, is the parking brake hydralic or mechanical?

That set up seems to be more complicated than it needs to be.
 
Hi Walt - the parking brake is purely mechanical, cable operated. Completely separate from the hydraulic disc system.
 
NutmegCT said:
...that got me wondering - when you apply the foot brake, the caliper piston pushes the pads onto the rotor to slow the wheel. But when you release the pedal, what pulls the pads *away* from the rotor?
Click to expand...
Been doing some reading on what would keep a piston applying pressure to the pad after the brake pedal was released. Seems it could be a brake hose that collapses. The brake is applied, the piston moves the pad to the rim, the hose collapses (which maintains the pressure behind the piston...getting everything hot). When foot is removed from the brake pedal, all three other calipers release...the fourth with the bad hose doesn't...then slowly it decompresses so that when you go to check if there is a caliper "hanging up", there isn't one. Sound like a stretch Tom but I think you're about at that point where you consider everything.
 
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