Rear Brake Resonance Killing Me

[turbomoose]

I know this is a common topic, and Im sorry for the post as there are several on the topic... but I have not found any thread with a final solution. Here's the story I have for you all, I hope this thread will be a productive one for us.

For the last three months I've had a terrible rear brake resonance! Came with the car actually. Was the only negative it had when test driving it. Well yesterday I bought new pads as they were quite worn. The originals were Volvo pads with a built in rubberish shim... and a stainless shim with a half cut out in it. (not a cut out window)

First thing I did was take out the stainless shim and reuse the pads. No real difference. I replaced the pads with the new ones I bought, greased them up well and used the same half shims on all pads and made the arrow point in the direction of wheel travel. It is absolutely unbearable!!! Every slow stop is like a swedish form of torture. The rear of the car groans and moans and vibrates. My sunroof even rattles!

If I make a quicker stop with steady brake pressure, there is nothing but silent, smooth and perfect stopping. I dont understand it at all!!

I have checked all brake lines they are unbent, and firmly clipped in place to the frame. No missing brake line clips. The rotors are not blue from heat and are not grooved.

Would it be a good idea to have them turned? I've read Volvos dont "like" to have their rotors turned. My front brakes are imperceptible. No vibration in frame or steering wheel or brake pedal.

One other odd form of brake behaviour, the brakes do not misbehave at all in any way the first few miles. No matter how I stop. Only once the brakes warm up do they become irate.

I wish I had quiet brakes!! :(

[prasamin]

use OEM pads (new) and get brand new shims....

otherwise, check the rotor surface...warped or maybe just need to be resurfaced...i had the same problem...first time it was new pads and shims, problem gone....this time around the pads/shims were fine and the resonance was very little, but I needed to resurface the rotors...no more problems.

[vasilli]

i have a similar problem w/ mine. they squeal like motherf@#$ers. light pressure = squeal, heavy pressure = silence. if you find a solution i'd be very much interested in hearing about it.

[Gilhuly]

I also have this issue and a new theory which I will be testing soon. Most agree that this happens when the brakes warm up. I get this on the worst hills slowing from pretty good speeds.

Here's the diagnostic question: Has anybody that has extensive rear brake vibration changed/flushed their brake fluid recently? My theory is that the vibration is partially catalyzed by boiling brake fluid(actually, moisture in the brake fluid) - think of the vibration that comes off a pot before it actually reaches a boil. Obviously the rear end of the car is very sympathetic to this vibration as anybody who's been subjected to it can attest. New pads will reduce it somewhat as the pad thickness and the grease act as thermal and vibration insulators, but will not eliminate the source of the vibration.

I'll keep you posted.

[Mesoam]

what you are describing can come from many sources...

most obvious is worn out rotors...

it is important to note that if you installed new rotors and did not propperly bed them in they can warp immediately

also if there where excessive contamanents on the rotors they could warp

also the number one cause for warped rotors is unevenly torqued lug bolts

your hub bearings could also be the source.

a poorly flushed brake system could also be the culprit

i say, start with the rotors and work from there

[Gilhuly]

Its possible that it is the rotors, but it's not warp and its not mistorqued nuts in my case. This is not a pulse like you get with a warped rotor this is an intense vibration the frequency of an electric toothbrush that will travel right through you. When you see war of the Worlds, it's like the "war cry" the enemy ships make.I have no idea how old or new my rear rotors are and the car has 100k. And why only hot?

[turbomoose]

Well I took the car into the dealer today, a dealer in which I hold very little trust in... They said it was the rotors. That they see tons of these models come in with the same problem. Im getting the rotors turned tomorrow morning while I rebuild both outer CV joints. *oh joy*

I looked closely at the rotors before I installed the new pads and they did not have any obvious wear on the outter edge where the brake pad doesnt contact the rotor, no lip I mean, So Im pretty sure theyre not overly worn. But we shall see tomorrow morning.

My experience with warped rotors is a pulsation. This is a vibration, very high frequency. Id say like turbulence. :angry:

[Tom]

Genuine Volvo brake pads.

[turbomoose]

Genuine Volvo brake pads.

←

....OBVIOUSLY DONT MEAN MUCH, since it was VOLVO pads that I took out of my car!

[Mike H. II]

I had a '98 V70 with the same problem - the loud howling rear brakes if you stopped at an average speed. I was able to solve it for about 6 months at a time by using BG brake quiet on the pads. My local Volvo dealer uses the stuff on all the cars they do brake service on. They sell the kits for about $16. Its a little can of brake parts cleaner and a little tube of solution shrink wrapped together. The solution is enough to use for about 3 sets of pads if you don't waste it. Its actually a black graphite/alcohol type of watery liquid that you apply to the acutual friction surface of the pad. After it dries, you assemble the pads into the calipers and thats it. Great stuff, I have used it one the last 4 sets of pads I've installed in all my cars. For the V70 the noise would come back about every 6 months so I'd pop the pads out apply the solution and reinstall the pads in back in the same location. Silence for 6 months.

FWIW, I think the root cause is a bad brake bias that works the rear brakes too hard under moderate braking which glazes the rear pads leading to howling because a resonance occurs.

[Gilhuly]

Some info that speaks to this phenomenon

Rigid Body Oscillations

As the name implies, rigid body oscillations are those in which the vibrating masses move with little or no deflections. These are usually at vibrating frequencies well below those of brake squeal, and most are below 600 hertz.

Rigid body oscillations can be of very low frequency, so they can be seen or felt, but not heard. Brake roughness, judder, and shudder are examples of these tactile forms of friction excited oscillations. They may have mechanical and thermal causes. Mechanical causes include machining errors (thickness variation, ovality, runout) and component wear (rotor thickness variation). Uneven heating of brake rotors can temporarily cause, or increase, thickness variation, and sometimes can produce a primary thermal buckling that warps the rotor into a washboard with three (sometimes five) high spots per revolution.

Brake roughness is felt at wheel rotation speed, one torque pulse per wheel revolution. Some brake roughness may be felt at a low multiple of wheel speed. Disc brake thickness variation and drum brake eccentricity induce pulses at one per wheel revolution. Disc brake runout and drum brake ovality produce two vibrations per revolution. Lug bolt and other wheel distortions can cause 4, 5, or 6 pulses per wheel revolution.

Groan and creep groan are also rigid body oscillations, but at higher frequencies, so they can be heard as well as felt. These usually fall in the range of 100 to 400 hertz. Groan vibrations occur at suspension and brake assembly natural frequencies, often involving 'windup' motions of the caliper and/or suspension arms. Creep groan is the name given to a groan which occurs at very low vehicle speeds (under 2 mph) that is caused by brake lining stick slip. Groan may occur at almost any speed, but is most often noticed between 5 and 20 mph with moderate braking. A pinchout groan is one that occurs as the vehicle comes to rest, usually with hard braking, and often following brake usage that got the brakes hot.

Hum and moan are rigid body vibrations which are mostly heard, in the 150 to 400 hertz range. These often occur in disc brake assemblies as the result of a dragging shoe that excites a caliper mount windup resonance. Hum and moan can occur at highway speeds, with rotor thermal distortion as a cause. They also can happen at speeds under 20 mph, due mostly to negative damping effects.

[turbomoose]

Well after rebuilding both outer CV Joints this morning... I tackled the rear brake problem. I took the rotors off... And on the back sides of one of them I had four dark blue patches. I knew right then I had probably found my problem.

I took the rotors into the Volvo dealer and they had them turned for me. Half an hour and $12 later... I have no more swedish massage when braking!

It is SO nice to have silent buttery smooth brakes again. I need to buy new rotors however. These are at the wear limit of 8.4mm.

Thanks for all of the excellent ideas gang! And if you've got this problem, try out some new pads and resurface your rotors or replace them. Its wasnt that expensive. Total I spent was $26 on pads and having the resurfacing done!

[Guest Guest]

I was able to find the BG brake quiet at a local Subie dealer after calling BG. This stuff is basically graphite held in suspensiion in two types of alcohol. Applied it to my existing very new pads, re-greased the pads and so far so good. The graphite must work into the ridges in the rotor surface. It certainly can't stay on the pads for long. I suspect that a very mild resurface and sanding would accomplish the same thing.

[rdiedrich]

Wife's 850 in the shop tomorrow for the cursed moan/vibration in the rear.

Did all the relube pads thing without success and found traces of rust on the pads and rotors. Possible piston seize.

Let you know tomorrow.