New tire psi question...

The tire manufacturers factor in the expected pressure change when determining the cold inflation pressure. The listed pressure is the minimum required to support that load.

The figures quoted for the relation between speed and weight capacity are very optimistic. In the Michelin Tire Manual, (Page 10) the chart shows that at 11-20 mph there is a 17% increase in load capacity if tire pressure is increased by 15psi over the maximum. An 8000 pound rated tire could carry 9360 pounds.
Michelin Truck Tire Manual

This procedure of carrying additional weight above rated load is for heavy truck and bus tires, it is not mentioned as a viable option for RV tires.

ClassAGeek wrote:
Good question. I have a F-53 spring loaded chassis, so using the minimum safe tire pressure is essential for a smooth ride.

Weight carrying capacity follows a relatively gentle performance curve near the low end of the scale. Being 1 psi over doesn't make you instantly safe, and 1 psi under is not suddenly dangerous. Many factors go into the weight rating. Know them and you will be safer than most.

Factor #1: Weight carrying capacity is speed related. Capacity increases greatly as you slow down. If a given tire can carry 8000 lb at 60 mph it will carry more than 16000 lbs at 20 mph and 32000 lbs at 10 mph.

Factor #2: Cold tire pressure ratings are given because tire manufacturers (and their lawyers) are worried that you might misuse the data ON THE HIGH end. Blow outs are far more common due to overinflation when the tire heats up and exceeds the rim / tire limits. Unlike the low end of the range, the Weight Carrying Capacity drops suddenly at the high limit.

Factor #3: Tires do not care if they are cold or hot. As you have noted, a tire feels cushy at 80 psi cold or hot. It feels harsh at 100 psi cold or hot.

How do I keep the lowest safe pressure? After measuring my coach on a CAT scale, I find 76 psi is correct for my rear tires and 80 psi is correct for the front according to tables provided by Michelin. (BTW: I ignore the insignificant differences between right and left. When moving, the distribution tends to equalize - so just pick the heavier side). Now here's my secret: I set my COLD pressure to 74 psi rear and 77 psi front and yet I am 100% safe. How is that?

I drive at less than 40 mph until my tires warm up and my TPMS shows that pressure has increased to my normal driving range of about 80 psi rear and 84 psi front which is well into the safe range specified by Michelin for 70 mph. It never takes more than 10 minutes - and I have the smoothest riding gasser you will ever drive.

ClassAGeek: Although I've received some excellent advice regarding my base question, your comments are exactly what I was hoping for. Thanks!! / Larry:B

Executive wrote:

goufgators wrote:
After having it weighed, let's assume that weight on one of the wheels requires that tire to be inflated to...say 90 psi. I inflate to that pressure and then when I start driving on these hot Florida roads won't the pressure still increase 10 to 15 pounds over the required pressure as is occuring now and again resulting in harsh ride and steering difficulties? / Larry

Larry, an assumption on my part is that you know this, but remember you would inflate ALL the tires on that axle to the SAME pressure, not just one tire....Dennis

Right, Dennis. That statement was only for discussion about continued pressure increase. / Larry

Daveinet wrote:
Buy a better tire that does not run as hot. My old goodyears (which eventually blew out) ran at ~180 degrees, where the current Firestones rarely get above 110 degrees.

Holy crap...I was getting worried when I would hit like 20 psi above filling pressure. I can't imagine 180 degrees...nuts.

goufgators wrote:
After having it weighed, let's assume that weight on one of the wheels requires that tire to be inflated to...say 90 psi. I inflate to that pressure and then when I start driving on these hot Florida roads won't the pressure still increase 10 to 15 pounds over the required pressure as is occuring now and again resulting in harsh ride and steering difficulties? / Larry

Larry, an assumption on my part is that you know this, but remember you would inflate ALL the tires on that axle to the SAME pressure, not just one tire....Dennis

goufgator: Try increasing the cold inflation pressure to 90 psi, and then check the increase after driving. If they do not increase as much it would indicate your tires were slightly underinflated.
However: Since the only pressure recommended is based on cold inflation, checking pressure after running (by the book more than 1 mile) has no reference so most don't check it. No idea what it should be....
I also check the tire temp with an IR gauge. I look for a temperature that is noticeably different from the other tires. My 22.5 Goodyears run 100-115 degrees in cool weather, I have seen the tires at 130 degrees, 100 degree day on fresh blacktop interstate.

I don't think your pressure increase is a concern, You could ask the tire mfgr.
I would not lower the pressure below the recommended. You should not lower the pressure in a hot tire, will be underinflated.

Here is Michelin article on RV tires,from MH magazine. Page 40 shows that tire pressure can increase 15 degrees in normal use..
Inflating Motorhome Tires

Good question. I have a F-53 spring loaded chassis, so using the minimum safe tire pressure is essential for a smooth ride.

Weight carrying capacity follows a relatively gentle performance curve near the low end of the scale. Being 1 psi over doesn't make you instantly safe, and 1 psi under is not suddenly dangerous. Many factors go into the weight rating. Know them and you will be safer than most.

Factor #1: Weight carrying capacity is speed related. Capacity increases greatly as you slow down. If a given tire can carry 8000 lb at 60 mph it will carry more than 16000 lbs at 20 mph and 32000 lbs at 10 mph.

Factor #2: Cold tire pressure ratings are given because tire manufacturers (and their lawyers) are worried that you might misuse the data ON THE HIGH end. Blow outs are far more common due to overinflation when the tire heats up and exceeds the rim / tire limits. Unlike the low end of the range, the Weight Carrying Capacity drops suddenly at the high limit.

Factor #3: Tires do not care if they are cold or hot. As you have noted, a tire feels cushy at 80 psi cold or hot. It feels harsh at 100 psi cold or hot.

How do I keep the lowest safe pressure? After measuring my coach on a CAT scale, I find 76 psi is correct for my rear tires and 80 psi is correct for the front according to tables provided by Michelin. (BTW: I ignore the insignificant differences between right and left. When moving, the distribution tends to equalize - so just pick the heavier side). Now here's my secret: I set my COLD pressure to 74 psi rear and 77 psi front and yet I am 100% safe. How is that?

I drive at less than 40 mph until my tires warm up and my TPMS shows that pressure has increased to my normal driving range of about 80 psi rear and 84 psi front which is well into the safe range specified by Michelin for 70 mph. It never takes more than 10 minutes - and I have the smoothest riding gasser you will ever drive.

My Toyo's on a hot day in Florida max out at around 115 F.

The rule of thumb is for every 10 degree Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi.

goufgators wrote:
I try to run 85 tire psi on my coach but, as I drive, pressure increases to, often, 100 psi. That's not the maximum rated psi for the tires so that doesn't bother me but what does concern me is that at 100 psi the ride is harsher and control (steering) becomes a problem due to a tendency to wander. The coach drives much better at the lower 85 psi. My question is: since 85 is the correct psi...can I (should I) inflate to...say 75 psi (cold) knowing that a few miles down the road the pressure will be at the required 85 or 90 psi? Thanks for any input! / Larry

Air pressure is always cold psi and calculated by weighing each axle and then according to the inflation charts. I always add 10 - 15psi over that for a cushion and/or load imbalance of either side, not to mention changes in ambient temp. while traveling. I doubt seriously if your tires are increasing that much with temp, but even if so that's the way it has to be.

Buy a better tire that does not run as hot. My old goodyears (which eventually blew out) ran at ~180 degrees, where the current Firestones rarely get above 110 degrees.