Brake Job - Necessary to Measure Rotor Runout?

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I had a car in for a front/rear brake service yesterday. On each axle, the car received new premium rotors, premium pads and new hardware. The hub flange was also thoroughly cleaned using a wire wheel. I measured the lateral runout of each rotor once it was installed. Prior to taking the measurements, the rotors were installed using conical washers and lug nuts that were torqued to the specified 100 ft-lbs. These were the readings that I obtained: LF 0.001” RF 0.0015” LR 0.002” RR 0.0005” Note: on this particular vehicle, the manufacturer allows for up to 0.002” of lateral runout. For the LR and RF rotors, I removed the rotors and re-mounted them approx. 180-deg from their original position. In both examples, the runout value was significantly reduced; the final reading for the RF was 0.001”, and the LR was less than 0.001”. My personal opinion is that the less lateral run-out you have, the less likely that you will develop disc thickness variation issues overtime. Disc thickness variation is what causes most pedal pulsation issues. However, I discussed this story with a person offline, and he stated that having a low lateral runout value does not tell the full story. In his opinion, even if the runout is low when the rotors are at room temperature, it is hard to determine “what happens” once the rotors are hot. In his opinion is that if there is a casting issue, it is impossible for the average technician to confirm this during initial installation. Supposedly, a non-uniform cast can cause the rotors to expand unevenly when they heat up? I look forward to hearing your thoughts on this matter. Do you feel that measuring rotor runout is a critical part of doing a proper brake job? If not, please share your feedback. Thanks!
 
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Originally Posted By: The Critic
..I removed the rotors and re-mounted them approx. 180-deg from their original position. In both examples, the runout value was significantly reduced; the final reading for the RF was 0.001”, and the LR was less than 0.001”..
Reads like at least some of the variation is occurring at the hub interface. .001" is certainly within my allowable specifications. Any judder and/or pulsations with its current rotors? In the spirit of full disclosure, I've never measured a hot rotor. smile
 
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Modesto,CA
You did a great job setting up the new parts. Break-in is very important as well. Most people just drive like normal & thinks that's fine. It's not. A good break in should be 7-9 stops with increasing speed ranges & increasing brake pedal pressure. Each one should never have a complete stop. Park without making a complete stop using the brakes, roll in neutral & use the parking brake. Let them cool off for a couple hours & the brakes will probably never have pulsation for the life of the pads.
 

JHZR2

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Wow, never have really thought about this at all... Can you show how you measured this to such exacting levels? I wonder what the measurement error is...
 
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Originally Posted By: JHZR2
Wow, never have really thought about this at all... Can you show how you measured this to such exacting levels? I wonder what the measurement error is...
I imagine he has a very good dial indicator setup. What sucks is when you've been doing something successfully for your entire life (40 years wrenching for me), THEN you learn on the internet that you have been doing it wrong all along. I have never used a dial indicator to check runout on a brake job. I guess that answers his question for ME (knock on wood).
 
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It is worth measuring run-out if you have a rotor that can to find the position that has the least run-out. But some rotors have one or two screws that secure it to the hub (instead of only being secured by the lug nuts). In these cases the rotor can only go on one way, so just buy good rotors.
 
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Quality isn't the problem, the best rotors can have run out when mounted. On Asian cars with studs the screw play no role outside the assy line. Euro cars with bolts need them to prevent a wheel changing fiasco. Sometimes shims must be used as just moving rotor position will not correct the run out enough. Checking run out is critical to doing the job right. Many times this rotor brand or that get labeled as junk because they pulsated right out of the box. In reality it was a DIY just throwing on a set of rotors and pads and calling the job done that's at fault. In my experience slightly over the limit run out may go unnoticed when cool but gets exaggerated when hot and worsens over time. Getting the rotors true on the hub and the caliper and pads true on the rotor are critical to smooth and quiet operation of the brakes for the life of the pads. Guessing and hoping is no way to perform repairs properly, throwing rotors and pads on without checking them for being true is doing exactly that. Many times you get away with it and other times it just gets chalked up to bad parts.
 
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All I ever bother with is to scrub the hub face with a wire brush, and some 40 grit sandpaper if necessary, to smooth it out. I've never had a problem. If I do, I guess that's when I'll get a dial indicator. And I've never had a problem that I chalked up to bad parts, either. Same holds true for all of my family, AFAIK.
 
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