Why do my brakes vibrate more AFTER I change new pads?

[vjaneczko]

Go to the dealer and have him install rear shims on your brakes. If he tells you what my dealership told me today:" I've never heard of shims on an 850", then RRRRRRRRRRRRRRUUUUUUUUUUUUUUUUNNNNNNNNNNNNNNNNNNNNNNNN!!!!!!!!!!!!!!!!!

Last week I helped a buddy with his 850 brakes and the pads we got at AutoZone (sorry, don't remember what brand) had a 'permanent' shim attached to the pad. Maybe the dealership has been using something like those.

And I'd say that installing shims is very easy to do - what would a dealer charge for labor, 75 bucks? The 30 minutes to do it myself is worth that kind of $$

[bronxnativ]

"And I'd say that installing shims is very easy to do - what would a dealer charge for labor, 75 bucks? The 30 minutes to do it myself is worth that kind of $$"





Absolutely, I agree. I only made mention of the dealer because OP stated that he was at the dealer. I have NEVER taken, nor do I ever plan to, take my dearly beloved brick to the dealership. I value my 850 (and my wallet) too much!!!

Al

[golfman484]

I think mhn is right and it fits perfectly with a vibration were having and in light of the history sounds even more right.

A few months ago our ABS starts coming on when under normal breaking - i.e. not emergency breaking. For about a week we had MAJOR vibrations every time we stopped until I got the time to look at the ABS sensors and clean them all out - they were full of 'magnetic dirt particles' that we'd picked up over the years.

The ABS vibration went away immediately and breaking was nice and smooth until almost nearly at a stop - then there was a very subtle but definite vibration over the last 5 metres of breaking.

I wasn't sure if this was still the ABS so I removed the ABS fuse (confirmed it was disabled by braking hard and noting that the ABS did not activate - wheels locked up). When I came to the last 5 metres of braking the vibration was still there so it was obviously no longer an ABS problem.

I think mhn is right in saying that over an extended period of ABS operation, subtle radial (not tangential) ridges probably formed on the disks and that is why there is a residual vibration after cleaning the ABS sensors.

I'm going to get all 4 disks lightly machined to remove any ridges that may have formed and hopefully that will fix the problem - and prove mhn right !

[jimmy57]

In order for ABS to have any influence on wear on rotors your ABS would have to be HYPER active.
If you fit rotors to the car and the lateral runout is not checked then you are gambling. Fortunately the greater retraction calipers make this less important but if you have a vibration problem the rotor needs to be placed on hub in the position where runout of hub and run out of rotor cancel each other and you have less than .06 mm runout. Tougher cryogenic rotors often help but it is only due to their reduced wear characterisitics. If the rotor has runout the rotor can swing over enough to contact the retracted pad and wear that spot(s) thinner. Disc thickness variation (DTV) is what everyone calls "WARPED" rotors. There are a few rotors that wear due to issues with homogeneity of the metal in the cast rotor blank. If the metal is softer or full of porosity in one spot then the wear will be different and you end up with a thin or thick spot and thus DTV and vibration. A truly warped rotor will not initially have vibration unless caliper mount is bound up (or piston binding on fixed calipers) as properly working brake apply hardware moves laterally. The warped rotor will end up with vibration causing DTV after a time from the pad contact at max throw locations on rotor and the resultant wear spots.
Most of the time the runout of hub and rotor is low enough to avoid this and random rotor location gets you a smooth brake set. If you have vibration repeated problems then you need to check and locate rotors. If the rotor runout can't be set at or less than .06 mm (Volvo says .04mm) then you likely need a hub or that rotor is REALLY warped. Without somespecial tools hub runout can be determined by doing lateral runout check with two different rotors, one that is known to be within spec on the other side of car.

[golfman484]

Well I said I'd report back after machining the discs and here's the result:

Bingo! No more vibration!!!

I took off all 4 discs and had them machined - even though they visibly looked really smooth with no warping and no 'tracks' (usually from when the brake pads wear too low and you get metal on metal grinding - like happened once or twice back when I was a uni student on a budget =] )

As I said before - we previously did have a large collection of magnetic dirt surrounding the sensors which was triggering the ABS *every* time we stopped for a week or two. (The ABS light had not come on - it thought it was doing it's job but the sensors were not sensing rotation even when there was and so activating the ABS).

Even after the sensors were all cleaned up and the ABS stopped coming on all the time (I even removed the ABS fuse to prove the vibration was no longer coming from ABS activation) we still got vibration - but of course it was only about 20% of the intensity compared to when the ABS was falsely triggering.

My conclusion is that the week or two of constant/excessive ABS activation due to dirty sensors impregnated some 'pattern' in the disc that was very small and undetectable with the human eye. Machining all 4 discs removed that pattern and the vibration went away.

I do not believe the discs were warped - I've had warped discs on another car before and the vibration can be felt as soon as you apply the brake - not just over the last few ten or so metres of braking. In fact with warped discs the vibration reduces in frequency and intensity as you drop speed - quite different to my symptoms.

[MoVolvos]

Hot spots or glaze which can not be seen from uneven braking regardless of source including tire vibration.

Blessings,

BKM

[instarx]

No, actually, I don't. No one you'd know, anyway. I could easily be wrong and / or actual *decades* out of date, but I'd been told a couple of times by a couple of different people that making the ABS do that juddering skip it does, hard enough or more than once or twice, was enough to put ridges in the rotors. They might be subtle, but they'd possibly be enough to account for the symptom. New pads might be less acclimated to the slightly altered shape of the rotors and therefore show up the vibration a little more.

Just a thought -- one of those bits of received wisdom you don't think to question any more, but probably should. I'd still talk to the dealer.

This just isn't true - you can try it for yourself - I promise - it won't hurt! It is regular ...

By the way, just to keep the record straight, I didn't write that! When you edited the quote you accidentally attributed that statement to me. I questioned it too.

[800artfreed]

I would like to offer that the interface between the rotor and the hub is very important to minimize "run out". This is the condition where the rotor is actually wobbling because it does not sit flush and parallel to the hub. After doing a rear brake job on a 1999 S70 but with low mileage (65K) I found a rust and corrosion on the hub. I used a polisher set from Harbor Freight. It has 3 grades of abrasiveness and the most abrasive was able to clean up the surface without removing any metal. This really helped the rotor run more true. This minimized the rubbing you hear from the parking brake or rubbing up against the disk pads. I found this article to be very helpful. https://www.motor.com/magazine-summary/ ... practices/