Hi Matt,
You will recall I started by saying....It all depends on the type of noise!

I agree that the shims are most likely to address squeal however you made no mention of them or the requirement for grease when assembling.

I'm beginning to run out of ideas and at the same time am becoming confused because the info you have provided conflicts.

You state that you are now considering something different to the brakes (ie tyres) yet you state that chamfering the pads made a difference?

OK. The noise must be coming from somewhere so lets try to eliminate the brakes.

You state that the noise can be turned on off purely by accelerating/deaccelerating at a particular speed WITHOUT applying the brakes. CORRECT? If so I don't see how it can be the pads since these SHOULD be retracted under these conditions.
Is the problem in any way brake temperature sensitive? You didn't answer this when I asked previously.
I'm still placing a lot of emphasis on your original statement that the noise occurs once per rev of the wheel.
Have you VERY carefully ensured that the rotors (check all 4 for good measure) are not in close contact with anything (eg the bridge of the caliper) other than the pads. Just to be sure are you sure the parking brake is effective and not binding? I will continue to assume that the noise is coming from the front.
Next the wheels. Are you on standard OE wheels and tyres? Have you checked clearance between the wheel and caliper? Look for a witness mark on the inside of the wheel. Is there a stickon balance weight catching on the inside of the wheel? Have you checked clearance between the tyre and body? eg. Mudflaps.
With the pads retracted and the car on jacks do the front wheels spin with the wheels fitted? If not why not? Can you feel any play/roughness?
Next the tyres. Never known tyres make a grinding noise but they certainly generate noise. Since you can get the noise on demand does the road surface modify the nature of the noise/occurence of the noise? If the tyres have been fitted the wrong way around for an extended period the tread blocks will have worn and correcting the rotational fitment will not necessarily correct matters. Note: Moving tyres from rear to front doesn't correct the rotational issue unless you also change sides.
OK what happens to the noise if you coast (Neutral/Engine off.) thru the problem period. This will eliminate most engine related noises and in particular the PAS. TAKE CARE WHEN DOING THIS PROCEDURE TO ENSURE YOU DON'T LOCK THE STEERING. IN ADDITION THE BRAKES AND STEERING WILL ALSO BE LESS EFFECTIVE WITH THE ENGINE OFF (NO ASSISTANCE)

Look forward to your next post.
Stuart

 

Stuart,

Yes, my original statements now conflict. It did seem to get better with the rotor and pad changes but now that I've noticed it without applying the brakes I'm starting to look elsewhere. Problem is not temp sensitive.

I will check clearances tonight but if something was rubbing wouldn't I still notice it at least at very low speed? The problem only occurs around 17 mph. Above and below that speed the noise ceases. In other words, no noise at 5 or 10 mph.

I'm really starting to suspect wheel bearings but it's not induced by turning and typically bearing noise is prominent when turning.

Matt

 

Stuart,

I just took the car to my local mechanic. He thinks it a bad tire....maybe broken belt. I'll start swapping out with the spare tire and see if one tire is the culprit.

Matt

 

Stuart,

It appears to be the tires. It took me a while of swapping them around with the spare but I finally got the best combination of correct tire in the correct location to minimize the noise. Although not completely gone it's very close. And it doesn't seem to be just one tire making all the noise. It's VERY suprising that the tires could produce this mechanical noise.

There is some minor scalloping on the tires due to worn shock absorbers. I've replaced the shocks since but I wonder if the scalloping could cause the noise? Anyway, it looks like I'll have to live with the little noise that is left until I replace the tires.

Your comment would be appreciated. Otherwise, many thanks for your help and I look forward to future correspondance. By the way, I'm a mechanical engineer for Caterpillar. I noticed you are also an engineer??

Thanks again, Matt

 

Hi Matt,
I'm glad you appear to be about to solve your problem.
In my view "refinement" noise is one of the most difficult faults to track down on a vehicle.
Noise is often only generated at certain frequencies and the noise path to the observer is complex.
Often noise will occur only at a given frequency range because the problem consists of an exciter and an amplifier giving resonance.

Forgive me but a little insight into NVH. (Noise, Vibration, Harshness)
Take road wheel shake as an example. (aka steering ****le, lack of balance what ever you wish to call it)
The root cause is the out of balance of the tyre/wheel assembly. The faster it spins the greater greater the OOB force and one would expect the greater the problem as perceived by the driver. But no. The problem may not be noticeable at speeds up to say 50, it then builds up to a max at lets say 70 mph, then dies away again. This is because what the driver feels is not the wheel OOB but the whole suspension assembly or even body shell going into resonance (which it does at a frequency coincident with 60/70mph). Only the resonant suspension or shell has sufficient energy to be observed by the driver recognising the attenuation of the vibration/noise path.
Resonance is why garages can sometimes change shocks, tyres, bushes, bearings etc. unnecessarily because each component is part of a system and may have some small effect on the outcome. However the real problem is the wheel/tyre OOB forcing which may not be due to a tyre or wheel fault but the assembled combination. Reposition the components and the assembly may be OK.
Just to make matters more complex road wheel shake often changes during a journey. Why? This is because although the OOB of each wheel will remain constant (at a given speed) the combination OOB will change since each wheel will rotate at slightly different speeds due to differences in OD. The forcing phases. Sometimes the force from one assembly is cancelled by that from another etc.

Something similar may well be what is going on in your vehicle. Your new "evidence" regarding scalloping on the tyres may be significant. Why are the tyres scalloped? They shouldn't be. They are the visual symptom of a whole system.
You state the tyres were incorrectly fitted (for rotation), bad shocks. What about geometry...not just toe-in. All tyres or just one? This may not be possible to decipher because the wheels have been moved around on the vehicle. OE tyre type and size?
I personally don't move tyres around on the car. Tyres don't like having their direction of rotation changed and worn shoulders on front tyres do nothing to help stability when fitted to the rear. I say leave them till they need replacing but others may have different views!!
Apologies for the "Take Care" warnings earlier but you have to be so careful these days even when just trying to help somebody.
Guess you don't suffer from refinement problems on Cats!!
Yes I am an engineer having spent 40 years in the european auto industry battling with manufacturing issues and customer complaints. I'm a trained statistical problem solver which is why I place a lot of emphasis on good data.
Stuart