Scionlife,

I'm having an issue with my brakes. I believe it's the rears that are acting up, but perhaps it's just a normal thing.

Under light braking, I'm getting a pulsating vibration noise. It's not high pitched, similar to a "ROOOOM room room roommmmmm" as I'm coming to a stop.

But, other odd points about it-
1) It doesn't always do it... just when the brakes are cold.
2) It never does it under heavy braking.
3) It never does it at high speed (30+mph).
4) Rotors do not appear damaged in any way.
5) Braking performance is still great.

I figure it's just natural oscillation of the caliper (isn't it designed to move a little?), and if it were a warped rotor, i'd see it at all braking strengths and speeds.

Other factors-
1) Just rotated the tires. Front-back due to directional tread. Cheap-___ tires.
2) 18x7.5 wheels.
3) 25,000 miles. 3 autocross sessions (15 runs).

Any thoughts?

 

A warped rotor will act as you state. It will only pulsate under certain circumstances if it is only slightly warped.

If it is warped very badly it will pulsate at most speeds and heavy braking.

I would personally take the rotor off and run it out to see if it is within spec. I have seen this dozens of times.

I would also look into the tires having internal seperation or a broken belt but I woudl give it a 90% chance being the brakes over the tires.

 

Warped rotor covered under warranty?

 

Brakes are considered a wear item and are not supposed to be covered.

They will cover them at dealers typically if you are under 36K and have a decent friendship with them and the car does not show abuse.

Best of luck. I would put on the stock wheels if you ahve them as well just to rule out the other set you have if they could possibly be the culprit and the dealer will think less of the car the more stock it is.

 

Yeah i'm thinking it's the rotors too. Hard braking from autocross could have warped them.

 

Have you changed pads/rotors since you've been autoxing? I'd be willing to bet that the rear pads/rotors are warped from the heavy braking done at events. Though I have seen tires behave similarly after not correctly adjusting the pressures after finishing four runs and driving on the street. [/quote]

 

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I've never changed the pads or rotors. In fact, i've only done about 15 runs in this car. I split my time between this (my daily) and my friends SM engine-swapped rabbit.

I never really had any plans to change the rotors... just perhaps better brake fluid, pads, and SS lines. The car brakes pretty damn hard as it is.


Another thing to consider is that I never had this noise until I rotated the tires myself about 1 week ago. Road noise dramatically increased, and this braking noise starting appearing on occasion.

In fact, it hasn't done it since I posted this thread... and road noise is starting to decrease.

I'm thinking it has to do with the wear/balance of the tire. Just to be sure, though, I'll be checking out the rear rotors with a micrometer sometime over the weekend / next week. I'd like to put off getting an alignment/tire balance until I do some suspension work, unless it becomes a serious problem. The tires are ____ anyway, so I dont care if they wear out more quickly.

Thanks for the suggestions. I'll check the tire pressures to be sure they aren't totally out of whack, too... I haven't done that lately...

 

just an FYI, rotors don't warp, that's a rash generalization, but they don't. Especially stock rotors. Especially the rears which only see about 25-40% of the brake distribution. Crappy pads, or pads that are overheated, leave deposits on the rotors, which cause vibration due to the pad material adhering to the rotor in an uneven displacement, and it could even show areas that are thicker than others under a micrometer, due to the now raised surface actively collecting more and more pad deposits in the same area.

You say it only happens when the brakes are "cold" do you mean, leave them to sit for an hour, or overnight? it could be oxidization on the rotors being ground off by the pads. Also, with 15 auto-X's on the stock pads, they are probably very worn down, to a point of replacement.

I would say, if it happened after you rotated your tires, you could check that your wheels are properly torqued down as well. If the wheel is loose, at low speeds, it could be vibrating against the hub, and after the brakes heat up from use, the heat causes the metal to expand, including the wheel, to fill the then empty space that was causing noises.

also, at your mileage, and the autocrosses, your hub bearings could start to be going, which could be very erratic until they heat up properly.

There are just too many factors that could be going on that anyone but a seasoned mechanic could say is the problem, but without a proper diagnosis, it would probably be easier to just say there ISN'T a problem.

 

^
Good points. I'll print this whole thread out and sort of make a checklist whenever I'm in the diagnosing mood.

An engineering friend and seasoned racer pretty much told me the same thing. He suggested getting pretty aggressive with the brakes, like I was bedding them in, and see if it goes away.

Turns out, it sorta worked. Sound hasn't come back for a few days. Perhaps it's the fact I'm a light, easy braker when i'm not inbetween cones... and i'm learning that it's just as much the "strength" of the braking as it is the "duration" of the braking.