HenryPcar

I was at an empty parking lot with very spacious unrestricted area and went testing the brake after changing pads and rotors. By turning the ABS off, the front always lock up first. Is there a way to adjust brake bias between the front and rear ? I'm sure there is a way with highly modified track car brakes, but for stock brakes, are the bias adjustable ? My car is a 997.2 CS2

Presto

HenryPcar

Turning it off via PSM :
Here's Porsche's explanation of PSM off :
The “PSM Off” mode is activated by pressing and holding the PSM button. This is in keeping with the Porsche philosophy of allowing drivers to deactivate the control system entirely when desired. However, in “PSM Off” and “PSM Sport” mode, powerful braking in the ABS control range re-activates the full stabilising effect of the PSM system, but only until the brake is released again.

Notice that ABS is reactivated again ONLY after the brake pedal is released again after turning the PSM off.

nwGTS

You're not reading that correctly. ABS is ALWAYS on. Once you brake hard enough to engage ABS (the ABS control range) PSM intervenes again. That's what that is saying.
Brake bias can only be controlled in our cars by juxtaposing different pad/rotor compounds.

HenryPcar

thanks for the clarification. This warrants another outing to test the PSM/ABS combo and its default control.

Petza914

You should have felt the pedal pulsing during the test, which meant the ABS was still active. I guess you could pull the fuse but not sure why you'd want yo do that. ABS will stop the car faster and safer than you can if stopping hard enough yo engage it, and still let you maintain steerability, vs. skidding through or into something.

Bruce In Philly

Changing the bias may be dangerous.... probably not for aggressive street driving, but for high speed, definitely.

When you apply brakes, the weight of the car shifts forward and more pressure is applied downward on the front tires. This increases the friction between the tire and the road (aka tire loading) with more friction on the front tires and less on the rear. Another word for friction is grip. So, if under really hard braking from speed, the front is loaded and rear is unloaded, and you increase the bias towards the rear, you run the risk of locking up or skidding your rear tires. Even with ABS, this is dangerous and the rear end will come around. That is why all passenger cars sold bias the front beyond what is needed... for safety.

Of course, you can improve your stopping distances under normal or aggressive street driving by removing some of this "margin of error" by shifting the bias rearward..... but you are taking a big risk should you be driving really fast and road conditions not be perfect. Real race cars allow the driver or pit crew to adjust bias to get it "just right" for tire, track, and other conditions. Ideally, you want both front and rear to kick in ABS at the exact same time, maybe just a bit sooner on the front, therefore optimizing braking capability.

I assume you know all of this and hence your desire to take out the safety margin and improve braking.... what scares me is these cars already grip so much that to fully tune the bias, you have to be doing some speed and hard experimentation to learn where the margin ends and danger begins. You may try contacting a real speed shop that race preps and runs their own cars... (not a shiny muffler shop).

I rented a big truck one time that had a bias dial...... when empty, I swung the back end around and at low speed..... I had a quick lesson on how to set bias.

Peace
Bruce in Philly

kisik

You can only control braking "bias" by installing a more aggressive pad compound in a rear for TRACK use only. If stock 330mm rotors are used even with GT2 brake cooling ducts we are trying to "help" cooling the fronts by engaging rears more. For instance, I place PFC 08 in front and 11s in a rear. The benefits in 997s are not as pronounced as with older 996s and especially '6 GT3s. There is ZERO evidence of any benefits on regular roads.

Pointbye

bhvrdr

Reminds me of the old, "I got my naturally aspirated 997 tuned by XYZ and it totally transformed the car.... much better acceleration in the top end and it really smoothed out the torque dip that the stock car had."

LOL, really??? You felt all that huh.

Mike

HenryPcar

My input to yours
Thanks for all your great inputs.....king of sarcasm !

Pointbye

chuck911

What you just noticed is in fact now and always has been the case with all cars, well for something like, well, ever. Braking transfers weight forward so for straight line braking you definitely want front brake bias. Braking while turning you want even more front bias in order to avoid the tendency to oversteer. Those are the performance reasons. Then there's liability. The understeer resulting from excessive front bias is much more benign and easy to control than the oversteer resulting from not enough. So no way is anyone going to make a car with even a little bit too much rear brake. Especially not the one making the one car with the engine out back. Can you say, "class action"? So they give it a little too much front brake bias.

But only a little. If you want less, the fix is simple enough: replace your factory fixed front/rear proportioning valve with an adjustable valve. Then you can twiddle to your hearts content.

But, even though all the knowledgeable drivers I know agree the front bias is a bit too much, its only a bit. Worth doing only for that last tenth of a second at autocross- and even then only once you have the skills to judge how to adjust it and how to use it safely. Which in my experience, the mere fact of "if you have to ask" means the answer is: I wouldn't recommend it.

Come to think of it, Azerbaijan GP the other day, whole bunch of F1 drivers- notably Sebo, but seriously, a whole bunch- were billowing huge clouds of smoke from locking up under hard braking. Want to venture a guess which end of the car that was? And those guys, trust me, are able to adjust that. So when I say "all cars", yeah, I mean all cars.

swoardrider

Half the time I wonder what's going on in Stuttgart. They are all over the map with their brakes systems over the last 15 years on a car design that hasn't changed that much. The only constant has been the fact that their brake systems have continuously gotten larger in diameter, especially in the rears. Which proves 2 things to me. 997's are under braked for their performance level (350/350mm should be on all base 911's, not 330/330, IMO). Rear brakes do a lot more work in a 911 than any other car design due to all that weight in front of the rear axle, and should be doing more work, especially because their is not a lot of weight on the front axle of a 911.

Their Motorsports division got it right. 380/380mm iron brakes that fit under 18" wheels.

Petza914

+1
and don't forget that PSM actually uses the rear brakes to help stabilize the car with that weight pendulum back there. The 350/350 PCCB setup on my 05 C2S is spectacular and well balanced (was the same setup used on the 996TT - later 997s got 380mm fronts and stayed with the 350mm rears).