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

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.

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.

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.

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

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 :)

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.

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

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.

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

Most new cars have some sort of retraction mechanism;
whether it's the return spring like Daox' Toyotas (all Toyotas for that matter, and most Fords and so on)
or the X square piston seal (which works quite well)
You need to make sure all the parts are moving freely and you are not hearing a bearing noise or other rotational scraping.

A larger master cylinder could compensate for the "double pump" effect.

Mech, thanks for that info - i didn't know there was a system that pre-loads the brakes after knockback. Sounds genius, but I wouldn't want it on my race car... automation is scary - especially when it is made by dodge!

Come to think of it, my pads have springs, too... they offer about 6-7 pounds of force (estimated, thinking back to my last brake change), which is definately not enough to push the pistons back.

although we're off topic.... it isn't quite that easy. I have spent a number of hours discussing this issues with my racing community at large and the solutions are rare and expensive. Your solution results in a different feel every time (not acceptable at 100mph heading towards a wall between two cars) and a heavier pedal. Defiantely a discussion for a different thread... a different forum, for that matter. :p

Kart calipers are twin pistons (one on either side of the rotor). A kart chassis experiences a lot of flex, which can put a lot of drag on brakes. I also would not want this on a full size race car.

Don

Karting isn't a good comparison to full-size "real" cars. Karts have to have chassis flex as they have no suspension. Having a suspension divorces brake mounts from chassis flex, but you still have to deal with bearing clearances and hub/rotor flex. In a street-driven car that does not see a track or Autocross you won't likely see flex inducing any perceptible knock-back.

A Side Note
 

The brake rotors on the front of my Dakota were warped pretty bad. Last Saturday, I replaced the rotors, cleaned and lubed the caliper slides, adjusted the rear brakes.

In my week of driving, my average mpg city went from 20.5 to 21.3--according to the on board computer. (It also makes using the brakes safer and causes less vibration.)

For those of us who try not to use the brakes too much, the dirt and crud accumulation is something to check out periodically.

It seems my Jeep's brakes don't touch at all when I let off the pedal. The pads have less than 50% on them though which could explain that, but, nonetheless, coasting is forever on that thing, even in neutral. They come off the discs with such a small gap between the pad and rotor I don't think I could see it without getting eye right up to it.

Reposting from a different thread:
A few days later:

When I loosed up my brakes car rolled so much better that I had to brake when I come to my home. I glide to stop. It is about 700 meters, first 300 hundred downhill and rest gentle uphill. When brakes were dragging I needed 40 km/h speed at the top of that last hill to be able to get to parking space. When I loosed my brakes I could drop the speed to 30 km/h and still need to brake to stop at my parking space ;). On the downhill I gain same top speed even starting with lower speed. Due to less friction I have much more speed at the end...

Piwoslaw, thanks for quantifying this... although I'm sure it varies greatly case-by-case. I always like to say that making sure none of the brakes are dragging should be one of the very first things to check when tuning up or ecomodding. I'd wager dragging brakes happens much more often than people think.

Nice to hear that you sorted it out.

A tip from a good mechanic I once heard:
From time to time (let`s say once a month) while standing still step on the brake pedal as hard as you can until it goes all the way down, do this three times. And use a lot of force, don`t worry about destroying anything.

What this does is makes the hydraulics work all the way and increases the life of brake cylinders, while it also makes for smoother brake discs and improves brake life.

I had some rust on the rear discs and after doing this it is all gone and smooth as ice. Doing this not only prevents corrosion buildup and improves disc and pad life, but also maintains the shortest emergency braking distance.

I recall a story about Tesla's design and development. They found that reducing the drag on the brakes like the go karts. Allowed them to go 2-3 miles in their total commute on battery.

Could you tell any difference in coasting? MPG? Braking? And do you have any pics you could show us? Thanks :D

I'm mostly thinking it could help my coast down

Oooo, pictures, pictures! My car doesn't have springs in the brakes, and it seems that the brake dragging keeps recurring:( Can springs be somehow retrofitted?

you've give me something to do today now

*grabs jack*

The setup is pretty simple. On the end of he brake pads there is some way to hold the spring. They insert a wire/spring into the holder to push against the other pad. Here is a google image. The spring would be item 12.

http://myzx2.com/howto/RearDisc/ExplodedRearDisc.gif

Are we sure those are r return springs and not j just anti-rattle clips? As I'm sure you know it takes a good bit of ossie to relieve the calipers when changing brake pads... S springs so small seem more like they'd just be designed to keep the pads from rattling at the onset and release of braking.

Googling around found this Miata forum, and this Miata how-to, which suggest they may be anti-rattle, but may be able to keep the pads off the rotors also. I think it's not about the springs using lots of force to pry the calipers far away from the rotor almost instantly, but just enough to barely move it away, or just enough to keep them from moving back on their own. Just my guess.

the wire springs are anti-rattle clips. one could argue that even an minute amount of beneficial tension is going to help the anti-drag cause, but my money bets the anti rattle spring's contribution is insignificant.

i think the best contribution one could make with reducing drag is making sure all pins and sliders are lubed to allow free movment of the floating caliper.

also... one more thing to consider is the parking brake adjustment of rear disc brakes.

On a ZX2 for example, behind an access bolt on the rear of the caliper, an allen set screw exists that limits retraction of the caliper piston.

this is done so the parking brake cable mechanism has a reference upon which to pull against. one can use this allen screw adjustment to gain additional clearance between the pads, much like one would use a star wheel adjuster on drum brakes.

ONE very important thing about disc brakes, make sure that you lubricate the caliper/pad pins(#14 in the above diagram) with silicone grease.the calipers are all of the "floating" design and the grease helps them to retract. once a brake job has been performed you should be able to move the caliper back and forth easy when installed. Also i use a file and remove any rough edges on the pads this also helps on keeping them from hanging up. Something to consider, back in the day drag racers would use only cars with front drum brakes and adjust them back so as to reduce front brake drag. That's how much effect front disc drag had on the difference between winning and losing. there will always some drag with disc brakes, but they are better now a days.

.

I can't get rid of my dragging brakes:mad: It isn't as bad as before the first major brake cleaning back in January, but it's still louder (sometimes MUCH louder) than it should be.
Today my bro-in-law/mechanic and I took off the rear wheels to see what is scraping. The discs had lots of rust and filth on them. They were worn down about 1mm where the pads touch, but terrible elsewhere. It turned out that the scraping noise was not from the pads, but from the gunk that was caked on the edges of the discs, which would rub against something. We knocked as much off as possible with a small hammer, then filed down what was left, on both sides of each disc. Then we filed down the pad edges so that they are easier to move. The wheels now rotate more freely, and are much quieter.

The left wheel looked much worse than the right, this is also the louder of the sides. There is water inside the left cable housing of the e-brake, which may cause the pads to slightly stick on frosty mornings. The reason may that the car is usually parked facing east, so the left (north) side doesn't see any direct sunshine, while the right side quickly warms up and dries during the day. Our dog also seems to prefer the left side of our car when doing his peeing rounds.

Back the car in the parking spot on alternate days?

Sure is room for improvement in parking brake activation systems. Locked-up cables are a chronic problem here in the Rust Belt. You either have to use the parking brake all the time, or not at all. If you tend to not use parking brakes, then one day decide to use them, they probably will not release. :mad:

Not an option: It's a very tight spot which is hard to maneuver into backwards, but impossible to get into forwards. That's what I get for having a long car.

Replace the cable, and stuff the new cable's liner with a water repellent lubricant.
WD40 would do temporarily.
A friend of mine used a home brewn mix of graphite powder and a liquid molybdene-lube (or similar) on cables on his motorcycles.
The graphite makes the lube stick to the cable, and is a lubricant by itself.

I have a 1992 LaBaron that had a LOT of brake problems which was cured after remembering what a friend told me to check the brace on the rubber hose feeding the wheel. It rust and shuts off the return but the hydrolics will apply the brake. All you have to do is expand the brace and stop safely. SORRY about the spelling.

Also internal failure of rubber brake line has been said to happen where it may look OK from the outside but inside a loose flap of rubber closes off the flow (I've not seen this myself though).

I can vouch for that, Frank. I had a rubber line that was only a month old separate and internally collapse the plastic line inside the rubber. The brakes would apply, but not release for several hundred feet on that side. Eventually, they wouldn't release at all. Caught the poss on fire momentarily on Gerald.

Seen the brake hose "check valve" syndrome back in the 1980s on Mercedes in particular.

Best test was hit the bleeder and release the residual pressure, then reapply the brakes and see it lock up again.

Eventually I got to the point where I would clamp off all four hoses. This allowed the master cylinder to be tested, then each wheel individually by releasing the clamps in sequence. Helped a lot to quickly diagnose the frozen floating caliper pins.

One time a friend brought his old tired 280 Z in the shop with a brake pull. The left caliper was practically frozen. Not paying attention I put my hand on the rotor and had "min dia 10.5 MM" branded on the callouses at the point where the fingers meet the hand. The callouses were so thick it took too long to feel the burn. Never blistered.

regards
Mech