Yesterday I p Brake disc maximum runout 0.05mm (0.002 in )
Now, I took my digital verniers ( a different tool ) and set it up to
0.05mm. Holding it up to my eye, I could barely detect LIGHT coming through the jaws, so then I pulled a HAIR OUT OF MY HEAD and measured that thickness.
Hair = 0.07mm in thickness.
So...If I got this right, and I can't see how I didn't, the maximum allowable rotor runout is a FRACTION OF THE THICKNESS OF A HUMAN HAIR.
This can't be right.
I bet my last nickel, which I have here on my desk, that if I measure the rotor runout on even a NEW CAR, the runout is going to exceed the thickness of a human hair.
WHAT'S UP WITH THESE SERVICE MANUALS? Who writes them? Are they under the influence of hallucinogenic drugs?
Try it and see, Glickster. My manual says 0.004 inches for my particular application. If you dont want them to wobble and shimmy, then they need to run true.
================ Just because it's digital doesn't necessarily mean it is accurate. I use a threaded micrometer, and dial indicator with mechanical sweep hand that I've had for years. IIRC the disk runout maximum on my import FWD is 0.0020, and the sealed bearing for the front wheel is 0.0010. Recently checked and both were within spec. With the mechanical gauges you can move something very slightly and see the movement on the pointer. .002" is correct. I use a brass dial type tire pressure gauge with bleed off valve and find it much more accurate that a digital pressure gauge I was given as a gift. Especially if you've got an inexpensive runout gauge, I'd be skeptical of the precision.
I don't know about the human hair, but .002" IS right. I use a dial indicator and typically find readings of .0015 or so on good rotors. Depending on the vehicle a pulsation may be felt if it gets up around .004 -.005.
That's correct. You normally will not feel "runout" through your brake pedal, but "runout" eventually leads to "thickness variation"....and that's what you feel when the brakes pulsate. Which is why the runout measurement is so important. It will eventually cause a brake pulsation, but if often does not show up for 10-12K kilometers.
That is the wisdom of the day, and there is truth to it, but at the end of the day, when you start feeling the wobble/shimmy, eventually something has to be done. Realizing there there is more to the issue than just disc warpage is helpful.
You CAN feel runout as buffeting/jitter at some level, because there is mass to the caliper. If you have to use energy to move that mass from side to side, there will come a point it can definitely be felt, but perhaps not in the brake pedal. (and perhaps so).
If the caliper were without mass, and if there were no viscosity effects to the brake fluid on dual and quad pistoned calipers, then you might feel nothing.
Thickness variation is another side to the coin. You will definitely feel it in the brake pedal. It adds to the jitter in a minor way.
So, what do you do? Normally if you machine the rotors, the problem goes away. It may not stay away.
If the hub has too much runout you may have to change it, or at least shim or index the disc so that the whole thing comes into spec. You have to clean the surfaces so that when you bolt it all back together, there is no rust or dirt or corruption to keep you from truing up.
Of course, the disc must be true, properly surfaced, and of adequate thickness. Some discs can be machined, others may have to be replaced.
Having good measuring tools helps you find out exactly what is going wrong, and for those who want to effect a long term cure, not just a cheap 'fix', it should be possible if you understand the nature of the system.
I'm going to disagree with you. What usually happens is that you "hear" runout if it's extreme. You will hear the caliper banging back and forth, but if the disc is completely parallel...you aren't going to feel any pulsation in the brake pedal.
I've experience extreme runout just by having a brake lathe that wasn't centering the rotor properly while it was being machined.
Very true....but the problems that come about because of even very minor runout usually don't appear until a number of miles down the road. Which is why we often used to blame the driver, the rotors being thinner because of machining them....etc.
How do you know the analog gauge is giving you the right reading and the digital the wrong one. I've rarely seen two different analog pressure gauges give the same reading but several different digital ones I've got all give the same pressure. Doesn't mean it's right but it's the same which is more then the analogs.
That is information, correct or incorrect, that was passed along on a Ray and Bob show. These guys do Car Talk. They have a website. They are graduates of MIT, and I ass/u/me that because of their higher level of education, they know what they are talking about. Many other people make this same assumption. They also run a car shop, and speak from their own experiences. In fact their *job* is to give car experience to other people.
Since they broadcast on radio, and have an open website, challenges to their ideas would appear from everywhere. I've never seen anyone challenge them on the analog vs. digital pressure gauge issue.
That is information, correct or incorrect, that was passed along on a Ray and Bob show. These guys do Car Talk. They have a website. They are graduates of MIT, and I ass/u/me that because of their higher level of education, they know what they are talking about. Many other people make this same assumption. They also run a car shop, and speak from their own experiences. In fact their *job* is to give car experience to other people.
Since they broadcast on radio, and have an open website, challenges to their ideas would appear from everywhere. I've never seen anyone challenge them on the analog vs. digital pressure gauge issue.
here they say *some* digital gauges are "surprisingly accurate" ================================================================== Dear Tom and Ray:
I have three different tire gauges, all of which give me different readings. They are all the old-style "pencil type" gauges, and all are made in China, if that makes a difference. How do I find one that can be trusted? -- Jerry
RAY: Well, the pencil-type tire gauges, with the pop-up plastic readouts, are notoriously inaccurate. You could buy five more of them, and you'd probably get eight different readings.
TOM: And the dial-up gauges used on air pumps at gas stations are even worse. You set it for 32 psi, you hear "ding-ding, ding-ding, ding-ding," and you drive away with 80 pounds of air in your tires, with your head bouncing off the ceiling.
RAY: We use a very precise dial gauge at the garage that cost us about
100 bucks, and it's a beautiful instrument. We keep it under lock and key, because if we didn't, my brother would use it as a hammer to free up stuck brake calipers, or to crack chestnuts.
TOM: But I'll tell you what. We got some samples of some inexpensive, battery-powered plastic gauges with digital readouts. Surprisingly, they were extremely accurate. You can get them at almost any auto-parts store now. They cost 15 or 20 bucks, and they use replaceable batteries.
RAY: Yeah. The batteries cost 14 bucks.
TOM: Nah, they're watch batteries; 3 or 4 bucks a pop. In any case, whoever makes these gauges (and I'm sure at least some of them are made in China), this style seems to be far more accurate than pencil-style gauges. So, that's what we'd recommend for you, Jerry.
You'll note the STANDARD they keep "locked up" as a reference is a dial type.
Humans being contrary critters, here is what Tom and Ray have to say about dial gauges: ===================================================================
Dear Tom and Ray:
My question is in regard to proper tire pressure. When you buy a new car, the owner's manual tells you to abide by the sticker inside the door post. My present car calls for 30 psi in all four tires. But when I get the car back from the dealer, the tires generally have 36-38 psi. Do they know something I don't? -- Howard
Ray: No, you apparently know something they don't, Howard. Either their equipment is inaccurate, or they're just being careless.
Tom: Or perhaps, after the Firestone tread-separation debacle, they're so afraid of underinflating tires that they're being overzealous and overinflating them.
Ray: But you're absolutely right, Howard. If you have the original tires on your car, or approved replacements that are the same size, then the pressure recommendation in the owner's manual (or on the door post) is the one to follow.
Tom: Just be sure YOUR measurements are accurate. You have to use a good-quality tire gauge.
We recommend the round gauges with the needle, rather than the ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "pencil" type gauges with the pop-up indicator. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Ray: And be sure you're measuring the pressure when the tires are relatively cool, since heat from driving can dramatically increase the pressure. So if you take a 30-minute drive home from the dealer -- including a jaunt on the highway -- and then hop out of the car and measure the pressure, it's likely to be inflated by the heat generated during the drive, and it will give you an inaccurate reading.
Tom: But if you're checking the pressure with a good gauge when the tires are cool, you have every right to ask the guys at the dealership (nicely) to shape up and be a little more careful from now on.
So here, they contradict themselves and "recommend the round gauges with the needle."
So which is it? On their advice, I threw away what I thought was a perfectly GOOD digital pressure gauge, and went out and bought an analog dial gauge. Now they turn around and say digital.
I love their show, it's very funny, but I have heard them give bad info occasionally. They may have had a bad experience with the first digital gauges years ago and just keep passing it on.
BUt you'll notice that they just assume the $100 analog gauge is correct. They also seem to be saying that the cheap digitals give pretty much the same answer. Most people will not pay $100 for a super quality analog gauge, they will either pay $5 for a cheap, and inaccurate, analog gauge, or $10 for a fairly accurate digital gauge - the opposite of what the start of this thread implied they should do.
Yup. I think they made the whole thing confusing by bringing the pencil type gauges into the discussion. Those things are horribly inaccurate. Then they seemed to suggest you should use a gauge with a "needle" but really confuse that issue by talking about their $100 gauge when everyone is only going to spend $5 on one. You would be lucky to find two regular analog gauges that agreed within 2 psi of each other. My digitals are all within 0.5 psi of each other.
I think I am through buying tire pressure gauges for a while. No matter which kind I get, analog dial or digital, I'm never going to know if it is ACCURATE anyhow. I have no STANDARD against which to measure them.
So I have an analog, and am using that, and when it breaks, ( I should live so long ) I might consider going back to the digital I had to begin with, which never gave me any problems anyhow. I was happy with that digital pressure gauge until I listened to their show. Bummer. If these guys suddenly like digital so much, maybe -they- can pay for a new one for me. fat chance.
I don't have the time to do this now. Car is in constant use for medical issues relating to the family. But sometimes when I'm out waking around, I take a look at the disc brake rotors on other people's cars. My experience shows most have rotors on the front, and shoes on the back. This is my setup also. Rotors on front, shoes on back. This cursory inspection problem I have might go along with my diagnosed Obsessive Compulsive Behavior diagnosis. It is becoming a very very weird habit.
Somebody might ask me why I'm looking at people's rotors. I don't have a good answer for that, but some of them look, quite frankly, like shit.
When I get to mine, I'll be measuring hub runout, rotor runout, runner thickness and variation, and measuring the caliper PADS to make sure they have enough stop-stuff left on them for the winter.
I believe this all began LAST WINTER when I read an article in the paper about a Van with kids and mother that ran into a creek, where they all drowned to death. The cops said the brakes were shot and that is why the vehicle went out of control...wouldn't stop. They were on a paper delivery route...IOW a huge amount of Stop & Go, which is sure to burn up brakes quickly. Now they are all dead. So yes, it matters.
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