sillydriver

A recent thread on this board included a heated discussion of whether or not AMG has installed the cross-drilled brake rotors on one side of every car backwards. The argument was that the spiral pattern of holes (and hence the spiral pattern of ribs inside the rotors) was oriented differently on each side of the car, so that the spiral ribs would act as a centrifugal blower on only one side, while the brakes on the other side overheat.

When I looked, this was indeed true of the rotors on my SL55: the spiral holes were oriented in opposite directions. And I saw it was also true of the cross-drilled brakes on my S500.

Clearly this is strange, because while the argument about centrifugal cooling is a good one, it’s also hard to believe that the engineers at AMG and Mercedes Benz are all sloppy or stupid. So I had a talk about this with my car guru, a good friend of mine who collects high-performance cars and is a connoisseur of all things mechanical.

His opinion was that there was no blower effect on either side and the different orientations of the rotors were equally good. He reminded me that centrifugal blowers have their air intakes at the center, while the centers of brake rotors are blocked by the hubs on which they are mounted.

But what about the holes drilled into the rotors? He replied that there isn’t enough air flow through those for a blower effect and added it is a common misconception that the holes in cross-drilled rotors are there to create air flow to cool the brakes. In fact, they are there to prevent brake fade in a different way. After repeated hard braking, the pads and rotors get so hot that resin that binds together the material in the brake pads begins to boil away. This creates a cushion of hot gas between pad and rotor - essentially an air bearing - which reduces friction and causes brake fade. The cross drilled holes are there to vent that gas from under the pads, maintaining friction. Air flow through the holes is secondary.

But what about the spiral ribs, I asked. He replied that they are not there to pump air, but to separate the halves of the rotor while stiffening the whole assembly. The rotor would be weaker if the two halves were separated by radial ribs, since lines of flexion going through the center of the disk would not cross a rib. When you have spiral ribs, any line across the disk does cross a rib, hence that is a stronger configuration. The ribs aren’t meant to be blower vanes at all.

I can’t guarantee that his views are true, but it does explain the mystery.

jco-amg

Thanks for the intelligent explanation...it is quite believable...I have had a hard time believing that AMG would have made such a blunder and felt that there was more to the story here...as I mentioned before...if you look at the AMG F1 Safety car brakes the pattern is different...looks like even more holes.

E55 KEV

Good Points. I always heard the holes are there to emit gases and dissipate the heat. The holes allow the internal heat to come out through the holes or escape instead of building up inside the rotor.

So maybe it does have a 'cooling' air effect but the holes do not bring or pull cool air inward but allow hot air to come out.

Instead of blowing air in the holes only allows hot air to escape. Hence the holes do not have to be in any particular pattern because they are not vanes just holes.

jswedberg

I'm pretty sure my '97 993 had the holes spiralling the "wrong way", as well.

I have always thought the holes were purely for wet weather and pad-dust (and resin gas) release, not necessarily for cooling.

matoy

Sillydriver

I am sorry but your friend (car guru) doesn't know that MB rotors
ARE NOT BLOCKED by hubs at the center, there are even air
intake scoops to increase air flow to the center of rotors!

THIS IS WHY MB AND AMG ENGINEERS ARE IN BIG TROUBLES
TRYING TO EXPLAIN THEIR BRAKE DESIGN.

regards, KM

matoy

[QUOTE]Originally posted by jswedberg
[B]I'm pretty sure my '97 993 had the holes spiralling the "wrong way", as well.

Porsche 993, 550 Maranello and some other sports and racing cars has non-directional rotors as showed on pic. It really doesn't matter if the holes are spiralling the wrong way.

sillydriver

I’ve got to admit your argument seems compelling. I wonder whether anyone from Mercedes perusing this board could come up with the company’s view on this controversy.