Tire pressure- Force HD or DX3

[bclemens]

Originally Posted by Invisibleman

A quick search of tire manufacturers recommendations does, in fact, suggest setting tp at ambient temperature. Even though that makes absolutely no logical sense to me at all. Tires run in Northern climates are going to be consistently in a totally different range. especially at different seasons, than those in the southern climates. It seems this just isn't a black & white subject.

It actually is a black and white subject. You check the pressure at ambient air temp...that is what all the tables are designed around.

And in norther climates 95PSI is the same as 95PSI in the south. Air shrinks when cold, expands when hot.

So if the recommended pressure is 95psi, in the north let's say it is 35 degrees, we set the pressure on the tires to 95PSI. The air molecules move less when cold so they take up less space. When driving the tires will heat up and the pressure will rise.

If the recommended pressure is 95 psi, in the south lets say it is 90 degrees before we depart, we set the pressure to 95psi. The air molecules are already moving more because it is warm...so in effect they are already preheated and the volume of air has expanded. When driving the tires will heat up more and the pressure will also rise.

Up here, if its 70 degrees one day (my Jeep tires ambient are 40 psi) and the next day it is 20 degrees (sometimes that happens in the same day) my tires might be 35psi when I get up and I need to add air and I did nothing...but the temps made the air take up less volume, thus lowering the pressure.

edit: I can't believe I chimed in on a tire pressure thread...I think I need counseling.

[JACA]

Originally Posted by Invisibleman

I watch my tire pressure live on the TPMS. Its new to me, even after 37 posts Bill, that the manufacturer recommended pressure is based on "cold fill pressure." Here in Az you can go from 70 degrees in the morning, to 100+ degrees and every pound of 10-15 psi increase after you get going. I would have thought the manufacturer, and everyone else, is talking about running pressure after the tire is up to temp. Now I am not so sure.

Regardless of whether you are in a cold or warm climate, the temperature increase from early morning to late afternoon is still negligible. You could start with 30 degrees in the morning and end up at 70 degrees in the afternoon, or you could start at 70 and end up at 110 degrees - it's the same level of increase in tire pressure - your air pressure should only go up by 4-6 PSI.

If you are getting 10-15 PSI increase in your tires just from driving them, you may have other issues causing excessive heat buildup.

[Invisibleman]

I assume neither of us are PE's? Nonetheless, it stands to reason the tire manufacturers are basing their tp stats based on things like the rigidity, and flexibility of the tire, sidewalls, etc. I assume the they want the tire to be "x" hard, for lack of a technical term. To say, fill-em up at ambient temp wherever you are and whatever that happens to be, and run them at whatever temperature they end up at, is preposterous. Personally, I would be inclined to air my tires based on approximately (emphasis added) where I want them to run when at driving speed after warming up. That should not vary much day to day, but it might season to season, or geographically. Then again, keeping them in a good range probably is close enough.

[Invisibleman]

I do believe 10+ psi increase was not unusual. Maybe I am not remembering correctly but I paid attention because the TPMS provided good live data. I'll check next time I am out. Although i did check with the manufacturer of the TPMS at one point and confirmed the temperature being reported can be distorted depending whether or not the sender was on the sunny side vs the shady side, etc. That was during a long drive headed due West on I-10 in Texas and the south side tires read measurably hotter then the north side. Can't say the pressure was any different.

[Invisibleman]

JACA. I think in part we are saying the same thing. I do know that my tp is notably different depending on ambient temp. And a lot more than 4-6 psi. If I go from 95 ambient in the phoenix valley, starting out at 95 psi, I would typically be seeing 110+ easily once up to road temp/speed. If I drove to Flagstaff and the temp was 45 the next morning the tires might be down to low 80s. Once I would get up to driving speed, if the weather stayed cool, I'd be on 90+ psi tp. I think on average a 10psi increase. That's just what I saw. I can't say I could tell much ride difference, and that was on an Isata-5. But this entire thread started on tp and ride. And here we are. I am thinking that if 95 gives a good ride, then I'd be looking to try to be around that, however I start out. Then again, having made a couple 2700 mile trips East to West and back, I never did anything with the tire pressures and never noticed anything even though the tp varied significantly. So perhaps Bill was right. Enough is enough on this thread. Thanks for all the input and suggestions.

[JACA]

Originally Posted by Invisibleman

JACA. I think in part we are saying the same thing. I do know that my tp is notably different depending on ambient temp. And a lot more than 4-6 psi. If I go from 95 ambient in the phoenix valley, starting out at 95 psi, I would typically be seeing 110+ easily once up to road temp/speed. If I drove to Flagstaff and the temp was 45 the next morning the tires might be down to low 80s. Once I would get up to driving speed, if the weather stayed cool, I'd be on 90+ psi tp. I think on average a 10psi increase. That's just what I saw. I can't say I could tell much ride difference, and that was on an Isata-5. But this entire thread started on tp and ride. And here we are. I am thinking that if 95 gives a good ride, then I'd be looking to try to be around that, however I start out. Then again, having made a couple 2700 mile trips East to West and back, I never did anything with the tire pressures and never noticed anything even though the tp varied significantly. So perhaps Bill was right. Enough is enough on this thread. Thanks for all the input and suggestions.

Any discussions about tire pressure are very much like discussions about engine oil and batteries. Lots of information out there - some facts, some myths, and a universe of opinions. In most cases, there's no right or wrong answer...even among the "experts."

[FOURWHL]

Originally Posted by Bill Davis

Only 37 post on this one so far. Let’s get with it folks.

[bclemens]

and on that, I'm out.

[Tireman9]

Originally Posted by Webefine

I run 95 all around also. Why, because I weighed my rig fully loaded and then went to the Michelin load/inflation chart for my tires and found 95 psi is what I needed with a little cushion.

Wondering how much "cushion" you have. Stated as a % of the tire load capacity when inflated to your 95 psi.
You many not realize but most card come with a "cushion" that is 30% or more.
When you set the inflation based on the Load tables that means you are looking at 0% cushion.

[BBC Cobra]

I weighed my rig during a trip (loaded as normal) then I went to the Michelin website and looked up their inflation guide and aired up per their recommendation based on my weight per axle. Drives and handles fine and I expect to get good tire wear. End of story...

[BBC Cobra]

Invisibleman, most truck stops have certified scales and for about $20 you can get weighed, just ask at the register, it’s easy to do. You will want to be loaded like you would normally travel when you do it.

[Invisibleman]

Thanks Ron. That sounds like a plan. I've always wondered, and it would be good to know. I usually have a pretty consistent setup so it wont vary much once I get the data.

[Tireman9]

Originally Posted by Webefine

Another reason to weigh your rig and adjust your air accordingly. If the door sticker says 110 psi that would be enough air in the tires to carry the rig at the full GVW. When my DX3 is completely loaded I am still almost 5,000 pounds under GVW therefore I don’t need 110 pounds of air.

Tires must be capable of supporting the GAWR. I dare say I bet your Front GAWR is not the same as the rear GAWR.


So what ratings are shown on your Certification sticker, AKA tire Placard?

[ProfessorBB]

I find this discussion enlightening. Picked up our new DX3 in January, then completed our inaugural one-month 3,000 mile trip in February. Temps ranged from the teens in the foothills of the Rockies to low 70’s in the California desert. Tires remained at exactly 110psi in the mornings, then went as high as 129psi during the day. All were consistently within 3 or 4 psi of each other despite which side was facing the sun. The ride was acceptable, even smooth for the most part, except for cracks and potholes in the road which are all over most roads in the west. I haven’t weighed the unit, but believe I carry the same 2,500# of stuff the rest of you carry, give or take 500#, and I was dolly towing a Mini Cooper. Talked to my brother who retired as a lead bus mechanic for RTD in LA and was advised that they ran their busses at over 120psi, although city transit busses never go much over 50 mph, and the weight they carry can range across the entire spectrum of capacity. He suggested that I just leave the tires at 110 as recommended by the manufacturer, but if I am inclined to lower the pressures as some of you have done, to do it in 5# increments and check the difference in ride. I think that’s what I’ll do.

[Webefine]

https://www.michelinb2b.com/wps/b2bc...s_Brochure.pdf
If you have Michelin tires in this link it has a load chart for each of there tires. They recommend weighing each axle and then setting the air per the load chart for the heavy end of each axle. This is what I have used plus some cushion that I figured in.
For round numbers I have about 1200 to 1500 pounds of cushion per axle at 95 psi

[Webefine]

Originally Posted by Tireman9

Tires must be capable of supporting the GAWR. I dare say I bet your Front GAWR is not the same as the rear GAWR.


So what ratings are shown on your Certification sticker, AKA tire Placard?

The RV is in storage at this time. As I stated I go by Michelins load chart for my tires per axel. You are correct my front and rear GVWR are not the same but I also don’t have dual wheels on the front. 95 psi works for both axels in my case with 1200 to 1500 pounds of cushion in the equation.

[dedobias]

Originally Posted by Webefine

I run 95 all around also. Why, because I weighed my rig fully loaded and then went to the Michelin load/inflation chart for my tires and found 95 psi is what I needed with a little cushion.

Great answer. Unless the weight changed, the sticker is the right pressure. Going higher is over inflation with all the negative effects
.

[Tireman9]

The inflation in the tables is ALWAYS the COLD pressure before driving and before the tires warm up from being in the Sun.
We tire design engineers know about pressure change and see that in ALL our testing.
The pressure in the chart is the MINIMUM required to support the stated load so running more is not a problem some seem to want to make it out to be.
With the exception of a few Michelin tires you will find that all tires have the same load capacity for the pressure shown in the tables, so you can use a Bridgestone table for a Goodyear tire and you will see the same numbers.

[Tireman9]

Originally Posted by ProfessorBB

I find this discussion enlightening. Picked up our new DX3 in January, then completed our inaugural one-month 3,000 mile trip in February. Temps ranged from the teens in the foothills of the Rockies to low 70’s in the California desert. Tires remained at exactly 110psi in the mornings, then went as high as 129psi during the day. All were consistently within 3 or 4 psi of each other despite which side was facing the sun. The ride was acceptable, even smooth for the most part, except for cracks and potholes in the road which are all over most roads in the west. I haven’t weighed the unit, but believe I carry the same 2,500# of stuff the rest of you carry, give or take 500#, and I was dolly towing a Mini Cooper. Talked to my brother who retired as a lead bus mechanic for RTD in LA and was advised that they ran their busses at over 120psi, although city transit busses never go much over 50 mph, and the weight they carry can range across the entire spectrum of capacity. He suggested that I just leave the tires at 110 as recommended by the manufacturer, but if I am inclined to lower the pressures as some of you have done, to do it in 5# increments and check the difference in ride. I think that’s what I’ll do.

What others run in their tires and especially when trying to compare city buses with your RV is definitely NOT the way to learn what inflation you should run in your RV tires. Get the weight on each tire position or at least on each axle. If you can only get the axle weight then assume a side to side load split of 1 to 2% imbalance so you might want to use 52% of the truck scale weight when consulting the tables and inflate all tires on any axle to the same level which is what the HEAVY end requires per the table.

[Invisibleman]

Tireman. The explanation that the manufacturers stated pressure is a minimum is excellent info. I mistakenly assumed that number was the desired running pressure. So, as can be seen in this thread, with pressures increasing 10-15+psi once getting on the heated roadways, that not being an issue is good to know. I have to say sometimes in the Az or Texas heat and pressures getting well above that "minimum" always concerned me a bit. Also, as stated several times here, we all seem to get to know our particular tires and rigs and know how things heat up, so again, knowing that minimum is good to know. I am assuming that for half an hour or so at 10psi +/- below minimum is no big deal? Thank you.