Question? Brake size difference

[hurtwj]

Do anyone know what is the difference in the size of the rotors for the 840 Brembo's rotors and the rotors for 1992 850i. I am doing a visual upgrade to the Brembo's. I have heard you have to switch out the rotors as well? I do see that the part number is different but the size 324x30 is the same on real OEM?

Thanks

[mr1173]

850 rotor has larger "working surface".

850 rotor:
850 rotor.jpg

840 rotor:
840 rotor.jpg

[hurtwj]

Is that the same for the BREMBO vs non? Thank you

[mr1173]

Short answer: yes. (for US market)

Long answer: It is a bit unclear.
According to RealOEM (ETK) some early EU 840s may have brembo calipers 34111161177/8 or ATEs 34111160325/6.
http://www.realoem.com/bmw/enUS/part...BMW-840i&mg=34
but have only one rotor option 34116757746 (324X30) - never seen that one.

[hurtwj]

Thanks, I am pulling the Brembo's off a 95 840ci. (My parts car)

[Hyper]

850 rotor has larger "working surface"...

I can't comment specifically about stock 840 v 850, but I can comment of this basic concept that might be in play here, based on my observations while developing the E60 rotor for both the Brembo and ATE.
The concept is that the single pot ATE pad is taller (def: stretching out along the radial, not thickness) than the Brembo pad, and incidentally barely fits between the perimeter and the hat, on a rotor designed for a shorter E60 4-pot.

Meanwhile, my Brembo on the same rotor is busy cutting an outer swept area, leaving an inner circle of unused iron.
Logically, the Brembo pad is shorter still than the E60 pad.


The basic concept...
On a rotor optimized for a shorter pad, the designer has the choice to leave out this inner material, making for a lighter rotor.
And eliminates the results seen above.

The same concept in reverse...
IF the 840 rotor is optimized for the for Brembo (shorter along the radial),
then it would be wrong to mix an 840 rotor with the ATE (taller along the radial).
IF this is the situation and nothing else, you could run the Brembo on the 850, and get the result you see above.

(The "else" could in theory involve hat diameter, and I've heard rumors about a mounting screw location, a non-issue if you are keeping an existing.)

[hurtwj]

Thanks!!