Vibration in brakes

[Tommymc]

'96 Legacy wgn. The brakes work fine, a little squeak now and then when I apply moderate pressure, but the rotors feel smooth. I replaced pads and rotors last year, and have put under 5K on the car since then.

One little problem: sometimes I get a pulsation and vibration as if the rotors were severely warped. Big time. I travel about 5 miles on the interstate on the way to work, and the times I've experienced this is when I get off the exit and try to stop. Then the problem clears up and might not recurr for days.

The only thing I can think of is if one of the calipers is sticking and causing the rotor to heat and warp.....but it returns to normal. I would expect warping that severe to ruin the rotor. Anybody experienced this or have any ideas? Thanks

[porcupine73]

Do you mean the vibration seems to be felt at certain speeds? Maybe this chart is useful:

[Tommymc]

Do you mean the vibration seems to be felt at certain speeds?

No not AT certain speeds but after driving certain speeds. So I could drive around town all day and the front rotors feel smooth as silk. I could take it on the interstate and usually it feels just as smooth. Every once in a while, after driving at highway speeds, the front brakes will vibrate like crazy when I slow down to get off.

For instance: today I drove to work twelve miles...5 miles on the interstate. When I pulled off at the exit, the brakes shook and vibrated the steering wheel. Everything I know about brakes says 'warped rotor'. Then I continue on to work and the brakes smooth right out again. On the ride home, reverse of the same route, I don't experience any problem. Whatever is going on seems to be triggered by speed, but isn't consistant. It may not recur for days.

[Blue Whale]

From the description, I'd suspect that your ABS (all '96's have it, don't they?) is causing the vibration. I have no experience in diagnosing and repairing ABS--I've only had a car with antilock brakes for three months, and I've had no need to tinker with the system--but maybe someone else here can help.

[tcspeer]

I also think it would be the rotors, but you might try getting your tires rebalanced, and having the shop check to see if one of the belts may have seperated. Or if you dont want to spend the money for that you might try putting the tires on the back to the front and the ones on the front to the rear.

No not AT certain speeds but after driving certain speeds. So I could drive around town all day and the front rotors feel smooth as silk. I could take it on the interstate and usually it feels just as smooth. Every once in a while, after driving at highway speeds, the front brakes will vibrate like crazy when I slow down to get off.

For instance: today I drove to work twelve miles...5 miles on the interstate. When I pulled off at the exit, the brakes shook and vibrated the steering wheel. Everything I know about brakes says 'warped rotor'. Then I continue on to work and the brakes smooth right out again. On the ride home, reverse of the same route, I don't experience any problem. Whatever is going on seems to be triggered by speed, but isn't consistant. It may not recur for days.

[Tommymc]

I thank you for the replies. I don't see how this can be a tire/balance issue because it is intermittant. Usually the tire thing happens consistantly at one speed. Oh yeah, this is a Brighton model=no ABS.

For all the world this feels like warped rotors....when it happens. But there again, warped rotors always pulse to some degree, just worse when braking at high speed. My daughter recently drove the car from VT to Boston at highway speeds and had no problem. The car sometimes will sit for a week or so without being driven, so I thought the calipers might be sticking. I can picture a stuck caliper making a pad drag on the rotor, causing it to heat and warp. What I can't make sense of is why the rotor would return to normal when it cools....with no permanent damage. My experience with rotors is it doesn't take much to ruin them. Tomorrow I'm going to pull off the tires and snoop around. I just wonder if there is anything else that might mimic a warped rotor.

[porcupine73]

I'm going to pull off the tires and snoop around.

Yes give all the suspension and driveline components a good shake down to check for any looseness. And check the wheel bearings as well.

[scoobyclimbs]

Yes give all the suspension and driveline components a good shake down to check for any looseness. And check the wheel bearings as well.

 

I 'll second that... check the lug nut torque before you pull the wheels off.

[Tommymc]

I 'll second that... check the lug nut torque before you pull the wheels off.

Well, I pulled the brakes apart and checked the front end, all looks normal.....for an 11 yr old VT car. BTW she just passed a state safety inspection today which included the regular shaking of the front end.

Lug nut torque....I don't have a torque wrench. Lug nuts were all tight but not overly so. Are you thinking that too tight could be a problem?

[nipper]

Rotors. Blu would do the same thing. I replaced the rotors and all is well.

 

 

nipper

[subaruplatt]

I thank you for the replies. I don't see how this can be a tire/balance issue because it is intermittant. Usually the tire thing happens consistantly at one speed. Oh yeah, this is a Brighton model=no ABS.

For all the world this feels like warped rotors....when it happens. But there again, warped rotors always pulse to some degree, just worse when braking at high speed. My daughter recently drove the car from VT to Boston at highway speeds and had no problem. The car sometimes will sit for a week or so without being driven, so I thought the calipers might be sticking. I can picture a stuck caliper making a pad drag on the rotor, causing it to heat and warp. What I can't make sense of is why the rotor would return to normal when it cools....with no permanent damage. My experience with rotors is it doesn't take much to ruin them. Tomorrow I'm going to pull off the tires and snoop around. I just wonder if there is anything else that might mimic a warped rotor.

Check the hubs too when the rotors are off. Was the car ever curbed? If you find your new brake job going south prematurely check the hubs.

[lyonsnet]

Hi Tommymc,

 

Did you ever resolve this? For a while I've noticed the samething, with my 96 OBW.

 

I usually only drive on the highway 1-2 times a week, but after driving 110-120 km/h if I use the brakes (either slowing down in traffic or slowing for an exit ramp) I really feel the vibration through my foot on the brake. But, not 100% of the time. Sometimes, no vibration, and other times it's there.

 

I replaced pads and rotors, front and back, about 8 months ago. Tires are about 1.5 years old, and the wear seems to be even, and tire pressure is normal.

 

I find it odd, like you describe, as I can drive around town or even rural roads for days on end, and never feel this vibration, until I go on the highway. And, like I said above, this doesn't happen all the time.

[ronemus]

That type of vibration is usually caused by pad deposits on the rotor, not warping. A hard stop from highway speed to a standslill will heat the pads and keep them in contact with the rotor. Subaru OEM pads use an organic binder that cooks out when they're hot, leaving a deposit that grabs with each revolution of the rotor under braking. You can get rid of it by making a few consecutive 60-5 stops (don't stop entirely), but the problem will reappear until you get better pads. I have Hawk HPS pads on mine and like them a lot, but they do dust noticeably; if that bothers you, Hawk Ceramics have very little dust and it's light colored.