Wider tires??
03-15-2006, 03:39 PM
#31
I think those things are WAY to tall. When a tire is 275, it obviously needs a lower aspect ratio, and maybe 40 is where you were supposed to be, butthey still look WAY too tall.
How many miles have you done since you jacked the car up? Maybe it hasn't settled yet. I'd look at those 18s on ebay with the 10s on the rear or even 11s. Just me.
How many miles have you done since you jacked the car up? Maybe it hasn't settled yet. I'd look at those 18s on ebay with the 10s on the rear or even 11s. Just me.
03-15-2006, 04:02 PM
#32
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Originally Posted by BrendanC
I think those things are WAY to tall. When a tire is 275, it obviously needs a lower aspect ratio, and maybe 40 is where you were supposed to be, butthey still look WAY too tall.
How many miles have you done since you jacked the car up? Maybe it hasn't settled yet. I'd look at those 18s on ebay with the 10s on the rear or even 11s. Just me.
How many miles have you done since you jacked the car up? Maybe it hasn't settled yet. I'd look at those 18s on ebay with the 10s on the rear or even 11s. Just me.
They do look very tall!! I had 255/40's before and they were fine. I needed more tread due to added power. I think 18's will be next, they offer 275-285/35/18 but no 35's for 17's
03-15-2006, 04:06 PM
#33
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I also put 275/40/17 on my 9' 996 rims and I am very happy with them. They are a bit larger in diameter and it works great for the short gears on the GT. With the 235/45/17 in the front, the car is perfectly balanced. No over or understeer.
Big difference in traction with the 275's in the rear.
In regards to pressure, if you use regular air I guess 29 PSI is OK. It would go up to around 35 PSI when hot. I get nitrogen put into the tires, which does not expand under higher temps... So I put it at 35 PSI.
Big difference in traction with the 275's in the rear.
In regards to pressure, if you use regular air I guess 29 PSI is OK. It would go up to around 35 PSI when hot. I get nitrogen put into the tires, which does not expand under higher temps... So I put it at 35 PSI.
03-15-2006, 04:28 PM
#34
If you did not change the aspect ratio, but added 20mm (255 to 275) you will have added more aspect ratio then you want for the car.
The only time people really feel handling differences is when there is an abrupt change. New tires, new wheels, new shocks.
None realizes that most 928s, if not recently upgraded, are handling like Buicks on thier old, tired shocks, springs, and bushings. Driving a newly redone suspension is the only time many will notice. Or when on a track, which few do.
The only time people really feel handling differences is when there is an abrupt change. New tires, new wheels, new shocks.
None realizes that most 928s, if not recently upgraded, are handling like Buicks on thier old, tired shocks, springs, and bushings. Driving a newly redone suspension is the only time many will notice. Or when on a track, which few do.
03-15-2006, 04:38 PM
#35
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My 79 is stock, the fenders were not "rolled" and it's sporting 285/30/18s (in the rear).
I bought a set of Kinesis Super Cups from Mark Robinson. Mark Robinson worked
with Kinesis to have the wheels custom built with the perfect offset to allow this.
You'll have to get Mark to "chime in" to get the details.
--Evil Patrick
79
82
84 (for sale - just PM me for details)
I bought a set of Kinesis Super Cups from Mark Robinson. Mark Robinson worked
with Kinesis to have the wheels custom built with the perfect offset to allow this.
You'll have to get Mark to "chime in" to get the details.
--Evil Patrick
79
82
84 (for sale - just PM me for details)
03-15-2006, 05:12 PM
#36
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so Sab, did you do a comparison of air cold/hot temp, to arrive at 35psi with nitrogen? My simple mindedness would think you'd want 29psi nitrogen (or whatever your ratio calc for your tires arrived you at); the nitrogen should only benefit your any temp psi reading that is "essentially" cold, and hence the benefit of running nitrogen to minimize expansion.
...perhaps your ratio calc IS 35psi cold??
...perhaps your ratio calc IS 35psi cold??
03-15-2006, 06:00 PM
#37
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I used to put 30 PSI of air in my tires and noticed that they get up to 36 PSI when hot.
So I figured I would be good with 35 PSI of nitrogen since it will not increase when hot.
Are you suggesting 29 PSI when warmed up? So about 24 or 25 cold?
So I figured I would be good with 35 PSI of nitrogen since it will not increase when hot.
Are you suggesting 29 PSI when warmed up? So about 24 or 25 cold?
03-15-2006, 06:08 PM
#39
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On the subject of nitrogen use to inflate tires, I'm confused by the comments above.
Both major components of air (~80% N2 and ~20% O2) follow the ideal gas laws re compressibility, volumetric expansion when heated, etc.
Pure nitrogen will behave identically to pure, DRY air.
Among the reasons that N2 is used to inflate tires is that it is generally 'bone dry', unlike compressed air; therefore, the pressure rise with increased temperature will predictably track the Ideal Gas Law. Moisture in compressed air is the killer, for it vapourizes from its liquid state as the tire heats - and really begins to jack the pressures disproportional higher.
Additionally, the absence of oxygen minimizes corrosive attack on the steel fabric components of tire belts, sidewall reinforcements, and beads. Also, bead seat corrosion is even an issue with aluminium wheels, leading to pressure loss.
The issue of pressure loss via permeability through the butyl rubber inner liner is less certain: N2 is a smaller molecule than O2 - so in theory, should exit first, leading to a more rapid decay of static pressure.
There are some other lesser points .... but the key issues are generally the absence of water ( erratic pressure rise) and oxygen ( corrosion of structural tire elements).
Both major components of air (~80% N2 and ~20% O2) follow the ideal gas laws re compressibility, volumetric expansion when heated, etc.
Pure nitrogen will behave identically to pure, DRY air.
Among the reasons that N2 is used to inflate tires is that it is generally 'bone dry', unlike compressed air; therefore, the pressure rise with increased temperature will predictably track the Ideal Gas Law. Moisture in compressed air is the killer, for it vapourizes from its liquid state as the tire heats - and really begins to jack the pressures disproportional higher.
Additionally, the absence of oxygen minimizes corrosive attack on the steel fabric components of tire belts, sidewall reinforcements, and beads. Also, bead seat corrosion is even an issue with aluminium wheels, leading to pressure loss.
The issue of pressure loss via permeability through the butyl rubber inner liner is less certain: N2 is a smaller molecule than O2 - so in theory, should exit first, leading to a more rapid decay of static pressure.
There are some other lesser points .... but the key issues are generally the absence of water ( erratic pressure rise) and oxygen ( corrosion of structural tire elements).
03-15-2006, 06:22 PM
#40
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Thread Starter
Originally Posted by Garth S
On the subject of nitrogen use to inflate tires, I'm confused by the comments above.
Both major components of air (~80% N2 and ~20% O2) follow the ideal gas laws re compressibility, volumetric expansion when heated, etc.
Pure nitrogen will behave identically to pure, DRY air.
Among the reasons that N2 is used to inflate tires is that it is generally 'bone dry', unlike compressed air; therefore, the pressure rise with increased temperature will predictably track the Ideal Gas Law. Moisture in compressed air is the killer, for it vapourizes from its liquid state as the tire heats - and really begins to jack the pressures disproportional higher.
Additionally, the absence of oxygen minimizes corrosive attack on the steel fabric components of tire belts, sidewall reinforcements, and beads. Also, bead seat corrosion is even an issue with aluminium wheels, leading to pressure loss.
The issue of pressure loss via permeability through the butyl rubber inner liner is less certain: N2 is a smaller molecule than O2 - so in theory, should exit first, leading to a more rapid decay of static pressure.
There are some other lesser points .... but the key issues are generally the absence of water ( erratic pressure rise) and oxygen ( corrosion of structural tire elements).
Both major components of air (~80% N2 and ~20% O2) follow the ideal gas laws re compressibility, volumetric expansion when heated, etc.
Pure nitrogen will behave identically to pure, DRY air.
Among the reasons that N2 is used to inflate tires is that it is generally 'bone dry', unlike compressed air; therefore, the pressure rise with increased temperature will predictably track the Ideal Gas Law. Moisture in compressed air is the killer, for it vapourizes from its liquid state as the tire heats - and really begins to jack the pressures disproportional higher.
Additionally, the absence of oxygen minimizes corrosive attack on the steel fabric components of tire belts, sidewall reinforcements, and beads. Also, bead seat corrosion is even an issue with aluminium wheels, leading to pressure loss.
The issue of pressure loss via permeability through the butyl rubber inner liner is less certain: N2 is a smaller molecule than O2 - so in theory, should exit first, leading to a more rapid decay of static pressure.
There are some other lesser points .... but the key issues are generally the absence of water ( erratic pressure rise) and oxygen ( corrosion of structural tire elements).
Never heard of using N2 for Auto Tires. We use it for all AF Aircraft so the tires won't freeze at altitude
03-15-2006, 06:46 PM
#42
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Cool stuff Garth. So the nitrogen isn't really holding the pressure constant or minimizing expansion, it is just ensuring it occurs at a consistent rate according to temp increase. Learn something new everyday!
Sab, always a cold read on tire pressure. The calc ratio method I use premisses on cold data, so 29psi is the cold value. I was thinking by you using nitrogen, you'd be virtually at 29psi even when hot, which would be ideal. But as Garth has educated, you will get expansion as the heat increases, but your temp/psi reads will always be consistent...I use compressed air, so my expansion could be all over the board depending on moisture content.
Real world results, when your tires heat up (assuming evenly heated up), you'll be at virtually the same pressure at all 4; high probability mine will be lopsided between any 4, which translates to potential disruption of handling.
Sab, if you're in the mood, try calculating the ratio for your tires and drive with it for a while. Remember to change the calc by 2 if different front/rear sizes (928's make it easy since it's 50/50 weight distribution). I bet you'll feel more planted than the 35psi you're currently at and the nitrogen should equate to consistent handling and tire wear (driving variables being equal of course).
[Appologies for getting a little OT...]
Sab, always a cold read on tire pressure. The calc ratio method I use premisses on cold data, so 29psi is the cold value. I was thinking by you using nitrogen, you'd be virtually at 29psi even when hot, which would be ideal. But as Garth has educated, you will get expansion as the heat increases, but your temp/psi reads will always be consistent...I use compressed air, so my expansion could be all over the board depending on moisture content.
Real world results, when your tires heat up (assuming evenly heated up), you'll be at virtually the same pressure at all 4; high probability mine will be lopsided between any 4, which translates to potential disruption of handling.
Sab, if you're in the mood, try calculating the ratio for your tires and drive with it for a while. Remember to change the calc by 2 if different front/rear sizes (928's make it easy since it's 50/50 weight distribution). I bet you'll feel more planted than the 35psi you're currently at and the nitrogen should equate to consistent handling and tire wear (driving variables being equal of course).
[Appologies for getting a little OT...]
03-15-2006, 06:48 PM
#43
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Welcome to the "slippery slope" of SC'd 928 ownership. 
When I had nice new 255's and my SC, no traction until 4th gear. So there came the Fikse's, 295's (255 up front), now time for a clutch, x-pipe.........
Jim
When I had nice new 255's and my SC, no traction until 4th gear. So there came the Fikse's, 295's (255 up front), now time for a clutch, x-pipe.........
Jim
03-15-2006, 07:21 PM
#44
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Sabs point of nitrogen use is a good one - and is used extensively by large class 8 truck fleets, monster earth movers ... and track cars. Tire pressure management is critical to fuel economy ( and longevity) for trucks, where 1/10% gain is mega bucks in the annual fleet fuel bill. Giant earthmovers rarely stop, and their tires operate at very elevated temps due to hysteresis - so internal corrosion is a real issue, especially as certain types are water filled .... and for track use - any advantage is to be taken!
I'd put nitrogen in my tires too .... if I had a source.
Water/moisture is almost always the thing that upsets pressure control ....and as far as freeze prevention in aircraft tires - again, I suspect that moisture is the issue: Nitrogen solidifies first at a cool ~ -210C, ahead of O2 at ~ -220C IIRC. To drop to those temps, earth orbit must be attained - but by then, the tires would have exploded due to the vacuum ...
Whoops ... even more OT ... stopped now!
I'd put nitrogen in my tires too .... if I had a source.
Water/moisture is almost always the thing that upsets pressure control ....and as far as freeze prevention in aircraft tires - again, I suspect that moisture is the issue: Nitrogen solidifies first at a cool ~ -210C, ahead of O2 at ~ -220C IIRC. To drop to those temps, earth orbit must be attained - but by then, the tires would have exploded due to the vacuum ...
Whoops ... even more OT
... stopped now!
03-15-2006, 07:34 PM
#45
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I've got the same wheels, but my fronts are 8" wide. When looking for tires there are specs on each size offered including minimum rim width. I ended up staying within specs and got 245/40 front and 265/40 rear. I will get 255/40 for the rear next time to even things out more.
p.s. shocks and ride height have everything to do with handling.
p.s. shocks and ride height have everything to do with handling.