jdistefa

Read posts 13 & 14 again.

jj1

This is throwing people off. Brake bias has nothing to do with dive. Rear brake bias could unsettle the rear though. But even if you had no front brakes your car would dive. Dive is controlled by geometry, spring rate and the shock I believe.

badabing

bringing up an old thread...

during this past off season, I replaced the master cylinder in my 987.2 CS with a GT3 MC and a PCCB brake booster.

All season the car has been extremely unsettled under hard braking. A few times I thought I was going to spin on the front straights at WGI or Summit Pt Main.

I was running RS29 on both axles before and after. The car did not do this at all before. I also tried DS1.11 and had the same issue.

Other changes made at the same time are GT3 front sway bar and monoball links, 2 piece LCA in front with monoball ball joints and rubber inners.

suspension is otherwise stock, non pasm

Alignment is approx -2.0 camber front and rear, caster and toe are stock.

I have received suggestions about using a less aggressive pad in the rear to compensate. Looking for some additional opinions on this as well as which pagid pad to use on the rear with RS29 on front.

Thanks

RickBetterley

I'm using RS29 all around and like them a lot.
I'm not a super hard braker, though, if that matters.
My shop likes the Pagid black (14s as I recall) with my front big brake kit.

Frank 993 C4S

Do you have the ability to look at your brake pressure traces to rule out operator error? How even are your pads wearing?

If you had a severe brake bias issue, you would probably be experiencing ice mode. You can only vary that around so much running a stock ABS.

badabing

I'm using Harry's Lap Timer at the moment for data. I don't think it has brake pressure traces.

My brother experiences the same thing and instructors who have ridden with me have observed it and don't seem to think I'm doing anything wrong.

I'm always commended for smooth inputs, especially on the pedals.

It didn't do this before when the only track mod was the Pagid yellow pads.

987part

Try pagid black rs 14 in front and yellow in back or even stock pad in back.

Frank 993 C4S

Understood - it wasn't clear to me in your original post. If the pads are not wearing even, you'd have a stuck caliper. I really don't think you have s brake bias issue as I used to run the same car with RS29s front and RS19s in the rear which has a more aggressive rear bias (which YOU don't want if your rear is unsettled under braking).

I don't know enough about your tires, ride height or suspension geometry so can't comment on that.

Sent you PM

Bill Lehman

OP, Were you running at Lightning yesterday and was the issue braking for turn 1?

A432

Agree with above to check/rebuild the calipers

altonj

Lots of interesting advice re pad selection and bias. These cars can sometime benefit from a more aggressive rear pad. The OP added additional front brake torque, a more aggressive rear pad would make some sense. If you using stock calipers/rotors, personally I like my Caymans with a more aggressive rear pad. I run the opposite of what Seb suggests. I run RS29 in the front and RS14 in the rear... Same on my Boxster S.

987part

Ive run all setup posted above

You want something that wont give ice pedal and a strong pedal for the complete race(45mins) in our case.

Myself, i want to avoid too much bracking in the back to kill the rear calipers. PSM is a stupid ****.

If your hard on brake, deep braking before turning, lot of trailbraking. Lot of things comes into play.

Jabs1542

Can you describe 'extremely unsettled' a little more? Weight shifting too far forward to start turn in with a very light rear is the exact opposite of rears grabbing too hard and making the rear end want to come around. Both are 'extremely unsettled'.

Since your pads seem ok, and you made changes to both the MC and the booster it would seem that your new brake system could be overloading your calipers or, at worse, your suspension. Remember this is all a balanced system and making one end mo-better may make the other end worse.

Brian C in Az

More rear bias will aggrevate the problem because the rear has less weight due to the front dive and weight transfer. You want to fix the excessive dive issue. You would want more rear bias or bite only if you are consistently locking up a front tire on hard braking in a straight line.

Higher spring rate in the front will help (but you need to revalve the dampers everytime you change spring rates). Higher spring rates in the rear will exasperate the problem.

Too much dive and and unsettles rear will not be corrected by increasing the rear bias. Increasing the rear bias will make the car more unsettled.

Something that I see missing from both jimk and badabing is what other mods have you done to your cars.
Did you add a splitter up front?
Did to add a rear wing?
Side skirts?
Canards/ dive planes?

Those aero changes will affect the initial dive and grip. Too much front aero and not enough rear aero will cause the car to be unsettled in the above described scenarios.

What dampers are you running? Is rebound adjustable?

More information is needed other than saying, " my rear is unsettled in high speed braking and turns"

badabing

Thanks for the PM. Will do. To answer your questions

Street tires. Same as always. OEM N rated ContiSportContact 3. 235/40R18 & 265/40R18

Stock suspension and ride height. Just GT3 front sway bar and 2 piece LCA.

Also as stated above, MC is GT3 and booster is PCCB. Also now running Girodisc front and rear but issue was there before.

I guess the real question is did the GT3 MC (while keeping calipers stock) modify the bias to the point that it's creating this issue?