Clunk when first applying the brakes

[MexicanFiestaST]

In my sig: 2020 GT, PP1 with staggered set up, Brembo front brakes, AE, currently sitting at 56k miles

For a long time now, I hear a clunk when first applying the brakes. This is most noticeable at low speeds, when there is less competing noise, such as when moving the car in the drive way, or shuffling your way through the drive through window. I was convinced that a bolt(s) was loose somewhere, and searched in vain through the following suspects:

• All the bolts between caliper and the frame(anchor bolts, sliding pins, suspension arms, etc)
• sway bar links
• Subframe connectors
• All suspension arms and related bolts
• Wheels(when attached the car)
• The hub rings in the wheels are still there(not missing), and there is no rotational vibration whatsoever to suggest anything is no longer round, or become uncentered.
• Hubs and rotors(looking for loose bearings, even though I do not hear typical bearing noise)

It seems to me that the noise is present when the pads first hit the rotors, and usually, but not always, when the pads are completely released from the rotors. If I apply, then modulate the brake pedal, the clunk happens at first application, whatever clearance there was has been taken up, and no further clunk happens despite modulations in pedal pressure. To reproduce the clunk, you must completely release the brakes and generate clearance somewhere.

I used to be absolutely convinced by the nature and timbre and feel through my body of the clunk that some large subframe bolt had worked loose(or fallen out), so that was the first place I looked when the clunk first appeared, but no dice. Then I thought maybe the calipers were moving in an unusual manner(bearing in mind the rears float on the pins, while the front Brembos are not supposed to), but nothing unusual there, either.

This makes me think it's merely the sound of the pads smacking the rotors, but I consider myself to be a pretty smooth operator(especially when I have a passenger who does NOT want to be impressed, like my wife, lol). BTW, she's also an engineer, yet we disagree which end of the car the noise is coming from. The passenger hears a different location than the driver.

My last attempt was to take a sledge hammer, and gently tap various bolts and joints, including parts of the subframe and brake systems, anything that I think could move to make the clunk, and found nothing but rock solid connections everywhere I tried. Yes, I'm particularly careful not to create the problem I'm trying to solve; I'm just using the mass of the sledge with low effort to try and detect something loose. I am a mechanical engineer with lots of experience, including NVH experience, which comes with the paranoia about systems not working like I think they should, and a mind full of the usual suspects, based on the characteristics of the noise. This makes it worse, lol.

For now, I've merely adopted an inspection routine every time the car is up(oil change, tire rotation, etc), and will keep looking. I'm slightly paranoid something is loose, so I want to catch it, and not ignore it. But so far, nothing has fallen off the car, nothing I could find is loose, and the severity of the clunk has not changed.

I've never owned a vehicle that does this.

Functionally, there is nothing wrong with the brakes. I love the brakes, and the instant feedback, and the immediate response, and how freaking solid they feel, and how powerful they are. I inspect the pad thickness every time it's on the hoist, and I'm surprised there is plenty of life left. I have complete confidence in the brakes. It just bugs the crap out of me that I can't find the source of the clunk.

Apologies if this was posted before, and solution found. If so, just point me to the link and shake your head, lol

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[GTP]

If you agree that the sound is a "knock" or "clack", then it is the front pads backing plates knocking against the caliper end slots.

There is a little slop between these slots and backing plate span. When you back out of the driveway, the pads shift and knock against the lower slots. Then when you start to drive forward, they shift upward and knock against the upper slots.

You can verify this with a friend. Listen while standing outside your car as they drive to and fro.

If you service your pads someday, you can shift them in the caliper by hand and you will hear the same character sound.

EDIT: You could also probably put brake grease in the slots. That might reduce or eliminate the sound. I prefer to run my brakes without any grease anywhere.

 

[sk47]

Hello; already some good suggestions. If the car has stock brakes setup then this is likely not the issue. Some time ago I knew of such a problem after a fellow changed the brakes on an older Mustang. A 1966 I think. He had changed the rear drums to disk. The mustang was for sale so I went to see it.
Sure enough when brakes first applied there was a loud thunk. Reapplying brakes quickly no noise . I made an offer which he turned down not being sure how bad the problem might be. I had a notion about the issue.
My guess was when he installed the support to which the calipers were attached that he had not used shims to center the caliper over the disc. That it was too far to one side by a few millimeters. This could allow the piston(s) to retract a bit much on one side. When the brakes were applied that extra distance allowed for the slam noise as the piston took up the slack in a hurry.
Anyway, he was asking too dear for a troubled car that might be expensive to fix if my guess was wrong. Before i left I told him of my guess about the issue. I cannot say if i guessed correctly. He never got back to me, but his add was soon removed. I do not know if someone else bought it or he was able to fix it. He did not really want to sell but was frustrated.
Good luck.

 

[boostmark]

I just had a problem like this on my Lincoln MKS. Here is how I found the problem. In the driveway, have someone look at the front wheels from the side and go slow and hit the breaks. Look for movement from front to back. I had a bad lower ball joint that did not make noise until you hit the breaks. Its a start anyway.

 

[ORRadtech]

I just had a problem like this on my Lincoln MKS. Here is how I found the problem. In the driveway, have someone look at the front wheels from the side and go slow and hit the breaks. Look for movement from front to back. I had a bad lower ball joint that did not make noise until you hit the breaks. Its a start anyway.

Yeah, your probably not going to find a suspension fault with it on the ground. You either need to get lucky by having someone see something move. Or get the front wheels off the ground and shake them hard top to bottom and side to side. Then get a long prybar and check each ball joint/bushing for even the tiniest bit of movement.

 

[StangTime]

My car does this but only seems to on the first drive when it's cold. After backing out of the garage I have a about a 200' drive through my condo area where I need to come to a stop before merging onto the road. This first stop presents me with a very audible clunk. After that, never again during the same day that I am aware of. I've had the wheels off a half dozen times, one of which was to replace the automatic rim dusters that Ford installed, and I've never seen anything that would be a concern. I'm going to say it's just the nature of the design.

 

[txgt]

Is this what you are hearing? He calls it a “popping sound” but it‘s more of a “clunk” to me.

A few days after my last track day (Oct 8, 2023) I noticed the same sound with my 2018 GT. She’s got 40k miles and 17 closed road course track days (14 track days on the Performance Pack Front Control Arm Kit that I installed in April 2022)

I tried the “rock the wheels and check for play” in the horizontal and vertical directions but didn’t notice movement on either front wheel. Like you, I also inspected, pushed, pulled, checked bolts etc. to no avail.

However, after hearing the exact same noise in that video, I did a more focused check on the control arms. Having the car up on four double-locking 3-ton jack stands, I confidently gave the control arms an aggressive shake and finally found some play in the driver side frontward lower control arm.

I now know that I’ve got a bad ball joint in that control arm and am now trying to figure out if that is serviceable by just replacing the ball joint or if I need to order a replacement arm.

Hope this is somewhat helpful.

 

[MexicanFiestaST]

@boostmark , @ORRadtech and @txgt , you were all correct! Today, when I was installing my winter tires, I gave the discs another tug(wheels off), and found one of the lower ball joints at the front of the car had play. I found it on the driver's side; of the two joints up front, it was the rear one.

Although I was looking for this movement all along, it seems that I didn't use the right "body English" on the discs to discover the play. Once I figured out how to move the wheel to make the joint move, I tried the passenger side, which didn't budge. Rear wheels were solid, both sides.

I came back here to post the update, and it seems I had missed the video from @txgt ; that was super useful to corroborate what I had found.

I guess I will order the kit on the video that @txgt posted and install it this winter. Many thanks!

 

[MexicanFiestaST]

Update: the braking clunk is gone, car sounds and feels like new!

At first, I bought just the LH lateral link, since it had a lot of play in the ball joint. Since my last post, I discovered that the RH tension link also had play, but it seemed at the time to be a lot less worse than the LH lateral link. I also knew that buying parts individually was going to be more expensive than the kit, but at the time, I was not convinced the M-3075-F kit had the correct parts.

After changing the LH lateral link, the clunk was slightly less loud, but still present. So I bit the bullet, ordered the kit, and then installed just the RH tension link. I had already decided that I had the time and resources to change one part at a time, and my retired-engineer-OCD habits insisted that I do this. Your OCD mileage may vary...

Changing the RH tension link cured the problem entirely. When I pulled the old one off, it was clear that there was a LOT of wear in the ball joint, which was not apparent as I reefed on the suspension to figure out which links had the worn ball joints in the first place.

On the way home, the car now seems as solid as it was when I drove it off the lot, even over our famous Michigan roads.

While I was at it, I was able to confirm the kit does have the correct parts, with matching part numbers to my original suspension, with the exception of revision to the PN stamped onto the forgings(from my engineering days, this might be a date code and not a PN). The paper Ford PN tags do match.

So now I have an extra LH tension and lateral links, and an extra RH lateral link. I'm obviously going to hang onto these, because expect the remaining original links will wear out before I want them to. I tend to drive my vehicles well over 100k miles...

Pro-tip(well, I guess it's an amateur tip): using a jack to raise the suspension to ride height will NOT raise it to ride height without also lifting that corner of the car off the lift, and even then, that corner will not be at ride height(it was about 2" shy when I decided it was a bad idea to keeping raising it further off the lift). I suspected this would happen, but OCD-me just had to find out. Fortunately, the place I use also has a drive on lift. So, after finger tightening the large bolt(making sure the threads were engaged enough the part would not fall off), I took the car off the 2-post, and drove it onto the other lift and was able to torque the new bolts to spec with the car exactly at ride height. Doing this for the tension link required me to remove several fasteners from the under tray so the torque wrench had room to swing, and even then, I had to drive the car onto the lift so that the inside edge of the tire was just on the inside edge of the ramp. The guys at this place(My Mechanics Place, Livonia for you guys here in SE MI) love it when I bring in my Mustang, so they were willing to guide me onto the ramp with my odd instructions.

Thanks again to @boostmark @ORRadtech and @txgt for pointing me in the right direction.

 

[boostmark]

Glad you found the problem. We have some major pot holes here in Chicago, These can destroy A ball joint and cause it to fail way too soon.

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