Steve Weiner-Rennsport Systems

Hi,

My first question is to ask if your mechanic ever measured rotor runout??? Until this is done, everything else is simply conjecture.

There are several ways to turn a rotor and some simply replicate a pre-existing condition and simply leave a smoother surface,..... Sticking caliper pistons can introduce excess heat into a rotor by preventing the pads from retracting and that leaves them dragging on the rotors a little bit. Again, if this is truly a warpage issue, this shows up on the dial indicator when checked for runout.

Surface finish simply affects brake noise.

2ndof2

Hey Steve,
Unfortunately he simply sent the rotors out to resurface and didn't ask the shop anything about their condition. Surprised to learn this was the first time he ever had rotors resurfaced as well. He said usually he just replaces them. So no chance getting runout info unless I ask to have it done this time. How can the pistons in the calipers be checked for seizing up and is this what I should hav done next or hav the runout on the rears checked first?

mhm993

I ought to let steve answer, but take a breath and start simple: reinstall rotor, bed in pads, see what happens.
A stuck caliper is going to give you a firey hot rotor after a short drive. Drive 5 miles around town. Touch the wheels. Is one brake stupid hot compared to the others?

It's gonna be brake pad build up. "They do that". Really.

2ndof2

Yeah, just not totally sold on the "rebedding" pads concept. Drove the car again tonight. Horrendous vibration over 40 or 50mph. Almost feels like its getting worse. At slower speeds hardly can tell there's any issue. Weird.

I just spoke with Mark Leech at rusnak in westlake and he said he's seen bad hubs cause vibrations like this. Then I spoke with my mechanic again who said i've been waaaaaaay over torquing the lug nuts. Putting two and two together he thinks maybe a bent hub may be the issue.

This assumes that the rotor was straight after being turned and within 150 miles it went bad again. Can't say I totally buy this theory either. He's ready to replace rotors and hubs in the rear. I need some guidance on what he needs to look for before replacing parts in an endless hunt for the missing link.

Steve???

Gunter

Like to add something to the excellent answers by Steve:

Caliper pistons move very little during regular use and that creates problems like getting stuck due to rust. Rust? Yes, brake fluid contains a small amount of moisture and that settles in the lowest point of the system which are the calipers.
I make it a point to "exercise" the pistons every time the wheels come off by pushing the pads back and then use the pedal to bring them out again; repeat 2 or 3 times. That keeps the pistons from getting stuck (rusted) in place.

The seals inside are square and designed to pull back the piston once pressure from the pedal is released. This action is very important because it removes the pads from the rotors preventing unnecessary friction and unwanted heat buildup.

Exercising the pistons regularly has served me well for decades on different vehicles experiencing no stuck pistons yet. And yes, I use a dial indicator for axial run-out on both sides of the rotor and don't want to see more than 0.005". Measure the rotor thickness in 4-5 different places because they can wear uneven giving pulse or vibration during braking.

I don't like having rotors taken off and turned on a lathe for a number of reasons: They may not be trued for axial run-out plus the machinist may remove more on one side than the other resulting in the rotor not being centered in the caliper.

When execising the pistons, I also crack open the bleeder briefly so any air bubble can escape plus it keeps the bleeder nipple from rusting in place. And, No, air cannot enter from an open nipple working agains fluid gravity unless the laws of nature have changed recently.

2ndof2

Gunter,
Mechanic said the pistons seemed okay. I change the brake fluid every two years and being in a very dry climate I doubt there's a high probability of much corrosion in the system. I also drive the car a few times every week so heat should help dissipate any moisture that gets in there.

New rear tires yesterday so we can rule out flat spots etc. The idea of a bent hub has come up. How does one check to see if that's the problem?

Gunter

All break fluid contains moisture regardless of climate and since the system is closed, no amount of driving dissipates it.
That's part of the reason why it is recommended to change the fluid every 2 years, which you're doing, more often depending on usage.

Pistons "seemed" okay? When was the last time the calipers were rebuild?

With third-party info and without seeing, testing or measuring the actual components, hard to determine what the problem might be.

Can you raise the vehicle and check the run-out of the hubs/rotors yourself?

NC TRACKRAT

Please excuse my sounding like the proverbial "broken record" but we still haven't seen any results of taking a dial micrometer reading to the rotors. If those readings show excessive run-out, remove them and take readings of the hubs as they turn. If the hubs are true, then the rotors are warped. BTW, what torque value were you using and did you tighten in an alternating diagonal pattern? You do torque the rears with the hand brake off, right?

Steve Weiner-Rennsport Systems

Risking some redundancy here,............

Theories abound, however its all conjecture/BS until someone gathers some facts and hard data. Your mechanic should stop speculating and drag out his magnetically-based dial indicator to get busy measuring rotor & hub runouts and see if anything might be warped. It really doesn't take very long to do this.

Until that happens, you folks should not be buying ANY parts before locating the source of the problem.

JMHO, naturally.

earossi

What a neat forum! I've been messing with cars for over 50 years and had never heard of brake pad material buildup as a cause of a pulsating brake. Its just difficult for me to visualise brake pad material bonding to a rotor. Once I get past that, though, the description is plausible. So, since I have always had success with getting rid of vibration by turning the discs.....I may have been just masking the real cause since turning the discs will remove both the pad material and the disc material at the same time.

So, I would like to experiment. My BMW 545 has new pads and discs on the front and within 2k miles of being "new", I developed a pulsating pedal. My indy believes that I used pads that were too hard (they are EBC dustless pads) and thus put to much heat into the discs and the added heat was the culprit. That's a difficult scenario for me to buy into, so I haven't done anything about it. By the way, I always torque my own wheels.....in steps and in a star pattern usually to 90-95 ft-lbs.

I do not have a second set of pads to put on the front wheels to "scrub" the brake material off the discs; so, is there another way to remove the material without machining the discs? For instance, could I use a small rotory sander to do this? I have a palm sander and could put just about any coarseness of sand paper on it to try this out. Or, can I chemically remove the bonded material?

We must be talking about very small amounts of material here since "visually" the disc looks pretty "shiney" to the eye.

Comments?

NC TRACKRAT

Will venture an opinion here...Not being a race compound, those pads aren't too hard. Unless you were driving with your foot on the brake pedal, took a trip to the mountains or did some track time, I seriously doubt the rotors got too hot. In all probability, the pads may not have fully seated to the rotors. Rather than pad build-up, suggest you do a "red neck" tune-up. Just go out on a lonely stretch of road and do about 8-10 really hard stops (to the point of feeling ABS kicking in) in succession from 60 or so to 30 mph. That will bed in the pads to the rotors. I'm betting that the pulsation will disappear.

2ndof2

Well, after some weeks the car went back in and came back with two new rear rotors. We really never 100% confirmed the source of the trouble but the rotors seemed to have fixed the vibration. Mechanic did temperature testing to see if a pad might be hanging, checked the calipers and hubs carefully, and in the end there really wasn't anything outside of spec that he was able to determine. I wasn't told he was ordering the rotors, but I think he decided to order them and throw them on to be sure that wasn't the cause. I guess if the vibration was still there he would have put my old ones back on and go on to the next thing.

Looks like the problem is solved, but the question is: What caused the issue in the first place? If the rotors weren't abused and only had about 10,000 miles on them, what caused them to go bad? I know that the lug nuts had been way over-torqued because I cranked them tight like an ape every time since my torque wrench doesn't go as high as whats required for the wheels and I didn't expect this to be an issue. So in the end this is the only thing that seems to be a likely cause. We'll see if the brakes continue to feel good and if they do, perhaps this will help others identify a possible cause of a similar issue on their car in the future. Still can't figure why the rotor would appear to be within spec on runout but caused a huge vibration over about 60mph.

vincer77

Thanks for the update. It will be interesting to see if the vibration returns.

The fact that the vibration only occurs over 60mph is a new piece of info (or did I miss something?). As I have experienced it, the vibration from "warped rotors" is speed independent. Speed related vibrations are normally due to imbalances that resonate at certain speeds. But I have not experienced a vibration that only occurs at or above a certain speed and only when applying the brakes. But maybe there was an imbalance in the rotors.

AOW162435

FWIW, our calipers & pistons are made of aluminum...


Andreas

Gunter

That's good to know.
Haven't opened any calipers for 993's but lots of others.

My older 911 has steel calipers and pistons subject to rusting.

Even though the 993-brakes may not have rust issues, the caliper pistons can gum up, IMHO, and I'll continue with the practise of exercising them whenever the wheels are off.

Same for the Volvo and Honda.