one-rennlist

Hi all

I have a 997 GTS Cab with fairly new Pirelli P Zero N2 tires (DOT 1114 front, DOT 1814 rear), tread at 90%. When I brake hard at low speeds (25mph, ca. 65 degrees F outside temperature, dry weather, warm tires, good road surface) the ABS activates, which is irritating. Braking with my former Z4 on PS2 tires felt like an anchor in comparison (and did not skid or need ABS activation).

Is this normal?

Nicola

Macster

Define "brake hard".

ABS kicks on under hard braking conditions and this can occur during a hard (enough) brake event from nearly any speed. I had a delivery truck make a sudden turn in front of my Boxster the other day as I was leaving the office to make a lunch run and I slammed on the brakes and the ABS kicked in. I was only going 35mph. Tires were "cold" as the car had been sitting a few hours since my arrival at work.

Absent any ABS (or other) warning lights on I'm thinking the behavior is normal.

When was the last time you had the brake (and if a manual and the clutch shares fluid with the brake system the clutch) hydraulic system flushed and bled?

one-rennlist

It's a manual, and had its last inspection & brake fluid change a month ago. Brake hard as in 'stop the car as fast a possible' (the same traffic situation you described). There are no errors in the dashboard.

Alexandrius

Try timing your 25-0 braking ability. With abs engaging I bet you get a very, very good number.

My abs engages and usually I hear the tires chirp slightly at the same time. Abs will engage when it detects you have exceeded the threshold of traction for braking. So maybe try seeing how fast you slow when abs engages, and how fast you slow down while on the verge of abs.

StormRune

I know that the electronic nannies relax a bit when the car is in Sport mode and even more in Sport Plus mode. This is most notable in its intervention level with traction control under acceleration and cornering. Now I find myself wondering if it has the same effect for ABS. I don't know. However, I spend most of my time in Sport mode and can't say that I've observed any level of unreasonable ABS actuation.

I would guess that alignment issues could also lead to this effect. Did you get your alignment checked recently? I'll typically get mine done when I get new tires. If not, it might be worth checking. If yes, maybe they made a mistake in alignment? Most alignment places will recheck their work for free if you feel like pursuing that idea.

Alexandrius

Some abs algorithms also take into account consumer behaviors...such as "panic" braking. If you are attempting to slow the car very suddenly with a sudden dab at the pedal, abs can intervene differently than usual. Rate of application of pressure can affect it. This is in general though, so I'm not sure if this applies to Porsche.

Aftermarket tires and wheels if improper in size can affect abs and traction control.

I'd imagine any modification that affects handling or traction would contribute as well; putting alignment in the mix for sure.

danit

Interesting. I notice this too in the 997.2 S and also have the P Zeros. I can't wait to get rid of them.

one-rennlist

I have the same plan, compared to the PS2 I had/have on BMWs these Pirellis are subjectively inferior. They brake like my (good) Michelin winter tires. What is your point of reference concerning braking performance?

MadScience

I've observed similar behavior on my 997 tt. Front only, on hard-ish, but far from panic breaking. Rapid pedal movement (even if it's not that far) seems to be more likely to provoke it. I've played with it in a parking lot, and I can only get the fronts to go, no matter how hard I brake, which seems odd.

I mentioned it during the last service, and they checked ABS and brake balance, and noted no problems. I've since observed that this may be intermittent, possibly only when I'm not going entirely straight.

My current suspicion are the substantial wheel spacers (15mm front, 18mm rear) that I recently discovered on the (CPO) car. I have not had a chance to pull them yet, but educated myself about what a substantial scrub radius can do to ABS.

Edit: My car has Bridgestone RE050 mounted.

Alexandrius

That is likely because most modern advanced cars allow ABS to control brake bias and balance.

MadScience

I'll follow up after I pull the spacers, might take me a couple of days to find the time.

MadScience

I just pulled the spacers, and will see if this ABS behavior went away when I drive it tomorrow, There's a stop sign in the parking lot by my office that triggers this more often than not.

The spacers were no fun to get off. I had to put the wheels back on after removing the spacer bolts and use them as levers to pull the spacers. While I was generally paranoid about chipping the PCCB rotors, it's not clear that wrapping them in towels like I did was necessary. That said, I don't see myself skipping that step either.

rjshar

I definitely get a pulsation from time to time in my parking garage if I have to heavily break going into a corner because someone comes zooming around the other way not paying attention. I have PCCB as well MadScience, maybe this is a characteristic of them? My car was recently purchased (April 2015) CPO too. I also had a PPI.

one-rennlist

I finally switched to summer tires (PS2) on my 5 series I use as DD and had to emeregency brake going 15mph yesterday. The grip was so high I thought for a moment my car would do a forward flip. Absolutely no ABS involved. Wonderful.

MadScience

OK, my ABS still activates, but it's no longer "irritating".

With the spacers, it resulted in a second or two of screeching between each activation. (I'm pretty sure I owe some pedestrians some clean pants due to this -- I was going 10 mph before hitting the brakes and never in any danger of hitting them, but the ABS made is sound very dramatic. -- this is what convinced me that I needed to fix this.)

Without spacers, it's just a near lockup, that results in less braking, that then cycles. It also seems to take stepping on the brakes a bit harder before it triggers at all.

So, spacers, or wheels that push the tires out further and result in a positive scrub radius, can have a negative effect here.

Additionally: I'm super happy with how the car feels without the spacers. It felt like to got smaller, more nimble, perhaps even lighter, and lost about 20 lbs of unsprung mass. -- Of course the spacers themselves are pretty light, however they move the tire out on a lever, so the impact is magnified quite a bit. Nearly every at-the-limit-behavior that I didn't like is gone now. I'm still not entirely confident throttle steering it in a slide, but I've now done it. Previously I was unwilling to let all the tires let go. -- The awd kicking on and off is still interesting.