Pulsating brakes

[TheSilentChamber]

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.

[Black Lotus]

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.

[534BC]

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.

[UncleBob]

Originally Posted by 534BC

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.

[534BC]

Originally Posted by UncleBob

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?

[UncleBob]

Originally Posted by 534BC

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.

[curtis73]

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.

[UncleBob]

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).

[534BC]

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?

[UncleBob]

Originally Posted by 534BC

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

[steelerguy]

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.

[TheSilentChamber]

Go to your local machine shop insted of parts stores.

[Austin8214]

Originally Posted by TheSilentChamber

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.

[534BC]

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.

[curtis73]

Originally Posted by 534BC

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.