Brake Wobble Help

"ZHP Sparky, the 5th" (e30s2k)
06/02/2014 at 18:12 • Filed to: None

Dear Opponauts,

As usual, resorting to the brilliant shared knowledge here for some help hopefully on a topic I'm absolutely lost on.

Since the day I bought it, my 2005 Legacy GT has had a wobble when braking that seemingly comes and goes for no known reason. Previous owner installed a big brake kit (sigh, brand unknown) - with large slotted rotors, and calipers much larger than stock. I love the performance and the feel...when they work right. Potatocam picture of my car from when I bought it, you can somewhat see the large rotors and calipers on here too:

Here's what happens - when I brake softly I feel a horrible wobble through both the wheel and the pedal. Definitely from the front wheels - my senses tell me it's the left wheel. Kicker is, the car will go months without ever feeling this, and then suddenly it returns - currently it's back and happens everyday - but only on soft braking. If I brake with a good amount of force it's smooth and not wobbly at all.

I've had my mechanic look at my pads and rotors - both of which he says look fine, no warping on the rotors. I had some pretty bad wheel bearing noise developing, I thought the worn out bearings might be causing this - replaced the hub assembly recently, but still no fix to the wobble. I've rotated the wheels and had multiple sets of tires mounted and aligned, still same issue.

Any ideas? Would it be worthwhile to disassemble the calipers and clean them out, to make sure they're working properly (would a mechanic even be able to fix this though, if it's not?). I know it's less than ideal, but my mechanic said he could try resurfacing the rotors to see if that would fix it as a last resort. Worthwhile?

Taking out the brake kit and replacing it with OEM parts would cost over a grand, and would result in significantly lesser braking performance - so I'd like to try salvaging my current setup if I can.

What would you do to try fixing this, OPPO? Anything you would do that I've missed at this point?

DISCUSSION (13)

Arch Duke Maxyenko, Shit Talk Extraordinaire > ZHP Sparky, the 5th
06/02/2014 at 18:15

How do your suspension components and bushings look?

CAR_IS_MI > ZHP Sparky, the 5th
06/02/2014 at 18:16

Tie rod bushings.

Garrett Davis > ZHP Sparky, the 5th
06/02/2014 at 18:23

If your rotors are good, jack up the car and see if there's any play in your wheels. Grab them at 9 and 3 o'clock and push/pull on the tire side to side as if you were steering it, then do the same for up and down at 12 and 6. If you have play in on both axis', you might need new wheel bearings. If you have play at just 3 and 9, it might just be tie rod bushings. If it's at 12 and 6 only, it could be ball joints.

While you're doing this, it helps to have a friend look for slop in the bushings or the spindle itself.

GeorgeyBoy > ZHP Sparky, the 5th
06/02/2014 at 18:24

I'll put money on pads and rotors. Make sure they are PROPERLY broken in. It's something that a lot of people don't do and leads to warping. Also, there's no way to tell if the rotors are good or bad, I don't care what any mechanic says (beyond checking for thickness that is).

ZHP Sparky, the 5th > Garrett Davis
06/02/2014 at 18:43

Interesting, thanks. Didn't feel any play the last time I was trouble-shooting this, but bearings were clearly loud and grinding - so fixed that. Will check what you said for tie rod bushings and ball joints. Had not considered that.

ZHP Sparky, the 5th > Arch Duke Maxyenko, Shit Talk Extraordinaire
06/02/2014 at 18:44

Hadn't considered that, another poster suggested this too - will add it to the list, thanks!

LuczOr > ZHP Sparky, the 5th
06/02/2014 at 18:49

I would put my money on some sort of suspension bushings causing brake shimmy. Maybe tie rod ends, control arm bushings or subframe bushings can even affect it. Also, make sure your tires are up to pressure and your shocks/struts are in good shape. Any looseness in the system between the ground, the chassis and the steering wheel can induce a cyclical loading and unloading of opposing front tires.

My experience was with my mazdaspeed3. I took it drag racing and aired down the tires FER SOME GRIP and forgot to air them up afterwards. On the drive home, I was getting some vibrations on braking so I thought I had warped my rotors while braking down from the 1/4 mile runs, but once I aired up the tires, the problem ceased.

case-sensitive > GeorgeyBoy
06/02/2014 at 19:12

That's my thought as well. My VW had a horrible brake shimmy almost from new. Replacing the front rotors got rid of the problem.

AMGtech - now with more recalls! > ZHP Sparky, the 5th
06/02/2014 at 22:52

1) Wow guys, automotive 101 in here. Any roadside grease monkey of a mechanic would have checked all of these things.

2) You can't resurface drilled/slotted rotors, they can chip or crack and the bit on the lathe will chatter making the original problem worse and maybe adding other noises as well. Don't do it.

3) It's the brakes, but they're not warped. The edges of the rotors are now further away from the center of the hub than ever before (not to mention are much heavier), thus every imperfection has more "leverage" over the hub and makes those imperfections more pronounced. Also, brakes like these are designed for aggressive use. So sometimes, when they are only used for light daily duty, pad material will start to build up unevenly on the rotors, causing your shimmy (the slots don't help with this). Most brake companies offer super aggressive ultra coarse race pads that can double as scrubbers for the rotors to get all of that old pad material cleaned off. You can also try braking aggressively on a more regular basis to keep the rotors clean. Poly control arm bushings could potentially help as well. Ultimately, it's the nature of the beast.

ZHP Sparky, the 5th > AMGtech - now with more recalls!
06/03/2014 at 11:11

Interesting - yes, a lot of the suggestions thus far have already been tried/fixed/not been identified in multiple inspections (I've had mechanics under there quite a bit - and specifically looked for play in the wheel etc.). Your theory is pretty interesting and makes sense. As I mentioned in my original post, for whatever reason the shake is far less pronounced under hard braking (as a result of which I have a new favourite off-ramp on my drive to work now) - and today I realized that the shake is far less pronounced than say when I got the car back from my mechanic a week or two ago. So maybe I'll keep attacking the brakes a bit more to see if it helps before my next run to the mechanic.

I suppose this is what you get when you buy a car with "upgrades" by previous owner. My LGT is a lot more steady-footed and firm than others, and for the most part I love everything about it - this is the one thing I've been struggling with. Given that it's a pretty big wagon it's less than ideal for chucking around and aggressive braking - but oh, well for now.

AMGtech - now with more recalls! > ZHP Sparky, the 5th
06/03/2014 at 22:28

Some cars have bulletins for the pad material issue, Mercedes included, I think there is an FAQ on the Wilwood website too. A few years back I was involved in developing/prototyping a Wilwood big brake kit for L320 range rover sports and ran into this exact same problem.

ZHP Sparky, the 5th > AMGtech - now with more recalls!
06/04/2014 at 11:39

Good to know, thank you for the information!

AMGtech - now with more recalls! > ZHP Sparky, the 5th
06/05/2014 at 01:38

No problem!