Bill Verburg

It's just not physically possible to put larger caliper pistons in place of smaller ones and then to have the travel reduced.

You will get more braking from less effort

you will get longer pedal travel

this of course is when then systems are nominal, if you corrected some other underlying issue you may have a harder pedal or perhaps you have tt rears which would provide a higher harder pedal

The only way to tell the difference between Rs and tt rears is to look at the pistons(or the pad spyders)

top tt w/ 28/28, bottom Rs w/ 30/36


pad spyders, which are marked w/ piston sizes

left big red 36/44, right RS 30/36


left big red 36/44, center RS 30/36, right 993 30/34

Cupcar

The longer effective radius of the TT/RS discs = +4% front and + 9% rear offsets the - 2.7% change in master/slave ratio so the brakes feel about the same or may actually have less travel and pressure for a given stop G than stock C2 given the same mu pads.

certz

You will get more braking from less effort

Maybe that is the key, but the pedal is definitely firmer with the TT/RS setup - btw I do have the RS rears, ordered them from Germany (or I got really hosed ).

I think you are in California which is unfortunate because I would love for you to come drive my cars. Then we could discuss why they feel the way they do.

Also, I cannot remember who brought this thread up again, but you would be welcome to stop by if you are in the Chicago area or visiting and see what you think.

Bill Verburg

Bob, you know better than that, I understand what you are saying and agree that for the same brake torque the actual effort is somewhat reduced by the larger rotor, but pedal travel to get that torque is not affected at all except that there will be less structural deformation in the system and thus a very tiny amount less pedal travel but compared to the gross motion due to hydraulics it is an extremely minor contribution. The much larger reduction is effort and thus increased pedal travel is by far more significant.

The best way to measure the differences in pedal travel are in max g stops, where max line pressures and structural deformation will be exhibited. In these situations line pressures, structural stresses and seal deformation will be maximized and roughly similar for both setups but he hydraulic ratio differences will not be.

the hydraulics and the linkage and the hysteresis w/ in the system from seals and material deformation are the sole sources of pedal motion, the slave/master ratio being by far the greater contributor to pedal travel

yes the rotors have different radii but that does not affect the pedal feel except to the extant that you get slightly more brake torque for the same leg effort because of the additional leverage. it does not materially affect pedal travel

The pedal only moves until the pads are in fully deformed contact w/ the rotors, by fully deformed we mean there is no more slack in the system from seal/pad/ structural deformation, the amount of pedal movement from deformation is extremely minor compared to the gross movement differentials due to caliper piston size differences(hydraulics)

Cupcar

Yes you are right I should know better on the lever motion not changing with disc diameter, I didn't think as I typed. I doubt that one can really feel the difference in travel stock C2 to RS brakes with the stock C2 master cylinder it should be only on the order of 3%.

However I think effort does go to brake feel with lower effort giving the feeling of stopping power - just like power brakes give the feel the brakes are "more powerful" when they really are not - they just take less pressure for a given G of deceleration.

If you look at the brakes on an Audi racing car for example, the discs are as large a diameter as possible to generate as much brake torque through disc diameter as possible and keep system pressures as low as possible to minimize deflections of system components and improve brake feel. Pedal foot force can be controlled with pedal lever length.



One thing though, I have seen brake systems improve in feel with stiffer calipers. I some years ago installed a set of ATE 911 2.5 ST aluminum rear calipers once and realized immediately why Porsche gave up on them - they were mushy and the pedal came back to normal when I put the original cast iron calipers back on.

Some brake calipers go to great limits in their design to promote stiffness and feel along with low mass such as below in an AP Racing "Radi-Cal" caliper.

Bill Verburg

Agreed, stock to RS w/ no m/c change will give a bit less than 3%(~2.7%) more pedal travel.

Agreed stiffer everything from calipers to hardware to mounts improves feel because it lowers hysteresis in the system.

Porsche specifies larger bolts in the built up calipers than Brembo uses in their own house versions of the same calipers for that very reason

TrackDays247.com

On another note, try the Pagid Yellows at pad changeout if you want further improvement - over the Oranges.

Cupcar

I am embarrassed to report a double brain fart in the statement I made earlier: "The longer effective radius of the TT/RS discs = +4% front and + 9% rear offsets the - 2.7% change in master/slave ratio so the brakes feel about the same or may actually have less travel and pressure for a given stop G than stock C2 given the same mu pads."

Bill already pointed out the fact the effective radius of the disc as theoretically nothing to do with pedal travel, but I should have said +2.7% in the master/slave ratio - so putting it all together, there is approximately a 5% decrease in pedal pressure required for a given deceleration with only about a 3% increase in pedal travel so RS brakes with a stock C2 master cylinder would probably feel "more powerful" because the pedal force for a given deceleration G is less than stock C2 although the pedal travel is a small amount more.