Weighed coach for correct TP - Methodology?

[Sandy Swede]

Finally had an opportunity to weigh the coach at a Cat Scale. Steer axle is 11,900 lbs & drive axle is 18,250. No opportunity to get individual wheel weights at this scale. Dividing, each steer axle wheel is 5,950 lbs and each drive axle wheel is 4,562 lbs. Then went to the Goodyear web site (tire care guide) to get Load/Inflation chart for the G670 RV 275/70.

http://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf

The chart shows a psi of 100 to 105 for each steer axle wheel and 85 psi for each of the 4 drive wheels. Am I correct that it is common to add 5 psi to each number from the chart, thus 105 to 110 would be cold inflation psi for the steer wheels and 90 psi for the drive wheels? I would appreciate any input on methodology and math.

Note: I added 1,350 lbs for passengers, 50 gal fresh water, etc to get a fully loaded equivalent weight, then calculated a 40/60 split. I had been going with the PO's 110 psi for all wheels.

[Philswrench]

If the tires are wearing even and not wearing more in the middle tread , I would run the max cold inflation

[FIRE UP]

Originally Posted by Sandy Swede

Finally had an opportunity to weigh the coach at a Cat Scale. Steer axle is 11,900 lbs & drive axle is 18,250. No opportunity to get individual wheel weights at this scale. Dividing, each steer axle wheel is 5,950 lbs and each drive axle wheel is 4,562 lbs. Then went to the Goodyear web site (tire care guide) to get Load/Inflation chart for the G670 RV 275/70.

http://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf

The chart shows a psi of 100 to 105 for each steer axle wheel and 85 psi for each of the 4 drive wheels. Am I correct that it is common to add 5 psi to each number from the chart, thus 105 to 110 would be cold inflation psi for the steer wheels and 90 psi for the drive wheels? I would appreciate any input on methodology and math.

Note: I added 1,350 lbs for passengers, 50 gal fresh water, etc to get a fully loaded equivalent weight, then calculated a 40/60 split. I had been going with the PO's 110 psi for all wheels.

Sandy Swede,
No, you're not correct in adding the "5 psi". There's a few on here that think they need to do it for whatever reason makes them happy. I've never worried too much about the four corner weight thing. I can't do anything about it anyway. My stuff fits where it's gonna fit, period. So, I weighed my coach, front, back, total. I too went to the web site for the tires I run and, have been running at that pressure now for about 6 years. My tires are doing just fine, albeit their the Michelen's and I've got the infamous cracking on the sides.

If you want to add psi for what reasons you think you need to, by all means, add away. I kind-a figure the tire folks have a pretty good, educated idea of what they think is appropriate pressure for a given tire and weight so, I run with that. Good luck.
Scott

[Mr_D]

Originally Posted by Philswrench

If the tires are wearing even and not wearing more in the middle tread , I would run the max cold inflation

The pressure on the sidewall of a Michelin RV tire and many others is not the "Maximum" cold pressure the tire should ever have (unlike car tires) it is the minimum to support the maximum rated carrying capacity of the tire. NHTSA defines truck tires as those rated for vehicles over 10,000#'s GVWR.

From the Michelin RV Tire Guide:

"If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."

From page 6 of the GoodYear RV Tire and Care Guide:

"How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load."

From TOYO:

Q: What are the consequences of inflating the tires to accommodate the actual loads?
A: If the inflation pressure corresponds to the actual tire load according to the tire manufacturer’s load and pressure table, the tire will be running at 100% of its rated load at that pressure. This practice may not provide sufficient safety margin. Any air pressure loss below the minimum required to carry the load can result in eventual tire failure.

But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

From the August 2010 Motorhome Magazine "Tread Carefully" tire article:

The maximum load capacity allowed for the size tire and load rating and the minimum cold air inflation needed to carry that maximum load are located on the tire’s sidewall.

From our owners manual:

Federal law requires that the tire’s maximum load rating be molded into the sidewall of the tire.
If you look there, you will see the maximum load allowed and the cold air inflation pressure required to carry that stated maximum load. Less air pressure restricts the tire to carry a lighter load.

[Crasher]

[QUOTE=FIRE UP;3615500]Sandy Swede,
I've never worried too much about the four corner weight thing. I can't do anything about it anyway.

That's not entirely true. When I weighed the axles on our 2013 Bus, they were all below the max rating when loaded. However, I noticed that the front right tire increased in pressure faster than the left front and continued to run at a higher pressure and temp. When a six position weight was done, the FR was at 8200# and the FL was at 7200#. the total was below the 15,600# axle rating, but the FR tire was overloaded by 360#s. When the ride height of all four corners was checked, the LR was 3/8" too high. When corrected, both front tires were within 50# of 7600# or 240# below max load. I don't know it I would have had a tire failure at some point, but I feel better about it now.

[Ed Headington]

I agree with what Crasher said. It is not uncommon, especially on the rear axle for one side to be as much as 1000 pounds heavier than the other.

Obviously the best thing is to weigh each side separately. However, when it is not convenient for me to do that, I usually add 500 lb. to the weight that I arrived at by dividing the axle weight (by 4 for rear tires, and 2 for steer axles). RVSEF recommended adding 5 psi to the pressure AFTER measuring each corner after we finally got our current coach measured at each corner.

Ed Headington

[Applefarmer]

[QUOTE=Crasher;3615541]

Originally Posted by FIRE UP

Sandy Swede,

I've never worried too much about the four corner weight thing. I can't do anything about it anyway.



That's not entirely true. When I weighed the axles on our 2013 Bus, they were all below the max rating when loaded. However, I noticed that the front right tire increased in pressure faster than the left front and continued to run at a higher pressure and temp. When a six position weight was done, the FR was at 8200# and the FL was at 7200#. the total was below the 15,600# axle rating, but the FR tire was overloaded by 360#s. When the ride height of all four corners was checked, the LR was 3/8" too high. When corrected, both front tires were within 50# of 7600# or 240# below max load. I don't know it I would have had a tire failure at some point, but I feel better about it now.

[Crasher]

Originally Posted by Mr_D

The pressure on the sidewall of a Michelin RV tire and many others is not the "Maximum" cold pressure the tire should ever have (unlike car tires) it is the minimum to support the maximum rated carrying capacity of the tire. NHTSA defines truck tires as those rated for vehicles over 10,000#'s GVWR.

From the Michelin RV Tire Guide:

From page 6 of the GoodYear RV Tire and Care Guide:

From TOYO:
But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

From the August 2010 Motorhome Magazine "Tread Carefully" tire article:


From our owners manual:

You have quoted this many times in different posts. When I called Michelin and spoke with a tech, he agreed that the pressure listed for loads less than the maximum was the minimum cold pressure to carry that load, however, when the load reached the maximum, the minimum pressure also became the maximum cold pressure. I just looked up the data for the XZA Energy and it reads "The Maximum Load and Pressure" . So if I understand all that, in no place can I find that it is allowable to inflate the tire higher than the value listed on the tire for maximum load.

[wildtoad]

Semantics at work. Since one should never carry a load heavier than what the tires can support, the minimum as stated on the tire to support the max load may also become the maximum pressure one should use. However, if there is someone who WANTS to run the tires at a higher pressure than their weight requires, the minimum stated on the tire is no longer the maximum.

My opinion is that the pressure stated on the sidewall is meaningless information in 90-99% of the time unless you know you are at maximum weight. Either use your actual weights if you know them, the GAWR as provided by the manufacturer and use the tire manufacturers chart to set the weights. If it helps you sleep better, add a 5% fudge factor.

When all else fails, your MH manufacturer has provided a nice federally mandated sticker that says what the tire pressures should be. Nowhere does it say to use the pressures as stated on the tire.

[Crasher]

Originally Posted by wildtoad

Semantics at work. Since one should never carry a load heavier than what the tires can support, the minimum as stated on the tire to support the max load may also become the maximum pressure one should use. However, if there is someone who WANTS to run the tires at a higher pressure than their weight requires, the minimum stated on the tire is no longer the maximum.

My opinion is that the pressure stated on the sidewall is meaningless information in 90-99% of the time unless you know you are at maximum weight. Either use your actual weights if you know them, the GAWR as provided by the manufacturer and use the tire manufacturers chart to set the weights. If it helps you sleep better, add a 5% fudge factor.

When all else fails, your MH manufacturer has provided a nice federally mandated sticker that says what the tire pressures should be. Nowhere does it say to use the pressures as stated on the tire.

I agree with you. What I was addressing was the implication that the pressure value on the side of the tire was "Just" the minimum and that it was ok to go higher. Is 125 or 130 psi ok if the tire says 120 psi?? I can not find where that is acceptable, but that is what Mr D's statement says.

[wildtoad]

It would of course beg the question why would you but that is not relevant. It seems that if you had a light load with such over pressure you're asking for trouble and poor tire wear. It is however a thought provoking question.

[Sandy Swede]

I should have put this in my thread post:

From the G670RV sidewall - Single max weight 6,940 lbs, dual max weight 6,395 lbs.

From the placard next to driver's seat: Front GAWR 13,200 lbs, rear GAWR 20,000 lbs. Inflate to 120 psi labeled next to GAWR info. I read that as the minimum pressure to inflate if you are at max, or GAWR, weight.

[Sandy Swede]

Thanks to all who replied. I found one of Tireman9's posts that I had missed in my search and that was also helpful.