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Old 12-05-2020, 02:59 PM   #1
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Tire pressures

I have a 1988 40 foot beaver marquis. I have been maintaining 90 lbs psi in all 6 michelin tires. When i called michelin, they wouldn't tell me what pressure to maintain, and said to contact the manufacturer.


When going over my instruction books i ran across a brochure that said to maintain the steering tires at 75 lbs psi and the drivers at 80 lbs psi. The tires themselves tell you 100 lbs psi. Thus i have maintained 90 lbs psi in all 6 tires.


I might stand to be corrected, but the brochure recommendations for the tires seems to be on the light side.
Some suggestions would be appreciated.
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Old 12-05-2020, 03:21 PM   #2
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The tire "told you" no such thing - the psi on the sidewall is what is required for the tire to carry its maximum load. How much you need for the actual load is determined by tire engineers and shown in a table of weight vs psi for a given tire size & model.


You need to identify the tire make, model & size and get the inflation table for that tire (most of the large tire makers publish it online). Then get the coach weighed so you know the actual loaded weight.
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Old 12-05-2020, 03:32 PM   #3
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The RV manufacture lists the tire pressure based on the RV weight, loaded to its max.

Your RV is not, and should never be, heavy enough to need tire makers max sidewall pressures.

What the RV manufacture states is fine and so is what your doing. Your not below their recomendations.

Many people weigh the rig and use tire makers charts to go below what the RV manufacture says. That's OK, if you want to go thru that, and don't add weight between trips.

If your happy, go with what your doing.
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Old 12-05-2020, 07:35 PM   #4
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Tire manufacturers publish load charts vs pressure. I went thru this process to find the minimum pressure I could use.
- So the first thing I did was to load the RV to it's max weight as I normally use it making sure it was also with full fuel, water and waste water. We do bring a lot of things, clothes and food so I did this just before an outing
- I took note of the 'unused' load to reach max Gross
- I've weighted front and rear axles and then each corner. My class A has a full wall slide out and is heavier on the driver side.
- going thru my tire charts, I looked up for the pressure required for the most loaded front single and rear dually. This is the pressure that I'm using. For my unit with 19.5 Continental HSR it's 90psi front and 100psi rear
- Double checked that my measured weight was under the RV's GVWR
Voila !
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Old 12-05-2020, 08:25 PM   #5
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Generally weights are done fully loaded with all gear, people, full fuel and water with empty gray & black tanks.
If all you can get is axle weights and not 4 corner wts you should also consider you could easily have 10% heavier one side vs the other... use the heavier wt or 1/2 axle wt + 10% to look up min pressure in the inflation tables. Many then add 5 PSI as a safety factor.
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Old 12-05-2020, 09:34 PM   #6
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Then the Tire Industry Association states to use pressures on the Federal Tire Placard.
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Old 12-06-2020, 11:30 AM   #7
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Quote:
Then the Tire Industry Association states to use pressures on the Federal Tire Placard.
Right - for legal liability reasons. The sole responsibility for recommending the psi for a particular vehicle rests with the vehicle manufacturer. Did a 1988 coach even have a tire placard with psi? The current federal placard with all the safety information went into effect in 2002, IIRC. Tire inflation wasn't a big legal issue until the Ford-Firestone tire debacle in the late 1990's.
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Old 12-06-2020, 01:00 PM   #8
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Tire pressure, my opinion

If the tire/wheel combination is sized correctly itís relatively easy to find the optimum pressure for each tire based on the heat generated across the tread. As a race car guy, I have spent many hours tuning the chassis of a car by adjusting suspension based on the temperature gathered during skid pad testing. I perform the same type of test on my tow vehicle and coach, sans skid pad. All that is required is a note pad and a digital infared thermometer. Do this test when the coach and tow vehicle are loaded.



After setting all the tire pressures to a base line, vehicle manufacturers specís are a good start, roll down the highway for enough miles to generate heat in the tires and bring them to operating temp. I then find a safe pullout where I can quickly scan each tire across the tread, inside-middle-and outside. It is important to remain consistent in which area you scan each time as tread blocks vary and will demonstrate different values. Also, too much time taken during readings will allow the temperature across the tread to average out and give untrue readings. My notebook will have each tire as positioned on the coach and tow vehicle and the letters I, M, and O below each position with enough room to record several tests as I adjust the pressures to each tire accordingly. After each test is recorded and pressure adjustments are made, I then again reheat the tires to operating temperature. The goal is even tire temp across the tread.


Recording even tire temperatures across the tread requires a very precise suspension adjustments that are likely unobtainable on a trailer suspension so you should not be too disappointed when that doesnít happen. Toe and camber influence tire temperatures and not easily adjusted. Where weight is positioned in the coach also will influence the temp so it is likely that all tires will record differently. Basically if the tire is hotter on the edges, it is underinflated, hot in the middle is overinflated.


So, if you are like me and love the set up and tuning, this is a great and accurate exercise. Now I only perform this when a significant change is made to tow vehicle and coach that will affect the value, ie, tire/wheel change or loading change.

Cheers
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Old 12-06-2020, 01:56 PM   #9
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For you, I recognize that this is both an intellectual and enjoyable exercise. For me, not so much. I'll do my due diligence by weighing my rig and sticking to RV and tire manufacturer recommendations for inflation vs. weight. The important thing for all of us, no matter the process we use, is to consistently adhere to it so we never put any Innocents in harm's way when we're on the road.
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Old 12-06-2020, 02:26 PM   #10
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Quote:
Originally Posted by Gary RVRoamer View Post
Right - for legal liability reasons. The sole responsibility for recommending the psi for a particular vehicle rests with the vehicle manufacturer. Did a 1988 coach even have a tire placard with psi? The current federal placard with all the safety information went into effect in 2002, IIRC. Tire inflation wasn't a big legal issue until the Ford-Firestone tire debacle in the late 1990's.
Even Goodyear states that in their RV tire guide.

Tireman9 just stated on FMCA forums that autos have 20%-30% inflation safety factor. I ask why not do the same for your MH?
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Old 12-06-2020, 03:11 PM   #11
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Limits always have margin because no manufacturing process in the world is able to produce with zero defects. So you design products to meet operational requirements for the intended application and set limits with margin to assure that nearly any outliers of the production process operate properly. Nothing is ever 100%, but this gets you very close.

The other side of this is that many feel a limit is an arbitrary setting established for liability reasons, and therefore can be pushed, just because. Margins by default absorb some of the risk of that push, but the more the push, the greater the risk that the variation of any production process is going to catch you. Stick to the limits, they're there primarily because of engineering and testing.
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