Vibrations when braking at 65mph. - JeepForum.com
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  • 1 Post By yjsaabman
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post #1 of 14 Old 08-01-2021, 08:07 AM Thread Starter
zjlockers
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Vibrations when braking at 65mph.

Vibrations when braking at 65mph.
They seem to be coming from the front.
Just measured the rotor-evenness at three locations giving the below numbers (will repeat for
other three rotors), but is there another common cause besides 'warped-rotor'?

upper-left: 2.520inches
lower-right: 2.527inches
top-middle: 2.524inches

max deviation from the average is: 3.666666666666707e-003

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post #2 of 14 Old 08-01-2021, 08:20 AM
yjsaabman
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Could be worn bushings in the front end, as well, but usually rotor issues. Does it it do it on initial application then go away? Or does it stay consistent until you get slow enough to not notice it?

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post #3 of 14 Old 08-01-2021, 08:28 AM Thread Starter
zjlockers
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Quote:
Originally Posted by yjsaabman View Post
Could be worn bushings in the front end, as well, but usually rotor issues. Does it it do it on initial application then go away? Or does it stay consistent until you get slow enough to not notice it?

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At 65mph, vibration starts as soon as I press the brake pedal.
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post #4 of 14 Old 08-01-2021, 08:56 AM
yjsaabman
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Quote:
Originally Posted by zjlockers View Post
At 65mph, vibration starts as soon as I press the brake pedal.
Yes, but does it continue to pulse until you come to a stop? Or does it pulse a handful of times, or so, then go away?

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post #5 of 14 Old 08-01-2021, 09:56 AM Thread Starter
zjlockers
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Quote:
Originally Posted by yjsaabman View Post
Yes, but does it continue to pulse until you come to a stop? Or does it pulse a handful of times, or so, then go away?

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Ohh, my bad.
It pulses til the speed (mph) gets low enough, then goes away; I mean it never does it at in-town speeds like 0-45mph.
Let me test it again after it's back off the jack stands, to answer you better.
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post #6 of 14 Old 08-01-2021, 10:51 AM
RedRiverT
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Quote:
Originally Posted by zjlockers View Post
Vibrations when braking at 65mph.
They seem to be coming from the front.
Just measured the rotor-evenness at three locations giving the below numbers (will repeat for
other three rotors), but is there another common cause besides 'warped-rotor'?

upper-left: 2.520inches
lower-right: 2.527inches
top-middle: 2.524inches

max deviation from the average is: 3.666666666666707e-003
What are you measuring that is 2-1/2"? Are those some sort of super aftermarket rotors or is that the face width of the brake pad contact surface, or is it a typo?

When checking rotors, you measure the thickness from one face to the other and lay it on a flat surface or use a straight-edge (ruler) standing up across the face to see if it is flat.

Vibrating after high speed driving could be because the rotors are getting down to (or below) their minimum thickness recommendation, so as the heat builds up from the pads naturally rubbing a bit, they warp. Or maybe a piston is sticking in the bore and not letting the pad release fully, so is holding it a little tighter to the rotor causing extra heat build-up at higher speeds. Look at your brake pads and see if one is worn more than the others, or maybe the inside pads on both sides are worn down way more than the outside pads. A little more wear on the inside pads is normal but they shouldn't be like down to 1/8" and have the outside pads still real thick.

When you try to lever the pad away from the rotor, is it real hard to move on one side? (or both sides for that matter) Have those rotors been turned (re-conditioned) before?
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post #7 of 14 Old 08-01-2021, 11:41 AM Thread Starter
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The ~2.5inches is from the face of the hub to the rotor (not the rotor thickness). I did this:
1.tighten two of the lugs
2.use a 3rd lug screwed on in reverse to place a flat piece of steel parallel to the hub, smack tight against the hub.
3.use the 'depth gage' part of a good-caliper (Mitutoyo) to measure the distance from the flat-steel-piece to the rotor.
Maybe this method is no good, but I saw one guy do it on youtube.
I will get back to the other questions later tonight.
Do you happen to know what is the MINIMUM rotor thickness, so that I can check that before I pull it off the jack-stands, or maybe that varies from one rotor-maker to the next?

Last edited by zjlockers; 08-01-2021 at 12:10 PM.
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post #8 of 14 Old 08-01-2021, 12:16 PM Thread Starter
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I think I found the min-thickness specs, from oreilly's web site: .374inches for rear and .894inches for front.
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post #9 of 14 Old 08-01-2021, 04:05 PM
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Have you ever changed the control arm bushings since you have had the Jeep?


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post #10 of 14 Old 08-01-2021, 04:51 PM
jeepjeepster
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Sounds like you just need some new rotors....
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post #11 of 14 Old 08-02-2021, 04:26 AM Thread Starter
zjlockers
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Quote:
Originally Posted by PolkaPower View Post
Have you ever changed the control arm bushings since you have had the Jeep?


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I just bought it used about a week ago or less.
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post #12 of 14 Old 08-02-2021, 04:28 AM Thread Starter
zjlockers
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The problem *seems* to have gone away "magically". Go figure.
The only thing I can think of that is different is that I bled the brake lines.
If the problem *stays* away, I'm happy, of course.
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post #13 of 14 Old 08-03-2021, 05:09 PM
dlk59
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Saw same thing on our WJ. Braking at +45 mph caused nasty vibe. Always heard rotors are only cause - period - from many. So went to replace seemingly fine rotors and pads. Inspecting as I pulled it apart, I found bad corrosion on caliper mounting hardware. Replaced hardware and lubed pins with ample amount brake lube. Reassembled and PRESTO no issues returned. Rotors and pads got returned though!!!!!
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post #14 of 14 Old 08-04-2021, 02:19 AM
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Unless metal to metal contact has occurred rotors don't usually vary much in thickness.. but heat cycles can cause them to wobble or not rotate evenly. You need a dial indicator to check. Be sure wheel bearings have no play, use 3 lug nuts to secure rotor to hub. Zero indicator then slowly rotate the disc while watching the needle. Check your FSM for tolerance value.
Now it's also common for rotors to be fine at slow speeds but vibrate at higher speed. A straight rotor can warp during the higher heat of a high speed stop.
What do I do? Since brake vibration is 95% caused by rotors, when vibrations start occurring, I just replace them. No need to over think it.
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