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Old 04-27-2005, 06:06 PM   #1
Mustangman25
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Pulsating brakes

For the past few months now, I've been having a pulsation in my brake pedal any time I step on it...the more I press, the faster it oscilates. When I bought the car about 7 months ago, the dealer replaced the pads, shoes and front rotors, so I figured it was the rear drums out of round, as they have 60,000 miles on them, but then I realized that there is a shaking in the wheel that matches the pulsations exactly which led me to believe that it was a problem with the front brakes...everything checked out; pads are still very good, and the rotors are still well within manufacturer's specs. Can an out of round rear drum cause the wheel the shake? Does anybody have any other ideas what it could be? I'm afraid of hard braking now with such a bad shaking pulsation.
BTW-2000 Focus with no ABS.
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Old 04-27-2005, 07:54 PM   #2
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Re: Pulsating brakes

Quote:
Originally Posted by Mustangman25
rotors are still well within manufacturer's specs..
Usually specs for rotors refer to minimum thickness. This is easy to check, but generally, 'runout' is more difficult to check. This is the sign of a warped rotor and your most common cause of brake pulsation.

I would suggest have the rotors machined. Installing a new set of pads is a good idea, because the old ones will have developed a unique wear pattern.
Also, check out the front end for out of round tires, alignment difficulties (often both show up as abnormal tire wear) and look for worn or damaged steering and suspension components, including wheel bearings.

Chances are the brake work will fix the problem; if not, take a look at these other problem areas.
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Old 05-03-2005, 10:57 AM   #3
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Re: Pulsating brakes

i've bought new rotors that are warped right out of the box & that could be the problem. i know mechanics that will actually take a light cut on brand new rotors for this reason. out of round tires/distorted wheels can cause similier problems that can make it seem like it's the brakes that are out of round..
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Old 05-03-2005, 12:39 PM   #4
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Re: Pulsating brakes

Rotors don't warp. It is the oldest and widest misconception in automotive history. They get depositions of abnormal material and the friction qualities are modified in certain parts of the rotor, but the runout you get is from load inconsistencies, not heat warping.

http://www.stoptech.com/whitepapers/...otors_myth.htm

You can temporarily solve the vibration by machining the rotors but if proper pad break in procedures aren't used it will come back very quickly, which is probably why SaturnSC2 had pulsating very soon.

Rust, pad material fusing, inconsistent pad application; all contribute to pulsating pedal, but make sure you attack the problem not the symptom.

Also, make sure you're diagnosing it right. In your post you say that the harder you press the pedal, the faster the pulsation. If that's the case, then it can't be a rotor issue since it would pulsate at the same speed the wheels turn regardless of how much you press the pedal.
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Old 01-30-2006, 12:12 AM   #5
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Re: Pulsating brakes

this sounds like a problem that usually happens on fairly new vehicles. anyone ever heard of it happening on a 10 year old explorer with 160000 miles?

maybe i just missed it on that site, but is it just fixed by replacing the rotors and pads and breaking them in properly?
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Old 01-30-2006, 09:27 AM   #6
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Re: Pulsating brakes

this is an old thread. try to avoid posting in threads where people haven't posted in while, especially if it's a few months old and the person was asking about a problem, it's probably fixed by now.

On a different note, welcome to AF, hope you find the info you need.
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Old 01-30-2006, 04:21 PM   #7
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Re: Pulsating brakes

Holy dead thread Batman! The problem was with rotors being out of spec...turns out that the micrometer that I used originally to check the washout on the rotors had been dropped fairly hard before I used it, and went way out of wack, making me think that the rotors were fine when they were actually not. On that note, old thread, time to let it die.
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Old 07-20-2006, 03:10 AM   #8
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Re: Pulsating brakes

Quote:
Originally Posted by curtis73
Rotors don't warp. It is the oldest and widest misconception in automotive history.
I'm feeling argumentive tonight.

If I take a torch to a thin chunk of steel, and it contracts and moves....is that warping is is it really getting deposits of torch gas that are magically transforming into carbon that matches the steel structure perfectly, making it indistinquishable from the original steel....and no weight is added....

and yes, its an old thread that I'm reviving. I could have started a new one, but it would seem a little weird to suddenly start ranting with nothing to point at

I always assume that these kinds of "theories" are thought up by the same people that think up other attractive conspiracy theories. They sound good on first blush, but when you get into the finer details of exactly how you would pull off such a complex set of events, it just becomes nearly impossible to be plausible. The fact is, if you heat up mild steel until its glowing, regardless to how you are heating it up, it will warp. If you can't find material deposits from the pad on the rotor, then whats this "deposit" stuff? Makes no sense.

If a brake system went metal to metal and THEN warped, what would be your rationalization for it?
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Old 07-20-2006, 03:27 AM   #9
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Re: Pulsating brakes

I think the biggest way to dispose of the myth is to point out the minimum thickness markings on discs.
Its there because everytime they warp, and you machine them flat again, you are removing material from the disc, therefore, making it thinner.
If the material being removed was deposisits from the pads, or some other epiphenomenal source, then machining the disc flat again would only remove that material.
Its not hard to test this theory, just machine a set of warped discs yourself, and stop just at the point they become flat again. Measure them before and after and note how much thinner they are.
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Old 07-20-2006, 03:33 AM   #10
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Re: Pulsating brakes

minimum thickness numbers are not a straight forward caluclation. There is a lot involved in calculating them. Not only does it involve heat removal (avoiding rotor warping) there are other factors, such as maximum safe piston travel for the given caliper. Piston retraction capability at a given piston extension. Brake fluid boiling due to heat retention in the caliper for a given fluid quantity....And probably many other examples that I wouldn't know since I'm not a brake engineer.

Its kind of like deciding what is the best tire pressure for a given vehicle. It seems like a simple question on the surface, but is very very complex and has a lot of compromises in the final answer. And then is ignored by most car owners anyway.
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Old 07-20-2006, 03:39 AM   #11
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Re: Pulsating brakes

I'v got some theorys on this, along with pictures, but I'm leaving to go to work, so I'll post them in the morning.
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Old 07-20-2006, 04:22 AM   #12
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Re: Pulsating brakes

Quote:
Originally Posted by UncleBob
minimum thickness numbers are not a straight forward caluclation.

Regardless of how they are calculated, every set of discs Iv work with has them stamped into usualy the outside edge.
They are there because discs warp, and when you machine a warped disc you reduce its thickness.
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Old 07-20-2006, 04:30 AM   #13
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Re: Pulsating brakes

Quote:
Originally Posted by Moppie
Regardless of how they are calculated, every set of discs Iv work with has them stamped into usualy the outside edge.
They are there because discs warp, and when you machine a warped disc you reduce its thickness.
you won't get any disagreement from me. But then again, I've seen more brake rotors warped that were well within their tolerance for minimum thickness than not. That by itself isn't a factor IMO. I think its heat that is what causes rotors to warp, and although that is effected by how thick the rotor is, that isn't the bigger factor. The bigger factor is WHY is the high heat being created in the first place....higher heat than the system was ever designed to deal with.

The linked article suggests that its NOT a heat issue, its a break in proceedure issue. This suggests that if you don't have rotor warping soon after installing pads, that there's something amiss, but this also assumes that you can't have rotor "warping" due to caliper issues or improper brake use....which is asinine IMO
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Old 07-20-2006, 05:07 AM   #14
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Re: Pulsating brakes

I think its just a side effect of repeated hot cold cycles on a piece of steel.
Iv seen it occur in other totaly differnt types of machinary that under goes repeat heat cycling. Hydrolic rams for example.
And of course we all know that rapid a rapid heat change can do bad things to metals, for example its a good way to shatter as casting.
Something as simple as heavy brake use, then driving into a puddle could easily cause that.


However, I was only offering further arguments against the idea put forward in the link provided by Curtis, which in all reality sounds deeply floored and lacks and understanding of what happens.

One thing should however be pointed out, there are systems in use today which will in theory never wear out. For example the brakes on the Lotus Elise in theory never wear out the pads, or the discs. The idea being material is transfered between the pad and disc during use. So it is possible for material transfer occur, however they are both made from the same Aluminuim based material. Very unlike a converional disc and pad which are made from very dis-similar materials.
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Old 07-20-2006, 05:16 AM   #15
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Re: Pulsating brakes

I can't comment on the Elise brake systems, haven't researched them, but chrysler likes to use a composite material for their rotors, which is nearly impossible to cut on a brake lathe, because it chatters like crazy.....

They also like to warp like crazy, and our shops solution is pretty simple....throw away the stock rotors and put some real rotors on there, aftermarket cast rotors....problem solved.....

maybe the brake pad material on those cars/trucks/SUV's don't transfer as well to non-composite rotors
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