Replacing front brake rotors

[Candide13]

I have a question for those who track on OEM brake rotors and do their own maintenance: practically speaking, when do you replace the rotors?

I know the theory, i.e. replace either when the rotors reach minimum thickness (28.4mm down from original 30mm for the E9x), or when there are long cracks connecting the holes. But what if there are no cracks but you are getting close to the minimum thickness? Do you proactively replace the rotors? Conversely, do you not mind going down below the recommendation to 28mm, say?

For reference, I tend to replace brake pads once they are noticeable thinner than the backing plate; I don't like to run them down further because of the increased heat load when they are over 60% gone. I am not sure the same applies to the massive rotors, where the theoretical minimum corresponds to only 5% of the material gone.

Anyhow, happy to hear experiences or practical tips, thanks.

[afadeev]

Originally Posted by Candide13

I have a question for those who track on OEM brake rotors and do their own maintenance: practically speaking, when do you replace the rotors?

When they are approaching min thickness, or show signs of damage.

Originally Posted by Candide13

But what if there are no cracks but you are getting close to the minimum thickness? Do you proactively replace the rotors? Conversely, do you not mind going down below the recommendation to 28mm, say?

a). Leave them alone (unless money is burning a hole in your pocket)
b). Sort-of, I want to finish the next DE at above min thickness (would order new rotors in advance)
c). Yes, I would.

Originally Posted by Candide13

For reference, I tend to replace brake pads once they are noticeable thinner than the backing plate; I don't like to run them down further because of the increased heat load when they are over 60% gone.

Agree with the first sentence, not sure about the second.
Backing plates are usually 4mm.
Full pad material thickness is 17mm (front) and 16mm (rear).

60% of fronts is 10mm, which is visually very different than 4mm (backing plate).
It's either one approach you describe, or the other. Can't be both.

w.r.t. to "heat load", I'm not sure what you mean.
Brake pad's coefficient of friction is in no way impacted by the thickness of pad material behind it.

Brake pad dissipates heat to air, rotors, and calipers behind the pad. Also, not much is changed as pad material thickness decreases.
As rotor get worn, the mass of that heat sync decreases, so that will negatively impact heat dissipation. But we are talking pads here.

Originally Posted by Candide13

Anyhow, happy to hear experiences or practical tips, thanks.

In practical terms, always buy spares before you need them.
There have been past (and may be future) pad shortages. There may not be a set of rotors in stock when you need them either. Get a new set 1-2 months before you project needing something.

a

[Candide13]

What I meant by heat load is that as the pad volume decreases substantially the remaining pad will get hotter. That is less clear with the much less significant reduction in the rotor volume.

Thanks all for the input; I already got the new rotors, so ready to install. I'll measure the thickness again and might squeeze in the first one-day event of the year before I make the change....

[afadeev]

Originally Posted by Candide13

What I meant by heat load is that as the pad volume decreases substantially the remaining pad will get hotter. That is less clear with the much less significant reduction in the rotor volume.

The effect would be marginal, if any.

The heat will be transferred to the backing plate, the piston, the caliper, and finally the brake fluid. I don't have any handy data on heat transfer coefficients for pad material vs. steel, but suspect they are similar, or steel's is higher.

Therefore, the smaller buffer of pad material might allow faster heat transfer to the backing plate and the piston/caliper, and thus marginally improve heat dissipation.

In practice, none of this matters with OEM pads - they will overheat and fade them regardless. And none of this normally matters with the track pads - they will withstand the DE-level heat levels either way.

a

[MRV99]

Also remember that pads wear out much faster than rotors. So I would not be that concerned about smoking your rotors at a 1 day event. It took me 50k miles and 4 track events before I got to min thickness.

[MRV99]

Also remember that pads wear out much faster than rotors. So I would not be that concerned about smoking your rotors at a 1 day event. It took me 50k miles and 4 track events before I got to min thickness.

[M-Pilot]

What is the minimum operating thickness for F80 steel rotors? My fronts are at 29.2mm and wondering if I should replace them before my next track day or not. Not sure if they can handle 1 more track day.

On a side note, the dealer refused to replace my front rotors under free maintenance (my car is a 2016 MY) because of the discolored calipers. My calipers are almost black all around the car and the shop foreman said they were exposed to severe overheating, hence cannot be covered by free maintenance or warranty. They have 7 track days and 20k miles.

[OhioRiderAaron]

Originally Posted by cntzl

What is the minimum operating thickness for F80 steel rotors? My fronts are at 29.2mm and wondering if I should replace them before my next track day or not. Not sure if they can handle 1 more track day.

On a side not, the dealer refused to replace my front rotors under free maintenance (my car is a 2016 MY) because of the discolored calipers. My calipers are almost black all around the car and the shop foreman said they were exposed to severe overheating, hence cannot be covered by free maintenance or warranty. They have 7 track days and 20k miles.

That's horse shit about the maintenance. I have a 2016 CP and just got my front rotors replaced under the maintenance program. Cracked all to fuck and the front calipers are very dark. Didnt even mention anything to me. Dealer to dealer I'm sure, maybe CA is worse?

7 trackdays or 7 events? How's the cracking? I got 5 events outta mine (approx. 12 trackdays some with extended sessions). Total mileage around 21k on them. That includes various pads from Hawk, Ferodo, to OEM. Post some pictures, but I'm sure you have a few more events left in them. I think we are always overly cautious when in reality the components can take more abuse.

Edit: just went and looked at the service record. My rotors measured 28.37mm by the tech.

[M-Pilot]

Originally Posted by OhioRiderAaron

That's horse shit about the maintenance. I have a 2016 CP and just got my front rotors replaced under the maintenance program. Cracked all to fuck and the front calipers are very dark. Didnt even mention anything to me. Dealer to dealer I'm sure, maybe CA is worse?

7 trackdays or 7 events? How's the cracking? I got 5 events outta mine (approx. 12 trackdays some with extended sessions). Total mileage around 21k on them. That includes various pads from Hawk, Ferodo, to OEM. Post some pictures, but I'm sure you have a few more events left in them. I think we are always overly cautious when in reality the components can take more abuse.

Edit: just went and looked at the service record. My rotors measured 28.37mm by the tech.

Wow unbelivable! Can't believe I got so much shit from the shop foreman about my calipers being black. He even went ahead and sent an email to my SA (my SA forwarded it to me) and said there are aftermarket stainless steel braided lines and the wear sensor is zip tied. I was like "so what? I already disclosed all that stuff when I dropped off the car. I'm not trying to hide anything". Still couldn't get them to replace my rotors. Sent couple emails to other SAs from other dealers to see if they would replace them. I'm currently planning to buy a pair of rotors out of pocket.

I don't have any pics right now but will try to take some tomorrow. I have 7 events but 3 of those were at Laguna Seca, each event having 7 sessions (20 min each). Interestingly, in the first six events I used RS29s and didn't notice any major wear on the front rotors. However, in my last event, I used PFC08s for the first time and noticed significant amount of small cracks on the front rotors (cracks aren't reaching edges or from hole to hole). I think PFC is harder on the rotors than RS29s.

Edit: Forgot to add that the shop foreman also wrote on the service order that the whole brake system needs to replaced lol

[jacobe92]

What year is your M3/M4? I'm guessing here, but it's probably because brakes, pads rotors etc, etc warranty has change. It's no longer covered for 2017 and up.

[M-Pilot]

Originally Posted by jacobe92

What year is your M3/M4? I'm guessing here, but it's probably because brakes, pads rotors etc, etc warranty has change. It's no longer covered for 2017 and up.

I mentioned in my original post, it is a 2016 so it is covered

[OhioRiderAaron]

Originally Posted by cntzl

Wow unbelivable! Can't believe I got so much shit from the shop foreman about my calipers being black. He even went ahead and sent an email to my SA (my SA forwarded it to me) and said there are aftermarket stainless steel braided lines and the wear sensor is zip tied. I was like "so what? I already disclosed all that stuff when I dropped off the car. I'm not trying to hide anything". Still couldn't get them to replace my rotors. Sent couple emails to other SAs from other dealers to see if they would replace them. I'm currently planning to buy a pair of rotors out of pocket.

I don't have any pics right now but will try to take some tomorrow. I have 7 events but 3 of those were at Laguna Seca, each event having 7 sessions (20 min each). Interestingly, in the first six events I used RS29s and didn't notice any major wear on the front rotors. However, in my last event, I used PFC08s for the first time and noticed significant amount of small cracks on the front rotors (cracks aren't reaching edges or from hole to hole). I think PFC is harder on the rotors than RS29s.

Edit: Forgot to add that the shop foreman also wrote on the service order that the whole brake system needs to replaced lol

Well I was a bit sneaky when I took mine in for that service. Basically I knew I wanted the brakes done so I put everything back to stock (happened to be the end of the season anyway). TCKline even gave me some extra OEM pads that were like 3mm left so it looked more legit.

If they're 29.Xmm and havent developed much of a "groove" I would consider getting one more event out of them. Alternatively you could buy some used rotors on the forum and keep these as backups.

[M-Pilot]

Originally Posted by OhioRiderAaron

Well I was a bit sneaky when I took mine in for that service. Basically I knew I wanted the brakes done so I put everything back to stock (happened to be the end of the season anyway). TCKline even gave me some extra OEM pads that were like 3mm left so it looked more legit.

If they're 29.Xmm and havent developed much of a "groove" I would consider getting one more event out of them. Alternatively you could buy some used rotors on the forum and keep these as backups.

Yea that's smart! One of my friends got denied for rotor replacement at a different dealer because his stock pads had more than 3mm left. His SA told him they can replace the rotors when his pads reach 3mm or less. I guess BMW replace rotors and pads at the same time.

I'm going to take my car to a different dealer on Thursday. My pads have way more than 3mm left and don't have time to put the wear sensors back on. If I get rejected again, oh well... I will order a pair of rotors from FCP Euro then.

[M-Pilot]

@OhioRiderAaron, here are some pics of my front left rotor although the quality is not that great. Btw my rotors were measured at 29.02mm not 29.2mm, my mistake.

[OhioRiderAaron]

Originally Posted by cntzl

@OhioRiderAaron, here are some pics of my front left rotor although the quality is not that great. Btw my rotors were measured at 29.02mm not 29.2mm, my mistake.

They don't look all that bad to me. Now this is a safety item so take what I say with a grain of salt, but personally I would probably be doing another event on those. The pictures could be deceiving me though.

Maybe some other guys will chime in. Where's CANdodoM3 when you need him?!

[OhioRiderAaron]

Here's my rotors after my last event. This is after some DD miles so the little cracks become more visible. Guessing you haven't driven the car much since the last event? You can see the groove around the perimeter of the pad area was getting pretty large.

[M-Pilot]

Originally Posted by OhioRiderAaron

Here's my rotors after my last event. This is after some DD miles so the little cracks become more visible. Guessing you haven't driven the car much since the last event? You can see the groove around the perimeter of the pad area was getting pretty large.

This is my daily as well so I actually probably drove it closer to 1000 miles since my last event which was about a month ago. I even drove it with PFCs on for week to clean up the pad deposit but didn't seem to help so much. I can still see and feel uneven pad surface under heavy braking after this much driving and it makes me think maybe they are not pad deposits but some damage. But yours look worse than mine in terms of cracks that's for sure

[Woodengun]

I just did front rotor change. It is very easy. If you change your own pads, changing the rotors is no problem. Wheels off. Pads out. Calipers off (2, 19mm bolts) - decided this was easier than removing studs. No studs, you can leave calipers on.

2, 5mm hex head bolts hold rotor on. Remove those and tap the rotor with rubber mallet to remove. Everything backwards to install new rotors.

~$250 each rotor.

16 Nm for the 5mm hex head bolts.

110 Nm for the 19mm bolts holding calipers to wheel assembly.

Torque values are from ISTA-D.

[nicknaz]

Find a good SA and get it replaced. I used to travel from SD to LA to work with Charlie at McKenna after Jessica left Encinitas

[M-Pilot]

Originally Posted by nicknaz

Find a good SA and get it replaced. I used to travel from SD to LA to work with Charlie at McKenna after Jessica left Encinitas

I tried three different dealers in Bay Area and contacted three most recommended SAs at those dealers. I'm not gonna name the SAs but I originally got denied at Fremont, then contacted BMW of Mountain View and Peter Pan BMW. All SAs told me that since my calipers are almost black, BMW NA may deny their warranty/maintenance claim and they may end up losing money. They basically consider discolored calipers as tracked or abused. One of the SAs even told me that their shop foreman can basically black flag my car in BMW NA's system if he thinks I track my car, implying they might void my warranty. We had the argument about how pointless and stupid to have an M car as just a highway cruiser that you are not allowed to track. Please also note that I didn't even mention or admit that I track my car to two of those SAs. As soon as they knew about my calipers they started talking about tracking and abuse. Maybe I'm just unlucky but already sick of chasing a new dealer to get this sorted. I'll buy a pair of rotors and be done with it.

[OhioRiderAaron]

Originally Posted by cntzl

I tried three different dealers in Bay Area and contacted three most recommended SAs at those dealers. I'm not gonna name the SAs but I originally got denied at Fremont, then contacted BMW of Mountain View and Peter Pan BMW. All SAs told me that since my calipers are almost black, BMW NA may deny their warranty/maintenance claim and they may end up losing money. They basically consider discolored calipers as tracked or abused. One of the SAs even told me that their shop foreman can basically black flag my car in BMW NA's system if he thinks I track my car, implying they might void my warranty. We had the argument about how pointless and stupid to have an M car as just a highway cruiser that you are not allowed to track. Please also note that I didn't even mention or admit that I track my car to two of those SAs. As soon as they knew about my calipers they started talking about tracking and abuse. Maybe I'm just unlucky but already sick of chasing a new dealer to get this sorted. I'll buy a pair of rotors and be done with it.

Sorry to hear that man. I would like to know how BMW NA service documents determine abuse or non-warranty coverage. Ex. "discolored calipers = abuse" or "deny for suspected track use." etc.

My argument to the service guys would be I drive in the canyons or in the mountains, these are just public roads. I'm pretty sure if we drove those types of roads for extended periods we could discolor the calipers.

[ferrarif1]

Originally Posted by OhioRiderAaron

Sorry to hear that man. I would like to know how BMW NA service documents determine abuse or non-warranty coverage. Ex. "discolored calipers = abuse" or "deny for suspected track use." etc.

My argument to the service guys would be I drive in the canyons or in the mountains, these are just public roads. I'm pretty sure if we drove those types of roads for extended periods we could discolor the calipers.

Tell them you rode your brakes down the Mt. Washington auto road, calipers were a discolored as a result.

As an aside, I always laugh when I see the "This car climbed Mt. Washington" bumper sticker. I assume they have no brakes left and keep my distance... Maybe you should put one of those bumper stickers on before requesting service.