Tire pressure vs temp?

[tireman9]

Comments on a number of posts.
1. Tire Industry standard as published by the "Tire & Rim Association" says to adjust tire pressure "When tires are at the prevailing atmospheric temperature and do not include any build-up due to vehicls operation".  The 57 members of the association include Accuride, Cooper, Goodyear, Bridgestone, Continental, Michelin, Pirelli, Sumitomo, Titan, Toyo, Kenda, Maxxis, Dunlop, Hankook, Kumho, Nexen, Yokohama, Sailun and others.

2. I don't know why some feel the "prevailing atmospheric temperature" is some specific temperature i.e. 70 or 65F as I have been reading in some other RV forum threads.

3. Cars have placard inflation numbers that yield a reserve load capacity of 20% to 30% while most RVs have placard numbers that provide 0% to 10% and this excludes the 52% that we know have overloaded tires and/or axles based on the tens of thousands of weighings done by RVSEF.

4 I know no no one in the tire industry that is suggesting you adjust your tire pressure multiple times a day. The general recommendation is to adjust, in the morning before the tires are warmed by the Sun or by operation.

5 Cross country busses generally set their pressure at the tire sidewall pressure and never really completely cool down enough to end up in overload.

6 I do not know of any car company since 2000 that intenionally under-inflated their tires trying to get better ride. Most have designed their suspension (springs, shocks & bushings & seats) to provide the soft ride they want. They still provide significantly more load Reserve than seen on almost any RV. CAFE dictates that they shoot for better MPG because they have significan financial incentive to get better MPG.

 

[fagnaml]

I'm with Roger's (Tireman's) guidance which is the same that my father who owned three Sinclair Service Stations drilled into me -- check tire pressures each morning when the tires are cold especially when big changes in weather occur.    As an example, a few weeks ago the Houston area had is first major cold front of the fall when morning temperatures dropped from a typical 75 F to 35 F.   I didn't take the time to check the tire pressures on my Jeep Wrangler and sure enough during my drive to work that morning the TPMS system alarmed with low tire pressures that were down to 28 psi versus the normal 35 psi.   Needless to say, I checked tire pressures the next morning and had to add air. 

For my coach, I keep the tire pressures at 125 psi for front and rear tires.  This inflation is quite conservative for the loaded weight of my coach with Jeep in tow.  I don't try to fine tune the tire pressures to match my loaded weight / tweak ride comfort.  Just simpler fo rme to keep the tire pressure at 125 psi (max pressure is 135 psi for my Hankook tires). 

Edited December 10, 2020 by fagnaml

[cleipart5]

On 11/16/2020 at 2:33 PM, wtbuck12 said:

Looking for opinions. Left south Louisiana with temperature in the 80's arrived in North Carolina with temperatures in the 30's. Needless to say, tire pressures in the morning are considerably lower, should I adjust back to 95/90 psi or run as is?

All tires pressures need to be set based on load inflation tables which in turn uses your scaled weight per axle. The manufacturer load inflation tables will give you cold PSI recommendations.

You lose or gain 2psi per 10 degrees. 80 to 30 is 50, that's a change of 10 psi. The load inflation table is more of a "range" of +/- 5 psi, if it says 100 psi and then you move it +5 to 105psi that will not make a difference whatsoever.

If you know you are going into a colder climate, gaining altitude (you actually gain 2-3psi for every 5,000 feet up you go if temp stays constant) or going into a cold front, add +5 or +7 more psi.

The sidewall of a Truck/Bus tire lists "minimum cold recommended pressure at the MAXIMUM LOAD". Read that again, the Truck/Bus tire does not give a recommended PSI on the side wall. ie; a Continental 315/80R22.5 HA3 LR-L has a max load carrying capacity of 9090lbs per tire in a single tire application AT 130psi and a max load carrying capacity of 8270lbs per tire in a dual application also at 130psi. In a 10 tire Coach that means (9090 x 2 =18,180) + (8270 x 8 = 66,160) = 84,340lbs max spread across all tires. I know a Class A American Coach 42 with 10 315/0R22.5's GVWR is 47,000lbs. If we could pretend all tires carried the same % of load that is 47,000/10=4,700lbs per tire. The load inflation table for the Continental 315 HA3 doesn't go lower than 5675lbs dual and 6175lbs single at 80 psi, essentially even 80 is a safe range. Run them at 95 or 100, see how they feel, do they wander, does it feel stable, does the wear pattern look even? Then adjust from there. Knowing this, why would we run the PSI at the max load rating on the sidewall? That would be 130psi and overkill.

The door placard will normally list a PSI based on a load inflation table but it's not reliable. Sometimes it's for different tires, sometimes the OEM is wrong, sometimes it's for UVWR and sometimes it's for GVWR. 

The best recommendation is 1) scale your axles, 2) look up the load inflation tables per manufacturer of tire and adjust accordingly.

Load Inflation Tables

[sharon1948rs]

hello everyone im looking for a new dealer i can buy my tire-safeguard tire montior system. the dealer im dealing with not been to good last couple of months.im not looking to get a different kind.just want another guy to deal with. thanks

[richard5933]

What brand are you getting? We bought ours directly from TireMinder at the rally in Gillette a couple of years ago, and my interactions with their customer service department since then have been great. These just screw onto the valve stems so no dealer is needed to install them.

Or are you talking about something mounted inside the tires? In that case, you need to find a tire dealer in your area that you trust.

[tireman9]

On 11/16/2020 at 9:41 PM, rayin said:

Look at the placard in your bus. The listed air pressure is adequate for temperature changes. Does anyone assume commercial bus drivers  adjust tire air pressure as they drive from CA to WY in December?

For RossBoyer; The NHTSA (another link) says to inflate to the Federal Tire Placard in the vehicle, that's what i do for all my vehicles. What do you do for your autos when you are the only occupant vs having 4 passengers, do you adjust tire pressure to the load ?

The main difference is that cars have about 35% Reserve load capacity while most RVs are lucky if they have 10%.  The placard inflation is reasonable till you confirm actual weights with scale weights.

I suggest a minimum of 15% reserve load if possible. Once you know the minimum inflation for YOUR RV you should plan on adjusting as necessary when there is a change in Ambient of more than 20F which would have changed your inflation by 4%.  Now if you run a 20% Reserve load as I do I can tolerate daily morning ambient changes on +/- 15F as I have a cushion built in. If I have the time and expect to run at the new Ambient I will probably adjust my morning "cold" inflation back to my goal.

 

FYI I will be at Perry next week. two seminars on tires.

Edited March 4, 2021 by tireman9

[tireman9]

On 2/9/2021 at 6:00 PM, cleipart5 said:

All tires pressures need to be set based on load inflation tables which in turn uses your scaled weight per axle (Most tables are per tire load not per axle load). The manufacturer load inflation tables will give you cold PSI recommendations.

You lose or gain 2psi ( its 2% so 2psi only works for 100 psi tires) per 10 degrees. 80 to 30 is 50, that's a change of 10 psi. The load inflation table is more of a "range" of +/- 5 psi, ( No the inflation in the table is actually the MINIMUM requires to support the stated load) if it says 100 psi and then you move it +5 to 105psi that will not make a difference whatsoever (No the load capacity does change for each change of 5psi).

If you know you are going into a colder climate, gaining altitude (you actually gain 2-3psi for every 5,000 feet up you go if temp stays constant) or going into a cold front, add +5 or +7 more psi.

The sidewall of a Truck/Bus tire lists "minimum cold recommended pressure at the MAXIMUM LOAD". Read that again, the Truck/Bus tire does not give a recommended PSI on the side wall. ie; a Continental 315/80R22.5 HA3 LR-L has a max load carrying capacity of 9090lbs per tire in a single tire application AT 130psi and a max load carrying capacity of 8270lbs per tire in a dual application also at 130psi. In a 10 tire Coach that means (9090 x 2 =18,180) + (8270 x 8 = 66,160) = 84,340lbs max spread across all tires. I know a Class A American Coach 42 with 10 315/0R22.5's GVWR is 47,000lbs. If we could pretend (not a good idea)all tires carried the same % of load that is 47,000/10=4,700lbs per tire. The load inflation table for the Continental 315 HA3 doesn't go lower than 5675lbs dual and 6175lbs single at 80 psi, essentially even 80 is a safe range. Run them at 95 or 100, see how they feel, do they wander, does it feel stable, does the wear pattern look even? Then adjust from there. Knowing this, why would we run the PSI at the max load rating on the sidewall? That would be 130psi and overkill.

The door placard will normally list a PSI based on a load inflation table but it's not reliable(Why?). Sometimes it's for different tires,(the info is different if the size is diffeerent but not if it is just a different brand) sometimes the OEM is wrong (How? When), sometimes it's for UVWR ( It should never be based on unloaded weight)and sometimes it's for GVWR. (The certification label AKA Tire Placard, by law is to have the inflation needed to support the GAWR)

The best recommendation is 1) scale your axles, 2) look up the load inflation tables per manufacturer of tire and adjust accordingly.

Load Inflation Tables

See comments in RED

[tireman9]

12 minutes ago, rayin said:

I'm happy you mentioned the 35% reserve capacity for cars. That should also apply to my MH tires if I choose to ignore load/inflation charts and go by the Federal Tire Placard in my MH, which is the minimum pressure for the GVWR.

I'm not clear on what you are asking. I believe you know the actual load on your tires. Simply calculate 135% of that load and then find the tire inflation needed to support that 135% load. You many need to go to higher Load range or a larger tire or a combination to achieve that goal. I am not aware of any regulation that prohibits you from upgrading your tires or wheels to achiever the 135% figure. Alternately you could reduce your actual load such that your tires can support 135% of that reduced load.  Note. I am not saying that achieving the goal of 135% in a MH is easy given they way they are currently designed i.e. limits of tire size.

[tireman9]

12 hours ago, rayin said:

I uprated from LR G(OEM) to LR H which have a sidewall listed pressure of 130 psi, however I cannot utilize that as my Alcoa rims are stamped @ 120 psi. I run the Federal Tire Placard pressures of 116 psi-front, 95 psi rear duals. I have to get the MH reweighed as I cannot find the old weigh tickets after I spent a month in hospital from Dec. 12-Jan 14 this winter.

Regardless of MH weights I always run by the placard anyway, I've tried it both ways, DW and I cannot detect any difference in ride (which IMO is a poor reason to reduce pressure anyway. And Goodyear agrees) As we know radial tires footprint remains nearly the same regardless of running  load/inflation or placard pressure. The old bias-ply tires were very different, over-inflation greatly reduced the footprint.

OK. After you get the axle weights we would be interested to learn what your actual Reserve load is.  Take care of yourself.

[rsbilledwards]

This thread is amazing and displays a lot lack of knowledge by several I thought had a handle on these sorts of issues. Rolling down the road with plackard tire pressures if fully loaded is one thing. It is a totally different thing if lightly loaded. Yes almost from day one I have run with load based, manufacturer suggested for a given load tire pressures. My very first rallie I had my coach weighed. This makes sense if one likes a smooth unjaring ride which takes a little effort in older coaches. So too will follow the advice of the TIREMAN!  By the way sir I finally have the pictures you requested but cannot now find your email request. Let me know and I will get them right off.

[urbanhermit]

This reminds me of discussions on motorcycle forums about which is the best motor oil . . . 😁

Edited April 8, 2021 by urbanhermit

[urbanhermit]

On a more constructive note, my method is to put as close to 100 psi cold in each (33,000 GVRW, six tires) as I can, actually ranging from 100 to 107, and watch the temperature readings on my TPMS.  I think 100 psi significantly over-inflates the front tires, but the coach steers and brakes well and we'll see about ride next trip after the shock absorbers are replaced. 

[urbanhermit]

On 11/18/2020 at 8:05 PM, wildebill308 said:

I don't care if you have oversized balloon tires 75 from McKinney to Durant will shake the dishes.

Bill

I-10 and offshoots, Dallas area popped my shower stall loose from the wall. 

[manholt]

What part of I-10?  Beaumont-to La border?  Lake Charles, La.  Lafayette to Henderson Bayou?  There are a lot of good Interstate and Loop roads in the Dallas/Ft. Worth area!

[hermanmullins]

Ok folks let's get on topic "Tire Pressure vs temp?"

Thanks, 

Herman 

[tireman9]

2 hours ago, urbanhermit said:

This reminds me of discussions on motorcycle forums about which is the best motor oil . . . 😁

Not quite.  I doubt that your motorcycle forum has discussions on how much oil you should run in your engine. 😁