Tire Inflation Tables Work

[wmiii]

Just to make sure I understand.
I have the Michelin 255/80R 22.5 XRV tires.
My last weigh was 8,620 front and 16,800 rear. My GAWR Front is 10,410 and rear 17,000.
According to the table, I think I see recommended weight 80psi front and 90psi rear. My placard says 105 front and 95 rear.


255/80R22.5 G
XRV® PSI 707580859095100105110 Maximum load & pressure on sidewall kPa 480 520 550 590 620 660 690 720 760 LBS Single 77508140860088809240961099501030010410 S 5205 LBS at 110 PSI Dual 141001482015440161601682017640181001874019220 D 4805 LBS at 110 PSI KG Single 352037003900402042004360452046804720 S 2360 KG at 760 kPa Dual 640067207000732076408000820085208720 D 2180 KG at 760 kPa

[BruceB61]

Ya"ll are "beating a dead horse", as we say in SC, contact patch is what matters, go with the recommended pressure and measure the tread depth after 1000 miles and see what it shows. I go with the truck says and then measure and adjust, I am new to this board, but maybe, just maybe, some of you are overthinking the process.
When I bought my rig it had 90 PSI , which is what the placard in the truck says, I have pretty much stuck with that based on the wear pattern.

[Crasher]

Originally Posted by BruceB61

Ya"ll are "beating a dead horse", as we say in SC, contact patch is what matters, go with the recommended pressure and measure the tread depth after 1000 miles and see what it shows. I go with the truck says and then measure and adjust, I am new to this board, but maybe, just maybe, some of you are overthinking the process.
When I bought my rig it had 90 PSI , which is what the placard in the truck says, I have pretty much stuck with that based on the wear pattern.

That's just what I am doing. The load inflation charts give you the most even "contact patch". That's why the tire manufacturers publish them. If you use the charts, there is no need to measure tread depth. In most cases, the placard will not give you the best results.

[JohnBoyToo]

In my cars I used the chalk test for contact patch, not feasible for the rv IMHO...
but some are saying you must check and fill immediately.... I never can since I have to drive to fill


So measure after being parked for a while, note the psi INCREASE you need (decreases can be done immediately) but for increases in psi, I note on my phone like RF +3 psi, RR +6, RTag +3, LTag +1, etc... drive to the air supply and remeasure and add the +3, etc... to the warmed up psi - then down the road we go !

[PCFriar]

Pretty new to the RV world. Have a 2017 Winnebago Vista. Tires say 110 psi cold on the sidewalls. Data plate by drivers seat says go with 82 psi cold for tires (have duals in back and only one axel there as it is only a 30' RV. I asked the dealership and they said probably go with the tire manufacturer and I called Winnebago industries and they said go with the data plate. First trip before I asked anyone I went with the 110 but on my first stop for the evening I checked the tires and they were up to 125+ so I reduced them some as I got scared. Rig handled poorly I was inexperienced but it felt like it didn't have great contact and bounced a lot more. After talking to Winnebago we went with 82 and it road smoothly and I had much better control even in windy conditions. My question is where do I find the tire inflation tables everyone is talking about? I assume it must be based on a loaded weight? And how do you use them? Thank you

[featherlite]

http://blobs.continental-tires.com/w...ok-en-data.pdf

this is for my front tires on the motor home 295 / 80 r 22.5

on page # 9

my front load on the scale when traveling is 12,800 lbs . I run with 108 lbs but could lower that to 102 lbs . my r/v manufacture want me to inflate to 115 lbs and at that psi the ride is like a log truck and on there plaque it is for 13,800 lbs max but not at the weight I run at

on the rear and tag I have a lot of extra carrying weight available

if you look on the continental chart the psi includes a 10 % extra weight in the air inflation table

ps I can adjust the weight on the front axle with the tag axle pressure ( but not much ) depending on the weight of the trailer

dave

[sajohnson]

Originally Posted by wmiii

Just to make sure I understand.
I have the Michelin 255/80R 22.5 XRV tires.
My last weigh was 8,620 front and 16,800 rear. My GAWR Front is 10,410 and rear 17,000.
According to the table, I think I see recommended weight 80psi front and 90psi rear. My placard says 105 front and 95 rear.


255/80R22.5 G
XRV® PSI 707580859095100105110 Maximum load & pressure on sidewall kPa 480 520 550 590 620 660 690 720 760 LBS Single 77508140860088809240961099501030010410 S 5205 LBS at 110 PSI Dual 141001482015440161601682017640181001874019220 D 4805 LBS at 110 PSI KG Single 352037003900402042004360452046804720 S 2360 KG at 760 kPa Dual 640067207000732076408000820085208720 D 2180 KG at 760 kPa

Yes, you are reading the tables correctly. I am looking at the table located here:
https://www.michelintruck.com/refere...tion-tables/#/

For 255/80R 22.5 XRV tires.

The placard is generally a "worst case" recommendation -- rig grossly overloaded and/or unevenly loaded -- and also takes into account that some owners rarely check tire pressure. It is the mfr playing CYA.

That said, it is not always that far off, and is a reasonable starting point if the rig has not been weighed.

In your case, 105 psi front is good for 10,300 lbs (evenly loaded), and 95 psi rear will carry 17,640 lbs. That's well over your measured weight. It appears the mfr was using the GAWR to set the pressure.

[wmiii]

Thanks for the confirmation.
Wm

[Tireman9]

Originally Posted by JohnBoyToo

In my cars I used the chalk test for contact patch, not feasible for the rv IMHO...
but some are saying you must check and fill immediately.... I never can since I have to drive to fill


So measure after being parked for a while, note the psi INCREASE you need (decreases can be done immediately) but for increases in psi, I note on my phone like RF +3 psi, RR +6, RTag +3, LTag +1, etc... drive to the air supply and remeasure and add the +3, etc... to the warmed up psi - then down the road we go !

^x2


I covered this in my RVTireSafety blog March 4 2013.

[FastEagle]

Originally Posted by sajohnson

Yes, you are reading the tables correctly. I am looking at the table located here:
https://www.michelintruck.com/refere...tion-tables/#/

For 255/80R 22.5 XRV tires.

The placard is generally a "worst case" recommendation -- rig grossly overloaded and/or unevenly loaded -- and also takes into account that some owners rarely check tire pressure. It is the mfr playing CYA.

That said, it is not always that far off, and is a reasonable starting point if the rig has not been weighed.

In your case, 105 psi front is good for 10,300 lbs (evenly loaded), and 95 psi rear will carry 17,640 lbs. That's well over your measured weight. It appears the mfr was using the GAWR to set the pressure.


Tires. The size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate for the GAWR.

The Vehicle manufacturer must comply with the rules as written above in blue. FMVSS are written to minimum standards. Therefore, the vehicle manufacturer has determined their recommended cold inflation pressures are appropriate for that fitment, thus making them the minimum safe inflation pressures for that vehicle.

[bobkathy95]

Originally Posted by featherlite

http://blobs.continental-tires.com/w...ok-en-data.pdf

this is for my front tires on the motor home 295 / 80 r 22.5

on page # 9

my front load on the scale when traveling is 12,800 lbs . I run with 108 lbs but could lower that to 102 lbs . my r/v manufacture want me to inflate to 115 lbs and at that psi the ride is like a log truck and on there plaque it is for 13,800 lbs max but not at the weight I run at

on the rear and tag I have a lot of extra carrying weight available

if you look on the continental chart the psi includes a 10 % extra weight in the air inflation table

ps I can adjust the weight on the front axle with the tag axle pressure ( but not much ) depending on the weight of the trailer

dave

Your solution makes way to much sense. Weigh the coach as you normally drive it (gear, fuel, water, etc) and then consult the tire charts for your particular tires. Add 5% (assuming you weighed with full fuel and water) inflate the tires - and go.

[sajohnson]

Originally Posted by FastEagle

The Vehicle manufacturer must comply with the rules as written above in blue. FMVSS are written to minimum standards. Therefore, the vehicle manufacturer has determined their recommended cold inflation pressures are appropriate for that fitment, thus making them the minimum safe inflation pressures for that vehicle.

I'd never heard that before but it does make some sense. In setting the psi specs to be displayed on the placard, the mfr might assume that some owners will ignore the GVWR and use the sum of the GAWRs (or perhaps ignore both and just load up and never weigh).

When considering what owners might do, it is prudent to assume the worst.

That said, it does not follow that those pressures are the 'minimum safe inflation pressures for that vehicle'. The min safe psi per axle is determined by actual weight, the heaviest end of that axle -- not the GAWR.

If the axle happens to be loaded to its max weight, then those pressures on the placard will be correct, but only in that case. For any weight significantly less than the GAWR, those pressures will create an over-inflation condition.

With the View, we are close to the GAWR in the rear so we run exactly what WGO recommends on the placard. The front is well below the GAWR so we run about 5 psi below the front psi on the placard.

With the View (and all Sprinter-based Class C RVs) there is not much OCCC to begin with -- 1,422 lbs in our case, some are <1,000 lbs!

With the Sprinter 3500, the sum of the front and rear GAWRs is an additional 1,100 pounds over the GVWR, or 12,130 lbs, but there still isn't that much difference between empty and loaded (compared to larger rigs). Even so, if the View/Navion is unloaded, or lightly loaded for a weekend trip, and the owner uses the WGO recommended pressures, their tires will be grossly over-inflated:

Empty weight = approx. 9,608 lbs. Michelin recommended pressure (assuming even loading) is between 40 and 45 psi.

45 psi is good for 10,070 lbs. So, an empty View, plus driver, passenger, and say 10 gal. of water.

GVWR is 11,030 lbs. Michelin recommended pressure (assuming even loading) is 51 psi.

Sum of GAWRs is 12,130 lbs. Michelin recommended pressure (assuming even loading) is <60 psi, but say 60. Very close to the WGO placard spec of 61 psi front and rear.

In short, the WGO recommended pressure is enough for a V/N that has been overloaded by 1,100 lbs over the GVWR!

It is 10 psi -- 22% -- over the Michelin recommended pressure for the GVWR.

It is 16 psi -- 35%! -- over the Michelin recommended pressure with a driver and passenger.

With other rigs the difference may be even more extreme.

[Tireman9]

Originally Posted by sajohnson

I'd never heard that before but it does make some sense. In setting the psi specs to be displayed on the placard, the mfr might assume that some owners will ignore the GVWR and use the sum of the GAWRs (or perhaps ignore both and just load up and never weigh).

When considering what owners might do, it is prudent to assume the worst.

That said, it does not follow that those pressures are the 'minimum safe inflation pressures for that vehicle'. The min safe psi per axle is determined by actual weight, the heaviest end of that axle -- not the GAWR.

If the axle happens to be loaded to its max weight, then those pressures on the placard will be correct, but only in that case. For any weight significantly less than the GAWR, those pressures will create an over-inflation condition.

With the View, we are close to the GAWR in the rear so we run exactly what WGO recommends on the placard. The front is well below the GAWR so we run about 5 psi below the front psi on the placard.

With the View (and all Sprinter-based Class C RVs) there is not much OCCC to begin with -- 1,422 lbs in our case, some are <1,000 lbs!

With the Sprinter 3500, the sum of the front and rear GAWRs is an additional 1,100 pounds over the GVWR, or 12,130 lbs, but there still isn't that much difference between empty and loaded (compared to larger rigs). Even so, if the View/Navion is unloaded, or lightly loaded for a weekend trip, and the owner uses the WGO recommended pressures, their tires will be grossly over-inflated:

Empty weight = approx. 9,608 lbs. Michelin recommended pressure (assuming even loading) is between 40 and 45 psi.

45 psi is good for 10,070 lbs. So, an empty View, plus driver, passenger, and say 10 gal. of water.

GVWR is 11,030 lbs. Michelin recommended pressure (assuming even loading) is 51 psi.

Sum of GAWRs is 12,130 lbs. Michelin recommended pressure (assuming even loading) is <60 psi, but say 60. Very close to the WGO placard spec of 61 psi front and rear.

In short, the WGO recommended pressure is enough for a V/N that has been overloaded by 1,100 lbs over the GVWR!

It is 10 psi -- 22% -- over the Michelin recommended pressure for the GVWR.

It is 16 psi -- 35%! -- over the Michelin recommended pressure with a driver and passenger.

With other rigs the difference may be even more extreme.

Pressure in the tables is the MINIMUM needed for the measured load on a tire. Using axle loaad assumes a 50/50 side to side load split which only a verey small % of RVs have.
If you only run the minimum you have no margin when there are corners, or wind side load or when a tire structure has been weekened through use or cumulative damage or if the ambient temperature drops.



As a tire engineer (40 years) I recommend people run a +10% on the inflation for their "set" pressure. This allows for day to day temperature (and pressure) variation. This +10% is not going to hurt the tire and I would certainly not consider it "over-inflated"


You might want to review my blog on RV tires.

[DoC808]

Okay I had a question on this topic. Since I have Duallys in the back my rear drive axle weighs 9020 lb. Correct me if I'm wrong but I think I'm supposed to divide that by 4. Is this correct? That would give me 2255 pounds when I look at the tire chart