Warped Brake Rotor Comment


#1

Every so often the discussion comes up about brake rotors being “warped” or not and some of those discussions have maybe reached the mildly heated stage.(kidding!:-0)

Since I am one of those who feel that cast iron rotors warp just like manifolds, engine blocks, heads, and even carburetor throttle bodies and water pump flanges, I believe they do warp. Micrometers, dial indicators, car makers, trade groups, and mechanics agree.



While “whittlin’” out a couple of carburetor venturis this weekend I decided to pick a rotor out of the pile and spin it up on the lathe. This was the worst offender out of the first 6 I picked up.



I made several cuts across this rotor totalling about .007 of an inch in all.

Note the rotor is not even yet and by my estimation will take at least another .006 or 007 to clean completely up.



Any non-warping believers care to comment on this particular rotor? I have some that are warped much less (probably 3-4 thousandths) and will be glad to make a few passes on one of those followed by a pic.

http://i28…rotor3.jpg








#2

Metallurgist may call it dimensional stability, or, some such. To determine the amount of dimensional variance, it would better to take the measurement “in situ” (in place). Mount the wheel on the vehicle and measure the run-out on the vehicle…


#3

Comments? Okay, when whittling you should always whittle AWAY from your thumb, never toward.

I recall the discussions, read the website links, and still believe that rotors warp. Those who disagree can read a few metrology textbooks, specifically as regards casting and machining, especially the chapters on “residual stress”. Residual stresses are normal with worked metal, and heat relieves the stresses.

Hey, that rotor you’re turning…by the time you’re done you’ll have to measure the thickness in angstroms!

  • mountainbike

#4

Sorry, I meant “metallurgy” books.

Doggoned lack of an Edit feature! Give me the old website back!

  • mountainbike

#5

I’ve posted on this topic before but will repeat much of what I said. Yes, rotors CAN warp but do not do so necessarily. Excessive heat CAN warp rotors but not necessarily. The thing that warps rotors is uneven cooling in which the heated metal bends to accomodate temperature variations. The main cause of rotor warping caused by uneven cooling is driving habits. Begin lecture here …

If you look at a disc brake assembly, you’ll see a caliper attached at the top or side of the rotor that holds the brake pads that clamp the rotor. Increasing pressure (braking) increases heat when the rotor is turning as the kinetic energy from the speed and mass of the vehicle is converted to heat at the rotor. As long as the rotor is turning and cooling evenly, it is extremely unlikely that any warping will occur. If you come to a hard stop from high speeds (ex: downhill exit ramp from highway to a stoplight) and sit still after the stop, the pads/caliper will clamp one small section of the rotor (maybe 15-25% of the surface area). The exposed rotor surface is cooling from convection, while the covered/clamped rotor surface retains the heat. This uneven cooling causes the rotor to warp. It is very common in police cars that must run hard to a scene and then come to a complete stop as the officer exits the vehicle to respond to a call. It is also very common in drivers who brake with the left foot, riding the pedal slightly, and with drivers who brake hard as a routine at every stop.

Rotors are made of metal. Metal is a crystalline substance that can be made malleable or even amorphous by heat. Normally the metal will retain it’s cooled shape unless heated to the point where the crystals begin to shift as bonds break. When those bonds break, the metal will take on the shape of least resistance. When it cools, it will retain the last shape it took.

You can all but eliminate rotor warp if you plan early to slow down (when possible which is most of the time), come to a stop slightly short of where you normally would stop behind an intersection or another vehicle, and then allow your vehicle to creep slightly forward while waiting for the light to change. This will minimize heat build up, and then allow the rotor surface to be completely exposed as it slowly rotates while creeping.

I saw this phenomenon many times when I worked at a dealership. On several occasions, customers would bring brand new cars in complaining of warped rotors and demanded new rotors. When we replaced the rotors (once only), they quickly became warped again. On one occasion I drove the car myself for a week waiting for the factory rep to inspect the car, and never had any warping after the rotors were turned. One week later, after the customer took the car back, she returned and demanded that we fix the car. It was impossible.

I drive a very high performance sport sedan as a personal vehicle. It has 40,000 miles and no rotor problems of any kind. I do not drive it hard, but I do stop from high speeds on occasion. I can intentionally warp rotors on any vehicle made if I want to do so. I can also drive any vehicle tens of thousands of miles with no rotor warping if I want to do so. It’s all a matter of habit and knowledge.


#6

I’m one of those who disagree, sort of. Rotors will warp, but every time the brakes start shuddering doesn’t mean that the rotors are warped. This is the main disagreement, some here believe that brake shuddering is always due to warped rotors.

The Toyotas brakes started shuddering last week. The brakes and rotors are only a few months old. I pulled the wheel and checked for runout with the rotors in place and there was NO runout. The problem in this case was the caliper pins were stuck. I had cleaned and regreased them when I put in the new rotors and pads, in fact this was a reman caliper I got because the old one had too much internal rust. The car is an 86 BTW. I used a different grease on the pins this time, hope it works better, but I may have to get new pins and bushings.

This is not the first time I’ve had the pins cause shuddering. I had it on my 90 Colt. I greased the pins on it and put in new pads, I didn’t even rough up the rotors, no more shuddering and that was after 114k miles.

Most of the time when a vehicles brakes start shuddering on me, a few hard stops have cleared up the problem. I had to do this in my Saturn about every 5k miles. At 135k miles, I did its first brake job. I used rotors and pads from a company called EBC. I read the literature that came with the rotors and they pointed out a few interesting things. First, even though these are high quality, American or British made rotors, they recommend doing a runout inspection right after installation because sometimes there may be some crud or rust between the rotor and the hub, or the hub may not be true. According to them, if the rotors start out not true, they will get worse very fast. That may explain why some people get warped rotors very frequently. Maybe the problem is in their hub.

Next, they point out the importance of bedding in the rotors. Their pads have a bedding compound on the surface to help bed them to the rotors so that the shuddering does not occur. When the pads are not bedded (British company), then shuddering can occur. The most common way to bed the pads, a couple of hard stops. Works for me.

Is it also possible that those who always find rotors to be warped on their lathe might be because the had to warp a little in order to run true on the hub? Maybe thats why manufacturers are recommending that the run out inspection be done in place, and any machining be done in place.


#7

How did you mount the rotor in the lathe? Did you reference from the inner and outer bearing shells?


#8

Oops, no hub. sorry. What surface did you use as a base reference and was it clean?


#9

There is a machine made that will turn rotors while mounted on the car. A dealer I worked for bought one and basically it was a POS.

The rotor I have shown is clean on the backside and mounted on the lathe chuck.
The center of the rotor is running true; the pad surface is not - as you can tell.
This rotor will clean up by removing around .025-.030, but it will be at or slightly under the min. thickness.

The lathe chuck is a much more reliable and solid piece than the hub it mounts to.
Keep in mind that the rotor as mounted on the car will more than likely be a lot looser than my lathe chuck, which has no slop. Ball joints, wheel bearings, tie rods and tie rod ends, can all have an affect when checking a rotor. That is also why extreme care must be used to not confuse suspension looseness with warped rotors.

I have about 20 sets of these things and I guarantee you every one of them will look the same after making a cut or two. The only difference is the degree.
The dial indicators says the center is running true and says otherwise on the pad surface.

(This particular rotor is from an 80s era Subaru and are the most notorious IMHO. They also use the park brake to operate the front calipers.)

All I can say is that every rotor I’ve ever checked before machining for a brake shudder complaint has shown excessive run-out or parallelism. No exceptions.
With a solid front end, even .002 will cause a shudder and most recommendations are that anything over .002 requires rotor machining or replacement.

As to “whittlin’” that’s my tech term for machining. The venturis are long obsolete, most are eaten up pot metal, and why use a Taiwan made pot metal, maybe it fits, maybe it don’t, when one can make good ones out of aircraft aluminum.

Thanks for the comments. Just curious. :slight_smile:
Also curious. Do any of you believe a 4" long, 1/2" thick cast iron throttle body can warp? (Less heat than a rotor.)


#10

One additional comment bears repeating as well. New rotors (assuming normal manufacturing tolerances) are equally thick around the disc. If a disc is turned to eliminate a rotor warp, that disc is forever subject to warping where the high “waves” were ground off to the low “wave” depth, and the overall thickness is reduced. This leaves less metal to absorb the heat, and it causes a greater tendency to re-warp. Rotor turning should be done to smooth the surface for new pads. It is only a temporary cure for brake pulsing which will almost certainly re-appear with poor driving habits as the rotors are repeatedly abused.


#11

If you mount the dial indicator on the hub, how do the ball joints and tie rod ends affect it?

I have no doubt that you lathe runs true, but what if the hub you mount your perfectly machined rotor on to is itself not true?


#12

Funny, my experiences are 180 from yours. My wife has problems with brake shudder on every car she has ever owned, and she hardly ever hits the brakes hard. when she complains about the shuddering, I take her car out and do a couple of high speed hard stops and the shuddering goes away.


#13

Let me add this to the argument

Turned rotors: $40

New rotors installed: $200

A couple of hard stops: $0

if it works: Priceless


#14

The dial indicator cannot mount on the hub since the hub rotates with the rotor attached. The dial indicator has a magnetic base and attaches to the steering knuckle, control arm, etc.

Loose wheel bearings or tie rod ends have an effect because it could possibly allow the entire knuckle assembly or hub assembly to move a few thousandths. That’s why I always turn the wheels to the stops and block them before taking a reading.

Yes, damaged wheel hubs can affect it to some degree, but this is part of doing the inspection; to make sure that problem does not exist. A hard curb strike can tweak one and in this case it’s either replace the hub or remove the lug studs and surface the face of the hub.

What I don’t understand is this. If you have a hub/rotor assembly running true at the center and you actually see the pad surface wobbling, how in the world does slamming on the brakes cure this?
(You should see a rotor with .010 or so warpage spinning.)

I’m also still curious about this scenario. Do you believe engine blocks, cylinder heads, manifolds, water pump flanges, throttle bodies, etc warp?


#15

The hard stops are likely removing the pad material that has transferred to the rotor. This is a well known phenomenon you can google for yourself if you’d like.


#16

I meant steering knuckle. I don’t know why I often call it the hub, I know better, but I still do it. Anyway, if the dial indicator is mounted to the steering knuckle or strut, then it is not affected by ball joints or tie rods.

As for your question about how does slamming on the brakes cure a wobble. It doesn’t. If the rotors are warped, then they have to be machined. My point was that brake shudder is not always due to warped rotors. There are other things that can cause the brakes to shudder, and a couple of hard stops often is all that is needed to cure the problem, for awhile anyway.

If that doesn’t work, then the rotors should be checked, but as I pointed out, the rotors could be true. I’ve had two cases where on or more of the caliper pins stuck. Once was right after a brake job where the pins were cleaned and regreased and new seals installed. The other was with the original brakes, but after 114k miles. One pin stuck in that case and caused the pads to wear on a slant. BTW, at a 114k, they were only half worn, but I replaced them anyway.


#17

I’m still asking the question; do any of you think engine blocks, cylinder heads, manifolds, and assorted widgets such was water pump flanges and throttle bodies warp?

Does anyone consider the rotor in the pic NOT warped?


#18

I believe those items you mentioned CAN warp, especially when super-heated and improperly torqued.

Some may disagree (I know they will) with my practice of changing the rotors when I have new brake pads installed.

The rotors aren’t warped and the pads are the second set.

Improper wheel nut torquing causes rotor warping as you well know, so I double check the torque after I get home.


#19

Any metal part can warp. Anything that causes metal to distort past its modulus of elasticity will be permanently distorted.

Just because something can be warped, doesn’t mean that it is warped. Assuming that a cylinder head is warped because the head gasket was blown or the engine overheated is just an assumption. It doesn’t always happen.

You are far less likely to see a Ford V8 head warp due to overheating than other makes. This is because Ford heads are heat treated and aged to remove the stresses before they are machined. Sort of “pre-warped” so they are less likely to warp again.

Anyway, what’s that got to do with brake rotors?


#20

I mention the other items because they can and often do warp, overheated or not.
The throttle body warpage I mentioned was a very common problem and this throttle body was actually cooled by the engine cooling system also.
Overheated or not, those things would warp like a bandit.

Brake rotor temperatures are also a lot hotter than those items I mentioned.
If a spell of 250 degrees will warp a cylinder head (usually does) then 400, 500 degrees, and more, can certainly warp a much thinner brake rotor.