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.

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.

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..

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/warped_rotors_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.

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?

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.

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.

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?

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.

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.

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.

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. :)

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

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.

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 :rolleyes:

I believe rotors do warp, plain and simple. I dont think its just material build up on the face of the rotor.

Most of your typical street car rotors are going to be cast iron one piece with a top hat. Cast iron is a very stable material, when heated (I consider rotors to be heated evenly even though one part is generating heat, it is spining so the entire surface is coming in contact and being heated) and allowed to cool naturually it should return to its same shape and dimensions. However the top hat is not being directly heated, and is also usually protruded from one side, and only connected to one side, so when the rotor expands it tends to mushroom tward the top hat, this isnt that big a problem because most of the time even agressivly driving your not going to get the rotor hot enough to cause this distortion. Even when heavly using the brakes while high speed driving this shouldnt matter, as it will cool and return to its origional dimensions as you let off the brakes. As the pad is applied and the mushroomed rotor passes between the pads, it is straitend back out again, but sence it is hot held into this position till it cools it doesnt effect it. Now where does rotor warping come in? When you use them so heavily that the rotor is in this mushroomed state, and you slow down to the point that the rotor accually cools while being clamped between the pads, as that part of the rotor is distorted from its naturual state for that tempature it stresses the rest of the rotor, pulling the sides out creating kind of a pringles potato chip shaped rotor, these internal stresses dont go away after the rotor is machined and at some point could very easily show back up again because the grain structure of the rotor has been altered. This is why high performance vehicals and high end aftermarket brakes are of a two piece design, as the rotor expands, the top hat has very little effect on it and the rotor simply expands in all directions, no mushrooming. Thats why you could ride the brakes so hard they glow, pull into the pit, with the brakes on the whole time the car is serviced, pull out and not have warped rotor.

Just some thoughts--
Check wheel bearings.
Check (hell, just replace) the suspension bushings.
Twenty-five years ago when i worked in an auto repair shop, I coulda swore I was turning genuine new and used warped rotors. Lots of them. Be advised that Carroll has probably never even seen a brake rotor that wasn't a multi piece race disc and hat assembly.
I think the pad has to be specially compounded to be able to transfer material to the disc. You also have to follow a specific bed-in procedure that you can't even hope to accomplish on many low powered cars and trucks.
Further, the transfer (bed-in) process can be totally screwed up by--loose wheel bearings and less than perfect suspension bushings--get out the garnet sand paper. Don't ask me how I know.
Now I'll get off my soapbox.

I never liked the term "warped" because that to me indicates a "bent" or "wobbling" rotor. It may give a shimmy, but I am thinking that a rotor with a varying thickness will cause a definate pulsating even at slows speeds.

I'm not sure if it is material or metal bending/contrating/expanding because I have never turned a rotor, but am sure it has nothing to do with minimum thickness and everything to do with heat/abuse.

I do know that many drivers will never have the trouble and yet others who use brakes aggressively and repeatedly will have the trouble.

Exact same with clutches, the problem shows up when slipped and it takes two surfaces to be out, a flywheel with runout by itself will not cause the pulsating. The mating surface has to be running out also.

I'm not sure if it is material or metal bending/contrating/expanding because I have never turned a rotor

I have cut probably thousands of rotors, warped rotors are nearly always truely warped, ie, the thickness is uniform, but when you cut it, its quite apperent that the entire rotor is "bent" as you say.

I have cut probably thousands of rotors, warped rotors are nearly always truely warped, ie, the thickness is uniform, but when you cut it, its quite apperent that the entire rotor is "bent" as you say.

Thanks Bob. Are those the ones who give a pulsating at any speed?

It seems the caliper/piston should be moving as a set, but the piston position should not vary? thus not moving pedal up and down?

Thanks Bob. Are those the ones who give a pulsating at any speed?

It seems the caliper/piston should be moving as a set, but the piston position should not vary? thus not moving pedal up and down?
this is the root of the question: why do they warp? IMO, most of the time, its due to a caliper problem. Either a hanging/sticking piston, hanging/sticking slider (when applicable) and some designs, the pad isn't free enough to follow the piston/caliper assembly.

Its definitely possible to overheat the brakes with heavy use, but unless you're hauling stuff through a mountain pass, or driving VERY aggressively, or forget to turn off your emergency brake, I don't think most average drivers come anywhere close to what I'd discribe as overtaxing their brakes.

So when someone has warped rotors, I check the calipers very carefully. If I can't find nothing wrong, I will suggest we attempt to do a brake job and see what happens. If it comes back again with warped rotors, I call out calipers. It always stops coming back after that.

Warped rotors or not, warping does NOT cause brake pedal pulsation. Pad deposition in the form of cementite inclusions DOES cause brake pedal pulsation.

Lets make sure we're defining things properly. Warping is a runout. The rotor's surface alters direction but maintains thickness. Inclusions on an otherwise flat rotor alter its thickness causing pulsation. You can warp a rotor severely and not notice any pulse, but a perfectly flat and straight rotor can pulse if you don't machine off the inclusions.

Cementite inclusions are rarely visible with the naked eye, but they are there. I've personally cut my own rotors to view them on a scanning electron microscope. Its there. The only way to make sure your pulse doesn't come back is to machine them flat, true, and straight and then properly bed in the new pads. If you don't, you'll burn pad material directly into the rotor and that spot will be forever altered in both thickness and friction qualities until you remachine it again. Ever have one of those times where you put new rotors (or freshly machined ones) on with new pads and within a few hundred miles you discover a pulse? The research has been done, the facts are there and tested, its just a matter of getting the "old" habits to die. For a hundred years we've been talking about "warped rotors" which may happen but its NOT the cause of brake pulsation.

I don't see how the phrase "warped rotor" means that it must have the exact same thickness at all points. If that was the case, then I've never seen a "warped rotor" because I've never seen one that was absolutely perfect thickness, runout or not.

Calipers put uneven pressure across the pad surface. Rotors will not necessarily heat up equal across the surface.

and all that is assuming the calipers are functioning properly.

Your comment about replacing pads and rotors and getting another warped in 100 miles is exactly what I'm talking about. That is bad calipers (or a restriction in a line).

At least we can agree that if a rotor has a "varying thickness" it will cause the pulsating?

It has to right?

Next can we agree that if both the rotor sides nave zero runout (regardless of the caliper) there will not be a pulsating?

There cannot be , right?

At least we can agree that if a rotor has a "varying thickness" it will cause the pulsating?

It has to right?

Next can we agree that if both the rotor sides nave zero runout (regardless of the caliper) there will not be a pulsating?

There cannot be , right?

thats what I just said :p :icon16:

Whatever is causing the pulsation, a good turning & new pads, properly broken in would seem to fix most pulsation problems.

My problem is finding a place that can do the turning correctly & at a reasonable price. Autozone has stopped. Checker has proven incompetent to me & NAPA has like a 1 or 2 day wait (and is expensive).

I guess I just need to call around the next time & pay the money for a good job. Have Roto-Tech drilled & slotted rotors that said on the instructions that they can't be turned. A mechanic a my favorite shop in Gilbert AZ said it can be done but must be done very slowly & with great care.

Go to your local machine shop insted of parts stores.

Go to your local machine shop insted of parts stores.

^^^^^^^^^^^GOOD ADVICE^^^^^^^^^^^

I have turned my rotors myself with the lathe at work and did a far better job than I have ever gotten out of any parts store.

Most of the rotors I have considered turning on my last brakes jobs have all been simply replaced as I can't afford to make 1 or 2 trips with a dirty rotors in a box in my "other" vehicle to a shop for the price of a new set.

The ones on my blazer have never been turned or replaced and I don't think they ever will even though they are doing all the stopping.

At least we can agree that if a rotor has a "varying thickness" it will cause the pulsating?

It has to right?

Next can we agree that if both the rotor sides nave zero runout (regardless of the caliper) there will not be a pulsating?

There cannot be , right?

Actually there can. If you machine them flat but don't remove all of the inclusions (which can't be seen with the naked eye) the part with the inclusions will have a different coefficient of friction than the bare iron.