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[Lotus99]

I know that you're supposed to measure the tires when they're cold, but I was reading the manual of my dad's Porsche, which was pointing out that the pressures should be taken at 68 degrees F, and it's at that temp that the tables are done up.

I know how pressure drops like 1 PSI for every 10 degrees in temperature.

So let's say that your tire pressure is supposed to be at 35 PSI. If the temp outside is only 50 degrees, does that mean you only need to have 33 PSI in the tire when you measure with your gauge, because 33 PSI at 50 degrees is the same as 35 PSI at 70 degrees?

His car actually read and reports back the temperature adjusted tire pressure, and tells you how much air you need to add / take out. The above seems to be how it does things, because it reported to me today that the tire pressure was 33 PSI (when I had filled it up to 35 a day before based on my manual gauge), but that it didn't need any adjustment, instead of telling me to put in 2 PSI (because the temp was actually about 20 degrees lower too).

[Mach V Dan]

You need to put the recommended cold pressure in at ambient temperature, before driving the car. (The pressure will increase some after you drive, since the tires and the air inside heat up.)

Think about it -- the tire needs a certain amount of air pressure to hold the car off the ground. It needs that pressure whether it's warm or cold out. As the seasons change and summer transitions to winter, the temps get colder, and the amount of air you had in summer that made 33 psi will not be enough, so you have to put in more air. In the summer, you will have to let some out, assuming it hasn't already leaked out.

--Dan
Mach V

[Lotus99]

Yeah, that's what I've been doing, as it seems like common sense.

The Porsche threw me off though, as it reports the actual pressure (this morning it was at 33 since it's cooler than a couple of days ago when I filled them to 35), but it says no extra air is required, even though it should be telling me to put in 2 PSI (it's actually been programmed that 35 is the normal PSI, and tells you how many PSI each tire needs to get to 35).

[tibra1]

Actually very curious on this subject as well...is there an "optimum" pressure to be running..I have scoured the net looking for answers but there really isnt anything usefull out there..

Im sure its dependent on the tire/wheel combo..size..diameter etc..up to till now I have experimented w different tire pressures to see how the car "feels"..but is there a more scientific approach?..formulas etc..I always run a higher pressure as a counter-measure of protecting the wheel from impact..

With my setup I currently run 38psi R / 35psi F..all measured cold.

Would really like to get a meaningful discussion on going about this

[HighlandPete]

Interesting topic, one I'm much aware of.

It really is the working pressure we are interested in, for best performance and a balance of wear and road feel. Also remember in cooler ambient temperatures, it takes more mileage to get to working temperature and pressure.

One key factor is, we must inflate with enough pressure to carry the load. We have to consider tire construction as well, for example, reinforced (XL) tires have the load index rating set at a higher pressure.

Another factor is, are we making short trips? If so, the tires will not heat up enough to achieve working pressure and we can get some premature wear. RFTs are particularly susceptible to strange wear patterns on lower pressures, as we run on the sidewall, if they don't heat up enough and gain the working pressure. Colder weather means even more attention to having enough pressure from cold. Even hitting wet weather chills a tire, so often we drop from ideal working pressures, even mid journey. In fact do we ever really heat the tires the same as in hot ambient's?

For me, winter/cooler ambient temperatures typically mean a bit more pressure, to ensure I'm running enough pressure to prevent premature and odd wear. Applies to winter tires as well.

HighlandPete

[D_o_S]

I say get a proper pressure gauge (will have a write up on my home DIY version soon, hopefully) and a tire pyrometer, and get find your own proper tire pressures.

What you want to do is set pressures slightly above what is on the sticker, say 0.2 bar. Then, go driving around, preferably an open parking lot where you can do sharp turns, braking, etc. Heat up your tires for a few minutes, then check tire temps with the pyrometer. Check inside (say 1 inch from outer edge), center and outside (also 1 inch fron edge) temperatures. Your temperatures should be equal (more or less) across the whole tire.

If they're too high in the center, you're over inflated - use your pressure gauge and deflate some air, say 0.05 bar, or as close as you want to get it...

Measure again, etc. until tire surface temp is even.

If they're too high at the edges, you're underinflated. Blow up the tire some more (this should not happen if you set out running higher than sticker pressures).

Once the tire shows equal temps from inside out (I'm expecting SLIGHTLY higher temps on the inside, since we all run some negative camber) you're done - note the tire pressures. These are your warm tire pressures.

Now, you must guess how much your tire warms up from day to day and base your inflation rates on it... like if its 10 C today, and 25 C tomorrow, and you inflate your tires today to 2.3, tomorrow, you'll have say 2.4 bar cold, which makes a difference...

[Lotus99]

Thanks for the different tips guys, especially what to do for track.

So bottom line, for "cold" pressure, you should adjust things by adding more air as temps drop. You don't aim for the recommended pressure at 70 F, for example, which adjusted at a lower temp would be 1 PSI less for every 10 F. So at 30 F, you would add 4 PSI (4 x 10 F drops) to your tire, so it's still at the recommended pressure.

Right?