does overinflation cause a floaty feeling on the road??

[FinallyGotOne]

my 79 SC has 40 series tires on 17 inch cup wheels. I had the shop inflate to 40 PSI. The book calls for 29-34 front and back. But the book assumes i have 16 inch with larger sidewalls. Anyways I am getting a lofty feel at over 50. don't like it. is it due to wrong tire pressure?

[jluetjen]

Ok. A couple of questions to help me understand what you've got.

1) Are you using a rear wing?
2) What is the wheel width and tire width that you're using?
3) Have you done any other changes to the rear suspension -- especially the geometry?

[McLovin]

I would still use the factory PSI for front and rear.

40 PSI is way high for normal street car driving. You're going to get a weird ride and weird tire wear with that much PSI.

[FinallyGotOne]

Yes a 3.8 RS Wing. I have a large front chin spoiler also.
The tire width is from total outside to inside 10.7 but for the part that actually contacts the road, about 9.5 inches.
The car was lowered and aligned a couple years ago by a air cooled specialists shop here in Denver.

[Nine9six]

Anyways I am getting a lofty feel at over 50. don't like it. is it due to wrong tire pressure?
I'd say so. Reset your tire pressure within the limits as indicated on the tires, and call me in the morning.

[FinallyGotOne]

yup 10-4 guys! i'll give it a go

[lupin..the..3rd]

Originally Posted by FinallyGotOne

my 79 SC has 40 series tires on 17 inch cup wheels. I had the shop inflate to 40 PSI. The book calls for 29-34 front and back. But the book assumes i have 16 inch with larger sidewalls. Anyways I am getting a lofty feel at over 50. don't like it. is it due to wrong tire pressure?

in a word: yes.

the tire size doesn't affect the pressure. ignore what it says on the tire. use only the factory recommended tire pressures.

[john walker's workshop]

some tires handle like crap. push the car sideways at the bumper repeatedly, front and rear, and watch how much the tire deflects.

[CCM911]

Originally Posted by john walker's workshop

some tires handle like crap. push the car sideways at the bumper repeatedly, front and rear, and watch how much the tire deflects.

+1. I had a cheapie pair of Dunlops on my car a while ago, and the sidewalls flexed so much that I was terrified of corners. The steering wheel would move back and forth in my had while approaching the apex. Now I have a set of Bridgestone Potanza's, and my confidence is back.

[jluetjen]

My original thought were as follows...

1) Those are awfully low profile tires given that the 911's suspension (including the SC's) was originally designed for 78+ series tires on 5 inch rims. As a result there is a fair degree of camber change designed into the 911's rear suspension.

2) 40 series Tires have very short sidewalls that don't provide a lot of compliance. Compound that with a tire with a wide flat contact patch and you have a tire who's contact patch is most likely changing quite a bit as the car goes up or down as the camber changes. In droop it may be moving outside, and in bump moving inside. In so much as the car is going up and down this may vary from side to side depending on the pitch of the road. This action could be creating a shifting torque forces on the rear end.

3) Over inflating the tire may be causing it to bow out in the center of the tread (You'd need a pyrometer to confirm this) which would make the whole thing even flakier.

So yes, running the recommended pressure is most likely a good place to start. If that doesn't resolve the problem, try switching to a higher profile wheel/tire combination.

[Nine9six]

Originally Posted by lupin..the..3rd

... ignore what it says on the tire. use only the factory recommended tire pressures.

I COMPLETELY DISAGREE
Hmmm, ignore what it says on the tire! Wasn't that what the guy was doing when he over-inflated the tire to 40 psi?
Never seen a performance tire rated to run @ 40 psi
The poster did not state that the tire recommended pressure was 40 psi. That was jus sum "jethro" type ciferin', what arrived us'in at that conclusion.

[dshepp806]

Never looked at the tire prints,..only what's stated on the car (1989)....and that's 29 psi in the front and 36 psi in the rear...one can move from this to tune,...but (again) I pay no attention to the tire info itself......I'd goo with the Porsache reccos' and tune from there,..all of this, OF COURSE, dependent upon the tire you get, especially the stiffness of the sidewall,....and I always go for that!


Doyle

[Flieger]

40 series tires are really low profile? Have you seen the 997s?

Porsche wrote their recommended tire pressures before these tires were designed. I would go by the tire, not Porsche.

FWIW, I use Advan A048s and run them about 28 front and 32 rear hot. It has been a while since I checked them, though.

They have stiff sidewalls and wear the center of the tire first, apparently due to centrifugal force.

[Nine9six]

Tire manufacturers safety rate their tires based upon construction, speed rating and intended use. How would Porsche know what tire preference each individual would select for their car? They don't...and thats why they toss out ballpark numbers based upon wheel size and "recommended" tire ratings.
I have NEVER seen a need to exceed recommended tire pressures; i do however, adjust my pressure based upon handling, driving, and tire feedback.