Strange phenomenon on my front rotors

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DaveInLA

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Mar 14, 2004
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263
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LA, CA
1989 BMW 325is

About a month (500 city/hwy miles) ago, I had my front and rear brake pads replaced with Hawk HPS. The rear rotors had tons of grooves and were replaced, while the front rotors had very minimal grooving and were therefore preserved.

Anyway, I just took a look at my rotors today through the weels, and I noticed that on the right front wheel only one side), the outer edge of the rotor seems to be a little raised to the touch. There's a concentric line about 1/2 cm from the outer rim of the rotor that marks where this raised portion begins. This line/mark doesn't wipe off (maybe burned in?) and I'm pretty sure it wasn't there before I got new pads. The left side is fine.

Is it possible that part of the pad on that wheel isn't contacting the rotor? If that were the case, I think hard braking would steer the car in one direction, and that's not the case here. I saw the mechanic put in the pads, and everything seemed to go in straight. I trust his work. What happened?
 
sometimes the face of the pad doesn't fully sweep the area on the rotors. this could be due to a slightly smaller or bigger pad area versus the area on the rotor, or a bent backing plate.

just because the front rotors had 'minimal grooving' doesn't mean they should be kept, even if they are turned. Did you have them checked in 5 or so spots around the rotor for thickness variation? How about the actual thickness, above or below min? Were the rotors at least resurfaced or did your mechanic just slap the pads on? Rotors are cheap enough that it's not much more to replace with every pad change.
 
The size of the pads is appropriate. I'm not sure why a bent backing plate could cause this, though.

Anyway, the rotors weren't turned and they weren't measured. Stupid of me, I know, but it really was extremely minor. I mean, you can't feel the individual grooves, but you can just feel that it's not a perfectly smooth surface. I figured the pads would eventually settle in anyway.
 
grooves dont cause brake unevenness, its the rotor becomming thinner on one half and thicker on the other half that makes the steering wheel shake when braking.

actually, grooves worn into the rotor increase the effective contact area of a rotor to a pad, assuming the pad has grooves which match that of the rotor.
 
I really doubt it is a big problem and there are many things that could cause it. If it doesn't shudder and doesn't pull when breaking and stops well, I think it is fine. I'd replace the rotors next time around though, shouldn't be much more than say $50 a piece. Atleast I wouldn't think so, but it is a bmw...

Btw, do you like the pads, any noticable differences to what was on beforehand?
 
My front ones both have minor ridges. Its not very wide tho, just enough I noticed it. I didn't look on the back rototd. I may need rotors before pads
shocked.gif
 
My car doesn't shudder or pull when I brake. The steering wheel doesn't shake, either. It's just strange seeing that one rotor like that. I'm just afraid that some of the pad might not be contacting the rotor and thus reducing my braking power.

I like my new pads. It's hard to describe, but they feel "heavier" than the old ones I had, yet feel grabbier.

Despite it being a BMW, new rotors are under $50 for the fronts. But even then, that's $80-100, which is quite a bit of $$$.
 
My car has 4 wheel disks and GM said not to turn them unless they had a certain amount of runout. The back ones needed replacing and the rotors were in spec so I did what was recommended. What I found out afterwards is that about double the effort is required on the emergency brake is now required to get it to hold. When the fronts finally come due (this is normal for this model, BTW, the rears are smaller and wear out first)I'm going to have the fronts turned or replaced regardless of the spec.
 
if you arer afraid of the pads not getting full contact, take a black paerminant marker and draw a line from the center of the rotor to the edge.
go out and make a few stops and see if the line is fully gone or if there are parts of the line which remain.
 
I tried the marker test, and it turned out to be pretty informative. It seems clear that the outer edge isn't getting the same contact that the inside portion of the rotor is getting, but there's still some contact. However, the space between the elevated and non-elevated part is getting NO contact with the pad, as can be expected with a flat material contacting a ridged surface. That "line" I was talking about... I think it's a collection of heated and compacted brake dust.

My question then: under normal driving (mixed city/hwy), about how many miles before new brake pads mold themselves to slightly grooved pads? So far, it's been about 400-500.
 
I've always been under the impression that new pads aren't bedded in for atleast 500 miles. And that is with new or freshly turned rotors. So with "slightly" grooved or used rotors, it could take even longer. For comparisons, I just did my front brakes (Bendix Import Quiet pads and turned rotors) just over 100 miles ago. You can still easily see the non-directional marks from the turning process. I haven't made any high speed panic stops yet, but have been impressed with the pads. Odd thing is they don't seem to sit as close to the hub (center of wheel) as the OEM's that I took off. Instead of about a 1/4 inch of non-contacted rotor at the hub, I have 1/2 inch. Maybe it's just a manufacturer's tolerance thing. But the car definitely stops better with these "smaller??" pads. And Maxima's have pretty good brakes from the factory.
I like that marker test. And you could do it about every 100 miles, and see if it is getting better. That would be a good way for you to judge if the pads are seating all the way.
Depending on how easy the access to your rotor is, you could take some 120 grit paper and see if you can "scuff" that line down. If it is something like burnt/caked brake dust, it'll come off easily. If it is the rotor itsself, it won't be easy, but could probably still be removed with some effort if you're worried about it.
If you aren't getting a pulsating pedal or other brake problem, it probably won't have any noticeable effect on braking. But only you can make that judgement call, as in if the car stops safely and to your expectations.

Hope this helps.
Dave
 
It only takes about 1 or 2 miles to bed in pads. You did follow the bed in precedure provided with the HPS pads?

3 45-5 light/mod stops to warm up the pads, prevents thermal 'shock'
5-8 60-5 moderate/hard stops, or until the pads get all smelly.

then cool for 15 minutes w/o using brakes.
 
think of how many thousands of miles it takes to wear out a set of pads. the pad matereal wears very very slowly. you cant expct youre used pads to be break into new rotors within so few miles.

give it a couple more thousand miles and itll probably clear up.
 
quote:
Originally posted by Master ACiD:
think of how many thousands of miles it takes to wear out a set of pads. the pad matereal wears very very slowly. you cant expct youre used pads to be break into new rotors within so few miles.

give it a couple more thousand miles and itll probably clear up.
Click to expand...
hawk (and many other performance pad manufacturers) actually has very spedific bed-in procedures for its line of pads. If its not done properly, you won't get optimal braking or pad life.
 
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