This guy has the right idea. It's almost always pad deposit. I race a BMW. Racers go thru more rotors in a year than most folks will go thru in a lifetime. Pad deposit problems are common. Less common is a rotor that is not on "square" and therefore causes pulsations. Warped rotors are quite rare.

-Warped- Brake Disc and Other Myths

To beat pad deposits you can either replace the rotors, scrub them a bit with some abrasive, or do a serious of very hard braking efforts like 80-20mph and "threshold" (just before ABS engages) 6-10x.

 

ok but how do you explain one that pulsates when it heats up but is fine when cool? Enlighten me.

 

Heat=expansion. Perhaps the issue you describe is too small to be detected when cool, and only is detectable after the trouble spot expands.

 

Describe symptoms in more detail pls.

It might be a mistake tho to get wrapped up in the intricacies of a specific example. Odd things happen. Sometimes they never really get adequately explained. Be that as it may, brake shuddering is almost always caused by non-homogeneous pad material deposits on the rotor. If you look closely at the rotor you can often see "blotches" on the surface which are further evidence of the cause. The various ways to beat this are clear enough.

Note that if a person chooses the multiple hard braking solution, you really have to brake hard. The average person has no experience braking at threshold from high speed. You want to brake so hard that ABS "almost" engages.

 

it simple really you are at the top of a big hill with a heavy trailer the braks are nice and smooth by the time you get to the bottom of the hill the brake pedal is shuddering violently and the truck is shaking under breaking drive ten more mi then at the top of the next big hill the brakes are nice and smooth go down the hill and the hotter the brakes get the more the pedal shudders repeat over and over(this is Idaho) someone tell me with scientific evidence how that is pad deposits. Obviously its not its a rotor that is warping when heated. Just like a head it happens all of the time here in big mountain areas.
Pad deposits do happen but are more likely to happen in stop and go driving or race conditions. To help avoid them when doing a hard braking like you suggest don't come to a complete stop go to like 5mph and keep driving until the rotors /pads cool. P.s. I'm sure you know better than me because I don't race a BMW. Lmao

 

No one is going to be able to meet your requirement for "scientific evidence" because the bar for that is too high. But we can make some educated guesses and recommendations.

Brakes just aren't that complicated, even if they are often misunderstood (no criticism of you, just a general point). A brake system needs to be nicely rigid, apply the pad parallel to the rotor, and be able to handle the heat via some combo of material choice and cooling.

What's probably happening to you is pad deposits that are worsened by heat because the pad is having a hard time handling the heat. DD pads don't tolerate high temps well and all sorts of bad things happen to include outgassing, crumbling, glazing, or just not stopping you.

Modern brake pads are designed to transfer a layer of their own material to the rotor's surface. So the pad is designed to attempt to bind with a layer of it's own mateiral on the rotor. Old school pads merely abraded the cast iron rotor. If that layer of pad material on the rotor isn't homogenous, you get the shuddering. It's easy to imagine a DD pad overwhelmed by the conditions you described, getting too hot and struggling in some way that creates non-steady state braking.

My suggestion would be to find a pad that is designed for higher temp operation and also take a look at how your rotors are being cooled. There should be an air inlet at the center of the hub. Consider ducting air to them if this is a serious problem.

Note tho that rotors sometimes don't take kindly to a change of pad material so you might have to abrade your rotors a bit to remove the old layer of pad material before putting on higher temp pads. I've pretty much dodged this bullet but sometimes it happens.

Re. pad deposits being mostly a race car thing. I don't agree. There's lots of ways for this sort of thing to occur.

Re. racing a "BMW". I know, that sounds obnoxious. But I don't have a 5000 post count in this forum and I was looking for a way to establish that I wasn't a 16yr old nitwit in a single sentance. Our race class uses 25yr old BMWs, it's not like we're driving $70k late model M3's. Of 4 BMWs I own this week, it's the '06 F150 tow vehicle that is the nicest vehicle I own.

 

First off your entitled to your opinion but I don't believe it's based on experience.

I'm not young or in experienced either and every part of my 4 years of automotive/diesel tec courses and 9 years in the profession says that said scenario is not a pad deposit in fact from memory I tried to copy this scenario as close as I could from a ford AAA competition qualifying test as I could. The answer was a warped rotor. The test was written by a ford engineer. I qualified and my team got a second place finish in the state competition my junior year in high school. As far as scientific evidence Sae course manuals or even just mitchell on demand troubleshooting pages would suffice but you probably don't know what those are. Obnoxiousness arrogance and bravado intended
Obviously getting rid of heat is the problem thanks for clearing that up for me. I'm curious if your opinion is based on other peoples opinions and regurgitated knowledge or if you have a bunch of years with a mic in your hand measuring hot and cold rotors. I can only say what I have read in textbooks and it has always lined up perfectly with what I have seen in practice.
If you notice I didn't say that pad deposits is a race only problem it's a problem any scenario where you brake really hard and get the pad material hot and then stop and hold the pad in 1 spot on the rotor it tends to cascade from there.



Ill give you credit at least you didn't just attacked my spelling because you had no point to argue.

 

Dude, take a chill. I was trying to help, remember. No one knows brakes like racers know brakes. 10-20x per lap we are standing our cars on their nose. I'm not saying we know more about brakes than brake engineers, but we are likely to know more about brakes then any engineer that does not specialize in brakes.

Speaking as a (non-brake specialist) mechanical engineer, sometimes engineers have the right answers and sometimes they don't. Very often there is one engineer at the table, 4 lawyers and 5 accountants. Sometimes the rotor is off kilter for one reason or another but usually that's not the problem. The #1 problem an off-axis rotor will cause is "knock-back". This is where the off-axis rotor pushes the pads back a bit. This is a pretty dangerous situation because pads don't move very far when you press the brake pedal so the single knocked back caliper can result in the brake pedal going to the floor until you pump it a few times.

I wouldn't get too excited about a test question and answer. I've taken an awful lot of Microsoft and Cisco exams and sometimes it seems like it was someone's secretary that wrote the exam. It's a real world thing, not a MS and Cisco thing. There is no profession where the stars write the exams. Book learning is good, but it really just provides a framework with which to better understand the lessons learned from experience.

Don't just accept an answer blindly. Ask yourself why off axis rotors would cause this problem worse when they were hot. If off-axis rotors are rare, how can pulsating brakes be common? What are the likely causes for the rotors going off-axis? Require explanations that fit all the information you have.

Experience vs. regurgitation. Well, some of both. I got a little obssesed by brakes 5 yrs ago and did a bunch of tests to gather data on all of the track pads available for late 80's BMW 3 series. So a couple thousand laps of trying different ways to see what pad behavior differences I could find. That included studying the output from the data logger, trying different pads front and rear and different pads left and right. We're not talking braking at a stop light, we're talking 125mph stand the car on it's nose or you are in the trees.

There's also a lot of regurgitation. As a mechanical engineer I've a pretty good nose for what explanations are plausible and what aren't. Someone simply making an assertions is meaningless if they can't provide an explanation for their theory that will survive scrutiny.

Wrenching isn't theory to me. I can handle the theory, I taught HS physics for a while, but I do my own work on my race car. That means multiple engine rebuilds, 3 engine swaps in the last 6 weeks, I had to R/R the tranny 5x, it never ends. Pretty much the only thing I've not done multiple times is rebuild a tranny.

I don't claim to be a Ford expert mind you, but don't forget...I was trying to help here.

 

Here's an example for the engineer to explain then.....

Brand new Pierce Arrow Class A Type I fire Engine, weighs 32,000 lbs, 4 wheel ABS Disc brakes. Fire engine comes back from a run (avg speed 35 mph), ave duration 6 minutes. Upon return to the fire station driver immedately takes hose and rinses fire engine & wipes down. Very next run fire engine is taken out of service due to heavy pulsations when brakes are applied, uncontrollable vibration. Vehicle is towed to shop, rotors & pads are replaced. During the next 30 days, 16 new fire engines experience the same condition, total of 40+ sets of brake pads & rotors are replace, mfg is paying the bill, ask for all parts returned to them and engineers from brake mfg arrive a repair garage to inspect vehicles. 1 week later......
"Enginners determined rotors are warping do to cold water applied to units which have not adequately cooled", recommendation is not to wash the vehicles for "30 minutes minimum after returning to station". No further issues for the past 8 years...........

 

I'm sure you are trying to help and I'm the a$$ but I'm ok with that. How many times have you ran into the exact scenario I was describing and when you evaluated the braking system what evidence did you come up with to make you think it was pad deposits and not a warped rotor in those scenarios? Since you don't forget do you remember what the run out was on those rotors? When you measured 8 or 10 different places around the rotor what was the max/min thickness differences. Please explain further WHY you think pad deposits would cause a rotor to shake violently when really hot and work perfectly fine under normal op temp?
I've put a mic to a lot of rotors and a lot of them were warped plain and simple. Is that real world enugh for you. But I do forget things on ocasion so...

 

The fact that the trucks became symptom free after changing their practice of cold water on hot brakes makes it likely (obviously) that the cold water was causing the problem. I say "likely" because few things in wrenching are for certain. Vehicles and symptoms can be diabolical and some things are never really figured out.

This scenario depends on whether or not it was real life or just a scenario. In real life this would be pretty surprising. Altho brake rotors can get damned hot, cooling is proportional to delta T so the hotter the rotors get, the faster they cool. The suspension and wheel offer a lot of surface area so once the brakes quit generating heat, the rotor temps would drop from "very" high to "pretty" high in seconds.

Can't be a metalurgical issues. Altho interesting things happen to cast iron at high temps, by the time Joe pulled out the water hose the rotors would have cooled enough that water wouldn't change the structure of the cast iron.

That's "structure" as in annealing and work hardening, not structure as in shape.

Since rotors are made of homogenous cast iron, heat expansion and contraction would be entirely symmetrical. So expansion and contraction alone would not be enough to cause the kind of off-axis rotation that would result in pulsating brakes. The likely culprit would probably be the combo of heat expansion and the exact nature of how the rotor was held in place by hub, studs/lug bolts, and wheels. Those fastening points would resist the rotor's attempts to expand and contract symmetrically. If there was some assymetry in fastenings, then rotor warpage is possible I suppose.

Why are we in this quiz show game? Some childish "stump the newby" thing? Why make this a contest of egos? You'll be 50 some day too. If I had a choice I'd much rather be 20. Man how I would burn thru the chicks if I could be 20 again <muses with a nostalgic grin>.

General thought aimed at no one in particular. Forums are anonymous. Some folks react to that anonyminity(sp?) by treating others poorly and looking for opportunities to declare how cool they are. Heck with that. Go spend your whole adult life as a Marine and Army Ranger. You'll get over the need to prove how cool you are each day, I promise. React to that anonyminity by being courteous and helping others where you can. That is the real measure of your worth.

 

We're not going to agree so maybe it's not worth tusseling over. I've never had your exact scenario. I live in coastal GA. Even when I'm towing to fairly distant tracks, I don't normally hit anything like they have in ID.

I've already discussed a scenario for shuddering being worse when hot. I've gone thru DD pads in 2hrs on the track whereas dedicated track pads will last 6-10hrs on the same track, depending on the track. If you have another theory that explains hot rotors shuddering better than "the guy that wrote my test said so", I'm interested to hear it (no sarcasm intended).

We don't normally measure our rotors. They last a couple weekends, crack all the way thru, and we put a new one on. Do it all the time.

Just because a rotor is a little out of perfectly true doesn't make it the cause of the shuddering. Brakes are a hydraulic system. Hydraulics are based on pressure, not on position. That's a really important distinction. Therefore brake hydraulics can accomodate small deltas in rotor position. Maybe that's an issue in your example, maybe not. There are few things known for sure. All a person can do is get lots of book learning and then lots of experience.

 

same questions. Still waiting. Please have mercy no more big words please my little brain will explode. Ask my employer /family if I have worth. I've worked two marines "to hard" there words, guess they couldn't keep up the owner eventually let them go.

 

This leave me wondering how you learn from your experience Without taking a mic out and checking to see if they were warped you say that you don't commonly check them after you replace them with a quote unquote pulsating problem but somehow you're sure it's not due to rotor warpage. How do you know? You didn't measure. Of course you have no certainty you didn't measure Yet you still have a strong opinion that warpage was not the issue. We routinely just out of curiosity put warped rotors on a lathe just to see where it takes material of first. Or to measure run out all I can say is sometimes it's a warped rotor there's just no way around it.

 

I'm not saying rotors don't warp (I've had sets brand new, out of the box that needed to be turned before they could be used) but I am saying it's not ALWAYS a warped rotor.

From what I've read on the 'net and in talking with my friend who owns a tire and brake shop locally to me, as well as my experience with my Ford trucks, there are many, MANY occasions where disk thickness variation or hot spotting/cementite creation (sometimes called "black spotting") is what is causing the pedal to shimmy.

http://www.powerbrake.co.za/download..._01_judder.pdf

EBC Brakes | Troubleshooting Information | Bedding in new brakes | Brake Noise / Squeal | Brake Vibration

If you Google "brake cementite" you'll get more reading.

Ease up a bit, skunk. We can discuss topics with each other and disagree without getting aggressive or condescending in attitude.

Stewart