[trakattack]

Hi All, recently picked up a low mileage M2 LCI (non competition) and just took it to the track for the first time. I'm not sure why but halfway through the first session I experienced brake shudder/vibration & extremely inconsistent brake feel. Braking points and brake feel would change each session depending on heat into the pads.

At the start of each session I would have a consistent feeling pedal then as I got heat into the braking system, I'd experience a lot of vibration and pulsing - not ABS for certain because I'd get this even with light brake pressure/input.

I was thinking that my rotors could be warped but seems unlikely as the car has <15K miles on it. Further research indicates that it could be uneven rotor deposits from swapping out OEM pads with the PFC 08 pads? I also have changed brake fluid to Castrol SRF. All else is OEM.

If so, what would be the most cost effective way to resolve this issue? Given that the car has low miles and has not been previously tracked before, could I just resurface the rotors and re-bed in my track brake pads? What could be causing this inconsistent and pulsing brake feel?

I've tracked an FRS before (5-10 days experience) and this is my first time on this platform and haven't ran into this issue before. Any input or guidance would be helpful here. Thanks in advance!

[dvas]

Was the car tracked by the previous owner?

I ask because I've experienced this type of pulsating and heavy vibration on my car when I've gone through two sets of front pads (PFC 08 on stock rotors with SRF as well) and now on a new, third set on the same set of rotors. My hypothesis to why this is happening is improper bedding and mating of the pad to the rotor. For reference, I'm on my fourth set of front rotors while the car has about 25k - but mostly track.

Wear on the rotor is never perfectly even and by the time you're on a third set of pads, the rotor has been materially unevenly worn. A new pad will not mate perfectly with that rotor and will create pulsation. In my experience this eventually goes away with some hard braking over a track event or even two. The other, but expensive option, is to get new rotors.

The other possibility that comes to mind is just rotor deposits by not cooling down the brakes after a session, or staying on the brakes while standing still in the paddock after a session and imprinting the pad onto the rotor. Again, this should go away with some time and hard braking.

With regards to inconsistent pedal, I would flush the system again with SRF and ensuring everything is tightened and there are no leaks.


Anyways, those are the things that come to mind.

[tux2005]

I'd also recommend trying to do a proper bedding procedure on each pad swap, if I don't do that between the stock pads and PFC08s with my OG I can end up with pretty horrible deposits and pulsing/shaking.

If you have a distance to drive to the track and have roads with safe speed limits and traffic you can do it on the way to the track or do it during the first session.

For the road, it's a case of build up heat in the brakes by braking from 120km/h to 20-30km/h with a medium brake application, then gradually accelerate back to 120km/h to do the same thing about 5 times, then once things are hot now it's time to apply the material which is more like 120+km/h to 20-30 with a hard brake and then quickly accelerate back up and stomp again, again 5-10 times, you'd feel the brake performance change during this cycle. Then try to drive around 80-100km/h for 25-30 mins to cool the brakes, avoid braking as much as possible at this stage. Then once you park, let the brakes cool entirely.

If you have to do it on track, then start sessions with a slow and easy pace for the first lap or two to build some heat, then do the same but this time with hard acceleration and hard braking, then do some cool down laps, ideally laps with zero brake at all, then pit and park the car for minimum 45 minutes to allow to cool.

I find I need to do either approach every time I swap. I normally do weekend events with 2-3 days but I only bed them in the first day.

[trakattack]

Thanks for the feedback guys - is it even remotely possible that the rotor itself could be warped? The car had 4500 miles on it before I took ownership and it probably sat in a garage/never driven before me.

[dvas]

Originally Posted by trakattack

Thanks for the feedback guys - is it even remotely possible that the rotor itself could be warped? The car had 4500 miles on it before I took ownership and it probably sat in a garage/never driven before me.

In my opinion, no. On older rotors, I've had some really horrible judder, where it felt like issues with wheel bearings or something. It took a month or so and maybe two track days, but it eventually went away. Which is why I think it's really just getting a proper mating of surfaces between the pads and rotors. I don't know, maybe someone with an engineering background can validate?

Having said that, I'm sure a new set of rotors would solve the judder.

Alternatively, you can try swapping the brake pads around to see if that changes anything.

[PackPride85]

If it's truly just uneven pad deposits, you should be able to put the track pads on and drive them normally on the street for a few days. The lack of heat in the track pads will cause them to scrape off anything stuck to the rotor (not getting them hot enough to activate the grip in the material). Probably takes a small layer off the rotor as well. This is typically the noise you hear when people complain about track pads being noisy in the street.

[jritt@essex]

It's highly unlikely that the discs are actually warped. Here's a video I made many years ago on swapping between street and track brake pads, and how to manage pad swaps. Below that is our bed-in video. Hopefully these help. If you understand everything in these two videos, you'll then understand how to manage your street and track pads and discs properly in the future.