Wobble when braking

subaru
brakes
imprezawrxsti

#1

I recently heard a Car Talk episode in which the caller had a problem with his car wobbling or vibrating while he was braking. The conclusion was that the rotors were warped. A quick google search would confirm that this is the most probable source of the wobble, but I’m not sure if it is correct for my problem:

I drive a 2002 Subaru Impreza WRX. For quite a while not the steering and what feels like the whole car wobbles or vibrates when I brake. This is imperceptible at speeds under around 45 mph, but at high speeds it is VERY noticeable. It seems especially bad if I am going around 70 mph and lightly applying the brakes. The wobble does not happen unless the breaks are applied. So, warped rotors right? So I decided to check them and see if they are warped. I took both wheels, calipers, and rotors off my car and inspected the rotors. My initial impression is that they looked as good as when I bought them. Of course an eye-ball diagnosis would not sufficiently answer my question so what I did was compare one rotor to the other. I put them together rotor on rotor to see if I could find any variation in shape. I figured that if one rotor was flat, while the other was warped, it would be noticeable. If both were warped, than the difference should be exaggerated. I did this multiple times to make sure I wasn’t missing any warp or distortion. So as far as I can tell, ther aren’t warped. Another thing that I noticed, however was the brake pads. They were in good shape for the most part, but the corners/ends seems to be worn of at an angle about a quarter inch from the edge. I reassembled everything and when I drove the car, the wobble was gone. Now after about 15 minutes, the wobble returned. And now I need help figuring this out.

Here are my Questions:
Is my method of figuring out whether the rotor is warped reasonable? am i missing something?
Why would the edges of the brake pads be worn down that drastically?
Could the wobble come from something else? Struts? wheel bearings? bent wheel? If so, how can I diagnose/test which one it is?


#2

We’re talking fractions of millimeters here. You aren’t going to be able to see surface variations on a rotor with the naked eye.

The wobbling can come from other stuff, yes, but by far the most common is “warped” rotors. I put it in quotes because the going theory these days is that the rotors aren’t actually bent, but that brake pad residue has cemented itself to the rotor unevenly and is causing the wobble - doesn’t really matter, either way the answer is usually to just replace the rotors.


#3

For quite a while not the steering and what feels like the whole car wobbles or vibrates when I brake.

If you don’t feel it in the steering, but rather the whole car is wobbling you should probably be looking at the rear brakes. But not with your naked eye. Try using precision instruments.


#4

“For quite a while not the steering…”

I’m pretty sure OP meant “now” instead of “not.” OP, the rotors may also just be slightly tilted and not “wavy” so, again, new rotors should fix it. Also, pads often come with the bevel you speak of. Not a problem.


#5

You mentioned that it took fifteen minutes for the wobbling to return. Warped rotors sometimes behave differently when they’re cold versus when they’re warm or hot.


#6

I agree with @JayWB‌

Have a mechanic measure runout front and rear

I wouldn’t be surprised if the rear rotors are warped

If they are, it’s probably more cost effective to replace them . . . versus machining . . . along with the brake pads


#7

One cause or warped rotors is if the wheel lug nuts are installed too tightly, or not tightened evenly in the correct order. Did you have any of the work done requiring wheel removal prior to noticing this symptom? If so, sometimes you can get lucky and all that needs to be done is to remove the wheels and put them back on, this time properly.


#8

ok. these are some very helpful comments. For the record, i meant to type, “For quite a while now, the steering…” Sorry for the typo.
I will replace the front rotors and let y’all know how it turns out. I actually just recently replaced the back rotors and am confident that they are fine.
Thanks!


#9

You can feel brake rotor runout as little as four one thousandths of an inch. You certainly can’t see that.


#10

Nope, can’t see it with your eyes.

It does not sound like tires but have someone check your tires for a broken belt.


#11

@subarudaniel‌

Instead of continuously replacing parts, I’d go ahead and measure runout

It’s possible you’ve got a warped hub, in which case a new rotor won’t help