Vibration when braking - finding the cause

[9designs]

Been around the houses with this one.

Drive around town or local trips, plenty of braking. No issue

Drive couple of miles down a dual carriageway brake at the end, I get vibration (front) and feel like a warped disk, ie thrum - thrum - thrum slows with speed.
Discs are next to new, runout around 0.07mm on drivers side.
Bushes in suspension all replaced.
Drivers side wheel gets hot, disc has slight blue tint. Passenger side doesn't get hot, looks normal colour.
Brake caliper doesn't appear stuck, piston pushes in and out. I've not replaced the seals and stripped down as yet to check.

Possible cause? I have an idea but asking first. (Don't want to miss Zedfest !!! )

 

[Jembo]

Had this twice on my 3.0
- first time was a sticking calliper
- 2nd time was some eBay Mintex pads which I think were were copies, as after then buying some OEM pads problem went away

Also suggest you check this out as can’t recommend the upgrade route more https://z4-forum.com/forum/viewtopic.php?t=8399

 

[9designs]

It has as new Borg and Beck pads and discs

 

[Jembo]

Do you often keep you foot on the brake when stationary rather than use the handbrake?

What procedure did you follow to bed them in?

 

[9designs]

They were already fitted when I got the car..... It's an Auto, so yes foot is more often kept on than say a manual. It has the 325mm discs being a 3.0Si.

Thing is hooning around an industrial estate use the brakes a lot, but not going far no issues.
Drive a couple of miles without braking at say 60-70mph at the end when braking will have vibrations.

My thought was either sticky piston so the pad is dragging a little on the disc on one side, but does work. It's not stuck I tried with the pads out and move the piston in and out.
Or
Wheel bearing is starting to go and generating a heat build up, that cause the brake imbalance and issue.

 

[smorris_12]

Manky guide pins can cause a caliper to partially stick as well as the piston.

 

[craiginuk]

I had this on my daily 330d. Try and get to a friendly mot station and get them to put it on the brake tester. They can check for pulsations on the brakes. In my case that was the problem and new discs and pads sorted it. Was fine at low speeds but anything 70 or over it pulsated on braking. Also checked out ok with a dial gauge (both within tolerance) but one side was worse than other., Same side (worse) pulsated on the brake tester.

 

[plenty]

Almost certainly the caliper given the wheel is hot.

 

[craiginuk]

I must have missed that. Is caliper getting hot?

 

[9designs]

One side wheel and calliper gets hot, the other warm.
You can brake hard with short distances without issues.
It's driving without braking that then will cause the issue to happen at the end.

 

[9designs]

Right, with the aid of the Mrs pumping the peddle, the near side grabs and releases the disc. Driver side doesn’t back off much, so drags. With pads out pushed in and out 10-12 mm, it’s not stuck, but seems not free enough. Seal kit ordered.
Couldn’t seals on Eurocar or GSF, only postal suppliers.

 

[Z4Mariner]

Good job of sleuthing, 9designs ! I once had a similar problem on my old 325xi. If the square-cut piston seal gets wonky, it will not retract the piston fully, the caliper then slowly builds heat, thermal expansion applies more pressure, and then pulsation, pulling, etc. And because the piston is not "frozen", it's harder to diagnose.

 

[buzyg]

Right, with the aid of the Mrs pumping the peddle, the near side grabs and releases the disc. Driver side doesn’t back off much, so drags. With pads out pushed in and out 10-12 mm, it’s not stuck, but seems not free enough. Seal kit ordered.
Couldn’t seals on Eurocar or GSF, only postal suppliers.

Sounds like you have it covered.