I've been trying to get a handle on "cold" tire inflation pressures, and found my myself chasing pressures all over the place.
Today I was told the industry standard is 70° My contact, an engineer at PressurePro told me.
I was also told tire pressures change by 1psi per ten degrees of temperature. My experience indicates 2psi per ten degrees.
If I fill a tire to 100 psi at 70°, I can expect it to appear 4-8 psi low at 30° or 4-8 psi high at 110°. So, depending on ambient temperature, tire pressures ranging from 92-108 psi can all be correctly "100 psi cold"
At least I can easily understand this and keep from unnecessarily adding and removing air. Fortunately my PressurePro sensors tell me ambient temperature, so I can take it into account.
Unfortunately, I have yet to figure how altitude and barometric pressure fit into this.
Another thing I learned is that all TPMS sensors, internal and external, do a very poor job of reading tire temperature. They are either spinning in the air or hugging the rim, far away from where the rubber hits the road.
Tom,
That is INCORRECT per all the tire manufacturer's websites.
"Cold temperature" for a tire is defined as "temperature before driving/having external heat sources such as SUN on it". Doesn't matter if it is in the middle of the Alaska winter of West Texas summer.
Yes, many times a year, you are chasing a "moving target". Boyles law/Ideal Gas Law for a LONG time have shown that PSI is very dependent on temperature.
I just chased this on a quick car trip to Kansas with ambient temperature from 84 degrees F to 34. My TPSM on the car went from 36 to 30 overnight!
And, when we head to Angel Fire, NM this summer I KNOW I will have to add air (Texas heat to temperatures at 8,400').
For those who may want to adjust tire pressures, here is a density altitude calculator. Very easy to just fill in the four boxes.
Density Altitude Calculator - English/Metric (https://wahiduddin.net/calc/calc_da.htm)
I use it mostly for high altitude runway length but it can come in handy for many RV applications.
Pierce
Pierce,
I didn't go to the link (Dianne is calling for dinner time) but altitude has a substantially smaller affect on tire pressure than ambient temperature.
If you have a minute, might calculate tire pressure at same ambient temperature at sea level and a mile high. Let us know.
Thanks, and, off to dinner before I get in "Time Out".
If you REALLY want it right here is a commercial OTR setup. Expensive. Worth it? Up to you.
My guru buddy only installs these.
https://www.bendix.com/en/products/tpms/tpms_1.jsp
Brett,
Good article here with real world pressures staying about the same from sea level to 5000 feet as the loss of outside pressure at the increased altitude is offset by the cooler air. Translation is at 5000 feet and the same temperature, the tire pressure gauge will read 2 to 3 psi HIGHER than at sea level but as the temperature is cooler (adiabatic temperature lapse rate is 3.5F or 2C per thousand feet) the air will be denser so close to the same indicated pressure on the tire gauge.
Here is the a partial quote from the page:
When it comes to measuring tire inflation pressure, it is important to realize there is a difference between atmospheric pressure and gauge pressure. Most pressure gauges (including all tire pressure gauges) are designed to measure the amount of pressure above the ambient atmospheric pressure.
Imagine removing the core from a tire valve and allowing the air to escape. Even after the air has completely stopped rushing out of the valve, the tire is still experiencing 14.7 pounds per square inch of atmospheric pressure. However, a tire pressure gauge would read zero pounds per square inch of tire inflation pressure because the pressure outside the tire is equal to the pressure inside.
Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi.
However, the differences indicated above assume that the tire pressures are measured at the same ambient temperatures. Since tire pressures change about 1 psi for every 10° Fahrenheit change in ambient temperature, the tire pressure measured in the relatively moderate climate typically experienced at sea level will change when exposed to the colder temperatures associated with higher elevations.
This means that in many cases differences in ambient temperature may come close to offsetting the differences due to the change in altitude. Depending on the length of their stay at different altitudes, drivers may want to simply set their cold tire pressures the morning after arriving at their destination, as well as reset them the morning after they return home.
Quote and chart thanks to www.tirerack.com
The problem is when the temperature up at altitude is normal or warmer than expected. Since the air molecules are further apart, the radiator, intercooler, radiator fan are all less efficient so overheating may be experienced. Diesels with a waste gate turbo may not experience any power loss until the waste gate is closed and the boost starts to drop so they will continue to produce sea level horsepower up to that point. This is called Critical Altitude. Food for thought: when the intercooler (CAC) is less efficient but the waste gate keeps sea level boost, the thermal load on the engine will be increased. Once the boost starts to decrease, a mechanical diesel will have a lower fuel air ratio (more fuel) so the EGTs will go up. Bottom line is warm days at altitude may have some surprises, not all of which may be pleasant.
Brett, I've ignored the first dinner call so it may be "time out" for me also. :D
Pierce
Rather than getting a college degree in tire pressures I would just ask my neighbor Larry ( a two million mile OTR trucker) what he does.
Bet he does like I do, pump them up to the max listed pressure on the sidewall and leaves them there. We kept them at max and checked them each week on service day at the dept at the recommendation of Firestone for emergency vehicles. Bus companies do the same.
I posted this before but this is from our MB sedan's cold tire recommendation. As you can see, they recommend 34 psi cold in the rear tires when loaded. When it's crunch time at 100 mph or higher and you need max handling, braking, the least amount of tire deflection, they recommend raising the pressure 7 psi. That's 20 PERCENT. Imagine raising our 295s 20%.
Pierce
Mmm..makes me wonder if passing gas could be affected by altitude. Anyone have a chart?
Reminds me of the one about the guy having a flat and while pulled over changing it another motorist stops and asks; "Have a flat?"
The guy replies; "No, Numbwell, I rotate my tires every 3000 miles regardless of where I am!"
Love y'all...................have a great night.
Yes I have that chart, but it all boils down to the mass out is the same. Boyle's law is technically for ideal gases but close enough for typical air, but humidity or passed gas does not move across a space at the speed of sound and is highly subject to air currents. Pressure gauges are notoriously inaccurate. +/- 3-5% for a good one, PSIG.
Passing gas is definitely affected by altitude. At flight school they put 10 of us in a altitude chamber to demonstrate hypoxia. By the time you started losing muscle function, you were
hoping you would pass out cuz it was stinking so bad in there.
Boy has this tread taken a WRONG TURN!
Pretty sure it was Randall ^.^d
Sorry - my bad. :not:
Published tire load charts based on undriven cold pressure should be adequate as the manf. Legal department most undoubtedly has adjusted those charts. IR temp gun while traveling is a good practice, I do a walk around the coach about every 2 hrs of travel to strech and check.
What Phred said, no need to split hairs or ain't that critical.
So before I leave Ruidoso and it's been 18F overnight and it's still in the low 20's, you want me to air my front tires to 110 psi? If I do that, when I get to Ft. Stockton and it's over 85F and the asphalt is over 110F my tires will be severely over pressurized and I am not seeing pressures for HOT tires. Do I stop and let my tires cool overnight?
Tires and wheels are rated for cold pressures. They are NOT over pressurized. Your example is seen a thousand times a day in the trucking/bus world. I would not touch them.
Pierce
Really ??? Are you serious ? As a commercial truck driver, I check the tires once a week... the rest of the time they get a thump with my hammer.
Seems to be the procedure for most of the truckers I know !!
Safe Travels
Dave
I agree. I don't add air when the temperature drops. I was just pointing out a scenario
Steve,
In those cold temperatures, how many PSI BELOW minimum recommended PSI (that is what the tire manufacturer's inflation tables give you) are you?
That would guide me to making my decision on adding air or not.
I mentioned "target pressures" a year ago and you all scoffed at the concept. Of course there is a sweet running pressure for your bus. Mine seems to be about 78-80# front, that is after running 2+ or more hrs down the road. In FL I start at 72# ish at 80 * Ambient.
The same tires will never get there in Canada starting from 72. The road and air temps are much cooler . I would need a couple more #s to hit the perfect running pressure( target pressure) . As it is, the Northern roads are so rough that a few pounds less works fine , so it seems to work it self out. :)
Is it a big deal? No . These busses will see maybe 8# variance over the normal range of temps that they gets used at. Starting at the same pressures.
Like I said, it seems to work itself out. The hot regions of the country have smoother roads and a few more #s of air wont bother most of us. While the cold parts have sucky roads and a little less dynamic pressure softens the blows.
Our tires won't pop until well over 300 psi and the Alcoa aluminum wheel are forged so they bend in an accident or any kind of overpressure and don't chunk off like cast aluminum wheels. Any worry about popping a tire or a wheel failing because of overpressure is unfounded.
I never have weighed the coach and just max the tires. Even if I did, the weight is always changing depending on how much water, waste, propane, etc is in the coach. When you live in the Sierras, you don't want any extra weight on-board so I carry minimum water and fill at the campsite. Since the propane tank and fuel tank are mostly on the right side, any change in level will effect the corner weights including the left rear corner. I can't even imagine chasing pressures in any case.
When I drove race cars, I would weigh a few times a season as the cars had weight jacks at every corner but watch tire pressures to keep them within a pound. As I remember, on the stock car, the left front got 17 psi, RF 43 psi, the rears about 41 RR and 36 LR. The rear tires were even measured for circumference with the larger always going on the right side.
But our rolling apartments are not race cars so tire pressures are at the operators discretion. I find the ride perfectly fine at the max pressure so leave it that way. Different tire brands may have a harder or softer ride depending on the construction. Too low pressures on a hot summer day in the desert can be disastrous. When you lower tire pressures, you always raise the tire temperature.
Vehicle and tire manufacturers want to sell more of their product than the competitor so if one has a stiff ride and the other does not, guess which is going to sell? Ride is the number consideration while keeping the vehicle safe is important but seems to be secondary in some cases. Yes, you should never go below their recommendation but how many drivers check their tires? Tires never add pressure by themselves but sure do lose air over a period of a few months. That's why Mercedes (for example) adds pressure for full loads and adds another 20 percent for high speed driving. So much for the myth of smaller contact area, etc.
I remember the higher than normal fatality rate of Ford Explorers back in the 1990's. Firestone at that time, produced a tire with a higher than average failure rate at high temperatures and Ford recommended 26 psi for the Explorer. A recipe for disaster. The tire may have come from Ford with 26 psi and perhaps it was checked during oil changes but I'm sure many of those tires that failed were below the recommended pressure and since many of the failures were in Texas, the combination of high speed, high temperatures and low pressures cost quite a few people their lives.
My take on tire pressures.
Pierce
Pierce, I find your response accurate. Years ago, both local mfgs Country Coach and Monacco as well as tire dealer Les Schwab went to putting max air press in tires for two main reasons. They found owners not checking tire pressures often, and also overloading after they weighed and changed pressure. This was primarily done to eliminate law suits and claims for underinflated tire failure.
I did not agree (RVs wandered and road rough), on my own rv I weighed individual corners often (had access to free public scale), and inflated to 5 lbs over truck tire mfgs air pressure guide.
I had a test long ago that worked then and have no idea if it still works but I have posted this before.
I found a piece of white concrete and dropped a large area of water on it.
Or drove through a puddle
Driving through it at an arc the tires as the dried out the foot print seems to show the actual contact area of the tire.
Adjusting the tire pressures a few pounds then repeating the test would show a different contact pattern.
Surely seemed to work on the older coaches and the first Unihomes.
I and a lot of owners back then drove in rain and snow and ice.
Having a correct contact patch made hydroplaning less of an issue.
Plus on ice traction issues are magnified
Are you saying that Firestone, my fire department and our Mercedes tire pressure sticker are putting the public at risk maxing the tire pressure?
When meeting customers in different parts of the country, I usually spent a least a couple of hours on the Autobahn every day with the speedo pointing straight down indicating about 155 mph (actually 143 mph true) With the normally poor weather, we still were over 160Ks or 100 MPH if traffic allowed in the rain. Everyone was. Driving that fast, checking tire pressures with a close adherence to the glove box/gas filler door sticker was done all the time. All the gas stations had mini pressure tanks to lift off the air filler and take to the vehicle to fill the tires. This is how important they considered the pressures.
After the wall fell, the Autobahn was filled with the tiny Trabants and the normal 2CVs. They would pull out to pass a truck while maxed at about 60 mph and when approaching them at 150 MPH, it was close to ABS activation for quite a few seconds before matching speeds. Scary feeling when you can't apply any more brakes but the vehicle keeps getting closer and closer in slow motion. Keeping the recommended pressures meant the shortest possible stops and best handling in all weather conditions.
While the MBs that the factory came to the states with Michelin XAS, in Germany we used the twice as heavy XWX V rated tires as they were mandatory for the speeds involved.
Now I'm using V rated Atturo tires from China. How the world changes.
Pierce
Well now, this appears to debunk what I was told about 70°F being the industry standard "cold" tire pressure.
Maybe it is, but just for testing purposes.
My take is adjusting so that the pressure will be correct plus 5 psi at 70° is still a good plan. If the morning air temperature is 90°, let the tire pressure be 2-4 psi over. That's still good. Las Vegas on a 110° summer morning, let it be 4-8 psi over. No problem. On my coach the placard and weight tables call for 115 psi in front. Adding 5 for safety and 4-8 for Las Vegas puts it at 124-128 psi. It's a darn good thing the maximum cold inflation pressure on the sidewall is not a "do not exceed" maximum. I use my PressurePro FX to check pressures every morning, and don't ever take air out.
On the other end of the spectrum, I've seen cold mornings in Quartzsite ( not Montana winter cold, but plenty cold for me ) were temps are in the 20's.
Here my front tires might read 5-10 psi low. I use my 5 psi safety margin to good advantage.
I need to go back and see how much pressure drops with a 50° temp drop. The industry says 5 psi. I seem to recall it being a bit more. But again, I'm not worried too much. For sure the weight tables have a safety factor built in, and both air and tire temperatures rise as the sun also rises and the tires roll.
If you are 20# over the tire chart pressure , there is a 98% chance that you have compromised the stopping distance . More pressure = less grip, less than optimal footprint, more dynamic load pressure. all bad.
I don't want to upset anyone but why do people overthink this? Wait for a couple of hours after driving coach then put in recommended pressures for the load on the axle. Tire company will supply this info if you ask nicely. Use wheel that has highest load as reference. I add a few pounds pressure to this. Make sure that tires are same temperature IE one side not in sun. The thing to avoid is under pressure. Truckers do not change pressures for the load and it seems OK, road alligators are mostly caused by running low or flat. When I was in the fleet business our vehicles would experience everything from empty weight to the occasional gross overloading (bad operators). We had very few tire failures, mostly on garbage trucks caused by constant curbing and damage at the landfill. Pressure will increase as the tire heats up when driving. Don't worry about altitude you can't get the coach high enough up to make a real difference in the US.
Keith
Contact patch is ONLY and I repeat the ONLY connection to the road surface from the coach, Technology or anything else can't overcome this fact. Overinflated tires for any reason are if fact overinflated with a reduced contact patch, our coach weights probably don't fluctuate much more than 5% on a day to day or yearly basis. Just call me crazy, but I like to be at least 20 feet from the scene of an accident. Do what makes you responsible!