Correct tire size question

[maytag]

Originally Posted by KRAM36

My Boxster is my DD and it's my only vehicle, it's been that way since 2014. I notice everything on my car when something changes. I personally do not like faster rotating front tires, it throws off the balance to me.

You remind me of my 1st wife, who swore she could smell cats, when there were no cats.
I think much of what you THINK you notice is what I call "confirmation bias", or a simple psychosomatic response. You expect to feel a difference (you even WANT to feel a difference) and so you feel a difference.
Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.



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[Topless]

Originally Posted by maytag

You remind me of my 1st wife, who swore she could smell cats, when there were no cats.
I think much of what you THINK you notice is what I call "confirmation bias", or a simple psychosomatic response. You expect to feel a difference (you even WANT to feel a difference) and so you feel a difference.
Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.



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Oh ya this is spark plug horsepower guy. nuff said.

[KRAM36]

Originally Posted by maytag

You remind me of my 1st wife, who swore she could smell cats, when there were no cats.
I think much of what you THINK you notice is what I call "confirmation bias", or a simple psychosomatic response. You expect to feel a difference (you even WANT to feel a difference) and so you feel a difference.
Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.



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Change it on my car and I will notice the change without being told.

Why are you even commenting? Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car? Or are you just being a chain jerkier like Topless?

[maytag]

Originally Posted by KRAM36

Change it on my car and I will notice the change without being told.



Why are you even commenting? Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car? Or are you just being a chain jerkier like Topless?

Because people use these forums as a knowledge base. If ridiculousness is left unchallenged, it gets defacto credibility.

That's why I'm commenting.

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[KRAM36]

Originally Posted by maytag

Because people use these forums as a knowledge base. If ridiculousness is left unchallenged, it gets defacto credibility.

That's why I'm commenting.

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You said nothing to discredit my recommendation. You've put nothing in check. You're just showing you want to argue just like Topless with zero bases to argue from.

[maytag]

Originally Posted by KRAM36

https://images.app.goo.gl/HiGdobdHKrLjaGnNA


.

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[Topless]

C'mon KRAM, put up or shut up.

Lets see hard data for all the 987/997/981/991 cars that came from the Porsche factory with identical or nearly identical (less than .3" difference) F/R tire diameters. Google is your friend and there are a few, and I know which ones. The numbers are out there for all to see. If you are going to make an argument AT LEAST be able to support that argument with hard facts instead of butt dyno mumbo jumbo.

You have 30 minutes to demonstrate your awesomeness to all who visit here.

Go!

[KRAM36]

Originally Posted by Topless

C'mon KRAM, put up or shut up.

Lets see hard data for all the 987/997/981/991 cars that came from the Porsche factory with identical or nearly identical (less than .3" difference) F/R tire diameters. Google is your friend and there are a few... very few and I know which ones. The numbers are out there for all to see. If you are going to make an argument AT LEAST be able to support that argument with hard facts instead of butt dyno mumbo jumbo.

You have 30 minutes to demonstrate your awesomeness to all who visit here.

Go!

I don't have 30 min to do anything.

You made the false claim. Two of you examples were not 1" different.

Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?

[Topless]

Originally Posted by KRAM36

I don't have 30 min to do anything.

You made the false claim. Two of you examples were not 1" different.

Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?

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. The 986 has a much simpler PSM and ABS that do not require precise matching of tire diameter. The OP can happily run the tires he is on with little or no effect. If he does change the rear to a 265/35/18 he will notice a bit better acceleration and his speedo will again be off by 4-6mph.

[maytag]

Originally Posted by KRAM36

Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?

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.

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[KRAM36]

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.

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.

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.

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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.

[Burg Boxster]

Originally Posted by KRAM36

Change it on my car and I will notice the change without being told.

[cough] horsesh_i-t [cough]

And I don't mean switching from summer to winter as you tried to squirrelgoat with earlier... b/c uhhh most anyone w/ a buttometer should notice that. I mean, heck even my wife noticed when I replaced RF tires on her car w/ non-RFs.

Anyhow, all other things being equal (brand, model, width, wheel diam, tire wear, etc), if someone switched rears on a vehicle you are to drive from 40 to 35 series 265s or vice versa without telling you which was on, I'll bet you a steak dinner w/ all the trimmings there's absolutely no way YOU could GUESS (because let's be honest that's ALL you would be doing - GUESSING) w/ more than 50% accuracy (with a statistically adaquate/reliable sample size) which was on after driving a kilometer.

I mean sheesh, after all it IS 13 less rotations per kilometer you insisted w/ great precision. That means every 252 feet you'd feel a rear tire rotate one less time than another. Easy peasy lemon squeezy apparently for you...

https://www.youtube.com/watch?v=fr9ppl2k_3c

I guess I stand corrected. My bad.

But that does beg the question with your F1 career in the balance, how do you find time to post w/ commoners like us here on 986forum.

Good luck in Monaco next week!

[piper6909]

Originally Posted by maytag

Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.

Driving blindfolded. Now there's an idea! Show us how it's done, Maytag!

[maytag]

Originally Posted by piper6909

Driving blindfolded. Now there's an idea! Show us how it's done, Maytag!

https://beta.washingtonpost.com/technology/2019/01/12/driving-blindfolded-bird-box-challenge-just-dont-officials-say/?outputType=amp


Can't make this stuff up, Haha

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[piper6909]

Originally Posted by maytag

https://beta.washingtonpost.com/technology/2019/01/12/driving-blindfolded-bird-box-challenge-just-dont-officials-say/?outputType=amp


Can't make this stuff up, Haha

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OMG! LOL I hope this doesn't spread like the Tide Pod challenge! Good lord, were we that stupid then?