[malickje]

Hello, 968 enthusiasts. Last month I purchased Michelin's Pilot Sport A/S Plus on the 16" standard wheels. The installer put the air pressure at 40 psi. I'm just wondering if that's the typical value that most 968 owners are running. These tires have proven to be very good and I'm glad that I made the choice after considering Hankook and Continental. I favoured the Pilot Sport A/S 3, but they are not available until this summer and I needed new tires immediately. My driving habits are not overly aggressive, but I do appreciate the responsiveness and ride quality that these tires exhibit. As far as adding nitrogen verses regular air, the benefits are negligible since I only drive the car three seasons so regular air is sufficient for me.

Happy motoring!

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[ds968]

I think 40 PSI ( cold ) borderlines on absurdly high . I never have my Michelin PS2 higher than 29 front and 31 rear ( cold ) and having measured the ideal temp of the tires when they were hot, that's actually just about the perfect setting for mine, but I would not hesitate going even one or two PSI lower than that. No doubt I'll get flamed for this but I simply don't buy into the alleged benefits of over inflating tires..

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[Rap]

What do the tires say about max air pressure? 40 sounds high. If you were to heat them up they could increase by about 4. 44 sounds real high!

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[94SilverCab]

I've never run more than 36 psi...Nitrogen is a racket! Jay

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[craigawoodman]

I used to work with earthmover tyres, and one of the biggest risks is tyre explosions. This is not the tyre simply letting go (contained air pressure), but internal chemical reaction (pyrolysis) which releases flammable gases, that build up and when in enough concentration, the tyre literally explodes. Same can happen in a fire, or if the equipment is struck by lightning.



One way of preventing this is by inflating with nitrogen. Unfortunately, you have to get to 5% oxygen for it to work. To achieve this, you need to inflate/deflate a minimum of 5 times to get to 5% oxygen. For aircraft tyres that can get to -50C, it may also make sense to remove moisture, but a good compressor with a desiccant filter can also do this.



The only other claimed benefit it a reduction in tyre liner degradation. Never seen this as an issue is car tyres.



Sometime soon, we are going to be told minor changes in Nitrogen levels in the atmosphere, is responsible for (insert your favourite issue here). Air is made up of something like 70% Nitrogen anyway.



So worth while worrying about - nope. And how do you adjust your pressures - you put air in, so it defeats the purpose.



As for 40psi, this sounds too high for modern tyres.

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[flash]

today's tires like more air than tires of old. their construction and material is different.



40 cold is high for any high performance tire, though not absurdly, depending on how hard you drive and how hot the road is.



for our car, on the kinds of tires we run, 29 and 31 is ridiculously low. it will cost you both fuel economy and adhesion. i could prove the latter of this to you on a skidpad.



air is 78% nitrogen. buying nitrogen for a street car is silly. not so bad for a 747 though.



an average target temp is 40psi HOT. the cold temp setting will vary depending on the weather, the way you drive the car, and the tire itself, even changing as it wears.

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[ds968]

[quote name='flash' timestamp='1368379940' post='142653']

for our car, on the kinds of tires we run, 29 and 31 is ridiculously low. it will cost you both fuel economy and adhesion. i could prove the latter of this to you on a skidpad.wears.[/quote]



That I'd like to see because I simply don't believe it - autocross drivers where adhesion is one of the most important factors are running low to mid 20s on their tires - never more than 25 cold or low 30 when full hot . Sure, you suffer in the gas mileage category but especially with the newer generation tires , no way, no how should you have more than 31 or 32 cold psi. Ask most any one that has worked tire stores or connect with Michelin ,s reps, they'll tell you NEVER to inflate your tires on any sports cars more than 30 psi cold.

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[flash]

autocross is an entirely different world, and has no application to street cars. for example, most of the fastest autocross drivers do not run sway bars, or run very small ones. they need to induce a lot of roll and weight transfer. for that reason, you run less air in your tires.



as for michelin specs, you are very wrong. i just had this very conversation when i got the super sports. not only did the guy i talked to recommend that i inflate them to 38psi, contrary to what the cars says to inflate them, but he went on to say that under-inflating them could void my road hazard warranty.



here is something from the michelin site:

http://www.michelinman.com/tires-101/tir...ssure.page



our manual specifies 36psi cold front and rear



here is something from tirerack - note what it says toward the end:

http://www.tirerack.com/tires/tiretech/t...p?techid=2

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[polliviere]

Nitrogen has a much lower tendency to generate moisture in the tire as it naturly heats and cools in response to use and climate changes. The moisture contained in normal pump air can damage tire pressure sensors over time. That is the primery reason car makers have gone to nitrogen.

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[Cloud9...68]

[quote name='polliviere' timestamp='1368405393' post='142658']

Nitrogen has a much lower tendency to generate moisture in the tire as it naturly heats and cools in response to use and climate changes. The moisture contained in normal pump air can damage tire pressure sensors over time. That is the primery reason car makers have gone to nitrogen.

[/quote]

That's true - the moisture contained in pump air, at the temperature seen inside tires, particularly if driven aggressively, can lead to accelerated corrosion, which, if it occurs at the tire bead, can lead to leaks. However, I would think a dessicator on the air tank would accomplish the same thing as nitrogen. It's just a matter of how dry you can keep the gas.

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[MB968]

Can't agree with either of the above comments on autocross. If you want to get into this form of racing attend some of the local SCCA events. Talk to the class winners, they are usually happy to share. And, most cities will have at least one person or so who attends the nationals and can speak with a lot of experience. I've known very specific cars where each of the above comments apply. But, neither are generalities that typcially apply. Each car has it's own recipe, and each weekend can change the best setup for the same car.

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[craigawoodman]

The moisture in the air is directly proportional to what the compressor sucks (well "draws") into the compressor (as there is no such thing as "suck" in Thermodynamics - air flows from high to low pressure). There is no additional moisture added to the air once it is compressed. As the air expands into the receiver tank, it can cool and condense moisture (hence why you need to drain air receivers).



The amount of moisture in compressed air is minimal. It is a pain if you want to spray paint, but not for inflating tyres. Air is assumed to behave like a ideal gas in most cases. It is only if you want to be super accurate, that y start dealing with vapour pressures, etc.



I have never seen a tyre stripped off a road car rim that has water in it (you would feel it as a vibration as it would eventually get "pinned" to the crown of the tyre) if it was simply inflated with air. If someone does not drain their air receiver, then different story.



Sorry to highlight it, but every time your tyre heats up and cools, it spells and draws in a small amount of external air, hence why tyres lose pressure. The liner and bead are not perfectly sealed, etc. perhaps water in the tyre might be a problem if your ambients get below freezing, but for most of us, it is not a problem. Nitrogen a gimmick - you bet.

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[flash]

uh - yes, there is moisture in the compressor. that's exactly why they have a drain. i had one that peed quite a stream when i would drain it every couple of months. that being said, nitrogen won't cure the problem. if you were willing to change the air in your tires every race, you may see some advantage. but, as craig points out, the osmotic tendencies of the tire preclude any long term benefit

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[craigawoodman]

Flash,



I was looking at the compressor rather than the air receiver/tank. The compressor heats the air which the cools off in the tank and condenses. That is why the receiver/tank/airlines have water drains. There is no additional moisture added by the compressor or compression process itself.



Any moisture comes from the air drawn into the compressor. So a day with zero humidity, will provide dry air.

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[flash]

fair enough. i was only pointing out that since the air comes from the tank, moisture generally goes with it. it isn't much, but it's there. certainly you won't have liquid floating around in your tires though.

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[gbarrier]

I run nitrogen in the Porsche and the wife's Beamer but mostly because of the smaller amounts of pressure changes from hot to cold.



On the other hand, before we had onboard air, we use to carry a co2 bottle on our offroad trips to air up the jeep tires when we came off the trails. that stuff is nasty with a lot of moisture and we never saw much of a problem tires when we pulled them off after four or five years. So perhaps just plain old air is good enough.

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[flash]

again, that benefit goes away pretty quickly. after about 3 months, you are back to atmospheric content. so, as i said, unless you plan to change your air frequently, you are wasting your money.



you can google the science and the realities on this. it's pretty much a waste of time and money unless you are doing something specific that can reap the rewards.

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[JTP]

Answering the OP question, I've experimented with tire pressure from 41 psi down to the Porsche specified 36 psi and 36 is the best from my experience. As the pressure was increased the car tramlined and became skittish much worse. Changing from 38 to 36 was a huge difference in ride and handling.

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[flash]

that is correct, depending on the tire, weather, how you drive, etc. it is the perfect starting point though, and i have yet to find a car that varies from that more than about a pound and a half without losing grip.

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[ds968]

Perhaps the butt-o-meter is very deceiving, but whether cruising on tne highway, whether driving on twisty roads and taking hard turns , whether driving 30 mph or 100+ mph, the car FEELS far more planted and stable with my tires a couple of psi below 30 than

at 36. Again, simply seat of the pants impression.

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