Help with violent shaking when braking pls

[SQ13]

My car was hit on the FL corner in March 2024, and I got it back from the body shop in late July. Test driving with the service advisor didn’t reveal anything bc I was lightly braking. However, later on I noticed that the car shakes under moderate to heavy braking at highway speeds. The shaking is felt thru the steering wheel but not the pedal. I thought maybe it was due to some rust buildup on the rotors from sitting outside for several months, so I tried to clean them up by following a bedding procedure, which completely eliminated the issue for the rest of the drive. But it returned when I drove the car a few days later, so I haven’t driven it since then. The body shop isn’t helpful and I’m hoping to try and narrow down what might be wrong and fix it myself.

The following was done as part of the repair:
Drop entire front suspension and subframe
Engine and trans removed (allegedly..charged for it but SA said they “lowered the engine”)
OE front control arms, OE FCABs, rebuilt ZHP steering rack, OE inner and outer tie rods, OE steering coupler
Alignment (allegedly..they didn’t give me a printout and didn’t charge for one)
The tires were fairly new at time of collision, but the car was moved around with the damaged suspension a few times.

I have gone thru all of the hardware they touched and did not find anything loose. The wheel hubs/bearings feel tight when pushing and pulling the top and bottom of wheels. Rotors look fairly clean.
What I did find was a knocking sound that seems to be coming from the steering rack, but I have no idea if that’s normal. It’s present with the steering wheel unlocked, both when manipulating from below as seen in the video and by turning the steering wheel.

I’ve come up with possible causes but not super knowledgable with diagnosing things, so I would like to hear how I should approach this or what else I should check out
1. Tires - will swap fronts and rears to see if anything changes
2. Alignment - will try to have someone check the toe tomorrow
3. Brake rotors - bed again? Replace?
4. Rebuilt steering rack might have internal issue - work something out with seller or buy used one off eBay?
5. Steering column damage that went undiagnosed..hard to hear where the knocking sound was coming from

Edit: Can I rule out the steering column and rack if I have someone hold the steering wheel tightly and feel no play in the front wheels?

Vids: https://drive.google.com/drive/folde...TY1UYEXllxRee1



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[bagger]

Just a shot in the dark - but how does the brake caliper look? Sticky? Does the car pull to one side under the heavy braking? Bleed the brakes just for fun?

[PSUEng]

Not sure if this could be a cause, but you did not mention your steering coupler. Is it tight/installed properly?

[bigjae46]

Body shops are notorious for putting stuff together wrong. I'd check that first.

Electric rack BMWs have a TIS that says the steering rack should be replaced if there is a significant front end impact. From that bent tie rod, I'd start at the steering rack, subframe, and the coupler. Did they replace the bolts that mount the rack to the subframe?

[bmwfnatic]

Electric rack BMWs have a TIS that says the steering rack should be replaced if there is a significant front end impact. From that bent tie rod, I'd start at the steering rack, subframe, and the coupler. Did they replace the bolts that mount the rack to the subframe?

I think we could logically extend that to the hydraulic racks from the older cars too, I have one car in mind that I service, it had an accident and it definitely needs a rack still, it has way excessive play in the middle position.

[SQ13]

I’ll pull the calipers later today to inspect, but they were totally fine before the collision. And nothing looked off when visually inspected while installed. Brake flush was done recently with SRF. No pulling at all while braking, and nothing out of the ordinary when steering.

I don’t think front subframe bolts were replaced, but I’ll double check today.

The steering coupler is tight..no play at all. I’ll make sure it’s torqued to spec, but I couldn’t find anything abnormal with it last night.

[Slideways]

What pads were on there when it rusted? Sometimes, it takes an abrasive race pad to get bad deposits off.

[Estoril]

You've cover the key things. I suppose I'd look at bushings again. And test wheel bearings - again. Rotor face looks even. Pad wear even? How about even tire face wear? Where specifically is the shaking felt under braking? Is it strong through the brake pedal? Does it brake in a straight line hands off the wheel?

[SQ13]

The car is currently at a shop to have the alignment and brakes checked. Beforehand, I looked over the brakes carefully and couldn’t find anything that stood out to me, but I’m not gonna rule out the rotors yet. I also had someone try to hold the steering wheel in place while I wiggled the front wheels, and both felt fine.

I’m using OEM Textar pads with the 996 calipers. Very possible that they’re not abrasive enough to scrub the rotor surface perfectly, but I would’ve still expected to see some kind of buildup on the surface? I don’t have race pads for the fronts yet, but I can order PFC 11 pads for them. A new pair of front ZCP rotors costs $500 thru FCP so I can order a set if the race pads don’t do anything.

Given that the issue disappears when the brakes are HOT, I think it’s likely a brake issue? I’m not sure if the behavior of any suspension or steering components would change after I follow a bedding procedure.

Once out of the shop I’ll get back on the highway again and do some more testing (haven’t driven the car since August). If the problem persists, I’ll retorque the suspension components.

Pad wear is even. Inner section of FL tire is a little scrubbed from car being moved around with the damage; it has been moved to the rear now. Shaking was most prominent thru the steering wheel, and I can’t remember if it was felt thru the pedal (will follow up on this in 1-2 hours). Will also check if the car brakes in a straight line with hands off the wheel.

[bmwfnatic]

~

Check brake disc runout with a dial gauge, or if it’s really bad might just be able to by eye.

I am also suspicious of your wheel bearing, especially on the side that got hit, if it got hit hard enough to break suspension parts that also means the bearing got a major hit.

[Slideways]

The car is currently at a shop to have the alignment and brakes checked. Beforehand, I looked over the brakes carefully and couldn’t find anything that stood out to me, but I’m not gonna rule out the rotors yet. I also had someone try to hold the steering wheel in place while I wiggled the front wheels, and both felt fine.

I’m using OEM Textar pads with the 996 calipers. Very possible that they’re not abrasive enough to scrub the rotor surface perfectly, but I would’ve still expected to see some kind of buildup on the surface? I don’t have race pads for the fronts yet, but I can order PFC 11 pads for them. A new pair of front ZCP rotors costs $500 thru FCP so I can order a set if the race pads don’t do anything.

Given that the issue disappears when the brakes are HOT, I think it’s likely a brake issue? I’m not sure if the behavior of any suspension or steering components would change after I follow a bedding procedure.

Once out of the shop I’ll get back on the highway again and do some more testing (haven’t driven the car since August). If the problem persists, I’ll retorque the suspension components.

Pad wear is even. Inner section of FL tire is a little scrubbed from car being moved around with the damage; it has been moved to the rear now. Shaking was most prominent thru the steering wheel, and I can’t remember if it was felt thru the pedal (will follow up on this in 1-2 hours). Will also check if the car brakes in a straight line with hands off the wheel.

OEM textar pads should not cause a vibration issue. As long as they were not Stoptech Sport pads. Those will fuse to the rotor and cause a rusted imprint so bad that it will take a race pad to remove them.

[Estoril]

Note: A wheel bearing test shouldn't require someone to hold the steering wheel. *If* they are going out a fair amount you'll hear it without braking.

[SQ13]

Okay so the body shop definitely didn’t align the car. Here are my findings after the alignment:
1. Cold rotors - no issues with any amount of brake pressure
2. Warm rotors (after some hard braking from 80 to 40ish - shaking moderate to heavy braking
3. HOT rotors (after more hard braking) - no issues with any amount of brake pressure
4. During cooldown, there was a short period when the brakes would cause shaking again. This makes me there’s a certain temp range when this problem occurs.
5. When brakes were warm, I would have to hold the wheel to the right a little to keep driving straight even when I wasn’t using the brakes
6. Letting go of the wheel while braking didn’t feel any different than letting go while cruising… for a car with zero front toe.
7. The most prominent issue is the steering wheel shaking and how loud it gets; the car as a whole shaking and brake pedal feel is negligible.

At the shop, the tech said the rotor surfaces aren’t heating up evenly which is likely causing this issue. It looks like the center section of the friction ring gets hotter, hence the darker bluish hue. Not really sure why this would happen in the first place. So uh, next step would be to change the rotors and pads?



Responding to the posts above:
Front wheel bearings have 110k miles and rears have about 20k. Probably a good idea to go ahead and replace the fronts. They are not making any noise currently; the rears made a distinct sound when they failed.

I don’t have a dial gauge and couldn’t really eyeball any unevenness by evaluating the depth. But the discoloration makes me think there may be some bit of unevenness.

If the Textar pads are good to go then the rotors are probably the culprit. I’ll look into replacing all four at $1k 😭 Will inspect the caliper pistons while the rotors are out.



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[Estoril]

Well - something is going on at that outer circumference of the rotor. If mine, I would pull the rotor off and ensure that it is seated and (RLB) mounted correctly. I'd also loo at the pad face to see if there is something going on at the outer contact patch. Also, measure rotor thickness in 3 places. While doing that, I'd measure the thickness of the rotor at 3 points on a straight line from the edge to the hub. Bum, if possible, a dial indicator for a run-out measurement.

[SQ13]

Well - something is going on at that outer circumference of the rotor. If mine, I would pull the rotor off and ensure that it is seated and (RLB) mounted correctly. I'd also loo at the pad face to see if there is something going on at the outer contact patch. Also, measure rotor thickness in 3 places. While doing that, I'd measure the thickness of the rotor at 3 points on a straight line from the edge to the hub. Bum, if possible, a dial indicator for a run-out measurement.

It’s odd that the outer part of the ring is like that on all four corners. While spinning the rotors, I did not see any wobbling, and I know the body shop didn’t remove the rotors from the hub. I’ll go ahead and inspect all four rotors, calipers, and pads tomorrow. Will take measurements and attempt to find a dial gauge as well.

Trying to decide if I want to order another full set of OEM ZCP rotors at $960, or switch back to slotted (Girodisc fronts and Centric or DBA slotted rears) for $1,500ish. This is my first time running drilled rotors on the M3 and it’s not going well 😂

Edit: The abnormal rotor wear might be due to the pad retaining springs if they’re rusty…