Safe tire pressures without weighing?

[Stump Jumper]

I know that the optimal way to set tire pressures is to weigh the corners of the motorhome and base pressures on that. For those of us who don't have convenient access to scales, what would most people here recommend for front and rear pressures?

[pkunk1]

Do you have a Mfg sticker anywhere in the MH that has a recommended pressure/ axle capacities and what size tires do you have?

[SteveLevin]

You might be surprised how close you are to scales... it's definitely worth checking it out. I live in the suburbs of the San Francisco Bay Area, and offhand, while perhaps not "convenient" I do have scales within an hour... which for what's generally a one off deal is really not all that bad.

And while it feels intimidating (or did to me) when you don't know the process, when I asked, it was amazingly simple (drive onto empty scale, press button to tell folks inside your license plate number, drive off scale, go inside, pay $9.00 and get weight slip) and I wouldn't hesitate to do it again.

Steve

[Mr_D]

In OR and WA most of the state scales are left on even when the scales are closed. Most have enough room to get each "corner" weight, so I do.

If you can't or haven't weighed then use the info on your rigs info usually posted on the wall beside/behind the driver. If you haven't changed to bigger tires then that psi is for the unit loaded at it's maximum.

[LVJ58]

Short of getting your coach weighed, I suggest following pkunk1's advice and look for the label in your coach that reflects the recommend tire pressure.

The label on ours is located on the drivers side wall just behind the cockpit window. Ours 38' coach has a GVWR of 22,000lbs and it calls for 80psi front & rear including the tag wheels.

Additionally the load inflation chart for our Bridgestone load range "G" tires require a minimum pressure of 80 psi.

[BryanL]

One of the best ways is to measure what tire pressure does for travel down the road.

Check the tire temperatures after you have been on the road for a half hour or more. This is a good reason to get one of those IR thermometers so you can check for any temperature anomalies when you do a walk around during any break in driving.

If one tire is hotter than the others, it is low on air. Add some.

Tire temperatures should not go above about 140F and will probably run (depending upon vehicle and ambient conditions) at about 120F +/- 10F

Mfg and such things are often towards a softer ride. It is better for your tires to err on the high side (up to sidewall max psi rating). With the pressure a bit high, the ride might be rougher but the tires will run cooler and be less likely for failure. Tread wear differences should not be significant, especially for typical RV use where age is usually the reason tires are replaced. Handling should not suffer and traction will be the same for braking.

So start high and maybe reduce if they run cool.

[wa8yxm]

Scales are fairly easy to find if you look for them.. Try the Yellow pages first.

One party suggested this

Chalk a wide chalk line on the tire (Sidewalk chalk should do it) Drive a bit and look at the line .. A nice even wear pattern is ideal

Worn ends, like new middle, Increase pressure

Worn middle, ends like new.. Too much pressure, decrease.

He has a point, the idea is to have the tread lie flat on the road

[wildtoad]

I look at the manufacturers label that states GAWR and use those numbers to compare to the tire manufacturers (Michelin in my case) suggested pressures which match the tire pressures on the manufacturers label. There is no way based on what we carry in our mh that we are over the GAWR and if we were, there aint a darn thing I could do about it anyway other than park it.

[RickO]

Quote:

Originally Posted by BryanL One of the best ways is to measure what tire pressure does for travel down the road. Check the tire temperatures after you have been on the road for a half hour or more. This is a good reason to get one of those IR thermometers so you can check for any temperature anomalies when you do a walk around during any break in driving. If one tire is hotter than the others, it is low on air. Add some. .

Seriously? This is one of the best ways? How do you compensate for the heat applied by the sun on only one side of the coach? Don't your inside duals run hotter than the outside? Would that lead to carrying different pressures in each?

Rick

[clubmed98]

scales can be found at gravel pits and such and also Flying J and Pilot off interstates..no reason to at min, get an axle weight..it is your life and fellow drivers around you...also plates on coach are based on you not being overloaded which you do not know..inflate when cold to the max pressure indicated on the tires...it hopefully is more than you need but best bet till you get coach weighed...it will ride rough and probably not handle well in rain..but a blow out doesn't handle well either..always weigh tires cold before you start trip..

[njs42]

[QUOTE=wa8yxm;906929]Scales are fairly easy to find if you look for them.. Try the Yellow pages first.

I did some checking about Pensylvania and it turns out the DOT only has (if what i read is correct) one stationary scale in PA---on I-80 by Clarion which is 113 miles from the OP. However I read of another on I-76 near Euphrata which is quite a bit further. Turns out PA sneaks portable scales into rest stops and suprises truckers. So it seems public scales are hard to come by---not like here in Washington where I have one about 2 miles from me on I-5.

So that would leave Flying J's or truck stops with CAT scales----they may not be so hard to come by.

Best to get it weighed and remove all doubt---people get surprised many times when they weigh their rigs especially four corner weighs. Other wise you are getting good direction by the others----good luck.

[Mr_D]

Quote:

Originally Posted by BryanL Mfg and such things are often towards a softer ride. It is better for your tires to err on the high side (up to sidewall max psi rating).

Go read the sidewall on a truck/RV tire, you will find that the pressure given is NOT the maximum the tire can have. It's the MINIMUM pressure required to support the maximum rating of the tire.
Also, pressure build up due to temps is figured into the cold pressure so no need for anything fancy. Weigh the rig on all four "corners" and use the provided weight/pressure charts. That's the ONLY recommended way to do it correctly.

[Mr_D]

Quote:

Originally Posted by wa8yxm Scales are fairly easy to find if you look for them.. Try the Yellow pages first. One party suggested this Chalk a wide chalk line on the tire (Sidewalk chalk should do it) Drive a bit and look at the line .. A nice even wear pattern is ideal Worn ends, like new middle, Increase pressure Worn middle, ends like new.. Too much pressure, decrease. He has a point, the idea is to have the tread lie flat on the road

Still wrong according to the tire companies!!
I can't for the life of me figure out why people have to invent another method that's more complicated and may lead to a blowout due to low pressure when the tire manufacturer says to do it one way. They're the ones that engineered and tested the tires, they certainly know how to inflate a tire.

[BryanL]

re: "How do you compensate for the heat applied by the sun on only one side of the coach? Don't your inside duals run hotter than the outside?"

I am serious about tire temperatures and base my suggestion on what I have found on the web as well as personal experience.

Indeed the sunside will be warmer than the shady side by a few degrees but the major source of heat for a tire on the road is the tire flex as it rotates so they'll usually be within ten or fifteen degrees or so after a while on the road. - this gets into precision and accuracy stuff about just how much is enough to be important when it comes to tire temperatures and pressures. - consider that many of the now required tire pressure sensors won't alarm until more than 10% pressure drop is detected.

- like a lot of things with the RV, we can make measures whose precision is significantly higher than the accuracy of the phenomena we are dealing with.

re: "Go read the sidewall" - See North American Load and Pressure Marking at TireRack.com to get the straight scoop. You can find other sources as well. Please don't mislead people!

As for the 'flat footprint' - I tend to agree with Mr_D. You can get a pretty good idea of the weight on a wheel by multiplying its footprint in square inches times the inflation pressure in psi - but tires are designed to keep the sideways footprint fairly constant over the nominal pressure range for the tire. This is one reason why center tire wear at 'over-inflation' isn't so much of an issue for RV tires as it takes lots of miles to get something visible or measurable.

An IR thermometer has many uses from kitchens to running gear (there are threads around about possible uses) so it is one of those things it is good to have. It is very easy to run the thermometer over your tire sidewalls or tread as well as hubs (trailers especially) or other areas where temperature might be a concern. By doing that when you stop (good excuse for a walkaround to get the blood going as well as just a safety check) you can answer most of your questions about what the sun does to tire temperatures on the road, what hot days and roads do, and differences between the inner and outer tires on duallies.

As for altitude and air temperature variations - search out why pressure should be checked in the morning when tires are at ambient temperature and then consider the accuracy and precision thing with the ideal gas law.

for those suffering cognitive dissonance: Do note that the presumptions here are for tires and wheels running within rated loads with tires properly sized and rated for the RV and its loads and for pressures and temperatures such as normally encountered by RV tires. I am also using a U.S. focus. If need be, I'll toss in a few more caveats and contextual descriptions if someone insists on (more) reductions to the absurd or similar tactics.

Also: if curious, do some searching and you'll find what I found. A major source of tire failure is heat. Many tires are under-inflated which means they will get hot. Running with over-inflated tires causes little or no harm.