Tire pressure - Cold/Hot

[wander2039]

On the RV Chevy 3500 chassis with 225/75/16's Jayco recommends 65 front /60 rear. Currently have Firestone Transforce D range's from 08. Planning on replacing probably in 08 at the 10 year mark.. but still in great shape mostly. At that time will upgrade to E range. Here's the problem I noticed this weekend. If I air up the fronts to 65 when hot it goes up to almost 72. 100 degrees by the IR temp gun. The max on the tires are 65.. is this a potential problem? I back it off to 64psi front and 62psi back (hot) I do notice the front center's are wearing quicker than outer 2 rings. And those 2 rings have a cupping. Rear's are fine.

Have some strange behavior from the passenger side. Feels like a bad shock but not ruling out a tire.. the other tire looks the same though.

 

[Inspecktor]

Not a problem. Tires will heat up when under load or at high speed. If you under inflated them the temp rise would be greater.

 

[SLATRON]

Fill tires when hot if worried about it. I prefer slightly harder tires on mostly highway vehicles like RV. But have the suspension checked if has odd wear pattern.

 

[splinter]

Originally Posted By: Eric Smith
..is this a potential problem?..

No. Manufacturers' "cold" specifications are for pressure differential at ambient temperature conditions (and will of course vary accordingly).

A ~7-psi change from sustained highway speeds should be considered entirely 'normal' and perfectly permissible according to my DIC's TPMS monitoring.

 

[LotI]

The rule is 1psi/10 degrees F rise or fall.

Set the tires cold and pressure will rise with temperature, your grade school physics taught this with balloons. GM knows this when they set the tire pressure above the load curve.

Use caution when increasing load range as you have to run higher pressures to get the same load capacity. If you run the same pressure on E tires the load capacity is less than your D load range tires. Rougher ride and poor wear may also result.

 

[jadatis]

I am able to calculate the needed cold pressure for you , with use of my made spreadsheet.
Need to know weights on axles or better seperate wheels ( or wheelpair).
So also need to know for rear howmany axles and total of tires on the road, front will be 1 axle and 2 tires on the road so single load.
If you never have weighed , give the maximum allowed axle loads, called GAWR or MPAW or MTA.
Also maximum allowed total weight of Motorhome, called GVWR, MPVW or MTM.
Also want to know your maximum speed you use and wont go over for even a minute.

From tires I googled Loadindex 118/116 and this is most likely D-load/8PR with AT pressure 65psi/4.5 bar. And Q and R speedcode seen so this maximum load of Single 1320kg/2910lbs and dual 1250kg/2756lbs can be carried AT 65 psi up to 160km/99m/h without any part of the rubber getting that high temperature that it hardens and damages at next bendings.
But look at the sidewall of your tires for these three data , maxload, AT-pressure and speedcode.

then about the pressure , once read that tiremakers test theire tires to can stand 2 to 3 times the AT-pressure and also read that AT -pressure is not the maximum allowed cold pressure.
That maximum pressure I found as 1.4 times the AT pressure.
If you fill at freesing point of water the tires with 1.4 times the At pressure of 65 psi, so 91 psi and when the tire inside reaches at any point 100 degr C , so boiling point of water the presssure rises to 130 psi wich is 2 times the AT pressure.

So dont be affraid of the pressure rising you have , they are peanuts to what the tire can stand to laws of nature.

Because of the lower pressure rear ( 60 psi) then front ( 65 psi) I think you have dual load rear axle or tandem axle , if both axles single, rear would give higher advice pressure.

 

[wander2039]

It's on my list to get it weighed some day. The door lists 12300 max but if I was to guess would be around 10500-11000 depending on gas and water levels... could be lower as I average 9+ mpg every tank.

The current load range D shows MAX 2335/2150 at 65psi and the range E in the same tire would be MAX 2680 at 80psi. Just going by that 60-65psi will be almost the same weight rating as the D's. D's are rare anymore.

Good points on the higher pressures.. guess I worry too much about the small things!

 

[gaijinnv]

Originally Posted By: jadatis
I am able to calculate the needed cold pressure for you , with use of my made spreadsheet...


You made that same offer in another thread weeks ago. I responded with all the info you requested, and you never answered.

I would advise others to treat this offer by jadatis as a hollow boast - not a genuine offer to help.

HTH

Edit to add:

Please see your previous offer of help here: jadatis offer

And also please note the lack of a follow-up response to my provided info.

 

[George Bynum]

The quoted portion is edited by George
Originally Posted By: Eric Smith
On the RV Chevy 3500 chassis with 225/75/16's Jayco recommends 65 front /60 rear. Currently have Firestone Transforce D range's from 08. At that time will upgrade to E range. If I air up the fronts to 65 when hot it goes up to almost 72.


As others have said, if the tire is rated for the load at a "cold" pressure, the increase with operation and ambient is ok. You've seen the approximation of 1 psi per 10 degrees F ... the calculation is P1V1T2-P2V2T1. Pressure and temperature are absolute; add 14.7 to gauge pressure and 459 to degrees F for absolute. Volume should not change enough to need to be in the equation, that is, you may use P1T2=P2T1.

Now lets look at the ratings. I'm pulling this from a Goodyear web publication, but it should apply to any brand.

225/75/16 D at 65 psi are rated 2150 lbs dual, 2335 single. At 60 psi, (single only ) 2190 lbs. 65 is the max for the D load range.

Assuming you use the same size in E, the loads are the same AT THE PRESSURES UP TO 65 psi. But you can pressurize the E up to 80 psi cold. At 80 psi, load rating is 2470 dual, 2680 single. So, round numbers, the E can carry about 300 more lbs per tire at 15 psi higher pressure.

If you are using E because they are more available and the ride and load is within that of the D, I would run the E at the pressures recommended by Jayco. If you've weighed that unit and it is heavier than Jayco's base, I'd add pressure. If you weigh it with empty tanks, don't forget that water and sewage weigh about 8.3 lbs/ gallon. Gasoline/Diesel are a little lighter, but you won't go wrong assuming 8 lbs/ gallon.

Frankly, I would not try to do any pressure work until I knew the weight. A truck stop won't charge much to give you the numbers on both axles.

I don't have the link where I got the Goodyear pdf, but the document I have starts on page 40 and the 1st page heading is "COMMERCIAL TRUCK TIRES AND DEFINITIONS OF TERMS". It was created in 2010. You might find it, or a newer version with Google's help.

 

[kr_bitog]

Originally Posted By: Eric Smith
On the RV Chevy 3500 chassis with 225/75/16's Jayco recommends 65 front /60 rear. Currently have Firestone Transforce D range's from 08. Planning on replacing probably in 08 at the 10 year mark.. but still in great shape mostly. At that time will upgrade to E range. Here's the problem I noticed this weekend. If I air up the fronts to 65 when hot it goes up to almost 72. 100 degrees by the IR temp gun. The max on the tires are 65.. is this a potential problem? I back it off to 64psi front and 62psi back (hot) I do notice the front center's are wearing quicker than outer 2 rings. And those 2 rings have a cupping. Rear's are fine.

Have some strange behavior from the passenger side. Feels like a bad shock but not ruling out a tire.. the other tire looks the same though.


The elevated pressure and cupping is indeed the sign of suspension problem. So you better sort that out first before troubleshooting the correct pressure. Make sure the shock is working and alignment is correct.

 

[CapriRacer]

A couple of thoughts:

First, the 10°F rise causing a 1 psi increase in inflation pressure is a rule of thumb for PASSENGER car tires, where the inflation pressure is about 30 psi. These are Light Truck tires where the inflation pressure is double that - so the temperature rise causes twice the pressure buildup.

Second, a 10% rise in temperature is a reasonable amount, but something worth keeping an eye on. 15% is an indicator that something is wrong.

So an LT tire going from 65 psi to 72 psi is just a bit out of range. Normal remedy would be to add pressure based on this alone. But I would also caution that maybe the readings aren't as accurate as they could be - and maybe the real pressure buildup is only 5 psi - a perfectly acceptable value.

 

[gaijinnv]

Originally Posted By: CapriRacer

Second, a 10% rise in temperature is a reasonable amount, but something worth keeping an eye on. 15% is an indicator that something is wrong.


I'm with you if you meant "pessure" instead of "temperature."

HTH

 

[ryanschillinger]

Hmm. So nitrogen filled tires aren't supposed to be as succeptible to pressure change with temperature, right? So would nitrogen filled tires do better being inflated higher than the tire sticker specs since the pressure wouldn't be increasing as usual? say my civic specs 30/29 and Costco puts them at that pressure. But with normal air inflation they would be closer to 33 or 34 on a hot highway drive. Should they just be inflated that 10% or so higher to begin with?

 

[CapriRacer]

Originally Posted By: gaijinnv
Originally Posted By: CapriRacer

Second, a 10% rise in temperature is a reasonable amount, but something worth keeping an eye on. 15% is an indicator that something is wrong.


I'm with you if you meant "pessure" instead of "temperature."

HTH


Yup, sorry!!

Note to self: Proofread your posts!!

Originally Posted By: ryanschillinger
Hmm. So nitrogen filled tires aren't supposed to be as succeptible to pressure change with temperature, right? ......


Nope! Not true at all. Nitrogen and air are virtually the same in this regard. I show that on my web page on the subject:

Barry's Tire Tech - Nitrogen Inflation

 

[Astro14]

Originally Posted By: ryanschillinger
Hmm. So nitrogen filled tires aren't supposed to be as succeptible to pressure change with temperature, right? So would nitrogen filled tires do better being inflated higher than the tire sticker specs since the pressure wouldn't be increasing as usual? say my civic specs 30/29 and Costco puts them at that pressure. But with normal air inflation they would be closer to 33 or 34 on a hot highway drive. Should they just be inflated that 10% or so higher to begin with?


Another advertising claim for N2 that ignores the universal gas law PV=nRT...

N2 is dry, that's good. Used in airplanes for a couple of reasons, but mostly because it's dry...but it's not miracle gas...

A car salesman once tried to sell me the "Nitrogen upgrade"...for a mere $150...

I told him I was good with 78% Nitrogen...at no cost...

 

[Garak]

But, we all know that science shouldn't stand in the way of good marketing or a great "theory" though, right Astro?

 

[ryanschillinger]

Glad I asked. Costco does nitrogen for free. Plus you get super cool green valve caps!

 

[Astro14]

Originally Posted By: Garak
But, we all know that science shouldn't stand in the way of good marketing or a great "theory" though, right Astro?

Never let the facts influence a good story!

 

[Garak]

That, too. Incidentally, I saw a buddy of mine with the green caps on his tires. I don't know why, because he's the cheapest person on the planet, and they don't give away nitrogen up here, generally speaking.

 

[HTSS_TR]

I saw some vehicles with green caps, I didn't know that green caps means nitrogen. Learn something new today.