Tire pressure

[homelesshartshorns]

I know this is been beat to death. I just got new back tires on my Suncruser and the tire boy scolded be about only running 90 pounds on the drives and 80 pounds on the stearing tires! The tires do say max pressures in the 110 120 range but I am still running the stearing tires at 80. If I put 90 in them my glasses will bounce off. So again I looked at the door plate and see that the manufacture suggestion for my 20,000 rig is 70 pounds!!! What a deal. Would like to hear the general consensus.

[Kirk W]

27 minutes ago, homelesshartshorns said:

The tires do say max pressures in the 110 120 range but I am still running the stearing tires at 80.

Your tires manufacturer should publish an inflation table that lists whare the inflation pressure should be for each common wheel weight. If you get the RV weighed by Smart Weigh or similar facility, then you use the weight of the heaviest tire on each axle and inflate both tires on that axle the same. Only if you are at miximum weight for the tire should you inflate to the listed maximum pressure. As a poor substitute, but better than no weight would be to visit a truck scale and get each axle weight as pretend that half of that weight is on each side. You would then inflate to what the inflation table lists, plus 10% to be sure you haven't overloaded any tire.

[rickeieio]

99% what Kirk said,  The only thing I would do different is consider whether there's likely to be a difference between sides before adding that 10%.

I can tell you that the door tag mean little.  The door tags on F-150 is generic, assuming a base model, unladen truck.  By using the method described above, I run my tires at 50 psi rather than the 36 as per the sticker.  I got 80k miles out of the last set.  Only 30k if going by the sticker.  Expensive lesson......

[Ray,IN]

Keep in mind, load/inflation charts show the minimum pressure to support the corresponding load. That means you are operating the tires at 100% capacity.

Tireman9/Roger Marble is a retired tire engineer. He recommends adding 15% to the pressure listed on load/inflation charts to add a small safety factor.

Auto tires on the other hand operate at close to 35% reserve capacity/safety factor.

A gas MH is built on a truck chassis, it will ride like a truck.

[jadatis]

Needed tirepressure is all to give the tire a deflection that wont overheat any part of tirematerial, when driving the speed for wich it is determined. 

So for that Load on tire, and max speed used( wich you wont go over for even a minute) is important. 

Maxload of tire is calculated by tiremaker for reference-speed (mostly 75mpg for your tire) and - pressure ( that 120 psi). But for lower weight on tire you can use lower pressure. 

Vehicle maker uses GAWR's for that and simply devide by 2 or 4, with no reserve mostly. 

I determined that using 90% of loadcapacity per axle given, for 99mph, gives maximum reserve with still acceptable comfort and gripp. 

Reserve is needed for next. 

Unequall weight R/L on the axle, always on RV. 

Pressure loss in time. 

Inacurate reading of pressure and load. 

Etc, etc. 

 

Can make a list for your tire, with my determined reserves and given per axle, so you dont need to devide by 2 or 4 or add reserves yourselves. 

Need tire-specifications for that. 

Made spreadsheet for that, in wich I use my own extra safe formula, even saver then used for official lists. 

Can also put in other or no reserves, yust write how you want it. 

 

 

Edited October 29, 2022 by jadatis

[Kirk W]

8 hours ago, Ray,IN said:

A gas MH is built on a truck chassis, it will ride like a truck.

While the F53 chassis does retain some of the same design as the Ford truck chassis line, it has not been a truck chassis since Ford redesigned it for introduction in 1999. There have been changes since and at least some of them improvements, but the main reason that gas chassis motorhomes do not ride as well as the majority of diesel chassis motorhomes is that most of the diesels have an air ride, which no stock gas chassis motorhome has. That is at least part of the reason that the F53 ride is so much influenced by proper tire inflation. The F53 that I had was the same generation as is the one in question here and my experience was that it is especially true for the front axle. The one that we had was also sensitive to proper weight distribution. I found that adding more weight to the front axle to get closer to the axle weight rating improved the ride.

Edited October 28, 2022 by Kirk W
Add information.

[Ray,IN]

Thanks for the correction, I should have said "modified" truck chassis.

[homelesshartshorns]

So I am to understand that I should weight all four corners and use a tire chart to establish the proper pressure in each corner! Does this mean I might run four different pressures?

[rickeieio]

Technically, yes.  But, you may find you can shift your load a bit to get axle weights close enough to run the same pressure across an axle.  That'd make life simpler.

[Kirk W]

1 hour ago, homelesshartshorns said:

Does this mean I might run four different pressures?

Absolutely not! But it does mean that you should use that information to balance the loading as nearly as possible and then use the inflation pressure of the most heavily loaded tire on both sides of each axle. It is typical to run a different inflation pressure in the front tires from what is in the back tires and dual tires share the load so each is carrying 1/2 of the weight there. It is unsafe to have different tire pressures on the same axle.

Quote

Inflation Pressure for Uneven Vehicle Weight Distribution:

• Select a tire with load carrying capacity designed to handle the maximum load point
• For each axle determine the correct inflation pressure needed for that size tire to handle the maximum load
• Inflate all tires on that axle to this same inflation pressure

 

Edited October 30, 2022 by Kirk W

[jadatis]

I dont know if its unsafe to have different tire- pressures on the same axles, but every car or tiremaker prescribes it that way. 

My idea is for dualload-axle to give the outer tires a bit higher cold pressure then the inner tires. 5psi max, and inner tires as prescribed or calculated for the load. 

Inner tires heat up more because of roadcurve more deflected, and worse cooling, because on inner side. 

Then warm pressure about the same. And better weightdivision between inner and outer tire, because outer tires lift up the inner tire a bit. 

Same effect as a tag-axle, if more suspension pressure on tag, more weight shifts to tag and front axle, and less weight on Drive axle. 

Tag system devides front rear direction, Dualload 5 psi higher devides sideward direction. 

[Ray,IN]

27 minutes ago, jadatis said:

I dont know if its unsafe to have different tire- pressures on the same axles, but every car or tiremaker prescribes it that way. 

My idea is for dualload-axle to give the outer tires a bit higher cold pressure then the inner tires. 5psi max, and inner tires as prescribed or calculated for the load. 

Inner tires heat up more because of roadcurve more deflected, and worse cooling, because on inner side. 

Then warm pressure about the same. And better weightdivision between inner and outer tire, because outer tires lift up the inner tire a bit. 

Same effect as a tag-axle, if more suspension pressure on tag, more weight shifts to tag and front axle, and less weight on Drive axle. 

Tag system devides front rear direction, Dualload 5 psi higher devides sideward direction. 

No,  you misunderstand. You cannot show me any tire load/inflation chart telling or showing or recommending different tire pressures on each end of the same axle.

reference: https://www.goodyearrvtires.com/tire-inflation-loading.aspx

If you still think you're right, please post a link to documentation of your statement.

You weigh the axle on each end, set both tire pressures for the heaviest end.

Yes, for dual tires on the same axle end, they both must be within a 5 psi window or risk a tire failure.

I am friends with another MH owner (also a lifelong mechanic shop owner) who also runs 5 psi higher on outside dual tire for the same reason. He said he's done that for over 40 years.

Edited October 30, 2022 by Ray,IN

[jadatis]

I only wrote that it might not be unsafe to have different pressures, yust my opinion, so cant show you confirmation of that. 

I could ask the same question to KirkW, if he could show a link that writes its unsafe. 

But your friend who does my dually idea for 40 years, what are his experiënces with it. Does he have tmps, and does he notice less pressure rising of inner tires. And does he experiënce different roadhandling? 

And in my spreadsheet for calculating tirepressure for motorhomes, I calculate one pressure for the axle. In part 3, where you can give axle-end weights, I make the  average percentage of the loadcapacity belonging to the calculated pressure 90%, but if heavyest side goes above 95%, it calculates for that 95% using, and then lightest side goes under 85% used, a border I determined to give bad comfort.  Then the heavyest side still has some reserve for things like, inacurate reading of pressure, and weight, pressure loss in time, etc. 

Lower then 100% used of loadcapacity for the pressure, gives higher pressure as outcome. 

To high pressure is never bad for the tires, but to low pressure can give overheating, so that must never happen. Can lead to tirefailure, and even accidents. 

 

 

 

Edited October 30, 2022 by jadatis

[Kirk W]

2 hours ago, jadatis said:

I could ask the same question to KirkW, if he could show a link that writes its unsafe. 

Quote

RVSEF offers two online seminars, RV Weight Safety and RV Tire Safety both available at www.rvsafety.com. RVSEF weighing schedule is also available on our website.

 

[Danfreda1]

When we had our smart weight done in bushnell Florida close to 2 years ago the man who did it told me to use the heaviest tire weight on each axle. Find it on the chart and go up one level on the chart. 

[Ray,IN]

On 10/30/2022 at 2:46 PM, Danfreda1 said:

When we had our smart weight done in bushnell Florida close to 2 years ago the man who did it told me to use the heaviest tire weight on each axle. Find it on the chart and go up one level on the chart. 

That seems to match with Tireman9/Roger Marble states on his website, a 15% pressure safety factor above the chart minimum.

[jadatis]

On 10/30/2022 at 6:55 PM, Kirk W said:

 

Link's went a bit strange, but about tirepressure they write next. 

The basic rule is that all tires on the same axle are set to the same pressure, based on the heaviest side.

And this does not state that its unsafe to use different pressures, only that the basic rule is to set the same pressure on the axle. 

 

Edited November 1, 2022 by jadatis

[bruce t]

If you have a spare what pressure do you put in it if your current pressures don't match?

Just throwing one more complication into the various theories.

 

[TXiceman]

For the spare, I carry the sidewall maximum pressure and check it every few months.  It will lose air pressure and I can always let it down if it is too high.

Ken

Edited November 1, 2022 by TXiceman

[Ray,IN]

3 minutes ago, TXiceman said:

For the spare, I carry the sidewall maximum pressure and check it every few months.  It will lose air pressure and I can always let it down if it is too high.

Ken

Yep! Sidewall listed air pressure. You can always let air out, or run over-inflated.

[Kirk W]

1 hour ago, bruce t said:

Just throwing one more complication into the various theories.

No complication at all. I inflate my spare based on the highest inflation pressure tires and then adjust if to match the other tire on the same axle if it is used. Reality is that I have owned RVs since 1972 and the last time that I used a spare tire on any of them was about 1985. But I have always had a spare and the others have all aged out without ever being on the ground. 

[Ray,IN]

Homeless, next time a "tire guy" says anything about sidewall listed pressure, ask why, since the federal tire placard listed tire pressures are for the GVWR, would tires ever need higher pressure than the vehicle should ever legally weigh?