BRAKE HELP!

[adam996GT3]

I want to apologize for the long post, but I want to be as thorough as possible since myself as well as many instructors at the track are truly stumped.

My car is a 2015 M3, with 11k miles and manual transmission. It is a weekend car so mostly just sits in the garage. My track set up is pfc 08 pads and apex wheels, 9.5 x18 and 11x18 on 275 and 305/35/18. Tires are nt01 that are nearing the end of their life. Other than that, the car is all stock and no suspension upgrades.

So here is the story, on Tuesday I do the usual track prep routine and swap the pads and wheels to track set up. I drove around the neighborhood a few times to bed the brakes in and then left the car alone until the track day. Saturday morning I make the 2 hour commute to the NJMP. First session go out, first few laps feel fine and then after laps 3 or 4 I start to feel a shudder under heavy braking. The best way to describe it is it feels like I am driving down a cobblestone road when braking. I continue, but after it progressively becomes worse my instructor advises that we come off the track. We visually inspect everything and all looks fine. The pads have ton of life, as they should since they were brand new pads when I put them on. Instructor advises I go bed the brakes in. I go to the road and perform about 5, 60 to 10 mph stops and the brakes feel fine while doing this, no shudder at all. Seems it's only happening when brakes heat up and under heavy braking. Go out for my second session, all feel well for the first 2 laps and then once I am warmed up and getting on it, same thing. After the session, the instructor advises I put the oem pads on. I do so and go out for my third session and same issue after the brakes warm up. Instructor then recommends that we call it quits which puts my track weekend to an end. I then drive home and under normal driving, no shudder at all. I even take some on/off ramps at speed and can't replicate the shudder. I can't seem to figure out what's going on. Some suggested my rotors are warped. Others suggested that there may be oem pad deposits on the rotor, but visually I don't see any build up on the rotor and I would assume I scrubbed those when I installed the pads, drove to the track, drove on track, and finally bed them at the track. Could it possibly be the settings I was driving on? I love the rev matching feature so I drove in sport engine, sport plus suspension, and sport steering. Any advice would be greatly appreciated.

[Psychotic Mouse]

Can you take a photo of your disks? You should be able to see pickup and high spots from a photo.

After a track day your disks should have a uniform clue layer on it. If there are deposits then may have spotty blue disks or dark spots.

[adam996GT3]

Left front and rear

[Psychotic Mouse]

I can’t see your disks too well there but it does look blotchy.

How far did you drive after the track on those?

I have a few other questions.
1) How often do you track your car?
2) Did you have Traction Control on completely or MDM

I’ve been giving your problem some thought and my best guess is that your overheating your disks. If you are driving with TC on or MDM the brake are applied on corners to assist. You will also be on the brakes more if you haven’t tracked before, or are still new to braking.

My best guess here is that at some point you parked you car with voter than usual brakes, either at a previous track session or after a spirited drive. (For example doing a twisty road and lots of braking and you see a look out brake hard and pull over. You like the view so much you spend and hour there. ok maybe not that extreme. It could also be as simple as being late getting home so you are racing from traffic light to traffic light putting your brakes through some serious heating cycles and because traffic lights are not that spaced far apart so you don’t get the cooling you need. This amplifies the temp over a short period more than you would think. You park the car when you get home and you get uneven cooling of the disk. The side under the calliper takes longer to cool. This can change the metal properties ever so slightly to a point whereby they now expand and contract at a different rate across the disk. This is made worse if after you had parked your car, a short while later you climbed back I. And rushed off somewhere else, you are not pushing your unevenly heated disks with minimal warmup and this actually maintains uneven temps across the disk aggravating the change in properties.

The above can be so slight that in normal or even light aggressive driving you wouldn’t notice.

Fast Forward to the track day. Now during your warm up laps your disks are holding up, but as soon as you start pushing harder your brakes heat up (and possibly overheating if you stability control keeps interfering) the slight difference in the metal compounds discussed above will cause them to expand in different areas at different speeds. This will creat a pulsing feeling and heavy vibration. So it’s not a warp so to say but rather Variable Disk Thickness causing the vibration brought on by the hotter brakes.

This will be evident in an non uniform discolouration of pad over your disk as the hotter spots will remove more pad and be darker in colour. Creating spots or areas where the disk expands more.

Unfortunately for you they will only get worse. Every time you feel the vibration it means those area are now catching the pads more than the other areas and will be heating up more so it’s changes the compound more etc etc etc and spirals. The change in compound if I recall correctly as well hardens the disk in that spot, so they will start to wear unevenly as well and is compounded.

When you replace your disks try just ensure that cool down properly after each run etc. At the track I always observe a cool down lap and head onto the road to do further cooling. Then when I stop I don’t use my handbrake and keep the car in neutral. Every 2-5 min I then bush my car back and 5 min later forward just to prevent one side of the disks from sitting behind the callipers.

Sorry for the long post. If there are lots of mistake it’s because I’m doing this on my phone. If something is unclear then feel free to ask.

If your going to go aftermarket for replacements I think (Just from a theory point of view) I would go for slotted rather than cross drilled.


Here is what mine look like with uniform pad layer. I have a photo somewhere with my Audi S4 that had the vibration issue and uneven layer of pad.

[adam996GT3]

Thanks for the response. In regards to your questions; the track is roughly 100 miles from the house, this was my 7th trackday, and yes I did drive with driver settings in sport because I like to utilize rev matching.

I do agree with you that there are high spots on the rotors, but do you think they aren't salvageable. I was thinking of using a rotor honing tool to clean the rotor surface. lol this one http://www.rotorhone.com

Or do you think it won't work and I should just pull the trigger on new rotors. Will I need to buy all 4 or just front 2? If I do replace the rotors, should I discard the pfc's or do you think I could re use on trackdays? They were brand new before Saturday.

[Psychotic Mouse]

Thanks. So it’s possible the “damage” is from the previous track day then?

I’ve never looked into Honing them so I can’t really provide feedback, but if it’s inexpensive to try that first then there is no harm. The only catch here is that if you don’t experience it in normal conditions it may be hard to see if it’s worked until the next track day when it will be too late if in your first session your feeling the vibration is still there.

I suppose you could ask a supplier if it’s possible to buy the disks and return after the weekend if unused. That way if you have the vibration you can swap out the disks at the track and put on the ones you have bought and if not then you could return them unused for a refund.

Alternatively after trying to hone them you can test for Vibrations by bringing them up to track temps through a similar process used to bed in your brakes. You find a quiet deserted road and do a series of hard braking from 100km/h (60mph I think) accelerating immediately back up to 100kph and braking again. Don’t ever come to a complete stop, go down to 10 or so kph and speed up again. A few of these will get your brakes hot. If you get a vibration then you know honing hasn’t worked.

As for replacement of fronts only or both pairs, I got rid of mine by only replacing fronts. As the car was still under Motorplan they were replaced by BMW, and the vibration went away. If you do replace I would replace only the fronts first and if it’s still there the replace rears. If you go aftermarket, I would keep the stock disks as back up seeming as you don’t feel it under normal driving.

Pad should still be fine to use. General rule of thumb is when replacing worn disks you replace the pads with them, but that’s because the disks don’t wear even and have radial ridges on them. These make grooves in the pads and then on the new disks creat radial hotspots until they wear even with the pad. Just check your current disks are still relatively flat across the surface and your pads are even. If not, then drive gently with those pads in for awhile until they have bed in properly.

One thing you could try before doing any of the above is drive a few times with your track pads in but don’t allow them to heat up. So basically a trip around your block with minimal to light braking and then remove them and run street pads again. Then do it again a month later. This way if it’s only deposits on the disk the cold pads should scrub them off. Ideally you would need a harder compound pad than your track pad to help clean up the layer off your track pad on your disk already. The way pads work is that when up to temp they will lay down a microscopic layer of pad on your disk that then bites the pad and offers the resistance. If this builds up unevenly due to improper bedding in then it could cause the vibrations. If caught in time just removing the layer will help. The reason you need a harder compound is that using the same compound will just bind rather than remover this layer. That’s why I say try with very cold brakes not allowing them to get up to temp. This might then scrub off the layer. If you know someone with a harder compound track pad on their F80 then ask to borrow those and do cold braking rather than spending money first. You could also try drive daily with your track pad for awhile as that will also clean up your disk and will eventually be squealing when clean. (Even though it’s highly unlikely to work it’s worth doing)

I just read your pads details online and they make reference to a “Fine grey slate like disk finish” so your disks should look more grey than blue like mine above.

Hope this helps.