Actually, you do. What you feel when you have pulsation at the brake pedal is not lateral runout, it's thickness variation at the rotor. Measuring a rotor for thickness variation is a waste of time....as you mention....you can feel "that" on a road test.
But you can't "feel" lateral runout if the rotor has no thickness variation. So lets say you had a brake pulsation, you machine the rotors....now we have no more thickness variation. But, if you install the rotors and don't check for lateral runout (which can be caused by rust on the hub, lateral runout at the hub, etc.) then eventually, if you do have lateral runout, you will end up with thickness variation and a brake pulsation again. Lateral runout will cause the rotor to hit the pads ever so slightly as the rotor turns. This will eventually cause thickness variation and a pulsation.
Which is why it's so important to clean the rust and crap off at least the inside of the rotor surface and the wheel hub surface that the rotor mates to.
I don't currently feel pulsation when I *apply* the brakes; but I do feel slight vibration in the steering at speed on smooth pavement. I didn't relate the two (brakes & steering) as I am doing brakes now (pads worn indicator just lit on my '90 Q45)... but may I ask what the slight vibration in my steering is due to in a later thread.
Does this only apply if the rotors are REMOVED from the car?
I don't plan on removing the rotors (unless I have to) and I've never done that. I suspect I'd have to remove the rotor if the brake rotor thickness isn't up to spec or if the rotor lateral runout is too great or if the rotor is badly grooved. Yes?
It may be more obvious to you (i.e., more experienced than I am); but, I've never done a brake job before so I'm more unsure of myself.
From this web page:
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The wording (verbatim) which I'm worried about is: ? With optional attachments check tire and wheel run-out
(BTW, those SAME description used to be on the description for the 3D103 yesterday, and I wrote to the web site, which they seem to have corrected between yesterday & today so those words only seem to show up on the #6450 today).
I guess if "brake run-out" is a different test than "tire & wheel run-out" (aren't the tires & wheel mounted to the rotor?) then I'm not worried I'll need more equipment.
Still, that $132.25 tool (#6450) seems indistinguishable (to me, from the picture anyway) from the $75.16 ($3D103) tool I actually ordered:
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On a different note, I LIKE you other astute suggestion for run-out tests:
What I like about this idea is the magnetic mount is 1/3 cheaper than the flex-arm dial-gage mount and it seems to be more generally adaptable (is it?) than the vise-grip flex-arm mount (which is better?).
But, uh, oh, again, the ambiguity of it all. Notice these two photos:
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Which one would you buy? The first is $43.87. The second is $105.27 The only difference I see is the arm off the magnet is either flexible (in the more expensive case) or rigid (in the much less expensive case).
The description for the first #FOW72-520-199 (43.87 dollar tool) says: FOWLER & NSK 72-520-199 - DIAL INDICATOR 0-1IN MAG BASE 0-100 CONTINUOUS READING Indicator features: ? 1" travel - continuous reading ? AGD group 2; 2-1/4" dial diameter ? Revolution counter ? Lug back mount Magnetic base features: ? 85 lb. pull strength ? On/off switch ? Fine adjustment
While the description for the double-the-price (#FOW72-641-300) tool says: FOWLER & NSK 72-641-300 - DIAL-IND 0-1/FLEX-ARM/MAG-BASE # Multipurpose tool with 12-1/2" flexible arm which can be set in any desired plane. # Great for precision measurements on engines # Dial Indicator measures 0-1" in .001" graduations # Includes: Gage, contact tip, locking flexible arm and On/Off magnetic base. Package in fitted case.
QUESTION: Would you buy the rigid arm mount or double-the-price flex arm? (which is better, all around, for jobs, including brake rotor run out)?
I do this for a living (repair cars) and we have the flexible arm with the vise grip type of dial indicator. By far the best set up...it is! It's adaptable to any situation, and it works well on aluminum....(which the magnetic base won't touch). I do a lot of work on aluminum front differentials which require backlash measurements. This style of dial indicator is the hands down winner.
It's also the preferred style of dial indicator for doing rotor runout measurements (it actually is the default dial indicator setup supplied with the GM approved tool for checking rotor runout)
One problem with measuring run out on a lift is that once the rotors heat up they are apt to distort much more. So, while a rotor that is out in the bay is bad, a rotor that is in spec is not necessarily good on the road. That's one of the reasons that cheap rotors suck.
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