930 Floating Rotors and 4 Piston Calipers - What's the reasoning?

[porche]

Sitting here, snowed in one more weekend in the NE. I have 930itis BAD!

The great thing is I’m allowed some reading time and I just finished up a great tread; Big red vs 930 brake trade. Kudos to the posters and especially to b5aar for the engineering piece from Stoptech that put a lot of interesting issues relating to pads and rotors straight with me for the first time.

After reading about all the extremely small changes that can make pad to rotor relationship critical to proper braking it has piqued my curiosity on floating rotors. Can it be that maintaining parallelism of pad to rotor may be the reason behind their engineering? Are floating rotors better because they help maintain parallelism between the pad and rotor whereas fixed hat systems might tend to create an angularity between the pad and rotor under conditions such as trail braking? Is part of the intent 4 piston calipers play to also enhance parallelism while providing a more even clamping force?

We know Porsche introduced floating rotors on the early 3.3 930’s along with the 4 piston calipers and I know they are the application of choice on many race cars but can someone explain the reasoning behind their installation? Did Porsche strive to finally get them installed on the production 930’s for homologation purposes to improve their race program? What are the advantages/disadvantages?

Bud
.

[IMR-Merlin]

You in Nebraska or NE New England....

The reason for my original post was regarding value aka dollar amount of the trade.

As for your question.

Floating rotors will "give" a little and stay "True" for a while. As the rotor heats and deforms, the "float" takes up the tolerance and you won't feel it in the peddle. Meaning the rotor, once deformed will still contact the "hat" in a stable manner.

at least that is what I remember learning.

P.S. I missed a TON to technical terms...

[porche]

Actually I was meaning the North East - the whole area is a icebox!

Yes, "the float takes up the tolerance and you won't feel it in the peddle", makes sense.

[rlane930]

Bud – one advantage for floating rotors is reduced brake fade. In the Porsche 930 Turbo 3.3 liter in the December 1977 issue of Christophorus, Porsche Engineer Heinz Dorsch, head of engine testing at that time described the new braking system and he mentions the joint between the hub and disc:

"Acceleration and top speed increase with engine performance. Thus the demands on Turbo 3.3 brakes are likewise increased. Experience gained from countless races was applied to development of the new brake system. Dissipation of heat produced during braking as quickly as possible is determinant for fade-free operation. To achieve optimum interior ventilation the cast-iron brake discs front and rear are now thicker to induce sufficiently large vent passages. These passages are evolvent in shape, similar to the blades of a pump impeller. Another measure for improved ventilation is perforation of these discs. This also improves response in the wet. When developing disc brakes particular attention must be paid to the joint between hub and disc because strong temperature variations at this point bring the danger of crack formation. In the front wheel brakes this was avoided by connecting hub and disc by finger interlock with accommodates expansion differences. Fixed aluminum calipers are descended from the 917 race car and have cooing ribs for improved heat dissipation.”

[IMR-Merlin]

Also the floating rotors on bikes these days are full free float so even it they warp a little. They won't back the pads off.

[IMR-Merlin]

And I'm buried up here in NH

[911nut]

According to Brembo, the primary advantage of floating rotors is to allow a centering movement of the rotor due to distortion caused by uneven thermal gradients in the rotor due to extreme use. A secondary benefit is the use of an aluminum hat, reducing unsprung weight.
This makes sense to me. If a rotor goes out of true by as little as .003 inch, it can be felt by pedal pulsation. I wish floating rotors came from the factory on my car.

[Tippy]

There was a huge debate on Rennlist that rotors do not warp, rather have pad deposits creating the pedal pulsation/shimmy.

I thought floating rotors were simply for heat dissipation?

[911nut]

Originally Posted by Tippy

There was a huge debate on Rennlist that rotors do not warp, rather have pad deposits creating the pedal pulsation/shimmy.

I thought floating rotors were simply for heat dissipation?

If that were the case, a one-piece rotor would be much better at heat dissipation than a two piece. A lot less conduction will occur at the hat-rotor joint on the two piece.

[porche]

Originally Posted by 911nut

According to Brembo, the primary advantage of floating rotors is to allow a centering movement of the rotor due to distortion caused by uneven thermal gradients in the rotor due to extreme use. A secondary benefit is the use of an aluminum hat, reducing unsprung weight.
This makes sense to me. If a rotor goes out of true by as little as .003 inch, it can be felt by pedal pulsation. I wish floating rotors came from the factory on my car.

Exactly. This is what got me going with my original thoughts after reading the Stop Tech article -Warped- Brake Disc and Other Myths, the ability of the pad to remain in total contact with the rotor as the suspension jacks around.

Originally Posted by Tippy

There was a huge debate on Rennlist that rotors do not warp, rather have pad deposits creating the pedal pulsation/shimmy.

I thought floating rotors were simply for heat dissipation?

Yes, another floating rotor advantage, smooth rotation through these deposits.

Originally Posted by 911nut

If that were the case, a one-piece rotor would be much better at heat dissipation than a two piece. A lot less conduction will occur at the hat-rotor joint on the two piece.

But by being separated isn't the heat retained and exited through the rotor rather than transitioning to the spindle/hub assembly? Maybe it's not that much, but it could be an advantage to bearing life in endurance racing?

[Dave 86 930 Fl]

I changed the fronts over to floating rotors on my 930 track car many years ago, working flawlessly and much cheaper than Porsche drilled rotors, they wear better too. Purchased mine from Coleman.

[Iciclehead]

I don't have a turbo, but I do have the brakes and did a back to back comparison when I changed over.

They are a lot more powerful and I certainly have more braking power than the M's they replaced, but there are two things that I noticed.

First is the noise. They do clatter a bit a low speed, it is annoying and I put a dab of high temperature silicone at the dogs which quietened them up a bit. I would love to find a noiseless floating set up, but according to what I read, all the floaters make some noise.

Second is the buildup. If I drive nice and civilized, they do build up, they do vibrate enough to notice and fear warping and the bloody things reject some much heat you need to really, really work them to get them hot enough to wipe off the deposits. Think six or eight 0-100-0 cycles at full acceleration and braking.

They can fade but it takes a lot of track effort to make them fade...way more than my M's

Dennis

[911tracker85]

I picked up a set of 930 calipers for my project. was told they came off a 79 with floating rotors/hats.

although I plan to build to track level, not sure I will track it. just finishing a 951 for that.

still debating on solid or floating rotors. have time as it will be a while before I have to make that decision.

[Miguel Antonett]

In a nut shell: that was the best brakes arrangement back then, and it is still one the best nowadays.