09mazda6

11 a6 avant - bought used in Jan with 88k miles and new tires. From what I can tell all new new pads and rotors were put on in the mid 60k range. Car came with 4 Goodyear new tires but they were w speed rated and not Xl load. Car had alignment @ Audi dealer in Jan/Feb and in 5k miles by April/May rears only were down to 4/32nds (perfectly evenly worn). Replaced with 4 new Conti DWS which were balanced twice -2nd time with Hunter Road Force and were @ 6, 7, 10 and 13 per that balance (going counter clockwise from D front), so far new tires (H/xl ratings) seem to be wearing very evenly.

I don't recall the shimmy/shake that I'm feeling now. It is also worse in the AM but never goes away completely. In my quest to find reliable Audi shop - had oil changed @ indy and they took car for test drive re shimmy. their diagnosis was warped rotors so new rotors and pads for $1100. However I'm concerned that this is not a definitive diagnosis for two reasons:

1) fact that it seems worse in AM makes me wonder about tire defect and
2) potential that wear on rear tires so fast caused substantially different diameter tires and somehow differential /diff fluid is part of problem.

So my initial questions are:
a) is there is a measurable/gold standard test for rotor warp? Also if rotors can't I just replace rotors.
b) if that comes back negative - could the differential be a problem and if /how to test that?
c) if neither rotor or diff - could tires have a belt or other defect and still show fairly good Road Force numbers?

Thanks for any logical suggestions/advice.

kelisko

When does the car shake? When you are driving or when you are braking?

09mazda6

Both - in the morning the shake seems to be mostly from the rear as I mostly feel it in the seat and at that stage it's worse than the shake I get in the steering wheel when I brake. As the car gets warmed up either I get used to the overall shaking while driving or it's reduced for some reason ( tires or differential warming up?).

My other question I forgot to post is if rotors are worn why can't I just replace them and leave pads. Is there a tool/method to check flushness of pads and if they are flat/square then they should work fine with new rotors.

KevinGary

kelisko

Definitely a tire or even a bent wheel.
Borrow 4 good wheels + tires somewhere, hook them up and test the car out to see if something changes.

kelisko

You can but it is not recommended. The old pads don't usually have an even surface like you said and they might damage your new rotors and cause other issues even if you have them evened out. Besides, the first layers of new brake pads contain stuff that improve the bedding-in process. Don't go the cheap route. If you are changing rotors, change the pads too. But you are not sure yet that your rotors are the issue so skip that for now. If the car is shaking even if you are not braking it definitely is not your rotors.

09mazda6

Thanks I did not know about a layer to help the bedding in process - that's interesting. I did a little more internet searching and found the following chart on Tirerack - seems like a reasonable set of questions to check into. Will share solution once it's found.

https://www.tirerack.com/images/tires/vibechart.pdf

09mazda6

So over the weekend - did things I can easily control - rechecked air pressure, re-torqued bolts to spec and then rotated rear (which were higher #s on Road Force test) to front. This morning's commute brought the following observations: - still shakes especially at higher speeds but I'm not feeling breaking vibration as much. This seems to lean towards what other's are saying that it's tire and/or wheels. Now I need to figure that out. no observable bends to wheels. I may get tires dismounted and have wheel balanced by itself and then add tire.

I've also learned that evidently I too sensitive to things. My wife is test driving a few used cars to replace her's and twice now I've taken them out for a spin and noted a suspension problem which the car dealer either did not pick up on or ignored and then they confirmed that tire rods, bushing or bearing needed to be replaced. This also lead me to find issues with my mazda's computer fuel/throttle mapping (hiccup/hesitation) and eventually when my engine mount started to rust. I'm beginning to think I may be just too picky.

beep-beep

at what speed do you feel shaking? Does it go away when you accelerate to the higher speed? I.e. my tires shake at 80MPH but do not share at <70 or >90. You can calculate harmonic ratio out of this and then figure out what causes it.


If you have problem with braking, it also can be a problem with the disk sitting on the hub. If the people who changed the rotors did not clean the hub, the piece of rust possibly can keep the rotor tilted.

09mazda6

The shake is all nearly all speeds - worse at higher and interestingly last night on long drive on highway when slowing from upper 70s it would go through stage where it was less as we decelerated but it never goes away. Also Brakes have not been changed since I bought car so that should not be the issue. I think it's tire and/or wheel - maybe the combo of the two are amplifying the issue, the tech dropped my rim when installing? Any ways - I'm trying to find extra tires/rims to see if those solve the problem then I can rule out brake rotor/pads.