Maintaining Tire Pressure

Pressure does change with temp. But not that much. They have portable compressors that run out of our 12v outlet in the car.
I check my tire pressure twice a year when I swap out the winter tires.
Peter

 

Check pressure at home and at station and just figure the difference. 2 miles probably wont raise it much at all.

 

I use a foot operated pump to add air to my tires, it doesn't make much noise, and as it doesn't require many pumps of the foot to add a few psi, I don't make much out of breath noise either.

 

No!! How about this:

Yep! Here's a link to the Sears site for a 12V compressor with a digital gauge. I was able to buy it on a great sale for about $20. Retail is around $34, I believe. It is a great little product, and I use it ALL the time. It also is very handy when helping people who have low/flat tires.

 

General FYI, though the example does not use numbers reasonably to what we are seeing nornally. I overfill by a few psig at the gas station and then adjust in the morning for where I want them set.



Ambient temperature is the temperature of the air outside the tire.How does temperature affect inflation pressure?

First, let's define "ambient" and "contained" air temperatures. Ambient air temperature is the temperature of the air outside the tire (also known as the "outdoor" temperature) while "contained" air temperature is the temperature of the air inside the tire's air chamber.

When a truck has been parked for about 3 hours, these two become pretty much identical. We often use the term "cold" inflation pressure, but what we really mean is the pressure when the contained air temperature is the same as the ambient temperature.
"Contained" air temperature is the temperature of the air inside the tire air chamber.The air inside the tire heats up as the tire turns, right?

Absolutely. If you've driven more than about a mile, friction with the road, flexing of the tire casing and flexing of the tread begin to heat up the air inside the tire.

How hot does it get?

It varies a bit with road and ambient temperature, but if inflation is correct for the load, contained air temperature probably settles in somewhere around 50 to 60 degrees Fahrenheit above ambient.

What do you mean by "settles in"?

After you've been running for a while, the contained air temperature becomes more dependent on inflation, load and speed than on outdoor or ambient temperature.
Inflation pressure controls the shape of the tire casing, a critical factor in controlling heat and performance.So if the temperature outdoors changes, it has no effect?

It has an effect, but it's much smaller than the effect of the mechanical friction and flexing.

Then why worry about temperature at all?

It's the air inside the tire that supports the load. Besides supporting the load, the purpose of correct inflation is to keep the tire the right shape, controlling the amount of flexing.

The chart gives you an idea of how inflation pressure varies with contained air temperature. If you inflate a tire to 100 psi at 70 degrees, it might rise to 110 to 115 psi at normal operating temperature.

Isn't that high?

Not really. Tire designers take that into consideration when they design the tire.

But what if we set inflation pressure when it's hot, then drive to where it's cold?

Let's take an extreme case: Let's say you start out in Las Vegas, it's 100 degrees, and you set tires to 100 psi. Then, you drive to Reno, where the temperature is 0. After the truck has been parked for 3 hours, you might find your inflation pressure is now at about 85 psi.

What should we do then?

You should add air to bring the tires back up to 100 psi. If you do that, then drive back to Las Vegas, and if it's still 100 degrees there, checking your tires again — after a 3-hour cool-down — you may find they're at about 122 psi.

Again, adjust to the correct inflation pressure when the tires are "cold." That's because what you're adjusting with inflation pressure is the shape of the tires. That doesn't have as much to do with temperature.
While temperature fluctuations can have a significant influence on inflation pressures, frequent checking and adjustment of inflation can cancel any negative effects.Why so much emphasis on "cold" inflation pressure?

Remember, we said the contained air temperature could be around 50 to 60 degrees above ambient after you've been running for a while. If you started out at 100 psi at 70 degrees, the inflation could now be at about 106 psi.

If you stopped and adjusted inflation pressure without waiting for the tires to cool, you'd be setting inflation pressure with a contained air temperature of about 120-130 degrees.

When the tires cool down to 70 again, the pressure will go down as well, and you could easily find your tires are now almost 10 psi underinflated.

So always check and adjust inflation pressure when tires are "cold," that is, after the vehicle has been parked about 3 hours, and before it's been driven any more than a mile.

Should temperature fluctuation be a big concern for us?
Checking and adjusting cold inflation pressure frequently goes a long way toward eliminating ambient temperature and altitude effects.If you frequently travel from a very hot location to another where it's very cold, you should check and adjust inflation more frequently. If you're doing that, you'll correct for these variations automatically.

So we don't need a thermometer?

You don't. Just check and adjust inflation pressures often, making sure you do it when the tires are "cold," and you should be fine.

 

TMI! TMI!

 

I check my tire pressure monthly; I rotate my tires every 5K when I also do my oil and filter change. My car has 116K on it now and has never skipped a beat. I get almost 50K miles out of a set of tires, which when replaced are worn evenly. Tire pressure is one of the single biggest things you can monitor to keep your car handling well and safe.

 

Measure it at home to determine how many pounds to add to each tire, then drive to the service station and add that much no matter what the initial readings are when you get there.

 

You're NOT suggesting 100 PSI as a normal baseline tire pressure--ARE you???

 

No, I'm not a hyper"inflator" practicer. This example unfortunately was for a truck tire. What I wanted to demonstrate though to someone completely unfamiliar with the concept was that ambient temperatures make up a small amount of the temperature change, but the flex of the side walls and constant rolling friction from driving will account for 50 to 60 degrees of temperature change from a cold temperature reading.

In an effort to make this as easy as possible, I would simply check my tire pressures often and if they look low, I'd overfill at a gas station in the evening (for most of you) and then check and adjust the next morning. However a double cylinder bicycle tire pump is handy to have for making adjustments. I can't imagine not having some way to add pressure to my tires at home. I've had too many nails over the years and flats first noticed in my garage to not have a small compressor or pump handy.

 

I copied the material from Bridgestone. The small compressors are somewhat slow but good if your tire is really low. However as I mentioned a bicycle tire pump is real handy for adding a couple of psig that you usually get from normal losses. About 50 pushes will typically do the trick.

Just thought of it, but a small compressor will probably fit well in the rear cargo storage organizers available from Toyota. (I know the weight will be a concern for some of you)

 

I know this is a bit tongue-in-cheek, but I do want to make certain people know that the little compressor I use (and suggested above) is small (as you suggested) and of very little weight. I've stored it in my cargo net (in the Gen2; not yet operable in the 2010), but I now store it in one of the side "compartments" of the cargo area. Also, it weighs 2.75 lbs., package and all!

 

Those small compressors are great. My only additional advice would be to use the compressor but still invest into a professional quality tire pressure gauge. I use a compressor but don't trust the built in gauge on most of them. I run it until it reads slightly over (very slightly) then use my "good" gauge to adjust (bleed) the pressure to whatever PSI I want. The small lightweight compressor is great, very nice to be able to add air to your tires anywhere, anytime.

 

My thought as well but apparently much to simple for the great minds of Priuschat. :rockon:

 

Can folks recommend some decent 12V air compressors? The reviews at amazon for the Viair 00073 are pretty good, but it seems to draw up to 14 amps. I believe I read on the forums here that the prius accessory outlets are only good for 10 amps. I'd like to pick up a compressor and quality gauge so I can stay on top of the tire pressures.

 

I was using a Campbell Hausfeld rechargable battery powered air compressor Model C1108; $45 at Home Depot. Great compressor and doubles as a DC12V battery pack using a standard auto (cigarette lighter type) power point.

However, a Nitrogen Fill really solves the air loss issues described above.

I pay $24.95 once and get a free Nitro re-fill at each 5,000 mile dealer service. Before NitroFill, I was checking my tire pressure cold and adding regular (78% nitrogen) air at least every 2 weeks to maintain 38/36 psi. I started using (100%) nitrogen in my Gen II about a year ago and I hardly ever added air between my 5,000 mile oil changes.

 

After several years of using a 12V compressor, I now use a bicycle floor pump. Mine needs about 7 strokes per psi, compared to 15 seconds per psi on the compressor.

The hand pump takes a bit more muscle energy, but creates much less noise and takes less time than was needed to re-stow the 12V cord into the compressor storage compartment.

 

I use the bike floor pump too and it works well.