[AMASIAN]

I have doubts.

two piece rotors are going to be pricey to replace for the average person.

Nissan/Infiniti made a similar update and now those brakes are plagued with fade/overheat issues. You need a certain amount of metal there...

[ZStig]

Can't wait until a retro fit brake kit comes out. Good job BMW. Sure they're increasing overall in weight, but if they weren't doing the weight reductions in the engine, suspension, and brakes as we're seeing, the weight gain would be even MORE. Over time I think newer BMW models will start to be lighter than their predecessors, as manufacturing costs of light weight items go down over time

[hks786]

Originally Posted by montreal red

they should just shrink the car down a little. Its ridiculous how big cars like the 3 series have grown over the yrs.

+1

Look at the size of the new A4

[Ghost170]

would luv that brake set up on my car!

[ENINTY]

Originally Posted by bayerischeperformance

cost on rotors would not be much more expensive if at all..from the looks of it instead of replacing the disc and hat like you do with tradition rotors you could replace just the disc itself and reuse the hat.

With the legal system in the US I doubt they'd let you reuse the rotor hat. And if they did it would only be after the hats were magnafluxed to check for cracking.

[blender]

CERAMIC option, please...

[AMASIAN]

Originally Posted by rclem05

CERAMIC option, please...

Lmao.

[abcommercial]

For further reading on unsprung weight and how it affects performance:

http://en.wikipedia.org/wiki/Unsprung_weight

[tarheel91]

Originally Posted by DDS2015

I wonder what size wheels they're going to have to put over these things because they look huge. Not that I'm complaining.

19+, I believe. That means more weight (not just unsprung, but spinning weight). Wheels spin, and that angular acceleration takes energy (1/2m*r^2*angular velocity) which means less energy for linear acceleration. Hopefully, the hollow wheel design will cancel out the added weight. Unfortunately, larger wheels means smaller sidewalls, which will hurt grip.

[Finnegan]

Originally Posted by Artmasterx

For some data:
Thermal conductivities are about (W/m-K):
Cast iron: 52
1% Carbon steel: 45
308 stainless steel: 16
Aluminum: 206

Heat capacities by mass and volume are about (kJ/kg-K // kJ/L-K):
Cast iron: 0.47 // 3.5
1% Carbon steel: 0.47 // 3.7
308 stainless steel: 0.46 // 3.6
Aluminum: 0.90 // 2.4
(Aluminum is about 35% the density of steel)

Aluminum is about 4x better at conducting energy than iron or carbon steel, but with the aluminum center they have also reduced the heat transfer area (from outer ring to hub) and probably introduced some interface resistance where the hub bolts on. You have maybe 1/2 the heat transfer area of a solid steel cap, so you better off in the heat transfer by 2x.

For heat capacity, it looks like about 1/2 the volume of the hub is gone, and on a volume basis aluminum can't hold as much heat as iron. Compared to a solid iron hub, you are probably 3x worse from a heat capacity perspective (in the hub; the friction surface will still hold a lot of heat since it is heavy and iron/steel). But then the new aluminum, swiss cheese hub is probably ~25% the weight of a solid steel one.

As mentioned the hub may be more efficient at removing heat since you have more area in contact with the air, but in reality, it is probable that most heat is dissipated through the venting in between the friction surfaces since those are the hottest areas and probably have the fastest air flow.

Regardless, I am sure BMW will have thoroughly tested these to make sure they are appropriate for their application (keep fuel pump comments to yourself).

Of course, you will probably now be paying $400 for a new rotor instead of $150.

Me, mention HPFPs? Never!

As for $400/rotor, I just replaced the front rotors on my Z4M. Not that far away from that (a little over half that each aftermarket). 400 each? These would have to save a hell of a lot of gas to offset the maintenance costs.

From a performance perspective I'm all for reducing unsprung weight. Wheels and tires are a good place to look as well (but lightweight and strong wheels are pricey too). But for all the unsprung weight reduction we could see how about not porking up the cars each generation? At 200-300 lbs. increase per generation unsprung weight isn't the real enemy--overall weight gain is a bigger issue (pun intended) as is the QE2 size most cars have "achieved" these days.

[royalblack888]

THATS BMW PORN!!!

[datbimmerdoe]

awesome!!!