Question [Closed] Tire pressure increase while driving
We have our first motorhome after being trailer people for 40 years. We have a 2015 Thor Four Winds 35SK with 6 Hercules Strong Guard H-RA 225/70R19.5 tires, Load range G 14 PR 128/126 - L Load /speed (75 mph), G ply rating, per the tire labeling
The Ford label on the driver’s side door jamb shows Front Axle GAWR of 7000# with these tires inflated to 95 psi cold. Rear GAWR of 14706# with these tires inflated to 110 PSI cold.
My CAT scale ticket with the rig loaded with everything we would possibly take, plus TOAD shows:
Steer axle: 6320#
Drive axle: 12340#
Trailer axle: 3640#
The Hercules website shows: Single Max Load @ Pressure 3970@110 & Dual Max Load @ Pressure 3750@110
Questions: Should I not be running these at 95 PSI cold on the front?
What pressure range & increase should I expect to see when running at 60 MPH on a summer day? Want to know what to set the TST Truck TPMS high pressure alarm setting to avoid unnecessary/nuisance alarms, but do not want to exceed any maximum pressure capacity of the tires.
Have read on some Thor Forum posts of people suggesting to run the tires at lower pressure than what the MFG label states to get a smoother ride. I prefer safety before smoothness. Your expertise would be appreciated.
You dont have to be afraid of the pressure rising, even above the 110 psi, wich is maxloadpressure cold, and not the maxcold pressure of the tire.
Warm pressure the tires have teststandards that they must stand 2x maxloadpressure, so here 220 psi. Mayby for the higher maxloadpressures the teststandard is les, lets assume only 1.5 x maxloadpressure, but this stll is 165psi in your case.
I will make a pressure axleload list for you in next post, but first see if I am allowed to answer in this forum.
Here my made list, made with tighter formula then the official lists, and build in maximum reserve, at wich comfort and gripp still acceptable ( list gives 90% of calculated axleloadcapacity)
So then your tires stay cooler, so pressure rises less.
Putted your given weights behind the pressures.
In front of psi for axleloadcapacity for singleload so your frontaxle, and behind for dualloadaxlecapacity, so your drive axle.
So you dont need to devide by 2 or 4, or add a reserve in load or pressure, all done for you. Front GAWR needs 110psi to my standards, rest I have given in the list
2804 lbs/ 40 psi / 5297 lbs
3122 lbs/ 45 psi / 5898 lbs
3438 lbs/ 50 psi / 6496 lbs
3753 lbs/ 55 psi / 7090 lbs
4066 lbs/ 60 psi / 7682 lbs
4378 lbs/ 65 psi / 8272 lbs
4689 lbs/ 70 psi / 8859 lbs
4999 lbs/ 75 psi / 9444 lbs
5308 lbs/ 80 psi / 10028 lbs
5616 lbs/ 85 psi / 10609 lbs
5922 lbs/ 90 psi / 11189 lbs
6229 lbs/ 95 psi / 11767 lbs
6534 lbs/ 100 psi / 12343 lbs/ given weighed
6838 lbs/ 105 psi / 12919 lbs
7142 lbs/ 110 psi / 13492 lbs/ maxloadpressure
7445 lbs/ 115 psi / 14065 lbs
7747 lbs/ 120 psi / 14636 lbs/ maxcold sometimes given and needed for GAWR drive to give max reserve
From an actual tire design engineer... Its good that you know your actual axle loading.
The Ford sticker 95 / 110 provides enough load capacity to support your measured axle loading so those are the number to go with.
You should NOT start with a lower inflation and plan on the temperature increase to give a pressure increase to 95/11. The ONLY inflation numbers you consider are the " cold inflation" which means when the tires are at AMBIENT temperature and not warmed up from being driven on or being in direct sunlight for the previous 2 hours.
Mr Jadatis means well but he is not familiar with US roads and driving so I would not get confused by reading his posts. He is NOT a tire design engineer.
RE people suggesting lower inflation to start. They are not the ones who will need to pay for any failed tires.
Your fronts are only running with a 10% margin and I would not run any lower infl. The rears have a better level of reserve laod so yould not run any lower than 100 psi. I doubt you will feel any ride difference of 110 vs 100 psi. You can read many more detailed posts on loading and proper inflation on my blog www.RVTireSafety.net
At 70bdegrF cold filled at 100psi, thevpressure changes 1 psi for every 4.5 degr F. if perfect dry gascompound in tire, wich it never is.
Driving 75mph ( max speed of tires) at 100degr F outside temperature, temperature in tire can rise to 160 degr F.
160 - 70= 90 degrF higher temp
90/ 4.5= 20 psi pressure rising it gives, so 120 psi. Then dont lower it to 100 psi, because then more deflection and more heatproduction, so tire inside, and with it tire-material, get even hotter.
Nit needed, but only way to get the presdure lower, and so lower temp of tire material, is to drive slower, then also lesser heatproduction a minute.
@roger Thank you very much. I was more concerned with running the fronts at 95 if the Hercules data sheet shows everything at 110. Didn’t know if Ford recommending the 95 for the front was similar to the Explorer fiasco/debacle some years back where they were recommending lower than what Firestone recommended. On our first trip of any distance, we experienced higher increases than we ever saw with our bumper pull trailers. Sometimes a 20 - 25 psi increase, but we also started on a cold morning in KY and were heading South and into higher temps. I could not find anything about the maximum safe pressure for the tires. That’s my biggest concern.
Do you only drive with TOAD?
Mind that if you drive without TOAD behind, that more weight on front axle and even lesser weight on rear/drive axle.
This will get the front axle closer to the 7000lbs GAWR, for wich I gave in my list the full 110 psi.
That 110 psi would still not give discomfort when riding with TOAD.
This because I made the list for max 75mph. Then more deflection then when I calculate for higher speed( only virtually, you are not allowed higher speed then 75mph) .
I once determined that using 90% of the loadcapacity for 99mph for the pressure, that then it gives a deflection of tire, that gives still acceptable comfort and grip.
Mayby for these larger tires the border of comfort is for lower speed. But could make a list for N speedrated is max 87mph, and then comfort still OK. Will guve higher pressure advice for same axleloads.
The weighing fully loaded per axle, as you did is how it should be done.
Better even if weighed per axle-end.
Then at the end 2 things.
1. Its difficult to edit posts in this forum. I tried but it gave edit, but when saving, capcha got in the way. So could not correct my writing errors.
2. What is exactly meanth by TOAD. I think a small trailer behind the motorhome.
Bottom line...If you inflate your tires to the PSI required to support the actual measured load and add +10% psi you should see the inflation increase that comes from driving to increase by only 15% to 20% of your cold "set" pressure.
The Ford numbers for psi are what is needed to support GAWR and I believe that Ford learned their lesson with the Explorer situation to not recommend low inflation to give a good ride.
Thats not specific for Ford. Every base vehicle maker gives pressure looked back in official tiremakers lists, for GAWR's with no reserve.
For RV, wich are mostly loaded to the max, and always weightdifference R/L on the axle, you yust need the reserves to prevent one or more tires overheating.
Thanks for your input. I can end this discussion thread now as I have the information that I need.