Strange brake behavior

[Solo_M_Tech]

Ok so here is the gist. Im on stock rotors and calipers. Cool Carbon pads for daily use.

I can brake hard from normal speeds, 70, 80, 90 mph etc without any issues. The car stops hard, consistent, vibration free.

On the track, the brakes perform perfectly. Zero issues.

When braking from high speeds, 130 mph plus ( I know I know) I get a hard sided to side vibration in the steering wheel. No pulsing in the pedal.

Is it simply pad deposits, or something else? I would think that if the rotors were warped, I'd notice the vibration under almost any braking condition, but I don't. When braking at speeds under 100 mph, there is zero vibrations.

If anyone has experienced this behavior specifically, or something similar, and has a solution, Im all ears.

[azndrunk74]

Sorry I’m of no help here but my buddy and I both get vibrations on semi hard braking on speeds above 100mph. We both have brembo 380/380 kits. Low speeds are a non issue.

[msan]

I Would guess light pad deposit. You'd feel heavy pad deposit at lower speeds. Since at 130mph I imagine the rotor is spinning so fast the vibration shows up easier . And is light...

[Solo_M_Tech]

Originally Posted by msan

I Would guess light pad deposit. You'd feel heavy pad deposit at lower speeds. Since at 130mph I imagine the rotor is spinning so fast the vibration shows up easier . And is light...

na the vibration is harsh enough to shake the steering wheel side to side about 15-20 degrees of rotation. At moderate speeds with normal or hard braking, it doesn't happen. Even high speed light braking, no issue.

Imagine driving in the left lane at 130mph and someone switches lanes going way slower than you. you have to brake hard, bit not hard enough for abs, and you get the shake. That's when it happens.

When I'm at the track, even with very hard brake abuse, there is zero issues, no vibrations or shakes.


Note I've installed new inner and outer tie rods, new tension arms and control arms, new shocks, mounts, bump stops etc. Pads have 10k Miles and rotors have 55k miles. I plan to change the rotors the next pad change.

[msan]

Wow did you measure the rotor thickness? 55k is definitely at or near reaplacement

[roastbeef]

they might not be warped... i think it takes something out of the norm to warp the fronts. i'd speculate you're dealing with deposits. what you're describing is similar to what i got when i made the switch to nt01 tires with oem brakes.
i bought ap's after that and haven't had to deal with deposits. lots of people will offer up suggestions and techniques for removing deposits, but i don't have any experience with that.
are you able to inspect the inside of the rotor for cracks? if they are starting to go, the insides are often hotter. look carefully around the drilled holes for cracks.

[Solo_M_Tech]

Originally Posted by roastbeef

they might not be warped... i think it takes something out of the norm to warp the fronts. i'd speculate you're dealing with deposits. what you're describing is similar to what i got when i made the switch to nt01 tires with oem brakes.
i bought ap's after that and haven't had to deal with deposits. lots of people will offer up suggestions and techniques for removing deposits, but i don't have any experience with that.
are you able to inspect the inside of the rotor for cracks? if they are starting to go, the insides are often hotter. look carefully around the drilled holes for cracks.

Thank you . I will do precisely that.

[bigjae1976]

Get the brakes warm (not hot), spray with cool water. Let the rotors rust up and then drive. It will remove the deposits if there are any. Make sure you get the back side.

I would remove the wheels and rotors. Clean all of the mating surfaces and apply a light coat of anti-seize or grease. Inspect the wheel bearing and control arms for excessive play. If you still have the brake dust shields, spray them down good since they tend to accumulate pad and rotor dust.

Pretty sure its not pad deposits or warpage because you'd have a consistent vibration braking from any speed. My bet is a bad control arm bushing or ball joint.

[dparm]

Could this be something unrelated to braking, such as a slightly loose wheel weight or maybe a tire that is a bit out-of-round?

[Solo_M_Tech]

Originally Posted by bigjae1976

Get the brakes warm (not hot), spray with cool water. Let the rotors rust up and then drive. It will remove the deposits if there are any. Make sure you get the back side.

I would remove the wheels and rotors. Clean all of the mating surfaces and apply a light coat of anti-seize or grease. Inspect the wheel bearing and control arms for excessive play. If you still have the brake dust shields, spray them down good since they tend to accumulate pad and rotor dust.

Pretty sure its not pad deposits or warpage because you'd have a consistent vibration braking from any speed. My bet is a bad control arm bushing or ball joint.

The only suspension possibility here would be the LCA. That's the only thing I haven't replaced. I agree the pad deposits would be present most if not all the time. That was my experience with EBC red stuff pads. They were terrible for leaving uneven deposits.

I checked the rotors with a dial gauge for run out and found 0.002" which is within tolerance and could be explained with the pad deposits I can see from the brakes sitting. ( I haven't driven the car for a week now.) Wheel bearings show no lateral or radial runout. Tension arms are new from BMW.

Tires are new P Zeros ( gross but needed something on a sunday and that's all discount tire had in stock) mounted on Apex ARC 8 18X10 ET25 squared. 275/35 at all 4 corners.

I've checked the basics. Im going to re-bed the pads as soon as I get my car back from the body shop. ( minor rear bumper fix and respray thanks to a hit and run)