myth of warped disks

[J Burns]

I vaguely recall an encounter with a pulsing brake pedal. I thought the disks would have to be machined or replaced, but the problem went away.

Now I've read it on the internet: warped disks are a myth! When they say a disk is warped, they mean it looks warped on a gauge because material from the pads has been deposited unevenly.

Disk brakes use two kinds of friction: abrasive (like walking on asphalt shingles) and adhesive (like walking on new roofing felt). New disks are bare iron. When the pads are at operating temperature, they deposit a little adhesive material on the disk. Disks are broken in when they have that thin layer.

If you come to a full stop with the brakes on and the pads good and hot, they'll leave a spot of material stuck to the disk. That will cause vibration in subsequent braking. If material gets built up in spots, it will cause pedal pulsing.

If you drive with only light braking, so the pads stay cool, abrasion will wear the deposits away. I guess that's why the pulsing I experienced went away as I drove. If your braking is always minimal, you can end up with bare iron disks, which is supposed to be bad.

When I see a red light or a stop sign, I like to brake so I could stop short of the mark, then glide in with only light brake pressure. That way, I won't overshoot if I have a little braking trouble. I see now that this gives the rotors a couple of seconds to cool the pads before I come to a stop. That should keep the pads from leaving uneven deposits.

[gfretwell]

I had a car that had all of the signs of warped disks after buying new tires. I re torqued the lug nuts and the problem went away.

[J Burns]

I believe calipers are free to move laterally. At first, I imagined a caliper could slide back and forth to ride evenly on a disk that was mounted a tiny bit cockeyed by uneven torquing.

But wait! The braking force on the caliper would interfere with lateral movement during braking. Braking would be uneven. If material from the pads built up on the spots of maximum runout, the runout would get worse.

[dpb]

...

Two separate problems...just because one _could_ possibly (I suppose) have such symptoms doesn't mean that over- or uneven torque'ing won't/doesn't actually physically warp rotors.

[Danny D.]

dpb wrote, on Tue, 04 Nov 2014 07:34:44 -0600:

The myth of the warped disk is renown (just google for it).

Here's my analysis of the problem:

1. People don't know what they're talking about
2. Why? Because they don't measure anything
3. If they measured a disk, they'd find it's not "warped"
4. At least, not like a potato chip, anyway

Rotors "can" and "do" warp, but, street use rotors almost never do (just google to find people who should know, who have MEASURED this!).

Note: If you don't MEASURE the warp, then you have no business ASSUMING it's warp.

But, then, why do people "think" the rotor is warped? And, why does "machining" (or replacing) the rotors always work?

Because the problem "is" the rotors, but the problem isn't warp! There are *many* problems that "can" happen to the rotors, but, the most common is disc thickness variation (DTV) and/or brake torque variation (BTV), both of which are often caused by hot pads sitting on a hot rotor for a period of time.

NOTE: DTV is NOT warp! (Many people do not understand this key point!)

I could go on (and on), but, the main point is that the short term solution (i.e., replace or machine the rotors) works for both "true" warp (which almost never happens on the street) and for DTV/BTV related vibration (which happens all the time on street machines).

The long term solution is simply to change your braking habits. Voila! All those cases of repeated warp go away, with a simple change in your braking habits.

What could be easier?

[Danny D.]

gfretwell wrote, on Tue, 04 Nov 2014 01:05:33 -0500:

That's most likely runout.

There are two "definitions" for the term "warped disk".

1. My rotor is doing something funny (causing vibration while braking).
2. My rotor is shaped like a potato chip (this is true warp).

The first use of "warp" is very misleading, because it leads to the right short-term solution but the wrong long-term solution.

[nestork]

I also had a problem with a pulating brake pedal.

I had an empty parking stall, so I bought a second car at an auction sale. Once the car was in it's stall, I checked it over and found that it needed new brake pads and shoes. I replaced the brake pads and rotors on the front disk brakes and shoes on the rear brakes.

A year or so later, when I put the car back on the road, it had a really bad pulsation in the brakes. I took the calipers off and found that the disk had rusted in a spot roughly the size of the brake pad.

I expect that water had somehow gotten under the brake pad and didn't dry up. It could have been a rainy day when I replaced the pads, I don't recall.

But, lesson learned. Make sure the brakes are DRY before putting a car into storage or you'll ruin a new pair of brake rotors.

[clare]

Well, having been a mechanic since 1968 I can say I've seen a lot more warped drums than warped disks, but I've seen my share of genuinely warped rotors as well. That said, uneven deposits and corrosion/pitting are MUCH more common

- along with collapsing of vented rotors (due to the "fins" corrodong in the center of the rotor). Generally you can see the patttern of the "fins" on the rotor in that case, and there is a high frequency rumble more than a pulsation.

MOST rotor warpage is caused by severe overheating - but a lot of it is just poor materials. Castings that are machined while "green" can warp all on their own. Used to be VERY common with chinese discount rotors. Often they were "pre warped" when they came out of the box!!! We would skim them to get them true, and within weeks they were warped again - very measurable warpage. We just stopped using the crap. Couldn't make enough on the job to cover the cost of the comebacks.

[Vic Smith]

I ran through a deep puddle with hot brakes. Unusual occurrence. Warped them. Only time I knew I had warped rotors.

[Danny D.]

clare wrote, on Tue, 04 Nov 2014 12:55:19 -0500:

As I said, I don't doubt that rotors "can" warp, nor, that they can be "warped" right out of the box; but I just said that (almost) nobody who says his street rotors warped actually measured any of that warp, himself.

It's always a guess, where, a *better* guess would be DTV or to BTV due to uneven deposits which, I might add, are caused by the driver himself (by repeatedly and constantly holding his foot on a pedal after a hot stop).

BTW, how did you measure the "warp" of the street rotors, as measuring warp is not a simple matter of just mic'ing it or using calipers on it (because the faces can still be parallel, even if "wavy", ever so slightly).

[Danny D.]

Vic Smith wrote, on Tue, 04 Nov 2014 12:25:06 -0600:

With all due respect, if you didn't MEASURE the warp, then almost certainly they were not "warped".

Look it up. It's all over the place. Street rotors just don't warp.

They "can", and I'm not saying they can't.

It's just that they don't.

Plenty of *other* things should be looked at first.

[Danny D.]

nestork wrote, on Tue, 04 Nov 2014 16:21:39 +0100:

Seems to me a good bedding run (10 near stops from 60 to 10, in sequence, if you can do that on today's crowded highways) would have had a good chance of cleaning that up.

In fact, that's a good test for DTV due to uneven pad deposits. If the vibration character (usually the speed at which the vibration starts) changes, and diminished (or sometimes goes away) with the rebedding process, then you *know* the rotors didn't warp.

That's the standard test and solution for many DTV/BTV problems due to pad deposits (which, IMHO, are the far more likely reason for people "thinking" that their rotors 'warped').

[Stormin Mormon]

Seems like in storage, wouldn't you use some kind of protectant on the rotors? Wax, or WD-40 (which everyone knows is a water displacer)?

[Mayayana]

| The long term solution is simply to change your braking habits. | Voila! All those cases of repeated warp go away, with a simple change in | your braking habits. |

So what should one do? Avoid hard braking? I've been noticing a vibration for awhile, though sometimes it goes away. It's especially noticeable when breaking long, like to slow down for a highway offramp. Not like the pulsing of anti-lock braking; more like a subtle oscillation I can feel through my foot. I had my mechanic look at it, telling him I wondered if I might need new breaks. He said they're fine. I tend to actually use the clutch a lot for slowing down.

[clare]

Ive even seen them SHATTER under those conditions. I believe it was in the Canadian Winter Rallye -and a Mini Cooper - but it's a LONG time ago. The brakes were almost white hot coming hast down a long hill when the car entered an ice cold puddle, and the car totally "lost it's brakes".

[clare]

I will have to dissagree with you. I have seen AND MEASURED rotors that warped on street driven cars. Actually quite a number of them. Used to be quite common on 4 piston calipers when one piston stuck, causing the pads to drag. Also seen it on single and double piston calipers. I'll agree there are more common causes of brake pulsations

- and warped rotors are more likely to be felt in the steering wheel than in the brake pedal.

[clare]

A dial indicator mounted to the strut or control arm indicating 60 thou or more runout generally is a good indication. I've seen close to

200 thou. Also REAL easy to tell when you throw them on the lathe to turn them and the cutter hits one face of the rotor at 6 oclock, and the other at 12oclock and after a couple cuts opposite halves of the rotor face are shiny on either side. I've turned hundreds - possibly thousands - of rotors in my day. Generally when they get warped they are not worth machining - and actually today I won't even bother machining a rotor any more. Just as cheap to replace - and then you are not reducing the mass of the rotor encouraging them to warp again. (or overheat and transfer material more quickly)

[clare]

Up here in the salty north the more common cause - by a LONG shot, is intergranular corrosion (possibly behind the pas deposits) caused by metallic and semi-metallic pad materials and the difference in composition between them and the rotors causing galvanic action - in turn causing huge "acne scars" on the rotors.. Totota had a HUGE problem when the metallic pads first came out and they had brass filings in the pads.The rotors virtually disintegrated on several models.

[clare]

For prolonged storage it is good to force the pads back away from the rotors - and store in a DRY location. (NOT under a plastic tarp!!!!)

[Vic Smith]

Well, I went into the puddle with perfectly functioning brakes. When I braked coming out of the puddle, my brakes were pulsating and stayed pulsating until I changed the rotors and pads. What do you suggest was the problem?