[steve]

When I pull my 3 month old, 2200-mile 2004 Prius out of my garage in the morning, or after its been sitting around for a while, or sometimes at seemingly random times, the brakes make a fairly loud squeaking noise. Does that happen to anyone else?

I took it in to the dealer, who gave me the following explanation. The rotors heat up quickly during normal driving and braking, and then they cool down very quickly when the car is stopped. As they cool down, condensation forms, and that moisture causes the rotors to rust - within a matter of hours. If the car has been sitting for more than a few hours, the rotors will therefore have some rust on them, which makes a squeaking noise when being rubbed off by the brake pads the next time they are applied. Does anyone know anything about the chemistry of rust? I thought rust takes weeks or months to develop -- how can it form in a matter of hours?

 

[snlcross]

Ours does the same thing, but we've learned to ignore it. We live in a very humid climate and have had the same situation with all of our previous cars. There is a built in warning to change pads when you hear them squeel every time they're applied.

 

[evil_attorney]

I have 1200 miles on my #9 and I just noticed the squeaking at different times. I'm not sure condensation is a cause...I live in Southern California where it's almost always dry and never rains. It's a little annoying, but I'm sure it's no big deal.

 

[BIF]

The rotors are bare steel. Bare steel will immediately begin to oxidize (form surface rust) in the presence of oxygen and water.

This phenomenon is easy to see on certain wheels that allow you to easily see the rotors behind. And yes, the brake pads press pretty hard against the rotors to stop a car. This action will naturally scrape off that surface rust...almost immediately, leaving bare steel exposed to the elements once again.

That leads me to ask a question of the engineers here. Does anybody know how hard a typical car's brake calipers have to press against the rotors (or drums) in order to stop a car the size and weight of a Prius?

 

[bradca21]

I have 1200 miles on my #9 and I just noticed the squeaking at different times. I'm not sure condensation is a cause...I live in Southern California where it's almost always dry and never rains. It's a little annoying, but I'm sure it's no big deal.

For what it's worth, I'm in So Cal also, have just shy of 8500 miles and yesterday was my 6 month anniversary of owning it
My brakes have NEVER made any kind of noise other than right after I first changed my wheels. It was a rainy day and they made a weird little shudder for the first 5-10 minutes of driving on them. The fact that it discontinued after that, I'm sure it was just a combination of new stuff and wet roads...

Last car we owned where the brakes squeeled was a 60 something Chrysler that my dad used to have...
Although I don;t think it's necessarily a huge deal, it isn't happening to everyone, even in your general area...

 

[mdacmeis]

The rotors are bare steel. Bare steel will immediately begin to oxidize (form surface rust) in the presence of oxygen and water.

That leads me to ask a question of the engineers here. Does anybody know how hard a typical car's brake calipers have to press against the rotors (or drums) in order to stop a car the size and weight of a Prius?

Automotive engineer's comments:

First paragraph: This is correct, except that it is actually a grade of cast iron. FeO3 forms quickly, depending on the absorbed heat, moisture in the air, content of the brake linings (some have corrosion inhibitors or sacrificial metals in them which slightly coat the rotor during braking), and a few other technical variables such as the exact metallurgy of the rotors. This has gone on for all time, however as lining compounds have changed to address environmental concerns and longer brake lining life, combined with brakes which are sized to be adequate for the mass of the vehicle rather than over sized (saves mass and cost), brakes can run hotter and be more sensitive than those of the past. Brakes made in the 80's are generally considered undersized, those in the 90's right sized but prone to noise or pulsation, those in the 00's much more technically advanced from a design perspective and perform much better. The rust particles are rubbed into the lining, acting something like grains of sand in your shoe, and result in non-uniform contact of the lining until they are worn away. Friction is simply high frequency vibration (high rate of stick and slip), and changing the friction changes the frequency, often resulting in audible noise or vibration which causes something else to vibrate which makes the noise.


Second paragraph: Typical automotive brake systems are designed to provide between 1600 and 2200 psi of maximum hydraulic pressure to the front brake calipers, with a proportion of this (hence the proportioning valve) pressure seen at the rear wheels, which generally require less pressure to lock the wheel. The resulting clamp force is then a function of the caliper pistion area the pressure is exerted on. I don't have the formula handy, so I'll give you a rough idea of the clamp force in a follow-up post.

 

[BIF]

Wow..."ask, and ye shall receive"....


That's a fantastic explanation! Thank you very much. I learned something today, now can I go home?

 

[steve]

thanks - but why reverse only?

Thanks for the responses - It sounds like the dealer's explanation makes sense. Anyone have a sense as to why the screeching noise would only occur when braking in reverse? Also, is there anything I can do about it? The dealer said its a feature of this car, and that some cars just do that. The only thing to do would be to upgrade the brake pads to higher-performance ones that have a different composition that somehow acts to resist the noise.

 

[mdacmeis]

Re: thanks - but why reverse only?

Thanks for the responses - It sounds like the dealer's explanation makes sense. Anyone have a sense as to why the screeching noise would only occur when braking in reverse? Also, is there anything I can do about it? The dealer said its a feature of this car, and that some cars just do that. The only thing to do would be to upgrade the brake pads to higher-performance ones that have a different composition that somehow acts to resist the noise.

The reverse answer is easy. You are rotating in a different direction than normal, thus the friction pattern changes. Also, when in reverse you usually use only light brake pressure, which combined with the friction pattern change results in vibration from friction which is less damped, thus you are more prone to noise. The other reverse issue is many back out of the garage or driveway after the vehicle sits for many hours. This is thus the first exposure to the "contaminants" which are quickly worn away, however in doing this you may generate noise.

The dealer advise is somewhat valid. There is no guarantee that a different lining will solve the noise issue. It would be a trial and error thing unless you know the frequencies the brake components are sensitive to and the friction frequency generated during most braking events. If they match up, you will still get noise. I would tend to believe your noise issue will go away with time, manybe 500 to 1000 miles. As the linings wear a bit, the friction frequency will change. Toyota brakes are generally quiet. Not always, but generally.

GM/Ford dealer policy for brake noise: Change or turn rotors, replace linings. When customer comes back again, do it again, and again until warranty period is up. Toyota/Honda dealer policy to is try to explain to customers in layman's terms that any noise that might occur is normal, nothing is broken, and that it should be infrequent. Unless it is near constant noise, Toyota rarely authorizes new brakes. Of course I always wonder why anyone complains about noise. According to Hollywood, all brakes make noise when the car stops. I'm surprised people don't line up at the dealers asking for the end of stop noise. FYI: the automotive miracle is generating all that friction to stop a vehicle without generally making noise. Similar amounts of generated energy require a muffler.

 

[mdacmeis]

That leads me to ask a question of the engineers here. Does anybody know how hard a typical car's brake calipers have to press against the rotors (or drums) in order to stop a car the size and weight of a Prius?

Follow-up: A typical vehicle of this size applies a maximum of about 6500 psi of clamp force to the front rotor. Some are a bit more, some less, it depends on the size of the components, as I mentioned earlier.

 

[BIF]

Thanks for all the info!

 

[Joel]

Usually, they put a high-temp grease on the pads (where they meet the caliper) to reduce the squeak associated with vibration - for disk type brakes. I'm not sure that anything can be done for drums - the drums are the only ones I hear (rarely).