Floating Rotor Argument

[CCM911]

We are currently engaged in our 2-Wheel Tuesday Porsche Discussion and we are having an argument regarding the true definition of "Floating Rotors". My three friends claim that any rotor that has a separate disc and hat is considered floating. I contend that a "floating rotor" is one that moves side to side, when used with a fixed caliper.

Who is correct?

[Bill Verburg]

Originally Posted by CCM911

We are currently engaged in our 2-Wheel Tuesday Porsche Discussion and we are having an argument regarding the true definition of "Floating Rotors". My three friends claim that any rotor that has a separate disc and hat is considered floating. I contend that a "floating rotor" is one that moves side to side, when used with a fixed caliper.

Who is correct?

Neither, though the second is closer. There can be some axial movement but the design goal is radial freedom of movement from expansion

a 993 tt front rotor is a typical example of a non-floating yet composite rotor

the rotor on the right is a 993tt right front rotor, it consists of a steel hat hard bolted to the cast iron rotor annulus, there is no movement between the 2 parts


A floating rotor allows radial movement between the hat and annulus, there several different styles, this is a Giro disk 993tt front replacement that floats and uses Mclaren syle anti-rattle bobbins





this is a 930 floating rotor, the lack of anti rattle springs is one of the issues w/ these older styles


these are 930 floating hats and t-locks, the t-locks are fitted into the rotor dogs and allow radial expansion in this case some axial movement too but most newer styles are spring laded to prevent axial play

[dshepp806]

Man,..that's wild,..good stuff, Bill. Good read!

BEST!

Doyle

[RWebb]

how much does a rotor expand during hard use?

do PCCBs have expansion issues?

[CCM911]

You Rock Bill Verburg!!!! Excellent tour!

[Bill Verburg]

Originally Posted by RWebb

how much does a rotor expand during hard use?

do PCCBs have expansion issues?

yes, the bigger the rotor the more issues
PCCB

[KTL]

Here's an example of a "fixed" two piece floating rotor that is not bolted together. E36 BMW M3 can be equipped with factory European Motorsport floating rotors, although currently no longer offered by BMW.



BMW also has a floater for the E39 M5, again a two piece non-bolted rotor for the European market

[KTL]

Here's a better picture of how the rotors are pinned together

[Bill Verburg]

Originally Posted by KTL

Here's an example of a "fixed" two piece floating rotor that is not bolted together. E36 BMW M3 can be equipped with factory European Motorsport floating rotors, although currently no longer offered by BMW.



BMW also has a floater for the E39 M5, again a two piece non-bolted rotor for the European market

Those are floaters, they are no more fixed than the 930s or Girodisks or PCCBs, the annulus floats radially on the pins. Fixed means there is a solid connection that has no play what so ever. Look at the 993tt rotors, the hat and rotor holes are circular and are a light friction fit for the bolts that hold the assembly together, the Girodisks and the like have oval holes, the 930s have the dog slots, the BMW pins serve the same purpose, they allow radial movement to accommodate thermal expansion.


here is a disassembled 993tt front rotor & hat, note the round holes in both, there is no float here

[wayner]

Most modern sport bike rotors are said to be floating but don't actually float. They are more looks than floating function because the fasteners are set so tight.

(most buyers don't the the rattle of floating disks on a bike..or dry clutches for that matter, although they like the idea).

[Steve@Rennsport]

If I may add something to the discussion,....

One advantage to floating rotors is the ability to float laterally, keeping the rotor centered properly between the pads and the ability to absorb knockback, something one can feel in your foot.

The main caveat to floating rotors is the need to maintain them. They must be periodically disassembled and all the drive dogs, slots and hardware cleaned of brake dust to keep them "floating". Failure to do so can result in some rotor runout issues which creates the same feeling as warped rotors.

[KTL]

By "fixed" (emphasis on the quotation marks here) I meant that they are permanently assembled. You can't take them apart and service them, as the floater pins are cast into the aluminum hub/annulus.

Yes the floating occurs on the rotor end of the pin. I suspect there must be a very precise casting & machining process to get those radial pins in the hub to respect the receiving holes in the rotor?

[RWebb]

more info about "knockback" - what is the definition or description?

[KTL]

Pad knockback is the rotor wobbling back and forth, which bumps the pads back and therefore bumps the pistons back into their bores a slight amount. You then go to press the brakes and you get a longer pedal travel.

In extreme circumstances you can have no brakes. At track speeds it can be really scary if you drive across a rumble-stripped curbing. The extreme shuddering you get by hitting the rumble strip shakes the rotors and really affects the brakes in a big way. I always check my brakes with the left foot after hitting one of those strips. Often times I have to pump up the pedal at least one time.

With a knockback caused by rotors that are constantly wobbling, it can make for brakes that seem like they need a new master cylinder because you can never get a high pedal after driving it.

[wayner]

Fry a slice of baloney and watch it curl. The same thing happens to thin, non floating sport bike rotors while hot.

[Captain Ahab Jr]

Originally Posted by Steve@Rennsport

If I may add something to the discussion,....

One advantage to floating rotors is the ability to float laterally, keeping the rotor centered properly between the pads and the ability to reduce knockback, something one can feel in your foot.

Totally agree, the less delay in the pads clamping the disc the later the braking can be left. Radially expansion benefits have less of a performance effect than pad knock back.

[Bill Verburg]

Originally Posted by Captain Ahab Jr

Totally agree, the less delay in the pads clamping the disc the later the braking can be left. Radially expansion benefits have less of a performance effect than pad knock back.

pad knock back is not a big issue w/ 911s, in 40yrs of 911 ownership and countless track days I've never experienced it, cracked rotors is a major headache for those that track the cars extensively.

[Deadeye]

I experience pad knock back at the track in my 911. I'm running boxster calipers on stock carrera rotors and I think floating rotors would reduce the effect. Does anyone make a stock carrera size floating rotor?

[Bill Verburg]

Originally Posted by Deadeye

I experience pad knock back at the track in my 911. I'm running boxster calipers on stock carrera rotors and I think floating rotors would reduce the effect. Does anyone make a stock carrera size floating rotor?

That's a potential issue when you use a caliper adapter, anything that reduces the stiffness of the components or increases the lever arm will exacerbate that sort of thing, for stock setups it's not an issue

[Steve@Rennsport]

Originally Posted by Deadeye

I experience pad knock back at the track in my 911. I'm running boxster calipers on stock carrera rotors and I think floating rotors would reduce the effect. Does anyone make a stock carrera size floating rotor?

As Bill said, thats a caliper adapter design/material issue. I've never seen a floating rotor made in those modest sizes (282x24mm) so I'd review the adapter design and find out what they are made of.