[johnung]

Has anyone measured their brake pad thickness at the time that their front or rear brake pad sensor has triggered? (I'm not talking about the computer screen estimate for how many miles left until pad replacement is necessary)

I'm looking for the thickness of the pad material remaining and also the thickness of the steel backer plate. For example, my Hawk 5.0 front pads came out of the box with a thickness of 16.9mm. Of that, 12.7mm was new pad material and 4.2mm was steel backer plate.

Since inner and outer pads can potentially wear at different rates, it would be great to obtain the measurements for the pad that triggered the sensor. There's a sensor on the F30 front left and rear right wheel.

My Bentley manual lists several values for when the sensor should trigger and what the wear limit should be. Often there's also confusion in wording as to whether the backing plate is being included in measurements, or not. So I'm hoping to obtain some responses with real world numbers when sensors had actually triggered. Thanks in advance!

[Billfitz]

I've only had one trigger, the rear inside, it was down to about 3mm. That was about half what the outer pad was. The thickness of the backer plate shouldn't matter, it doesn't affect the distance between the sensor and the rotor.

[visionaut]

Isn’t the min thickness defined physically by the protrusion on the sensor? (and why the different brake types need different sensors).

It wears down until the circuit detection triggers. So measure the protrusion on a your brake pad wear sensors (f & r)….?

[Billfitz]

My advice is don't trust the sensors. There are eight pads, but only two sensors, so that's six pads that could totally fail without tripping a sensor. Regular visual inspection of all the pads is a must.

[johnung]

Originally Posted by Billfitz

My advice is don't trust the sensors. There are eight pads, but only two sensors, so that's six pads that could totally fail without tripping a sensor. Regular visual inspection of all the pads is a must.

I agree not to fully trust the sensors. Bill's example of inner and outer pads wearing differently proves that. They were obviously designed by BMW to just be ballparks. My curiosity just wants to know at what thickness the sensors are set to trigger.

My Bentley manual, which usually has identical information to Newtis, is a bit confusing. See attached photos. It says that the minimum pad thickness is 2.4mm.

For context, brand new Hawk 5.0 pads not including the 4.2mm thick steel backing plate, measure Front (Brembo 340/370mm brakes) 12.7mm and Rear (Brembo 345mm brakes) 11.6mm. Minus a 2.4mm wear limit, that means that total wearable pad material equals:
Front 10.3mm
Rear 9.2mm

BMW actually sells Tool #341260 to measure the remaining brake pad material with the wheels still on the car. I've never used or seen the tool but the diagram seems to show that it would measure the combined backing plate and remaining pad material. See photo. I can't tell if the tool somehow manages provide a value with the thickness of the backing plate deleted. Or if it also allows for the minimum 2.4mm pad thickness. Maybe someone who has used this tool can chime in on these questions.

An odd thing written in the manual (see photo) is a claim that the brake pad sensor is two stages that is triggered first with 6mm remaining and again with only 4mm remaining. Everything I've read and heard has said that the sensor is single stage that must be replaced after it is triggered. This post is trying to find out the thickness that triggers the sensor.

Note that it's not unusual to find an error In Bentley manual or in Newtis. These shop manuals are written by third parties, not by BMW. So maybe the incorrect paragraph was cut and pasted from say the E-Series manual. That would make sense if the E-Series has two stage brake sensors. I've also seen the shop manuals not be updated for LCI changes.

[visionaut]

FWIW, my 4-series Bentley manual only shows the ‘BMW pad-thickness tool’ in the section on single-piston calipers. It’s not shown in the Brembo brakes section.

Those single-piston brakes use a very different pad wear sensor design than the Brembos. (more like a stick who’s end wears down, maybe that’s the 2-stage?)

In the Brembo section there’s no discussion of checking/measuring pad thickness.

In the section on checking/replacing Brake Rotors, it lists the “wear warning from residual lining thickness” occurs at 3.7mm/0.15”, for F & R.

[johnung]

Originally Posted by visionaut

FWIW, my 4-series Bentley manual only shows the 'BMW pad-thickness tool' in the section on single-piston calipers. It's not shown in the Brembo brakes section.

Those single-piston brakes use a very different pad wear sensor design than the Brembos. (more like a stick who's end wears down, maybe that's the 2-stage?)

In the Brembo section there's no discussion of checking/measuring pad thickness.

In the section on checking/replacing Brake Rotors, it lists the "wear warning from residual lining thickness" occurs at 3.7mm/0.15", for F & R.

Cool, thanks! My Bentley manual is the earlier 335i manual that only goes up through my 2015 335ix and doesn't include the 435i.

So that means that the sensor triggers at about 3.7mm, leaving about 1.3mm of pad material before reaching the wear limit.

Plenty of pad especially for those of us who remember the days when there were no sensors. The iron rotor grinding on the steel pad backing plate was your indicator that new brakes were necessary!

[visionaut]

1-stage brake pad wear sensor: When the conductor path is worn through, approx. 5% of the remaining brake pad thickness is reached. This corresponds to a remaining distance of approx. 2500 kilometres.

So for the Brembos it’s a “~5% pad thickness remaining” thing…

[johnung]

Originally Posted by visionaut

1-stage brake pad wear sensor: When the conductor path is worn through, approx. 5% of the remaining brake pad thickness is reached. This corresponds to a remaining distance of approx. 2500 kilometres.

So for the Brembos it's a "~5% pad thickness remaining" thing…

Does your 435i manual say that the wear limit is 2.4mm for both the front and rear pads, like my previous manual says? Please see the previous photos of manual pages that I posted.

Because if the wear sensor on the front Brembos triggers at 5% of the total original pad material, that only leaves 0.64mm of material, far less than 2.4mm

[visionaut]

It mentions that 2.4mm “Brake Pad Lining Minimum Thickness” only in the single-piston caliper section. Nothing in the Brembo section.

Brembo-related there’s just that 3.7mm wear warning lining thickness in the rotors section I cited.

Looking at the way the Brembo sensor fits/works (centered directly on the pad backing plate with retention springs), like I said initially - it’s that protrusion on the sensor that wears down on the rotor to break the circuit wire inside.

Eyeballing the pic, the protrusion looks more than 2mm tall (poking inward off the backing plate). The internal sensor wire is likely more than .64mm below the surface. There’s instructions on replacing just a bad sensor (without changing pads) that tells you to grind the new sensor down to match the bad one being replaced…

Got a spare/old un-triggered sensor you can cut up?

[Markyboyt]

Some did use a 2 stage sensor, I can’t recall what models I feel like it was E60 possibly. I think the F30 is back to a single stage but a quick search for a picture of the sensor by part number will show if it’s only got a small dimple or a longer leg onto the pad.

I’ve used the bmw brake pad tool, it takes into account the thickness of the backing plate but whether that’s still accurate on today’s pads I’m not sure.
It sounds random but I’ve always used hex Allen keys, where possible putting between the lip of the disk and the pad backing. Gives a pretty accurate indication of how much material is left before metal on metal contact.

[johnung]

Originally Posted by visionaut

It mentions that 2.4mm "Brake Pad Lining Minimum Thickness" only in the single-piston caliper section. Nothing in the Brembo section.

Brembo-related there's just that 3.7mm wear warning lining thickness in the rotors section I cited.

Looking at the way the Brembo sensor fits/works (centered directly on the pad backing plate with retention springs), like I said initially - it's that protrusion on the sensor that wears down on the rotor to break the circuit wire inside.

Eyeballing the pic, the protrusion looks more than 2mm tall (poking inward off the backing plate). The internal sensor wire is likely more than .64mm below the surface. There's instructions on replacing just a bad sensor (without changing pads) that tells you to grind the new sensor down to match the bad one being replaced…

Got a spare/old un-triggered sensor you can cut up?

I just checked realoem for my car, a 2015 335ix. It's showing the same rear brake sensor (34356792292) regardless if the car came with the Single piston Bosch caliper over 330x20 rotors, or the Brembo 2-piston calipers over 345x24 rotors. My car came stock with 330x20 rears and I later upgraded to rear 345x24, but the pad sensor was reused.

Maybe the other single piston Front 330mm, or 312mm, or the rear 300mm calipers use a different pad sensor but it appears that the rear 330x20 brakes use the same sensor as the Brembo 345mm.

[visionaut]

2-stage sensor pic:

[johnung]

Originally Posted by Markyboyt

Some did use a 2 stage sensor, I can’t recall what models I feel like it was E60 possibly. I think the F30 is back to a single stage but a quick search for a picture of the sensor by part number will show if it’s only got a small dimple or a longer leg onto the pad.

I’ve used the bmw brake pad tool, it takes into account the thickness of the backing plate but whether that’s still accurate on today’s pads I’m not sure.
It sounds random but I’ve always used hex Allen keys, where possible putting between the lip of the disk and the pad backing. Gives a pretty accurate indication of how much material is left before metal on metal contact.

Allen wrenches is a great idea!

[dave14x28]

my rear brake wear sensor experience on my 2014 X1 (which appears to have brakes in common with some 3 & 4 series) is over here:

https://e84.xbimmers.com/forums/show....php?t=1955076.

The warning light came up and I ignored it for a little while, but only hundreds of miles not thousands. Yet you can see the wear difference from driver's side to the passenger side where the sensor is.

Definitely do not understand the wear behavior...

[Billfitz]

That wear is probably related to insufficient lubrication of the pad to carrier and caliper contact points. It results in the pads not retracting fully, so they drag against the rotor rather than floating. My original pads were shot at 11k miles due to inadequate lubrication from the factory. The pads were practically rusted in place. I had to take a Dremel tool to the carriers to get the rust off.

[dave14x28]

Thanks Bill.

Going to have to keep an eye on the wear this time. I thought I did a careful job cleaning and re-lubing those contact points the first time I did them 50k mi ago. Interesting that the TIS instructions warn against "mechanically cleaning" those points, saying to use solvents only otherwise "the coating could be damaged". Personally, it wasn't obvious that there's a coating present, but maybe it was gone by then anyway. I cheated a little and used a nylon brush and then applied the requisite "thin coat" of the BMW lube that came with the factory pads.

Also, what's the consensus on the caliper guide pins? It appears BMW does not indicate to lube them, otherwise the rubber bushing could degrade. Is there a rubber-safe lube that has been found to work well for these bushings?

[Billfitz]

I had to mechanically clean my pad carriers as they were rusted. I've lubed my guide pins with brake grease going back 40 years or so, never had a problem yet.

[cfm56d7b]

Originally Posted by Billfitz

My advice is don't trust the sensors. There are eight pads, but only two sensors, so that's six pads that could totally fail without tripping a sensor. Regular visual inspection of all the pads is a must.

100% accurate. Periodic inspection is highly recommended.

[dave14x28]

Originally Posted by Billfitz

I had to mechanically clean my pad carriers as they were rusted. I've lubed my guide pins with brake grease going back 40 years or so, never had a problem yet.

I did a little reading (amazing what one can learn online, some of it is even right!), and found that the elastomers that are good for brake fluid (EPDM, nitrile) aren't so good with petroleum-based fluids (hence the apocalyptic warnings about not getting brake pad grease on the caliper boot).

I discovered several special "brake system" lubes that are silicone-based (AGS Sil-Glyde) or plant-based (Castrol red brake grease) or fully synthetic (Permatex makes one) and are made to play nice with the elastomers commonly found in brakes (i.e. piston seals, boots, & guide bushings). Assuming this is the kind of stuff you've been using without issues.

So I think next time I'm doing brakes (which may be sooner rather than later) I'll be sure to have some of that stuff handy. Thanks!