mark kibort

i think the best bet would be to use the 345mm rotor ring.. that way, you just put on a 7mm spacer on the radial mount, and have the extra torque, which isnt the biggest deal here, its the increased thermal mass to fight fade if you are in that range. if not, yes, the extra torque would give an easier feel to the pedal for equal slowing pressure

Bill Verburg

brake torque is proportional to effective rotor diameter amoung other factors
nominal rotor diameter/effective rotor diameter
964RS 322/268
993RS 322/259.6
993 304/251
964 298/248

the area of the pad has no influence on brake torque, it's only effect is distribute mechanical and thermal stress over a larger area, basically increasing mean time between failures

Modern rotor design is to use a relatively narrow friction annulus supported by a light aluminum hat. The resulting assembly is relatively light compared to the older designs like that used on 964/993

quacktoduck

very true.

consider the Brembo brakes in the latest offering from BMW the pad annulus is huge to compensate brake life for an average BMW must meet certain lifespans. The size of the pad is increased for extended life not for increases in friction applied.

think of it as this way imagine pads were to fill just the piston bore diameter. it will apply just the amount of pressure of the bores. no more as friction is constant in our mathematical model.

Im not an engineer but ive worked in the braking industry. you will only increase clamping force by increasing pistons but it is a balance for how much force is applied by a master cylinder as a bore too big will affect pedal feel and modulation. however you can assist the braking by increaing torque by spacing out the rotor as it works as a lever effect as a breaker bar or a long torque wrench.

cgfen

I have some aftermarket slotted front rotors that have ~ 2k miles on them.
mellow street miles, i never ended up doing a track day after install.
"big red" sized.
Don't remember the manufacturer.
If interested, PM me and I'll post up a few pics and a price.

NYC993

Interested pm on the way

mark kibort

Bill, the larger diameter rotor shifts the average radus outward.
the larger the pad, the more some of that force is shared by the inner diameter, but the outer radius does most of the work. (and sheds more of the heat). the same diameter rotor GTS/ 964T ... both 322mm rotors, but the GTS has a wider pad, so it will have more friction for the same pedal force (all other things equal) a smaller proportion of force will achieved by the inner radius of the larger pad. this would then allow for less pedal force for the same slowing power and force. (and yes, the power is dissipated over a larger pad, so it lasts longer and so does the rotor)

so yes, pad width will influence braking torque because of the additional friction the larger area would have. but, this additional force is not worth it due to the ability of the smaller pad doing the majority of the work anyway, with more effective cooling toward the outer radius as well.

mark kibort

actually, that is not true. think of some pad grabbing the rotor , at the width of the piston bore, from the outer edge down to the wheel bearings. if you had say 300ft-lbs of force total, what % would come from the big , all rotor covering pad, at what points at the rotor?
it would be progressive, from the point closest to the axis, to the outer radiius.
the speeds would be progressive and proportional to diameter in the same way
the power dissipated.. would equal that force x rotor surface speed at that point.
the total force would be the additive sum of all points along the radius.

(all for a given pedal pressure to achieve 300lbf of force.)

Now, while its true that the life is extended vs a shorter pad, it stands to reason that the bigger pad can generate more force on the rotor. not as efficiently as the smaller pad set up, so thats why racing brakes have shorter pads.

Bill Verburg

mark, this is the last reply
yes, if that was the only change, but it's not. The effective radius also depends on the center of the pistons which is usuall aligned w/ the center of the pads.

on 993RS big red 322 this is 252mm effective diameter w/ a 75mm wide pad
on 964RS big black 322 this is 268mm effective diameter w/ a 71mm wide pad

the 964RS effective diameter is larger and so w/ the same line pressure, pad material and piston sizes it will generate more brake torque

If you don't believe me, please go check the Porsche literature. It's written in black and white in the spec books.

RacingBrake

Bill is correct in brake torque arm which is the distance from the axis of hub to the center of caliper piston - the longer the better mechanical advantage.

Brake force = Hydro pressure x COF (Coefficient of Friction between pad and rotor)

Effective brake torque = Brake force x brake torque arm (166.8mm)



The brake pad (in green color) turns the pedal pressure (psi) into force, so a larger pad will generate the same amount of force with less pedal effort (less pressure required) due to larger area:

P=F/A

P= Hydro pressure
F= Brake Force
A= Friction (lining) area

It's always advantageous if you can run a larger pad because it helps to lower the hydro pressure --> lower brake temperature resulting extended life of brake system/components, preferably with compatible set up (larger calipers and rotors etc., not just a bigger pad)

Meatball964

Just to update this thread: went with OEM rotors all around. Still the most cost effective choice for me.
Lots of promising lighter/longer wearing/racy looking products soon to be in the pipeline.
I'll revisit them when these are toast.

trophy

Looks nice and tidy under there....

You should stop playing with your nuts, they are getting red

Meatball964

Gold !