Anyone else warp rotors ALL the time?

[Gawernator]

Using Textar pads and OEM Zimmerman rotors I have no issues lol

[flash968]

the only times i've seen warped rotors, the calipers were not lined up correctly. when the rotors then got hot, and then cooled down, they did so off axis, causing warping.

again, i would check for runout to see that they are actually warped. then i would check to see that the calipers are putting the pads down parallel.

[Slam]

Thanks again, all. After taking into consideration all your replies I believe my course of action will be to determine if they are in fact warped. Perhaps they're not. I'm going to deglaze them if I get time this weekend and see what results I get. From what I've read, a good sanding with 120, from edge to centre, should remove pad material. I'll do the pads as well.

FYI, when I rebuilt the front calipers I ensured I had the right 20-degree piston offset.

Cheers, gents!

[flash968]

that may work for a short while. i have never been successful at completely removing the deposits though, and the problem always came back. even surfacing the rotors did not do the job. the pads always had to be tossed.

i was not aware you rebuilt the calipers. maybe i missed that. in that case, that would be the first place i would look. never assume that a rebuilt or new part is working like it should. remove the pads and look carefully at them for uneven wear.

[Slam]

Roger that.

One interesting point: my 84 924 never had a problem like this, and I rebuilt its front brakes and drove it for a couple of years before selling it. Different design, but food for thought.

[cockerpunk]

Originally Posted by Slam

I seem to be changing rotors every year. Last time I made sure to rebuild the front calipers in case they were binding. No go. Rotors are warped. Front bearings are good and adjusted - checked today. Tie rods and ball joints are good. Struts less than a year old. I know, I know, there's a school of thought that says rotors don't warp but rather material builds up on the disc. But these are warped.

I think winter had something to do with it. From -35 to operating temp and then back to -35 on an average day may be doing it. Thoughts?

Anyone have good things to say about Frozen Rotors for our cars? This is driving me nuts.

no such thing as a warped rotor.

[Slam]

As I am discovering: -Warped- Brake Disc and Other Myths

However, I followed the bedding in procedure as outlined in that article. Obviously I'm doing something wrong and need to find what it is "so's I can don't do it."

[djnolan]

Originally Posted by flash968

the only times i've seen warped rotors, the calipers were not lined up correctly. when the rotors then got hot, and then cooled down, they did so off axis, causing warping.

When I rebuilt front calipers, what I found was the caliper slide rail grooves could get rusty and hang up. This would cause the calipers to get mis-aligned and not release fully and "possibly" cause hot spots on the pads. I used a small triangular file and emery cloth to lightly dress the caliper grooves and slide rails to remove any rust. Then grease with synthetic caliper grease and reassemble. The brakes worked much smoother, then.

As far as the caliper bore went, I did the absolute minimum which was to clean with brake clean and replace the rubber seals. No reboring or anything as I didn't want to damage the fit.

[flash968]

well, if you did the correct break in (i.e. didn't let them cool down between stops, etc) then i would say that your calipers are not putting the pads down evenly, either because they are not square, or the piston is hanging up, or something like that.

you will likely need to take it all apart and do it over, carefully measuring everything prior to reassembly. you will also need to start with fresh rotors and pads again.

[Slam]

I agree. Time to do over. One question: shell out for rebuilt calipers? I wonder if my rebuild is suspect, regardless of how careful I felt I was. Rotors don't exhibit pad imprints and aren't blued.

[flash968]

first, pull the pads and carefully measure them. see if they are going down crooked.

if they are, then you need to look at the calipers. first carefully measure things for squareness and fitment to see if they are bent, have something preventing them from seating properly or some other reason that the frames of the calipers could be the culprit.

if that fails to show the problem, then you need to look at the pistons and how freely they move. if the frames are not the culprit, then likely you have a piston hanging up a bit, causing the pad to go down unevenly. this is a more likely culprit than the frames, as it would be hard to imagine both frames doing the same thing, and it is far more likely that the pistons are just not traveling well.

if none of that shows anything, then you need to measure your rotor temps, and if you find that the front rotors are getting really hot, i suggest that you increase your rear braking by changing the hydraulic bias toward the rear (which is not a bad idea to begin with on any of these cars, as they all need more rear brake, not front). we do that in the 968, with outstanding results. for us, it's a simple valve change. i don't remember how the various models of the 944 are set up.

of course, it could merely be that the brand of pads you have chosen do not play well with the particular brand of rotor. i am going through this right now on the SL550. the same brand of pad i use on everything (porterfield R4-S) is not playing well with the brembo rotors i have on the front of the SL-550, and are making noise. fine on the rears though. it happens.

[Slam]

Gold. Thanks, Flash. Didn't get to autopsy the car his weekend but now I know what to go after.

When I rebuilt the fronts I did polish the pistons, as I found slight evidence of scoring. The bores weren't pitted but i polished those too. I think it's time for fresh calipers. I'll do the fronts first - rotors, pads and calipers. Bearings are less than a year old and maintain adjustment.

[Slam]

Well, when last we left our brainless mechanic he'd -

Oh, wait!

Okay, I took my cross-drilled rotors to be resurfaced and the shop tech said he removed less than 5 thou to true them up. He also commented that material just literally flaked off. Not warped. crudded up, as you gents had said. Good. Fresh pads on order. Rotors look good - not blued.

Stand by...

[flash968]

sounds like a great start

[djnolan]

On the calipers, the top rim of the piston gets dirty, and when you retract the piston for new pads, the crud will bind the piston and could damage the bore.

[Slam]

Well, here's the latest in my brake saga:

I rebuilt a couple of spare rear calipers I had lying around and this time made a template out of sheet styrene for the 20-degree piston angle. These I installed on Saturday in a 'let's get this ***** done befoe the weather improves' blitz on the car.

When I autopsied the rear calipers I found a host of issues (note that these were untouched by me - they were on the car when I got it). The frames were sticking, 1, the pistons were sticking, 2, the 20-degrees was off slightly on both, 3, and the pads were worn unevenly, 4. Okay then... I swapped rotors with a pair I had on hand, put in matched used pads, deglazed everything with 100-grit sandpaper and reassembled. Before you lot flame me for this, just know that my thinking was that I had to have a car to drive on Monday and I had to have the good rotors off the car to be machined. I'm getting new pads for the rear but won't install them until I get the rotors done.

On to the fronts! I pulled both calipers and adjusted the piston angle slightly on each. Template made all the difference. I could easily press in the pistons with my thumbs so didn't pop them out. What I did do was file, sand, polish and grease the slider contact area before putting the calipers back in the car. The pads had worn evenly, BTW. After all this it turned out my local guys had got me the wrong new pads...so... In the name of experimentation and learning I decided to leave the rotors on the car - no point putting in the good ones with old pads - and deglaze the pads and sand the rotor face with 100-grit Garnet paper. This is recommended. I also swapped over to my summer wheels and tires.

So now on to the results.

I have a power bleeder, and verified no air bubbles. I took the car out to bed the brakes as per the plethora of articles floating around the web. They all seem to agree on one thing: heat up the brakes to get material transfer. Okay, I can do that! My first three hard stops showed the judder had gone. I find this proof that warping is (as many of you have said) a myth. I sanded material off the rotors and now they weren't grabbing. Good. My next three hard stops and I was reaching more and more for the floor, but I understand fade in these circs is normal, so I carried on. By the 9th stop the fade was pronounced! And the smell was just as fun.

I gave them another once over on the way to work this morning.

The net result is I still have smooth braking but the pedal is still too soft. I'll rebleed on Wed when I have the time, but at least I won't have to pull the calipers again.

So, brake gurus, what's the score?

[djnolan]

It all makes sense to me. I bedded my brakes on the way home from Hershey due to some chatter on the LF. Was soft for a few days but firmed up and is smoother now. Soft pedal may be air from initial oxidation or not. Did you use synthetic fluid?

I have never heard of the 20 degree angle, though. Is this an N/A or S concern, too?

[Slam]

No, I didn't use synthetic fluid. Just Dot 4, fresh from an unopened container. I've read quite a few stories about synthetic fluid and since I never track the car I decided just t go with plain, as it were.

The 20 degres refers to the angle the raised flange/lip on the piston meets the oncoming rotor, measured in reference to the frame. I think it's designed so the piston puts the pad down evenly, without the pad chattering against the rotor. I don't know if this is an issue with the S cars but I know the NA cars need this done - the single-pot calipers.

Here's a link with a good pic: http://forums.rennlist.com/rennforums/924-931-944-951-968-forum/128110-piston-orientation-during-brake-caliper-rebuild.html

Hershey... That'd be nice!

[333pg333]

I'd dump the drilled rotors and just go to stock.

[fiat22turbo]

Could be your MC is failing.

Since you've got a power bleeder, you could try reverse bleeding the system to ensure no air is trapped in the lines near the MC. Carefully connect a hose to a bleed nipple, pump up a little pressure to get fluid to the nipple, crack the hose to get rid of the air there, then crack the nipple open and up the pressure. Watch the Retrovir to ensure you don't overfill it.