Brake bias / ABS / PTV issues after upgrades and running slicks.
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Thread: Brake bias / ABS / PTV issues after upgrades and running slicks.

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    Brake bias / ABS / PTV issues after upgrades and running slicks.

    To make my car more race-worthy, this winter I fitted 996 Turbo/GT3 rear calipers and 350mm GiroDiscs to my car with 991S + 350mm Giro at the front. I've had no issues with regards to brake endurance but I am struggling a bit with the initial braking phase where I have to be (for my feeling) too gentle and slow on the brakes to avoid massive ABS interference. My feeling is that it's more likely the rear wheels that are activating it.

    I'm running Endless ME20 pads both front and rear, which I had before on the stock brakes as well but obviously in this case the rears have gotten quite a bit larger. I used to be able to spike the brake pressure pretty hard and early on the old setup but then I was also on Cup2 tires with stock suspension and now it's Yokohama slicks with JRZ pro coilovers.

    Does anyone know if programming the car to PCCB brakes would make a difference with how it approaches the brake bias? Or perhaps I should go to a less grippy brake pad for the rear axle?

    After my upgrades I also feel that PTV is slowing me down out of corners. I never felt it working before but now with bigger rear brakes and much grippier tires I can feel it intermittently grabbing a rear wheel which almost making the car bounce up and down on a little on corner exit. Which is annoying but it's mostly the braking action that seems to be reducing my corner exit speeds. Turning PSM off does solve that problem, but I think this symptom is somewhat relevant to diagnosing the other issue.

    I feel that I'm losing a bit of time by not being able to hit the brakes as hard as I used to, and having experimented a little I don't *think* it is a shock setting issue. There have been quite a few 981s converted to race cars so I'm thinking someone has come across all this before?
    2015 Cayman GTS
    2008 Audi RS4
    https://www.youtube.com/user/JCviggen1

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    I believe that PCCB rotors are always the same diameter and there is a bias change to support that. It doesn't hurt to try changing the bias. I bet you'll know if it helps with a little aggressive street driving. It might take bias change and different rear pads to get it right.

    Years ago, Fords were known for a huge bias so the front of the vehicle would dive when the brakes applied because marketing found that people thought that meant they had great brakes (when they didn't).

    My Cayman, with Brembo GT 380/345mm rotors and 6/4 piston calipers was left by BGB Motorsports at standard bias due to the rotor size and piston difference. I never heard or felt PSM kick in on track, but I am sure it did early on as I boiled Castrol SRF brake fluid the first two times on the track and darkened slightly the rear red calipers. I added the GT3 rear brake cooling ducts to the LCAs and the extra cooling solved the boil problem. I also have PTV. Last year, because of the Cup LCAs, I went without the GT3 ducts but did not boil the brake fluid, so improvements to my braking technique reduced PSM/PTV intervention. I have since found there is a mount for the Cup ducts and tried it, but it will require some minor sanding as the Tarett Cup-style LCAs are ever so slightly wider than the official Porsche Cup LCAs.

    Did you change the master cylinder? If you did, the extra flow may be helping create the hard pedal 'full on' feeling. If not, going to the GT3 MC may make the feeling worse.

    V6
    Last edited by Voyager6; 05-24-2019 at 07:50 AM.

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    Wow fascinating thread.

    I don't know about braking but I have always felt PTV slowed me down in the Spyder coming out of tight low speed corners. AMP has a turn I think 10 which is a downhill tight kink and I feel like I can't can't accelerate outta there like the other cars can. I have to be a bit patient and then do get on the power. In the OPs post about the PTV is that a similar feeling?

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    Quote Originally Posted by Voyager6 View Post
    Did you change the master cylinder? If you did, the extra flow may be helping create the hard pedal 'full on' feeling. If not, going to the GT3 MC may make the feeling worse.
    Hey V6, no I kept my old mushy stock one. To keep things simple (not change too much at once) and save a bit of money too. Feels not too bad actually, I'm sure the GT3 one would be nicer but it's not very high on the list of things that still need to be improved. The ME20 pads do have a LOT of bite which probably isn't helping.

    Quote Originally Posted by digitalrurouni View Post
    I don't know about braking but I have always felt PTV slowed me down in the Spyder coming out of tight low speed corners. AMP has a turn I think 10 which is a downhill tight kink and I feel like I can't can't accelerate outta there like the other cars can. I have to be a bit patient and then do get on the power. In the OPs post about the PTV is that a similar feeling?
    Hard for me to sense what your car feels like at that point, could also be traction control kicking in I suppose. I feel PTV is doing weird stuff in tight 2nd gear corners mostly. On exit, grabbing one of the rear brakes does successfully get me a tighter line out of there but at the expense of losing speed. I never felt that this was the case when I was running stock brakes and road tires. I'm thinking the system just wasn't designed with this much grip in mind.
    2015 Cayman GTS
    2008 Audi RS4
    https://www.youtube.com/user/JCviggen1

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    I'm not sure it's as much PTV as PSM kicking in to keep the back-end in check. There are multiple levels of intervention by PSM. Only at it's most severe will the light blink on the dash. However, I can tell from experience it definitely intervenes more often than that.

    I've been driving more with PSM off recently, and on corner exit of fast corners where I used to think the back tires were hooking up, I'm now discovering the back-end wants to step out. PSM was keeping that from happening and keeping grip in the rear tires. With PSM off, in that same situation, if the back tires start losing grip, the back-end will come right around and send you on an off-track excursion.

    The car has considerably different handling characteristics with PSM off. You have to be much more attentive to what the back-end is doing, and get on power a little earlier in corners to get that weight transferred to the back end earlier to keep the rear tires from losing grip on corner exit. You also have keep committed to your line with equal or increasing power level on exit. If your line is a little ragged and you start thinking you're going to run out of track, any lift-off at corner exit and you're done. My fastest laps are still with PSM on, because I can throw the car around more and know it's not going to break loose. If I took the same line and throttle/brake combinations with PSM off, I would not be able to hold the car on the track. Eventually, I think that will change as I continue to learn and adapt to the movement of the back-end.

    As far as PTV goes, I've found it to be less intrusive. When my Pagid pads are cold, I can hear PTV chirping the rear pads, and it's more on turn-in: early-to-mid-corner. At corner exit, the wheel is mostly opened up I don't hear PTV chirping.

    digitalrurouni - what OPs post are you referring to?
    Last edited by ledbette; 05-24-2019 at 10:40 AM.
    2014 Boxster S - Racing Yellow, Sport Chrono, PASM, PTV, PDK, 3rd radiator, Cobb Stage 1 tune (Pagid RSL29's & Nitto NT-01's on the track)

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    I do think what I'm feeling is (mostly) PTV as it feels like it's pulling the car to one side. It's not just slowing me down, sometimes I end up leaving too much space on the outside of the turn because the car is working so hard to try and get me to the inside.

    Anyway, on slicks the car is very controllable so I'm likely to drive with everything off most of the time so not my biggest issue. The early ABS activation is a lot harder to figure out...sadly I can't dial down the sensitivity like is possible in a clubsport
    2015 Cayman GTS
    2008 Audi RS4
    https://www.youtube.com/user/JCviggen1

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    That description pretty much nails it. That is what I experience as well. There's no traction control light flashing. So definitely it's not the TC.

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    Option A is to try the PCCB programming. Option B is to try a lower coefficient pad in the rear.

    You added a little more clamping power and 20mm greater lever arm to the front; you kept the same clamping power but added 50mm to the rear. My napkin math says you are more rear biased than stock.

    I'm quite interested in what you determine because your setup is one upgrade option I'm investigating.

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    Quote Originally Posted by imstimpy View Post
    I'm quite interested in what you determine because your setup is one upgrade option I'm investigating.
    Will start with A because, well, it's the easiest thing to toggle and test.

    Looks good to have proper brakes in the rear, that's for sure. The paint on my refurbished calipers didn't hold up even 1 session though.

    Last edited by JCviggen; 05-26-2019 at 09:45 AM.
    2015 Cayman GTS
    2008 Audi RS4
    https://www.youtube.com/user/JCviggen1

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    Quote Originally Posted by imstimpy View Post
    Option A is to try the PCCB programming. Option B is to try a lower coefficient pad in the rear.

    You added a little more clamping power and 20mm greater lever arm to the front; you kept the same clamping power but added 50mm to the rear. My napkin math says you are more rear biased than stock.

    I'm quite interested in what you determine because your setup is one upgrade option I'm investigating.
    For what its worth.. with my stock calipers/rotors, it was suggested to use Pagid Yellow front, Black rear —- which is the “softer” rear pad idea. I’m not a wildly aggressive braker — I’m a 85% and have fun kind of a track guy — but this set up seems to provide a good balance/bias on braking..

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    I run the Brembo GT-S 380mm 6 piston front with GT-S 345mm 4 piston rears and Brembo RE10 pads all around with a GT3 master. Brakes are fantastic at the track. Initial bite is great. I don't run any nannies except ABS.

    I'm not familiar with your pads but something like a Pagid Yellow works great, although my last set squealed like crazy on the street.. I bet you can work this out with some pad swapping.

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    Totally agree with Ledbette on the PSM. Car feels more pure with it off and comes out of corners a little stronger. Conversely with PSM on you can toss the car around much more, carry more speed into corners, and know that the car will stick when the suspension finally catches up to whatever maneuver you executing. Very different animals at the limits of the car. Mine has been set up to run with PSM on over the years, and would take some adjustments to give the back end a little more grip in order to run with it off all of the time. I could not carry the same speed into the SSs at COTA with PSM off. The car became unhappy trying to finish turn 3, hit the brakes, and then roll into turn 4, all at about the same time. PSM is one smart cookie.

    Just my 2 cents, but the GT3 master cylinder did not do much for my car. Go figure? Mechanic said that all the complex electronics in the car compensated for it. Feels about the same in my case with PCCB calipers and 350 Giro discs.

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    Re: Brake bias / ABS / PTV issues after upgrades and running slicks.

    Quote Originally Posted by lovetoturn View Post
    Conversely with PSM on you can toss the car around much more, carry more speed into corners, and know that the car will stick when the suspension finally catches up to whatever maneuver you executing. Very different animals at the limits of the car. Mine has been set up to run with PSM on over the years, and would take some adjustments to give the back end a little more grip in order to run with it off all of the time. I could not carry the same speed into the SSs at COTA with PSM off. The car became unhappy trying to finish turn 3, hit the brakes, and then roll into turn 4, all at about the same time. PSM is one smart cookie.
    I used to have a similar way experience when using Michelin Cup 2 tires. However on the slicks now it feels like the PSM doesn't really know what it's doing. It's cutting in hard and early and makes you literally fight the car as it resists doing what you'd like it to do. Very annoying and very slow, too. I can see why for the GT4cs Porsche specifically mentioned the stability management and ABS had been re-calibrated for slick tires. They are too far outside the window of the stock software it feels like.

    On Friday I ran the old tires I used for a single day in mid-may and the grip levels were horrendous. But it gave me some time to get used to the car moving around at higher speeds and gain a little confidence which came in handy the next day when we put on different tires with slightly larger fronts which meant PSM refused to work anyway.

    Slicks are ****** awesome when they're new. But they go downhill quick and my lap time on the old tires was actually no better than I had done on this track with Cup 2.

    Here's one low grip lap followed by one where we had stuck new rears on:



    I also drove my buddy's car who has a big rear wing and a different suspension setup (KW competition vs my JRZ pro) and his car was so much more composed and efficient under braking. He did have the stock rear calipers and smaller discs than me.
    2015 Cayman GTS
    2008 Audi RS4
    https://www.youtube.com/user/JCviggen1

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