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Old 05-14-2015, 12:21 PM   #1
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Tire pressure

I was checking the air pressure in my old Class C, with 16 in. tires.

I checked the door jamb and found the tag that said 65 front and 80 rear. While filling my tires, I found that they are marked " max psi 80 lbs "

I have seen many discussions about tire pressure and weighing the rig and got to thinking, why would I put in less air, then the max, for the tire. Especially the rear ones.

Wouldn't a tire, filled to the max pressure, flex less and handle pot holes better.

If its just for a smoother ride, I don't know if can even tell the difference, on the rears.

I only filled the fronts to 65, like the tag said. I suppose I could bring them up to 80 as a test.
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Old 05-14-2015, 12:28 PM   #2
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I don't have the C anymore but as I recall 70-75 in front gave good ride and tracking and I always ran 80 psi on the rear axle. Mainly due to the fact I was about maxed out on weight. It handled much better with that pressure on the rear. Max on the front gave a harsh ride.


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Old 05-14-2015, 12:33 PM   #3
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First, I am not a proponent of load/inflation charts, neither is the Rubber Manufacturers Association (see pg 55)
The 80 psi is the minumum air pressure to support the tires rated maximum load.(thus no allowance to go higher)
Running automobile/light truck tires at the the air pressure for the greatest allowed load unnecessarily, makes tires more susceptible to damage from hitting a large rock, curb, or pothole, as the tire cannot flex much. That is one reason to follow the vehicle mfgrs tire placard recommended tire pressure for the vehicles maximum GVWR.
The exception to this is ST trailer tires, which should be at sidewall-stated pressure because almost all trailer tires are operating at their maximum anyway.
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Old 05-14-2015, 05:13 PM   #4
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Quote:
Wouldn't a tire, filled to the max pressure, flex less and handle pot holes better.
What do you mean by "better"? More pressure makes a stiffer tire, which tends to bounce more and shock the suspension more. Neither are desirable. As long as the tire is inflated with enough psi to carry the actual weight load, adding more doesn't make it better in any way I know of. If in doubt about the loading, though, more psi is a safer choice than risking inadequate inflation.
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Old 05-14-2015, 05:20 PM   #5
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When I say better, I am talking about the tire deflecting enough, to pinch the sidewall between the road and wheel, damaging the side wall.

I just replaced a wheel on my toad, due to a NYC, highway pothole.
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Old 05-14-2015, 05:30 PM   #6
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Originally Posted by twinboat View Post
I was checking the air pressure in my old Class C, with 16 in. tires.

I checked the door jamb and found the tag that said 65 front and 80 rear. While filling my tires, I found that they are marked " max psi 80 lbs "

I have seen many discussions about tire pressure and weighing the rig and got to thinking, why would I put in less air, then the max, for the tire. Especially the rear ones.

Wouldn't a tire, filled to the max pressure, flex less and handle pot holes better.

If its just for a smoother ride, I don't know if can even tell the difference, on the rears.

I only filled the fronts to 65, like the tag said. I suppose I could bring them up to 80 as a test.
I have a Class-C with LT225/75R16 LR-E tires but do not run the tire max.

You should get the unit on a truck scale to confirm you actual load. The door sticker is based on an estimate of how much "stuff" you will carry.

You need to have the Cold Inflation Pressure high enough to carry the measured max load (with the RV fully loaded).
You might add 10% to that inflation so you are not chasing around when the temperature drops (lower pressure).

Running much higher than that will give harsher ride (feel road joints and small bumps) It can also increase the potential of an impact break if you hit a big pot hole and lead to center wear of the tread.

I have had corner weights done and know the minimum CIP needed to carry that load. I run +10% above that pressure and have my TPMS set accordingly.

Check out the info on my blog to learn more about tires.
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Old 05-14-2015, 06:18 PM   #7
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I'm new to motorhomes so bear with me. I don't have a sticker on the door jamb to tell what psi is recommended. The tires are 215-85-16, it's an 1988 ford E 350 20 ft. class c. The tires state max psi of 80 but I will have to replace them since I found out they are 10 years old. I wonder if someone could give a recommendation for pressures front & back? Also, a tire shop said I could use LT 245-75-16 instead since they are approximately the same diameter but slightly wider. Just have to make sure they are D or E load rated. I have seen that size on a 16 ft. moving truck before. What say you all?
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Old 05-14-2015, 06:48 PM   #8
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Ford Motors came up with tire pressure, so I would hope that the chassis would be able to withstand the impact, at that pressure.

I don't understand your statement about wearing out the center tread, at the tire manufactures, stated pressure. Over inflation will do that, but I would think, that would be above the max pressure. I would also be concerned with outer edge wear, with lower pressures.

I did some research, last fall, when I purchased the MH, on my tires and could not find any charts for inflation pressures, according to weight.

They are 7 year old, off brand tires ( China bombs? ). but only had 500 miles on them.

I will read your blog later this evening.

PS: I have a 1999 30ft, E450, Gulfstream, Yellowstone.

How do I find your blog?
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Old 05-15-2015, 06:50 AM   #9
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Quote:
I don't have a sticker on the door jamb to tell what psi is recommended. The tires are 215-85-16, it's an 1988 ford E 350 20 ft. class c. The tires state max psi of 80 but I will have to replace them since I found out they are 10 years old. I wonder if someone could give a recommendation for pressures front & back? Also, a tire shop said I could use LT 245-75-16 instead since they are approximately the same diameter but slightly wider. Just have to make sure they are D or E load rated. I have seen that size on a 16 ft. moving truck before. What say you all?
Since you don't have the coach weight or axle load ratings to go by, inflate the tires to the psi shown on the sidewall for max load. That way you are sure to have enough to be safe, even if the ride is a bit rougher than it needs to be.
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Old 05-15-2015, 08:43 AM   #10
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[QUOTE=Ray,IN;2556129]First, I am not a proponent of load/inflation charts, neither is the Rubber Manufacturers Association (see pg 55)

Ray....Very interested reading. I went to the Rubber Manufacturers Association web page and looked at the list of members and all the big tire manufacturers are listed. Now why would those same tire manufacturers produce load charts when it would go against the Association which they belong to's publication? I hope Tireman comments on this.
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Old 05-15-2015, 03:38 PM   #11
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Originally Posted by Ray,IN View Post
First, I am not a proponent of load/inflation charts, neither is the Rubber Manufacturers Association (see pg 55)
The 80 psi is the minumum air pressure to support the tires rated maximum load.(thus no allowance to go higher)
Running automobile/light truck tires at the the air pressure for the greatest allowed load unnecessarily, makes tires more susceptible to damage from hitting a large rock, curb, or pothole, as the tire cannot flex much. That is one reason to follow the vehicle mfgrs tire placard recommended tire pressure for the vehicles maximum GVWR.
The exception to this is ST trailer tires, which should be at sidewall-stated pressure because almost all trailer tires are operating at their maximum anyway.
Dd you miss this on pg 51? "Determining Proper Inflation Pressure - With actual weights of the loaded RV acquired by weighing, it is possible to compare them against the GAWR, GVWR, and tire capacities posted on the vehicle tire placard or certification label. These actual weights are also what should be used to determine any increase in inflation pressure for the tires, if required. (See “How to Determine an RV's Actual Weight” on p. 53.)
Inflation pressure recommendations may also be determined based on the tire manufacturer's specifications, which define the amount of inflation
pressure necessary to carry a given load. These inflation pressures may differ from those found on the vehicle tire placard or certification label.
However, never use inflation pressure lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall
."

I also have different interpritation of the info on pg 55 that advises to use the Load Inflation tables in "Note #1"

Could you explain what is wrong with using the tables to establish the inflation needed to carry the actual load on a tire in RV application?
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Old 05-15-2015, 03:57 PM   #12
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Originally Posted by jeeper54 View Post
I'm new to motorhomes so bear with me. I don't have a sticker on the door jamb to tell what psi is recommended. The tires are 215-85-16, it's an 1988 ford E 350 20 ft. class c. The tires state max psi of 80 but I will have to replace them since I found out they are 10 years old. I wonder if someone could give a recommendation for pressures front & back? Also, a tire shop said I could use LT 245-75-16 instead since they are approximately the same diameter but slightly wider. Just have to make sure they are D or E load rated. I have seen that size on a 16 ft. moving truck before. What say you all?
My Class-C is 21' so we probably have similar weights BUT just because I run xx psi is not the best way to learn what you should run.

I strongly suggest you use the chart on pg 55 of the RMA guide.
I doubt you have a slide on yout Class-C so you are probably reasonably balanced and probably within 48/52% side to side.

With your Rv loaded as you would normally travel (Food, fuel, water, people) get to a truck scale. Use these links to find nearest

Truck Scales and Weigh Stations Locator for Truck Drivers and Trucking Companies at Truckstops

Locate truck scales, truckstops, truck service centers, and diesel prices

Get the individual axle loads then assume one side has 52% of the load. Using that load you can use the Load Inflation tables for your brand tire (list on THIS link)
find the inflation that is rated to carry at least the load you calculated for your LT215/85R16 LR-E tires.

The LT245/75R16 need 11.34" Dual Spacing which is a lot more than your OE LT215/75R16 that only need 9.88". This controls the clearance between the duals and is controlled by the wheels. Tire to Tire contact or "Kissing" can lead to tire failure so unless the tire store is willing to put in writing that the LT245's will not have dual spacing issues or they will replace the tires for free I would stay with your current size.

I suggest your CIP - Cold Inflation Pressure) be that inflation + 10% so you don't have to worry about the need to add air if there is a big temperature drop.

There is more you can learn about tires on my Blog.
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Old 05-15-2015, 04:02 PM   #13
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I read from Ray,IN's post;

" never use inflation pressure lower then specified by the vehicle tire placard, certification label or owners manual. Nor should inflation pressure exceed the maximum pressure molded on the sidewall"

In my, case both of those numbers are 80 psi, for the rear tires, on my MH.

Why would I need to get weights and tire pressure charts, for a recommended pressure and then add 10% to it.
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Old 05-15-2015, 05:44 PM   #14
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For what it's worth I also took load rated D tires off and installed E rated Coopers. Installed at Les Shwab in Lewiston Idaho and the installer recommended 80psi on the duals. Worked for me.


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