Yup, you have a warped rotor. If you have dentures, you can estimate how bad the rotor is warped by how far your dentures fly! Hahahaha. Sorry, ... Couldn't help myself.
Rotors can warp two ways. Most commonly the friction surface turns at an angle relative to the mounting surface. That causes the caliper to slide back and forth once each way once per wheel revolution. If it's bad enough, it can cause the brake pedal to pulsate up and down too, but to be that bad it is almost always caused by improper setup on the brake lathe, then the warp is actually machined into it. It's more common for the warpage to occur from unevenly-tightened lug nuts and repeated heat-up / cool-down cycles. Most people can't put up with that for long and they get it fixed before it gets bad enough to be felt in the brake pedal. You'll feel it in the steering wheel because the mass of the calipers causes the spindle and steering arm to tug back and forth on the steering linkage.
Always use a click-type torque wrench when tightening the lug nuts. That insures, 1) the nuts wont work loose, 2) the threads wont be pulled or "peeled", 3) a 90-pound weakling can get them loose to change a flat tire, and 4) the rotor will have even clamping forces all the way around to reduce the chance of this happening. Every repair shop and especially every tire and alignment shop will have a wall chart listing the torque values for almost every car and light truck. They'll be happy to tell you what to set your nuts to! A common value for front-wheel-drive cars with steel wheels is 95 foot pounds. Cast wheels usually call for around 80 foot pounds. Light trucks can be a little higher, like perhaps 110 foot pounds. By the way, number 2, peeled threads is always blamed on the mechanic who is trying to remove the nuts to take the wheel off, but the damage was actually caused by the previous mechanic who severely over-tightened them.
I should also point out that a lot of people run into problems with Chinese rotors warping. There actually isn't anything wrong with them, and getting them replaced under warranty wont help either. When we cast parts out of iron, we set them aside for 90 days to "age" before they get their final machining. The Chinese cast 'em, pack 'em, and ship 'em, then they age on your vehicle. Warping is a common result, typically after one to three months, but all that's needed is a light machining on the brake lathe. Machining does reduce the life of the rotor a little, but they're all so inexpensive now compared to 20 or 30 years ago, that it isn't worth sniveling over.
You may also notice that the two "plates" of a rotor aren't the same thickness. That's the plate on either side of the cooling fins. You might think that would lead to warping due to the thinner plate heating up faster, but that doesn't seem to occur. All that's important is the overall thickness. There is a published legal limit a rotor can be machined to, and a slightly thinner limit it can be allowed to wear to on the vehicle.
This type of warpage is called "lateral runout". It is identified by using a dial indicator with the probe resting on the friction surface of the rotor while it is spun slowly by hand. Most mechanics just place it on the brake lathe and watch where the cutting bit hits and misses to verify it's warped. The only time we're interested in how much it's warped is when we're reporting it to the vehicle manufacturer for a warranty repair. When you're paying for the repair, time is money and we don't have time to waste measuring the parts. The amount of warpage is irrelevant. We just want it gone.
The second type of warpage is called "thickness variation". This is found with a micrometer and taking a number of measurements at various places around the rotor. Since the rotor is thicker in one area, it pushes the piston back into the brake caliper. That pushes brake fluid back up to the master cylinder where it pushes on the brake pedal. You often won't feel thickness variation in the steering wheel but you definitely will in the pedal.
As a final point to this wondrous story, there is a new problem that has been showing up mostly on light trucks, starting in the mid '90s. That is an extremely hard brake pull that doesn't show up until the pads have reached a specific temperature, typically from five or six rather hard stops in close succession, as in when coming down a steep hill. There will be absolutely no hint of a pull one pedal application, then the next time you press it, you end up on the sidewalk or in oncoming traffic! This is caused by the pads and two rotors reaching different temperatures which alters the coefficient of friction unequally. The uneven heating is caused by a difference in thickness of the two rotors. We never used to see this or even consider it. In fact, I had an old '70s Chrysler with one front rotor that was.125" thinner than the other one, (and below the legal limit, but I'll never admit to it), and never had a problem. Now, I've run into two trucks with this problem and I've read about others. These were fairly heavy pickup trucks with rotors that were almost 1 1/2" thick. One had 0.020" thickness difference between them, and the second one had 0.007" difference. That's the thickness of two sheets of paper but it was enough to cause the brake-pull-when-hot. Standard procedure now is to machine both rotors just enough to clean and true them up, then continue machining the thicker one until it matches the thinner one. The common clue to this problem is it is always noticed for the first time shortly after a normal maintenance brake job was done. A lot of mechanics still haven't run into this or even heard of it. If this happens to you, simply return and explain this and have them machine the thicker rotor again to match the other one.
Forget looking at the rotors to identify a problem. You'll feel thickness variation as little as 0.002", similar to one sheet of paper. If lateral runout is real bad, you might see the brake caliper walking back and forth when you run the vehicle in gear with the wheel off, but I've had many where that happened but there was no noticeable pulsation or vibration.
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Thursday, April 9th, 2015 AT 5:10 PM
