Tire Pressure data - 2019 Cornerstone - pressures when traveling

[Gary.Jones]

There have often been questions about typical tire pressures, tire loads from coach weight, and common tire pressure settings. I have collected my data every year but recently collected the hot tire pressures while traveling down an interstate at ~ 68 mph. I thought I would scan all the data and post it up here which might give others a point of comparison for their own calculations.

The data is attached.

Page 1 is a chart of my 4 corner weights on my 2019 Cornerstone, loaded for a typical 3 month trip. These weights and pressures were set in April 2021 and then updated. You can see my weights (loads) on each tire position and the pressures that I have chosen to run my tires given those weights. I also have noted at the bottom of the first page the baselines that I have programmed into the Valor TPMS and the fixed pressure points which will trigger a warning and then a red critical warning on the dash./

Page 2 shows the voltages of the Valor tire pressure sensing units that are located in all my coach tires and also in my four Honda CR-V tires. Now, it is important to note that these sensors were replaced at all wheels on my coach in November 2020 because almost all the batteries has failed after a little more than 3 years. The best guess of everyone is that I got old batteries installed.... my coach was one of the first 2019 coaches to come down the line and most likely received old batteries at Spartan used in testing and developing this new technology on our K3 chasses.

Page 3 is a continuation of notes from page 2

Page 4 is the Michelin Inflation Chart for "truck" tires which includes the tires on my Cornerstone. My steers are 365s and al the rest of the tires are 315s.

Page 5. The pressures I have chosen to run in my tires is shown on page 1. Page 5 shows the pressures shown on the dash for each tire that has been running down an interstate at 68 mph for approximately 90 minutes when sampled. Below the dashed lines are the same pressures measured an hour or more later after more driving. The pressures were from the durations indicated, in moderate environmental temperatures (somewhere in the 70s and 80 ambient)

Hopefully, this will give you comparison data on what you have seen in your coach. My chosen pressures in the coach are simply a value that was somewhat above the minimums recommended by Spartan/Michelin and shown in the Michelin tables..... I don't want my tires below their appropriate pressures for the loads carried by each tire, but also not substantially higher than necessary (ride gets rough) to safely handle my loads (some owners use the max posted behind the driver seat but that makes no sense to me and most people, so my pressures are set to be safe.

Gary

[vsayler]

It would be interesting to see the pressures after driving for several hours on Interstate 15 into Las Vegas in August with outside temps in the 115 range..

[LWBAZ]

Gary, I’ll offer a few comments for what they may be worth.

On your steer tires, the heaviest side weighs 8,920 pounds. If both sides of your steer axle weighed that much your total steer axle weight would be 17,840. Based on the inflation table for 365 tires, 100 PSI would almost be enough for that weight, and 105 PSI would be plenty. If you want to run 115 PSI just because or for any other reason you are of course free to do so, but I’m not sure why you feel that much air pressure is necessary.

On your drive tires, the heaviest side weighs 9,760. If both sides of your drive axle weighed that much your total drive axle weight would be 19,520. Based on the inflation table for 315 tires in a dual configuration, the minimum recommended pressure of 85 PSI is capable of handling 23,230 pounds. In other words, 85 PSI is more than sufficient to carry the load on your drive axle tires. There is, however, another consideration, which I’ll outline below.

On your tag tires, the heaviest side weighs 6,320 pounds. If both sides of your tag axle weighed that much your total tag axle weight would be 12,640. Based on the inflation table for 315 tires in a single configuration, the minimum recommended pressure of 85 PSI is capable of handling 12,830 pounds. So similar to your drive axle situation, 85 PSI is sufficient to carry the load on your tag axle tires.

Now let’s discuss the other consideration. There is a school of thought which was conveyed to me by a 40 year veteran of the truck tire industry. This individual held positions ranging from entry level to up to and including CEO of a Japanese tire company’s U.S. operations. The theory is that for “high cube” vehicles, the drive tires (and tag tires if a tag axle is present) should be set to a minimum of 75% of the maximum recommended inflation pressure.

The classic example of a “high cube” vehicle is a regional delivery truck, where the box behind the cab is typically 12 to 13 feet high, and is often loaded with heavy cargo up to or near the full height of the interior ceiling. With such vehicles, the idea behind running at least 75% of max inflation pressure is to minimize tire sideway flex when cornering or driving in crosswinds. As you might expect, big motorhomes qualify as “high cube” vehicles.

With 315 tires, the maximum recommended inflation pressure is 130 PSI. 75% of 130 is 97.5 PSI. Rounding up, let’s call it 100 PSI. So if you subscribe to the 75% of max for high cube vehicles rule, you’d want to run 100 PSI in your drive and tag tires. If you don’t subscribe to the high cube rule, 85 PSI would be sufficient. You’re running 90 PSI in your drive and tag tires, which is kinda-sorta in the middle ground, but closer to the “don’t subscribe to the high cube rule” side of the choices.

For what it’s worth, the corner weights on my 2018 Cornerstone are similar to yours. I run 105 PSI in my steer drive tires and 100 PSI in my drive and tag tires. Not suggesting you should do the same – just related what I’m doing and why.

[Kid Gloves]

130psi is the maximum sidewall pressure for the 315’s. It is not a maximum recommended inflation pressure.

Tire manufacturers provide load and inflation tables which show a minimum recommended pressure to support a given load. The tables do not provide maximum recommended inflation pressures.

[Gary.Jones]

Larry

As always, I appreciate your opinion and your ideas. You may remember that we have "argued" this issue before. I do not ascribe to the high cube concept.... I don't believe that it applies to our situation. I used to drive freight and also furniture semis (as driver/loader/packer) for North American van lines. In both cases, most drivers that are any good, load their loads from floor to ceiling and heavy in the nose. At some point, a driver tapers his load down as it moves back to the rear of the trailer. You dont overload the tractor drive axles. This is certainly true for furniture. It is usually true for less than load (LTL) freight lines.... you load the heaviest items in the nose of the trailer.

That is simply not how a coach is loaded or how the weight is distributed in a coach, so although your buddy might be right on based on trucks and trucking, I dont think he knows much about motorcoaches. I have never heard anyone at Spartan advocate that logic when I have talked to them about tires and loads, I have never heard that argument from anyone at Entegra or anyone at NIRVC, and finally, I have never had anyone make that argument at tire dealers that I have talked tires to. That is just a difference in opinion of what rules apply. I just dont see that logic for our coaches.

Now, the operative word that I used in my description when I stated my set tire pressures, is that that pressure is arbitrary. I dont want my tire pressures set to the maximum plaque pressures (although there are owners on here that do that !!??!!) and I see no reason to set pressures to the absolute minimum pressures of the tire inflation tables. I choose to run my tires a few pounds above the minimum pressure. I am well within the safe pressures (actually more conservative than the Michelin tire engineers). Now, if my ride was bad (hard), then I might reconsider dropping a little but my ride is just fine, also my tire wear is great.

So, at this point, my coach is set up in a safe way, and it is set up to be comfortable and not harsh. I see no reason to rework my pressures. My calculations were done with empty black and gray tanks and full water, however, a lot of time, I am heading down the highway with black and gray tanks that are not empty so my loads are actually higher in the rear of the coach than these tables, but there is plenty of "margin" on the drives and steers.

Gary

[LWBAZ]

Originally Posted by Gary.Jones

although your buddy might be right on based on trucks and trucking, I don't think he knows much about motorcoaches.

The last I knew he still had his 2009 43 foot Tiffin Allegro Bus which he bought new.

I'm not suggesting anything you're doing is wrong or dangerous. Just offering my perspectives and opinions.

[LWBAZ]

Originally Posted by Kid Gloves

130psi is the maximum sidewall pressure for the 315’s. It is not a maximum recommended inflation pressure.

Tire manufacturers provide load and inflation tables which show a minimum recommended pressure to support a given load. The tables do not provide maximum recommended inflation pressures.

What exactly is “sidewall pressure?” How is it measured?

If the highest pressure shown on the inflation tables isn’t the maximum recommended inflation pressure, what is? And how would an owner make that determination?

Below is a photo of a portion of the sidewall on one of my 315/80R22.5 Michelin tires. I think the photo is readable, but in case not here is the key information:

Max Load Single 9090 lbs at 130 PSI COLD
Max Load Dual 8270 lbs at 130 PSI COLD

So if 130 PSI isn’t the maximum inflation pressure should we similarly presume that we could load a 315/80R22.5 tire to more than 9090 pounds in a single configuration or more than 8270 pounds in a dual configuration by increasing the air pressure to 135 PSI, 140 PSI, or some even higher pressure?

[Kid Gloves]

Originally Posted by LWBAZ

What exactly is “sidewall pressure?” How is it measured?

It is the pressure that is stamped into the sidewall. It can be measured with a pressure gauge

If the highest pressure shown on the inflation tables isn’t the maximum recommended inflation pressure, what is? And how would an owner make that determination?

It is actually the minimum recommended pressure necessary to support the maximum rated load. While also being the maximum pressure to which the tire should be inflated.

Below is a photo of a portion of the sidewall on one of my 315/80R22.5 Michelin tires. I think the photo is readable, but in case not here is the key information:

Max Load Single 9090 lbs at 130 PSI COLD
Max Load Dual 8270 lbs at 130 PSI COLD

So if 130 PSI isn’t the maximum inflation pressure should we similarly presume that we could load a 315/80R22.5 tire to more than 9090 pounds in a single configuration or more than 8270 pounds in a dual configuration by increasing the air pressure to 135 PSI, 140 PSI, or some even higher pressure?

No. But your mixing of the terms “maximum inflation pressure” aka: sidewall pressure, and “maximum recommended inflation pressure” dare I say a nonsensical term, could be the source of your confusion. Tire manufacturers recommend minimum inflation pressures in their charts. The maximum pressure shown on the sidewall is not a recommendation.

[LWBAZ]

Originally Posted by Kid Gloves

No. But your [you’re] mixing of the terms “maximum inflation pressure” aka: sidewall pressure, and “maximum recommended inflation pressure” dare I say a nonsensical term, could be the source of your confusion. Tire manufacturers recommend minimum inflation pressures in their charts. The maximum pressure shown on the sidewall is not a recommendation.

So the "tire manufacturers recommend minimum inflation pressure in their charts" but the highest PSI listed in the same charts isn't a recommendation?

I’m willing to call it whatever you like, but as I understand it the manufacturer is saying the cold inflation pressure of the tire should not exceed that specified PSI. At the very least I’d consider that a recommendation. If you prefer to call it a stipulation, requirement, or some other non-discretionary term I have no issue with that.

[andy29847]

Tires hold the air. The air carries the load. I run the recommendation on the tire chart plus 5 pounds. The 5 pounds is to allow for tire pressure fluctuations caused by outside temperature.

My pet theory is that if a tire doesn't hold air, it is no good. To clarify, if you have to add air frequently, something is wrong. The China bombs on my 5er several years ago did not hold air.

Air pressure gouges are generally accurate to plus or minus 4%. I have multiple tire gouges. Deciding which one is right today is bothersome.

I also have a tire pressure monitoring system on my coach. My sensors are on the valve stem on the outside. I find that the temperature function has little value. The indicated temp fluctuates a lot with exposure to the sun and the outside temps. I was attentive to the rise in pressure shown on the TPMS when I first installed it. I used the info to set the alarms on my TPMS system. These days, I look at the tPMS monitor occasionally to verify that the system is working but I mostly rely on the alarms to let me know if I have a problem.

My rear tires are 6 1/2 years old. I have been pricing new tires. DURN! Tires have gotten expensive.

[Kid Gloves]

Originally Posted by LWBAZ

So the "tire manufacturers recommend minimum inflation pressure in their charts" but the highest PSI listed in the same charts isn't a recommendation?

I’m willing to call it whatever you like, but as I understand it the manufacturer is saying the cold inflation pressure of the tire should not exceed that specified PSI. At the very least I’d consider that a recommendation. If you prefer to call it a stipulation, requirement, or some other non-discretionary term I have no issue with that.

On the charts, 130psi is a recommendation when the load calls for that pressure.
On the tire, 130psi is not a recommendation.

Additionally, your editing of my use of the word “your” in the post above is grammatically incorrect.

[RV_TireMan]

Regarding tire pressures -- a good rule of thumb is.....
Air the tire to 90% of the max pressure listed on the tire.
Safe travels to you and your family

[Kid Gloves]

Originally Posted by RV_TireMan

Regarding tire pressures -- a good rule of thumb is.....
Air the tire to 90% of the max pressure listed on the tire.
Safe travels to you and your family

Why would anyone follow a rule of thumb that ignores readily available information, such as the manufacturers recommended pressures on the placard?

On many RV’s, following that rule will produce under inflated tires.

[Kash]

Gentleman (and ladies)..... Tire pressures are extremely important and the importance goes up with the bigger the rig and the larger the tire needed. It is a huge safety issue. The tire pressure tag or label that the RV manufacturer puts on your RV or coach or whatever is required by law. The sidewall markings on every tire are required by law. Now, what your RV actually weighs, the weight on each axel, is also important and can only be determined when you load up your rig with all your stuff, full fuel, full fresh water and get it weighed at a truck stop scale. That will get you close to the weight on the front and rear axels, not side to side which is another discussion. Take that weight and add a few percent because of other factors like passengers or other variables like maybe some stuff in your gray and black tanks, and hopefully you are within the recommended axel weights and total recommended by your manufacturer. Then knowing the make and size of our tires, I suggest you go to that tire manufacturers web site, find the inflation tables, and see what the recommended tire pressure is for your size tire and axel weight. It is your responsibility to keep your tires inflated properly. Everyone has their own opinion on following the recommended pressures exactly or a little higher or whatever. Tire maintenance is another discussion and pertains to safety. Be knowledge about your RV, its requirements, your tires and their requirements and knowing your RV's axel weights needs to be known before you can make the proper decisions. Be safe, Be well and Happy New Year to all.

Dick and Sandy near Buffalo. NY
2015 HR Ambassador 38 DB
2000 Ford Explorer Toad