Brake Rotors Slightly Warped

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Old 02-01-18, 07:13 PM
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Brake Rotors Slightly Warped

Last brake job, I had new rotors put on. I'm not half way through the pads and I have a slight wheel shimmy when I apply the brakes at 65mph. I have not tire issues. Should I be able to get a finish cut on these rotors and not replace them. Then just new pads. Thanks
 
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Old 02-01-18, 07:43 PM
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Yes, but you need to find a shop with the right machine. I went to two places and asked if the cutter floated on the rotor or if it would take out warp (since I had a similar issue to you). Both said it would just take out groves and ridges, not remove warp. Finally was sent to a machine shop where they cut them flat.

You may not even need new pads depending on wear. You can take any glaze off with sandpaper.
 
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Old 02-02-18, 04:56 AM
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Here's an alternative diagnosis to warped rotors . . . . rusty ridges which may mimic warping. It may pay to carefully examine your Rotors before turning or replacing them; especially at the stage of wear that you're at with these rotors and pads.

I had a brake pulsation develop on my Volvo S60 last spring, and just presumed that I had warped a rotor during the winter (often happens when hot rotors encounter ice cold puddles of water or snow banks . . . . or so I thought). I could feel it in the steering wheel, so I concluded it was in the front brakes, not the rear.

So I bought new front Rotors and Pads and some Dot 4 Fluid and set myself up to swap then out one Sunday afternoon.

Upon dis-assembly (which for me is always more difficult than re-assembly), I discovered that II had two ridges of rust on one of the rotors: one along the outer circumference of where the pad was making contact with the rotor; and another along the inner circumference . . . . and the ridge of rust around the inner circumference was broken and missing for about 180. And when I examined outer pad for that wheel, it had a wear groove in it that corresponded precisely with the ridge of rust that I was seeing.

I made note of those discoveries; but proceeded to complete the installation of my new rotors and pads that day; but later, I showed that rotor to a friend (who's about 85 years old) with a machine shop to turn turn rotors and re-finish drums (among other things), and I asked if he had ever seen that kind of ridge, and he said no; but it was probably the underlying cause of my "pulsation" felt at the pedal. He simply took a ball-peen hammer, and bumped off all of the remaining rusty ridge and said "There, that should end that pulsation; and there's still plenty of thickness in that rotor ! "
 
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Old 02-02-18, 05:21 AM
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What year/make/model vehicle do you have ?
 
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Old 02-02-18, 09:25 AM
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Have the runnout of the disc measured by the shop, should indicate if there is a warp in the rotor. It is probably a rotor warp, but it can be other things, eg. like a wheel/tire problem, a wheel bearing, a tie rod, when you brake from high speed this puts a high load force on the whole front end and can surface emerging wear issues. While the rotor warp is most likely, it would be a shame to replace and still have the problem.

Also, before doing anything, check all your tire pressures, a low tire can cause this. Also just take a look at the rotor, does it look smooth or is there some "pad ghosting" pattern on the rotor? Sometimes the pads glaze the rotors, especially cheaper quality ones that can't take the heat and can mimic the effect of a warped rotor. Skimming the rotor and replacing pads would solve for this. If mild glazing, some sandpaper by hand will solve for the problem.

Many new rotors and pads are not that great a quality. The Centric Premium line is widely available and has a higher grade of iron that makes them more warp resistant. If you are on a budget, just use these on the front wheels, the backs can use a cheap rotor as they are only responsible for about 20% of the braking forces and don't get the same heat load. I generally go with OEM pads, even if a bit pricier, as they have the better initial grip character that most drivers benefit from and meet the necessary quality control.
 
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Old 02-02-18, 10:33 AM
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So the first thing everybody needs to understand is that brake rotors do not warp, even a hot rotor hitting cold water!

The pulsation you feel is due to uneven wear which is caused by the pad. The pad lays down a layer of material and with the low friction characteristics of pads used in the US (because we dont like dirty wheels) it doesn't get removed evenly thus pulsation.

Cutting the rotor essentially removes this material, and metal, to provide a flat surface. If pads look flat and straight you can give them a good sanding and reuse.

OEM rotors are better, good metal, good dimensional control, good coatings to control rust!
 
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Old 02-02-18, 10:52 AM
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Raybestos Brake Tech School, Part One: Rotors Don't Warp | Hendon Publishing

Excellent and easy to understand about "warping" rotors, fixes, pads, etc. I think most people know that rotors don't really warp like a potato chip (somehow the idea of a 3/8" or more thick, ventilated and slotted disc of cast iron warping seems kind of silly when you think about it)...but it's an easy way to explain the problem. "Out of true" is the new term we should push from now on
 
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Old 02-02-18, 12:20 PM
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or the tire was improperly torqued to the hub during the last tire change

Drives me up a wall when you see Dealerships/repair shops using their impact gun at the highest settings driving the lug nuts down.

Lug nuts should be torqued down to around 100 ft/lbs.
 
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Old 02-02-18, 04:53 PM
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If a shop isn't using a at least a torque stick, then I ask them to please do it by hand with a torque wrench. It's worse when they think they are the front tire changer on a NASCAR team and don't crisscross when tightening.

I still re-check when I get home.

Of course I also check tire pressure. I've had them overfilled by up to 8 lbs.
 
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