Ventilated Brake Disc Thickness: How Thick?

Hey fellow mechanics,
Unfortunately, I have to replace the front brake pads on my friend's 1990 Citroen XM, which has a diesel engine and an 80kW output. The vehicle has internally ventilated brake discs.
Does anyone know how thick the windshields are when they are new, and at what minimum thickness (wear limit) new windshields must be used?
It seems like they left about 1mm of material on each side.

Regards,

Eike.

Does anyone know how thick the discs are when they are new, and from what minimum thickness (wear limit) new ones have to be used?

Hi!

Unfortunately, there is no 'standard value'; it always depends on the manufacturer's specifications... }

'1mm per page doesn't sound too bad to me...'

In my opinion, it's much more important that the worn surface doesn't have too many (or too deep) grooves. If you put new pads on there, you might need 1,000-2,000 km of break-in time, with reduced braking performance. Otherwise, the disc might develop uneven wear and brake 'unevenly' within 3 days due to partial pad glazing and subsequent uneven heating.

Regards,
Michl.

@Michl!

The surface of new brake pads has a softer initial friction layer that quickly adapts to the rotor.

@Eike!
Depending on the manufacturer, there is a clearly defined wear limit for brake discs, which should be taken into consideration. I don't know what this limit is for your specific vehicle.

Best regards,
Uwe

Hi,

For the Xantia, the internally ventilated windows have a new thickness of 22mm, and those with a thickness of 20mm are considered due for replacement.
For my C4, the new brake disc thickness is 24 mm, and the wear limit is 22 mm.

Olli.

@Michl!

The surface of new brake pads has a softer initial friction layer that quickly adapts to the rotor.


@Uwe: Ah, so... that's why I can always press the surface of new coverings with my fingernail...

Sorry, just kidding, but...

'This is definitely not something you find with all brands and pad types, and even less so with standard pads. I've only read about it so far in relation to performance pads (like Ferodo and others).'
2. Why do renowned manufacturers like BMW fundamentally always replace both pads and rotors?
3. Why is a break-in period of up to 500 km required even for NEW brake discs and pads (meaning they have absolutely no scratches)?
4. Why have I, on several occasions with different cars, experienced a clearly noticeable impact in the brake disc after approximately 1500 km with new pads on a grooved disc, resulting from exactly a single emergency braking maneuver from a higher speed, when that impact was not present before ???

Regards,
Michl.

PS: That's exactly why, since then, whenever I replace the brake pads, I also replace the rotor (which is unfortunately almost always ), and since then I haven't had any more problems with disc warping.

At BMW, it's not always automatically the case that both parts are replaced.

I've never changed windshields unless they were below or above the minimum acceptable thickness. With three sentences of text, the adhesive always ends up being damaged. I drive a bit faster, and I've often had to brake hard because RV drivers seem to be fundamentally blind.

There are absolutely no issues with warped discs or anything like that...

'And at least with BMW, the rumor about windshield replacement seems to be just that – a rumor. The reality in (independent) workshops is that for all brands, when cars come in after the warranty period, the windshields are just as old as the vehicle, but the brake pads are already on their (at least) second set. Many windshields have the manufacturer's date stamped on them, which makes it very obvious.'
Drum brakes are apparently not opened in the contracted workshop, either.

CU Gremlin.

At BMW, it is not always automatically necessary to replace both parts.

@ Kenny: I can only repeat what a friend of mine, who is a workshop foreman at a large BMW company, told me:
He said they had put new pads on the old rotors, but because of too many customer complaints like 'I got new brakes and after 1 week they're already bad again...!' (rotor damage!), they started replacing the rotors as well. This is because the rotors are usually so worn out after one set of pads that you can't put another set on them.

But maybe things are completely different in our Austria.

@ Gremlin:

1.: It depends on how deep the grooves are... I'm talking about something like 0.3-0.5 mm .

2.: I didn't say that if the brake pads eventually wear down and then lie completely flat against the disc, the brake will no longer be 'fully effective' afterwards!

3.: It really depends on the car; in some cases, the brakes are simply undersized. Additionally, sometimes you find very hard 'long-life' brake pads (like the ones on my Seat ), which make the whole situation even worse...

I can only speak from my own experiences, which doesn't mean that others don't have different experiences...

Okay, I think I'm going to exit this thread now, because Eike actually just wanted to know the minimum thickness for his brake discs...

Maybe someone still knows.

Regards,
Michl.

'Do you need new brake discs every time you replace the pads? That seems like a rip-off! I mean, there are certainly cars, like Porsches, where it's necessary due to certain factors. But for a regular BMW, I wouldn't necessarily think it's required. One disc lasts me for about 2-3 pad changes!'

Hi everyone,
This morning I measured 20.9mm. According to CITROEN, there are two versions: 26/24mm (new/limit) and 22/20. Since mine still look quite good, I probably have the latter ones. And on this old car, they should probably last a while longer.
Thank you for all the answers, nonetheless.
Regards,

Eike.

Hi!
'With the Golf IV, a maximum wear of 2 mm is allowed, which is only 1 mm on each side (I personally think that's a bit too little). I replaced my brake discs and pads after 90,000 km, and the discs had worn down by about 2 mm. However, they had no grooves, and I think they could have easily lasted for another set of pads.'
Usually, a very thin ridge remains on the edges of the tiles at the outer edge, which is not covered by the facing material. You can often see the wear and tear on this ridge.

Best regards,

Hmm....

Regarding the BMW disc replacement: At the dealership where I work, brake discs are generally only replaced if the customer reports issues like steering wheel vibration during braking or similar problems. Otherwise, they are only replaced if the minimum thickness for new brake pads has been exceeded. While there is a minimum thickness specification for the discs, it doesn't matter if the discs need to be replaced if the brake pads are still in good condition.

There was a time, during the introduction of the E46, when aluminum control arms were installed that had the exact same shape as the steel control arms. However, aluminum has completely different properties.
This steering wheel damper caused excessive steering wheel vibration.
This was particularly noticeable when braking.
The only solution until new handlebars with a different shape were installed was to replace both brake discs and pads, regardless of how well they were still aligned. Perhaps that's why there was talk of a complete replacement.

Check out DuW.de for your disc; they sometimes include wear specifications.

For my A4B5, the new brake pads are 25mm thick, and the worn ones are 23mm thick.

Best regards,