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Originally Posted by Topless
Yes, glad you asked. There is a very valid reason. On 987,981, and most 997 cars the PSM and ABS are precisely calibrated to stock tire stagger in diameter (typically 1" differential). Put the same diameter tire on these cars and the PSM overreacts in corners applying the rear brakes and cutting the throttle. It can also confuse ABS creating an "ice pedal" condition under heavy braking causing a crash.
Getting the factory tire diameter stagger is very important in Porsche cars after 2005 so that PSM and ABS function properly. Mounting the same diameter tire on all cars without consideration of factory designed stagger is foolish and dangerous.
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I said street cars, not race cars were "ice pedal" is a common issue and I have yet to read any racers blaming it on the tire circumference size. I've read racers having a too aggressive master cylinder setups along with running tires that are basically slicks, to pads and rotors are not stopping the rotation of your wheels enough to reach lockup.
PCA even made a 2019 the proposed rule change. "Every ABS whose PWIS programming includes an option for a PCCB flash may use that flash. Doing this seems to help with "ice" pedal and is inexpensive."
I thought you were a "PCA-GPX Chief Driving Instructor"? You do give breaking lessons to help keep racers out of this issue as much as possible, right?
On a street car "ice pedal" can happen with just hitting a bump in the road at the wrong time and confusing the ABS. I have never had a "ice pedal" issue in my car, ever, and I'm running smaller wheels and tires then my car came with from factory. Not by choice as someone swiped the Carrera Light 18" wheels off the car, probably one of the dealerships the car was at as I'm the 4th owner of the car.
People change the wheels and tire size on their trucks, SUV and cars all the time. Going from 18 up to 22" on trucks and SUV, some people will even do it on cars lol and never ever do you hear about people having "ice pedal" issues. Heck you can walk onto a dealership lot and they will sell you a customized Truck/SUV with way bigger tires then stock and the dealership won't break a sweat over it because it's just not a street issue. If it was, no dealership in the USA would sell you a vehicle setup like that as they would get sued for putting the people in danger and them crashing the vehicle. Way too much fear mongering with that post.
Quote:
Originally Posted by maytag
Hey man, don't start throwing out squirrels here..... you're changing the direction to deflect....
This is about whether or not you can FEEL it..... hahaha.
Sent from my SM-G970U using Tapatalk
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I can feel the difference. The car is more balanced through the corners when the front and rear tires are as close as can be to rotational speed. There is a lot less steering input needed. I'm not fighting the car around the corner, the balance is great. Maybe you should try it? Oh never mind on that, I see you have had a track alignment done to your car.