PDA

View Full Version : Knocking Noise While Braking



B5_2000
11-20-2018, 11:25 PM
Hello all. I own a 2000 Audi A4 Avant 2.8 5MT. I have owned it for about 8 months now and it has been great so far. One issue that has popped up since my ownership is that starting about a month ago, it will make a soft clunking/knocking noise while braking under about 15 mph. The noise is coming from the front driver side and happens about 50% of the time I am braking. The clunking happens while going straight and while turning. The noise does not happen at any other speed over 15 mph and the brakes feel relatively strong during braking. The noise corresponds with a slight pulsation in the brake pedal, so I believe that it has to do with a brake, axle, or control arm issue.

The brakes (pads and rotors) are in fine condition, with plenty of life left and no rust. The rotor doesn't seem to be warped as during higher speed braking, there is no vibration. The caliper piston seems to be moving freely and the pad sliders seem to be free of debris or roughness.

The control arms have about 60,000 miles on them, but from what I can tell, seem to be functioning properly.

The cv axle has about 40,000 miles on it and the boots (inner and outer) don't have any cracking or sign of wear.

It's kinda been driving me crazy the last couple of weeks and I want to get to the bottom of it. Any suggestions are welcome.

Thanks,
Alec

walky_talky20
11-21-2018, 08:53 AM
If the rotors were resurfaced ("cut" or "turned") it would have been done with a lathe which cuts in a spiral pattern. If the final surface cut was a bit rushed by the operator (ie: a deep cut and/or done at a high speed so Joe Mechanic can get to Lunch already), then the surface is not glass smooth like it should be. Evidence of the spiral pattern is left on the surface of the rotor. Under light to moderate braking, the pad will want to follow that groove like a record needle. When it gets to the end of its possible movement (the pads can only jiggle just a little bit) it will kick back and repeat the pattern. This can create a repetitive pad chatter that only occurs whilst braking. The fix is to do a better quality "final cut" (ie: take it back to Joe and have him do a nice slow cut on both front rotors) or to replace the rotors if Joe can't be bothered.

Other possibilities would also relate to the rotor surface, like a rusty section or other damage to the rotor surface which will cause similar pad movement or noise with pressure applied.

Note: These are not the only possibilities. Please don't go replace your rotors based on my comments alone and then be upset that you spent monies that did not fix the problem. See my signature.

B5_2000
11-21-2018, 02:34 PM
Thanks for the response walky_talky20. I took a look at the rotors earlier and didn't notice any grooves or spirals. I don't believe that the rotors have been resurfaced at all either, but I'll go ahead and check them out tomorrow.

henri12
12-03-2018, 11:52 PM
Aside from the brake system, I would check the suspension components.

mobildetroit
12-05-2018, 12:21 PM
Thanks for the response walky_talky20. I took a look at the rotors earlier and didn't notice any grooves or spirals. I don't believe that the rotors have been resurfaced at all either, but I'll go ahead and check them out tomorrow.

Hard spots can develop even on new rotors, particularly if the steel used to make the rotor is poor quality or the hardening process is not done well (i.e. India). China is much better at refining and machining steel these days. Hard spots will cause pulsations, a kind of thumping may radiate via the car body/frame as well. This can happen when oil is left on the rotors after changing them (glazing from the oil burning and sticking onto the rotor, this could maybe be cleaned off with break cleaner and scrubbing with a rag), if a rotor is of poor quality or when they wear down past the hardened breaking surface (cheap Indian steel with less then adequate temper hardening). It can also happen on a poor quality rotor after a single instance of hard fast high speed breaking, like having to emergency stop completely while doing 100 mph on the highway.

Hard spots can literally be hard spots or could also be due to the rotor warping from uneven heat distribution, also due to poor manufacturing.

There isn't much you can do about it. You could maybe find a machine shop to resurface your rotors, but frankly, it would be less expensive and time consuming just to buy a new rotor and replace it all in one shot versus running around getting your shit cut. Auto parts stores used to resurface rotors, but they don't anymore because India and China make cheap rotors now. Rotors used to be expensive back in the 80's and early 90's before free trade agreements of the 90's. I remember taking rotors to the parts store on the second break change to get them resurfaced. But today, because the hardening process isn't that great, you would end up cutting off whatever you have left of the hardened surface and the rotors would just eat up your pads and wear down to paper thin quickly.

Also, hard spots are not necessarily noticeable by looking at the rotor...but usually you will kind of see evidence of uneven wear in that some spots may be shinier then others on the rotor. Also, the spots can be on the back side of the rotor instead of the front...so you would really have to take them off and look at them under a light to see any irregularities. Maybe take a straight edge to examine if the rotor surface is warped anywhere. It's possible it's not the front rotors as well, could maybe be back rotors....but usually this sort of thing happens in the front as front brakes usually take all the force due to the engine weighing down on them.

grmnmusl
12-12-2018, 09:43 AM
check caliper/ pad wear may not be retracting,so when you brake it jumps on the slider.
My wife's Infinity did that, chased it for a while.

Also jack up wheel and hold back of tire at 12-6 and again at 3-9 and shake for looseness.
Lastly walk next to car as someone is driving and applying brake and see if there is movement could be control arms.

Fasterd
12-18-2018, 09:39 AM
If the wheel bearing is going bad it could cause the axle joint to fail prematurely (conditional/intermittent symptoms)

mobildetroit
12-18-2018, 03:56 PM
If the wheel bearing is going bad it could cause the axle joint to fail prematurely (conditional/intermittent symptoms)

In my experience failing wheel bearings do not exhibit intermittent symptoms and you don't feel them through the break pedal.
They just get louder and louder over time until your wheel falls off.
If you mean CV joint, that would be a clicky clacky sound and you wouldn't feel it in your breaks. Usually you would only hear the clacky sound while turning.

Also the thought that a bad wheel bearing can make a CV join go bad is the kind of think an auto shop crook would tell you in order to convince you to do more work to your car then you need to do.
I guess it's possible a CV can wear out about the same time a wheel bearing might (that kind of shit is likely with an Audi) but i've never witnessed it and would more likely be coincidental if it occurred then directly related. I say this because CV's are designed to move in and out, up and down...I think the only way a CV could fail due to a bad wheel bearing is if you let the bearing get so bad your wheel fell off....at that point your issues are larger then a damaged CV joint from loosing the tire

Fasterd
12-18-2018, 04:06 PM
The CV would fail because the inner race is allowed radial play putting undue stress on the joint as driving forces (and downward pressure)

Crook? Because I'm in the parts/service business and benefit one way or another right?.... spare me.

A warped rotor would in fact cause this if my posed circumstance is in fact true

mobildetroit
12-20-2018, 12:30 PM
The CV would fail because the inner race is allowed radial play putting undue stress on the joint as driving forces (and downward pressure)

Crook? Because I'm in the parts/service business and benefit one way or another right?.... spare me.

A warped rotor would in fact cause this if my posed circumstance is in fact true

Auto shop salesmanship aside....If the inner race has enough radial play to cause the CV to mechanically fail the wheel would likely come off first.
CV's are engineered to transmit torque from multiple angles so minor/normal radial play in a bad wheel bearing isn't going to make a CV operate outside of it's typical design specifications.

Pulsation in breaking is usually caused by a rotor issue of some kind in my experience and is much more likely to be the issue....especially if your wheel isn't noticeably flopping around while you are driving.

Fasterd
12-20-2018, 03:25 PM
But when it is at an angle constantly vs. proper operation where its only excessively angled when turning....



Nope not even in the automotive business, but some of the things you have said make me sure I would never visit yours.....


EDIT: what you are posing becomes more evident with speed, this symptom was self described at <15 MPH where CV axles tend to give feedback