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Piwoslaw 06-15-2009 09:09 AM

Disc brake drag
 
There is a similar thread on reducing drum brake drag, but it seems discs and drums are two totally different worlds.
Ever since it got warm enough to drive with the windows down, I can hear the drag on my disc brakes. It's more like scraping. The rear discs are flat, the front aren't, so there is a continous scraping from behind and pulsing scraping up front. In the winter I burned my rear pads when the parking brake froze solid and had the pads replaced (the discs were OK). I noticed recently that the brake fluid is just over the max line in the reservoir, but when I asked a mechanic-friend he said that raised after putting in new pads and to just leave it, it should get lower with time.
The scraping noise is pretty loud and I'm wondering if maybe I should get rid of a little bit of the brake fluid? Would it reduce the force with which the pads are rubbing against the discs? I believe that it's louder just after starting up, that after driving for some time it's quieter, but I'm not totally sure. I've tried pumping the brakes after a cold start, I've tried hard braking, but it doesn't help. Can anything else be done to reduce the drag?

Thanks,
-- Adam

The Atomic Ass 06-15-2009 09:51 AM

I can't quite tell from your wording, but are you saying your front discs aren't true? Have them ground down or replace them if so.

As for dragging, disc brakes are in contact with the disc 100% of the time by design. It's normally not something you can hear, and when spinning the wheels by hand while the car is jacked up, there should be no noticeable increase in drag with the calipers on vs. off.

First thing to check would be to see if the caliper cylinders are sticking. If they are then replace the seals and clean the pistons thoroughly, replace if pitted or worn.

Also, the level doesn't affect anything, but if the fluid is dark, change it.

MazdaMatt 06-15-2009 10:20 AM

disc brakes only push against the disc, they don't pull away. generally they are bumped back on their own. Two things can make them not bump back - your wheel bearings show zero flex (good) and your discs are perfect (good), or your calipers aren't sliding well (not bad) or stuck (bad).

Try this... go to a parking lot, roll in without using your brakes and ensure that you can hear it. Find an open space and do a full circle left, then right as hard as you can at neutral throttle. If the scraping sound goes away, it means that the pads were knocked back and your sliders are good. This means that the friction is so light that it doesn't matter.

If not, then as above, take apart your calipers and lube the sliders, if they are bad, then replace them.

bikin' Ed 06-15-2009 11:39 AM

I agree...
 
with what Matt said, but want to add that the exact sme test will give indication of a bad front bearing. If you do get noise, make sure that both the brakes and the bearings are working properly.

Frank Lee 06-15-2009 12:51 PM

Actually the square x-section piston seals are supposed to retract the piston a bit upon release.

MazdaMatt 06-15-2009 01:01 PM

Thanks, Frank. Didn't know that.

In racing this is actually a really bad thing. I can see how it would be on the street too if it was taken too far. You can actually buy a 1 or 2 psi checkvalve that ensures that your pads stay against your discs, for use on the track. In racing conditions, the hub can flex so much that the pads get "knocked back" and the driver has to push the brake pedal twice before he gets full brake pressure - one push to get the pad back to the disc, another push to not smash into the wall :)

evolutionmovement 06-15-2009 04:21 PM

It's not likely your problem, but thought I'd mention it anyway—if the scraping only happens in turns, check that there's more than 1/8" between the rotor and the backing plate. It doesn't have to be actually touching the rotor when static for it to scrape. If there isn't, you can bend some more clearance in with a screwdriver.

Don't know how you normally are with brakes, but I drive hard (or used to) and found cryogenically treated rotors to be worth every penny in terms of longevity. For normal drivers, it's probably unnecessary.

DonR 06-15-2009 04:50 PM

Racing kart brake calipers have spring loaded pads that pull the pads back to eliminate rotor friction. It does lead to a longer pedal & cooler brake temps. Karts have so little power this actually decreases lap times. You may not want cooler temps on a road car.

Don

MechEngVT 06-16-2009 12:52 PM

I don't think you generally want to reduce "normal" levels of disc brake drag for the aforementioned increase in pedal travel. I don't think that normal drag results in any appreciable temperature benefit on street cars.

Another thing that can cause binding brakes is an internal failure of the flexible portion of the brake line. An internal separation can cause a flap of rubber to retain fluid at the caliper and prevent it from knocking back. I see the OP's car isn't that old, so this shouldn't be an issue.

MazdaMatt: the SRT8 Challenger utilized a patented algorithm where the ABS/EBD system "pumped up" the brakes to compensate for high-g cornering induced knock back, preventing the double-pump problem in hard driving.

DonR: a similar spring could be used but only on pin slide floating calipers. Without knowing the particulars of the poster's car it's hard to say if this would work, but it might. It might also require a double-pump or increased pedal travel. I've worked on vehicles that had non-pin floating calipers and it would be hard to make such a spreader spring work, and with twin-piston (opposing pistons, one each side of the rotor) calipers I don't think it could be done.

Daox 06-16-2009 01:02 PM

Both of my cars have springs on the pads to move them back after you're done braking...


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