what could cause a single rotor to warp?

[kolosy]

i've got an odd situation -- my passenger rear rotor is super badly warped. take a gander, and just listen to the sound at 0:08

I thought the caliper was stuck, and when I was having the shop do the corner balance on it, I had them take a look. He said it didn't appear to be stuck, but that apparently the rears can get wedged out of alignment and get stuck as they heat up because it's a large single piston (haven't heard of that on here, not sure what I think about that).

alternatively, the rear wheel bearings do seem to be going out, not sure if that can contribute somehow?

if it was straight up user error, I'd expect both rotors to warp, not just one corner. And I generally try to not do stupid things with them (I take a cooldown lap, and drive around the paddock if needed, I don't touch the brake when parking, I don't use the e-brake at all, etc.)

any ideas? is this a reason to upgrade the rear calipers?

[spammysammich]

How do you know it is warped? Have you measured runout on it? If your wheel bearing is bad enough they might be chattering and battering around in the race. Didn’t you post pictures of badly wedged pads at some point? Might be a sign of bad bearings allowing the hub face to go out of alignment with the carrier.

[kolosy]

Originally Posted by spammysammich

How do you know it is warped? Have you measured runout on it? If your wheel bearing is bad enough they might be chattering and battering around in the race. Didn’t you post pictures of badly wedged pads at some point? Might be a sign of bad bearings allowing the hub face to go out of alignment with the carrier.

It’s a fair point, I’m assuming it’s warped because the sound and vibration is only happening under braking and coming from that corner. Rotating the wheel by hand encounters variable resistance similar to what I would expect if the rotor were warped. The wedged pads were from the fronts.

I do intend to pull that hub this weekend so we’ll see how that looks regardless but I think the rotor *is* warped also.

[spammysammich]

A lot of people assume warpage when there are myriad other reasons for weird vibrations. Only way to be sure is measuring runout or possibly visualizing it. My cousin always claimed his Q5 was warping front rotors until I showed him how to bed pads in. Poor pad transfer and bad braking habits on his part.

[Estoril Blue]

Without runout measurement and hard numbers, I think the shop is wrong. More likely pad deposits, a pad problem - or problem elsewhere. What does the hub look like? Post a pic of the affected rotor.

[kolosy]

Originally Posted by Estoril Blue

Without runout measurement and hard numbers, I think the shop is wrong. More likely a pad problem - or problem elsewhere. What does the hub look like?

Can you explain how a pad issue could manifest in a thrumming noise like that? Also when I rotate it by hand, I feel like it drags on the pad part of the time, and spins freely the rest of the time?

I’ll pull it apart this weekend.

[spammysammich]

Thats exactly what was happening to my cousin because of uneven pad deposits. You could actually see them on his rotor. He had a nasty habit of braking hard and standing on his pedal while he was in stop and go traffic. Especially bad coming down from mountain passes during snowboarding trips. Red hot rotors and improperly bedded pads just eventually left pad prints you could actually see in places but he never bothered to look. Two good rounds of bedding in and long cool downs sorted it out. Not saying that’s your problem. But it is a problem.

[Ximian]

What brake pads are you using? CarboTech pads don't agree with my ST60 kit and rotors and always ended up with crazy amount of shaking due to pad deposits. It didn't sound as bad as this, but looked similar.

[kolosy]

Originally Posted by Ximian

What brake pads are you using? CarboTech pads don't agree with my ST60 kit and rotors and always ended up with crazy amount of shaking due to pad deposits. It didn't sound as bad as this, but looked similar.

The Winmax R5s. No issues with deposits/shaking on the fronts, same pads.

[Estoril Blue]

Originally Posted by kolosy

The Winmax R5s. No issues with deposits/shaking on the fronts, same pads.

That assumes all slide pins are functioning well/equally.

[kolosy]

So I pulled the rotor off and looks like you guys were directionally right. The pad transfer layer is uneven on the inboard side. That’s likely causing the shudder, and I think the knocking / loud noise is still from the bearing. Will need to re-bed the brakes.

[jritt@essex]

I agree with everyone else in this thread...uneven pad transfer layer.

Below are some vids I made many years ago on the topic of properly bedding in brakes and how to swap between track and street pads.

Here's a bonus article on how to get the most out of your track pads: https://www.essexparts.com/news-blog...d-owners-guide

[bigjae1976]

Originally Posted by kolosy

So I pulled the rotor off and looks like you guys were directionally right. The pad transfer layer is uneven on the inboard side. That’s likely causing the shudder, and I think the knocking / loud noise is still from the bearing. Will need to re-bed the brakes.

A front BBK will cause the rear brakes to get a lot hotter or at least cause the pads to wear faster. I assume you have the dust shields in the rear? Could be overheating the inner pad which is causing the pad deposits. You can see the rotor is getting very hot with the blue heat marks.

[kolosy]

Originally Posted by bigjae1976

A front BBK will cause the rear brakes to get a lot hotter or at least cause the pads to wear faster. I assume you have the dust shields in the rear? Could be overheating the inner pad which is causing the pad deposits. You can see the rotor is getting very hot with the blue heat marks.

Can you explain why? I will say, my rears are barely used. I’ve gone through 5-6 sets of pads in the front and have maybe 60% pad life left in rear. I ended up getting the matching rear bbk (got a good deal on it so it wasn’t materially more than just new rotors) so hopefully that helps a bit.

[bigjae1976]

Originally Posted by kolosy

Can you explain why? I will say, my rears are barely used. I’ve gone through 5-6 sets of pads in the front and have maybe 60% pad life left in rear. I ended up getting the matching rear bbk (got a good deal on it so it wasn’t materially more than just new rotors) so hopefully that helps a bit.

Why? Not sure. E46 data...those who run a front only BBK have seen increased rear pad wear. Might get two front pad changes to every one rear. I get 5-6 front pad changes to one rear with a 4 wheel BBK. My buddy has since installed a rear BBK (stoptech) a couple of years ago and I don't think he has had to change his rear pads.

One common pad setup earlier in the E9X M3 life was a pagid yellow front and pagid black in the rear. The rears would wear as fast as the fronts.

There's some data. I don't know why. Maybe ABS related? I have cracked 2 rear rotors on my E46 although those discs were at or maybe even a tad below the min thickness. I found the cracks by a visual inspection, didn't feel anything. Looks like a heat crack that just spread to the outer edge.

Another data point - if you are wearing out a pagid yellow pretty quickly, it indicates that the pad is overheating. I don't think the rear brakes get much airflow/cooling.

Conclusion? I don't know. Might be ABS related. Could have had a corded or heat cycled rear tire which caused the ABS to activate? Or maybe a track that overheats the right rear tire which caused more ABS activation?

Probably need to ask a nerd that can do brake bias calculations and stuff.

[kolosy]

Originally Posted by bigjae1976

Why? Not sure. E46 data...those who run a front only BBK have seen increased rear pad wear. Might get two front pad changes to every one rear. I get 5-6 front pad changes to one rear with a 4 wheel BBK. My buddy has since installed a rear BBK (stoptech) a couple of years ago and I don't think he has had to change his rear pads.

One common pad setup earlier in the E9X M3 life was a pagid yellow front and pagid black in the rear. The rears would wear as fast as the fronts.

There's some data. I don't know why. Maybe ABS related? I have cracked 2 rear rotors on my E46 although those discs were at or maybe even a tad below the min thickness. I found the cracks by a visual inspection, didn't feel anything. Looks like a heat crack that just spread to the outer edge.

Another data point - if you are wearing out a pagid yellow pretty quickly, it indicates that the pad is overheating. I don't think the rear brakes get much airflow/cooling.

Conclusion? I don't know. Might be ABS related. Could have had a corded or heat cycled rear tire which caused the ABS to activate? Or maybe a track that overheats the right rear tire which caused more ABS activation?

Probably need to ask a nerd that can do brake bias calculations and stuff.

Yeah I’m wondering if the particular bbks you’re seeing change the brake bias and shift braking rearward.