It would be interesting to know why though. I know the 120 psi max on my Michelins is correct for all driving conditions up to and including the RV’s Max gawr. But the minimum pressure for the axle weights according to the Michelin Tables is correct as well.
H Rated Tire Pressures
- Thread starter KFX450RXC
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Ray-IN
Well-known member
"Heated" tire pressure is immaterial, tire design engineers have that accounted for in their design. Roger Marble, a retired tire design engineer covers that.
Reference: rvtiresafety.net
Tulecreeper
Well-known member
No idea. But, since the truck seems to ride pretty smooth with them maxed out I figure unless I start to see unusual tire wear I'll just leave them as is.
Ray-IN
Well-known member
Remember the Ford Explorer roll-overs? The final anysis showed Ford put incorrect pressures on the door sticker/Federal Tire Placard. They reduced pressures to increase occupant ride to below what Firestone determined the minimum pressure for the vehicle.
reference: Ford Explorer-Firestone Tire and, https://www.researchgate.net/public...ity_An_Examination_of_the_Ford-Firestone_Case
This is an excerpt from the first document.
Quote: "Internal Ford memos from the late 1980’s when the Explorer was designed showed that it had a rollover propensity as bad as Bronco II which it was replacing. Although Ford engineers recommended major changes including widening the track, these were rejected. One decision that was made was to have a recommended tire inflation pressure of only 26 psi when Ford modeling showed that the Explorer could roll over if tires were inflated to 35 psi. With tires losing about 2 pounds of pressure per month it was foreseeable that the Explorer could easily reach dangerously low levels at which tire damage and failure would occur."
reference: Ford Explorer-Firestone Tire and, https://www.researchgate.net/public...ity_An_Examination_of_the_Ford-Firestone_Case
This is an excerpt from the first document.
Quote: "Internal Ford memos from the late 1980’s when the Explorer was designed showed that it had a rollover propensity as bad as Bronco II which it was replacing. Although Ford engineers recommended major changes including widening the track, these were rejected. One decision that was made was to have a recommended tire inflation pressure of only 26 psi when Ford modeling showed that the Explorer could roll over if tires were inflated to 35 psi. With tires losing about 2 pounds of pressure per month it was foreseeable that the Explorer could easily reach dangerously low levels at which tire damage and failure would occur."
Gary RV_Wizard
Site Team
It's a bit more complicated than that. The change is 1.8% of the current inflation for every 10 degrees F. So a tire at 100 psi gains or loses about 2 psi, whereas a 50 psi tire gains/loses only about 1 psi.
If an elephant walked across here someone would counter that it could be a mouse with a glandular condition. Thanks Bill Nye, but it’s 1 to 2 and contingent on a number of factors of which beginning psi is only one, and otherwise the pt. is no tire is under ordinary conditions going to fail due only to ambient temperature variations
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Ray-IN
Well-known member
The problem with running minimum air pressure is, the tire is operating at 100% capacity all the time.
Which begs the question, If one is a believer in running tires by load/inflation charts: How often do you adjust your car tire air pressure when you are driving alone, then next day hauling 5 passengers?
Gary RV_Wizard
Site Team
Agree heartily! My method of dealing with that is to add a modest fudge factor to the scaled weight, thus allowing some amount for changes in load. Not a large amount, though. It's somewhat of a SWAG, based on where my actual weight coincides with the table ranges. The goal is to make sure the psi is enough, not how little you can get away with.
I also add a few psi to provide some cushion for temperature changes, especially if it happens to be a hot day when I set the tire pressure. That's simply a matter of convenience, so I don't have to add air on chilly weather. My TPMS will warn me if climate conditions have made my psi low, but I'd still rather avoid having to air up tires when I'm getting ready to depart.
Passenger car tires are routinely set to handle maximum load and that's practical for the usage. The difference between min & max load isn't all that much and the soft suspension can absorb the stiffness of the higher inflation. Light trucks (pick-up and fullsize vans), on the other hand, have a larger loading spread and the suspension is stiffer to handle the higher max. That makes it more appropriate to adjust psi for load. Most motorhomes are medium or even heavy duty trucks, so ride benefits even more from tuning tires to the actual weight.
Gary RV_Wizard
Site Team
In what way?
Ray-IN
Well-known member
Tireman9/Roger Marble of rvtiresafety.net recommends 10%-15% above the load/inflation tables for the same reasoning as yours.
So hypothetically, if a tire’s max psi is 120 and the scale weight indicates it should be run at 120 psi you should set it to 139 psi?
Has any tire, in the history of tires, ever failed because its psi was set in strict accordance to the mfg. tire inflation table?
The fact is any psi between that prescribed in the load/inflation table and the maximum sidewall pressure is correct for that tire. My tire shop set the psi on my new tires to 120psi because that’s the maximum cold pressure Michelin recommends and it’s perfectly fine to run them from now on at 120psi.
Has any tire, in the history of tires, ever failed because its psi was set in strict accordance to the mfg. tire inflation table?
The fact is any psi between that prescribed in the load/inflation table and the maximum sidewall pressure is correct for that tire. My tire shop set the psi on my new tires to 120psi because that’s the maximum cold pressure Michelin recommends and it’s perfectly fine to run them from now on at 120psi.
NY_Dutch
Well-known member
Running a tire inflated significantly above the load chart setting can cause excessive wear to the center of the tread. Running high also makes for a stiffer ride. The 10-15% mentioned is a good suggestion.
Running the tires at what the mfg. load/inflation table indicates is also a good suggestion. No tire is going to be materially affected regardless.
The best suggestion is forget the 10-15% and adjust the psi to wherever between the psi prescribed in the load/inflation tables and the max indicted on the sidewall which results in the most efficient performance. That is for operator to determine.
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NY_Dutch
Well-known member
If the operator chooses to run the tires at max psi when the load table calls for a significantly lower psi, he runs the risk of premature wear in the center of the tread.
So?
NY_Dutch
Well-known member
You have suggested that running anywhere between the load chart psi and max is acceptable, and as you can see from my post, that's not the case when max psi results in over inflating the tire for the load.
Gary RV_Wizard
Site Team
But "Acceptable" isn't the same as "Desirable" or "Optimal", right?
A tire isn’t over inflated unless it’s inflated to a psi exceeding the maximum inflation indicated on the sidewall. Otherwise, a tire inflated to 1 psi over the psi listed in the mfg. load inflation tables for the axle weights would be over inflated. Meantime adjectives ( opinions) like acceptable, optimal and desirable aren’t in the mfg. load inflation tables, not Michelin’s at any rate.
NY_Dutch
Well-known member
It's really pretty simple. If the load/psi chart calls for 75 psi and you inflate the tire to the sidewall max of 110 psi, the center of the tread will bulge out more then the edges, causing more wear. Every tire company will tell you the same thing, just as under inflating will cause more wear on the edges of the tread. Believe it or not, that's your call, I'm done.
