GregWormald

I may be overthinking this but I'd like to know what to do as the temperature changes.

The temperature in Adelaide can easily range from 5° to 40+°C (winter/summer), and being a desert in the summer the temperature can change from 15° to 45°C in any 24 hours. That's about a 6 psi tyre pressure change!

So--the question:
As the ambient temperature changes do I keep the tyres at the recommended pressure (29 psi for my tyres) whatever the temperature, or adjust the pressure so that it would be at the recommended pressure at the standard temperature (20°C, 68°F)?

(Assume the car is for street use and the pressure measurement is made on tyres not driven for several hours, i.e. "cold" in tyre parlance.)

Thanks,
Greg

H82BL8

use nitrogen as it is less prone to changes in pressure due to temp changes

GregWormald

My high school physics (ideal gas law) says this is unlikely or at least a very minor difference, despite what the nitrogen sellers proclaim.

Greg

H82BL8

might I suggest, that when you ask a question on Rennlist.... you want an answer.
Don't smack the ones giving you answers.
Racers have used for years

http://www.popularmechanics.com/cars...a3894/4302788/

I think this explanation is better. Its not just Nitrogen.

https://jalopnik.com/heres-the-deal-...res-1795659391

GregWormald

I appreciate the extra details about moisture--that makes sense.

Sorry to be so fussy, but I was hoping for an answer to my question.

Greg

Pep!RRRR

The short answer is that you should check and adjust "cold" tire pressures over the course of the year to maintain the recommended manufacturer values. This will typically require adding air in the cooler months to account for lower ambient temperatures, and possibly bleeding some air in the warmer periods. The recommended pressures provide safe levels of sidewall rigidity and contact patch area for traction purposes.

If you track your car, tire pressures should be checked and adjusted every session. I typically bleed about 5 psi over the course of a day, and refill each evening.

GregWormald

Thanks Pep!RRRR.

I persisted in my reading, widened the search, and discovered both the Tire Rack's Tire Tech section and some advanced info on Barry's Tire Tech that agrees with your answer. They also imply that the recommended pressures are close to minimum pressures and say that increases due to reasonable ambient temperature increases and driving are expected.

The information also implies (as does your tracking suggestion) that if adjustments have been made due to very large temperature swings then tyres must be re-adjusted before driving re-commences.

I think I now have what I need.

Greg

JCtx

Indeed. Plus seepage occurs anyway, and what a drag having to air tires every time with 'Nitrogen' (probably 90% at best). In my case, the difference is probably meaningless since I live in the dry desert, so minimal water vapor, which is really the issue, not the oxygen (aircraft tires use pure Nitrogen, which requires purging tires from air).

Anyway, for folks like me that go from the 100ºF+ desert for a mountain run at 70ºF, I use just 1 psi more than the comfort pressure of 30 (33 is normal). I don't like my tires above 35, and on the way there that's what I get with 31 'cold' (which is like 80ºF). At altitude the pressure drops, but with the workout the curves provide, they get between 33 and 35, which is perfect for me. Wish our cars had the ability to also read temperature, like McLarens; that'd be perfect, since that's a more accurate figure to keep track of. And yes, for those tracking, they have to lower pressure quite a bit more than my 31, or it'd easily exceed 35, killling performance (and the tires) in short order. The other issue is 'heat cycling'. Once you did that enough, tires are done, even if there's a lot of tread left. Not much of a problem with just canyon carving. I religiously check tire pressures every week, and before any trip, even if short.

Dr.Bill

Yes, race cars use Nitrogen.
They also have wheels with two stems - one to purge the air in the tire, the other to add nitrogen. On a regular street wheel, you would need to fill & purge a few times. Or just drop by the desert here & use our dry air. It's free!

To answer the original question - you should adjust the pressure so that it would be at the recommended pressure at the standard temperature (20°C, 68°F)? The TPMS system does compensate for temperature. Not with the the pressure it reads, but if you go to the 'info' screen, it will indicate a zero next to the tires that are at the appropriate pressure and have either a positive or negative number next to the tires that are either too high or too low for the conditions.

JCtx

No, it does not, but I think it's a semantics issue. Let's discuss it, but I'm not very good with words, so let's see if I can make my point. TPMS is 'adjusted' to 20ºC indeed, but it does not 'compensate' for temperature (constantly); otherwise, TPMS settings wouldn't move while under way, like with BMW motorcycles. In other words, with 'temperature' compensated readings, TPMS always shows your 'cold' temperature setting, whether tire is 1, 3, 5, or whatever amount of PSI higher, due to hot asphalt, etc., because the computer is 'compensating' those extra PSIs due to heat (temperature increase, in this case), by deducting them from the actual reading. In our cars, tire pressures change constantly due to temperature fluctuations, as evidenced by reading the TPSM screen while driving. And that's a great thing. The only annoyance is you have to determine how many PSI low is your 'base' reading, which is 2 in my case. So when I adjust my tires to the 31 cold I like, TPMS reads 29. When it reads 33, it's at the 35 I prefer when hot. As mentioned, it'd be awesome to also have a temperature reading, like McLarens (and other high-end sports cars) do, but it's not really necessary if not tracking. Why the hell does anybody do that, especially on a sports car or motorcycle, is beyond me. But at least in our cars, we can read relative pressure increases, to make adjustments necessary. And that's important if you push your car in 100ºF+ ambient temperatures; too much pressure is not a good thing. Take care.

Dr.Bill

Here's an easy way to prove what I explained above.

Set your cold tire pressure at 30psi when it's 70 degrees out. Look at what the 'FILL INFO' screen says. (NOT THE PRESSURE MEASURED BY THE TPMS - that will show 30psi) Do the same when it's 50 degrees out and again when it's 90 degrees out. In each case, the TPMS will show 30psi, but if you look at the 'FILL INFO' screen it will show how the pressure varies from ideal. Or just read the manual, but you knew that already.

http://www.porscheownersmanuals.com/...-pressure-menu

danschy

I'm no expert, but from what I've read an advantage of using Nitrogen (maybe the main advantage) is that the moisture level is very low. On my air compressor I have an air filter that allegedly "filters unwanted dirt, debris and moisture from air lines", which I mainly use when spraying paint. I have no idea how much moisture it really removes, but maybe it is at least a tiny step in getting dry air in my tires, although a road trip to the southwest sounds like a good alternative.

JCtx

Just like I suspected, you're talking about something else. That doesn't tell you anything unless you need to CORRECT tire pressures. If your tires were at 30 psi cold, and you go run 100 mph on 130ºF asphalt, that screen would read '0' adjustments (if you can even invoke it while running, which I doubt), because temperature increases would be deducted. What we really need to know is the real pressure of our tires when driving aggressively (about 35 in the case above), fast, or both, especially on hot days, to make pressure adjustments either on the spot, or for future ocassions. And all we need to do is click the lever a few times and see actual pressures (plus 2 psi, in my case).

On my first trip, I left the pressure at the normal 33, and when I saw 36 (actually 38) on the highway, pulled over and bled 3 psi from each tire (that's when the screen you posted helps), and marked the 'comfort psi' box. That left them at the real 35 I wanted the rest of the trip. If I go to the high mountains, I put them at 31 cold, since it's a lot cooler/higher up there; otherwise, they stay at 30 cold all the time.

Kevin Rohrer

Converting Celsius (which I don't know) to Fahrenheit yields a range of 59F to 113F. Impressive range.

Your car should have a TPMS built into it that lets you watch the change in pressure. If you start out at the lower range (29psi seems a bit low; mine is 32psi) and it does not exceed the upper limit for your tires, everything is good. Even then, there is a safety margin built-into the tires. I don't believe I have ever driven my car at a temp of >105F (40.5C), and my tires have never gone above 40psi (max is 50psi), a range of eight.