Correct tire pressure

[wolfe10]

CORRECT TIRE PRESSURE

To determine the correct tire pressure for your coach, neither Michelin nor Goodyear recommend carrying the PSI imprinted on the tire sidewall UNLESS YOU ARE CARRYING THE MAXIMUM WEIGHT which is also imprinted in the sidewall.

Go to Michelin or Goodyear's RV Tire Guide and you will see that the tire manufacturers do NOT have a single recommended tire pressure for each size tire, but always show a chart where correct PSI is based on actual weight.

So how do you do this correctly: BEST way is to load the coach as you go down the road. Get all 4 (or 6) wheel positions weighted. RVSEF does this at all FMCA events and many other places. Of course you can just weight axles, but here you are ASSUMING perfect left to right weight distribution-- something that is NOT true on the majority of coaches.

Taking the heavier wheel position on each axle, go to the tire manufacturer's weight/PSI chart for your tire and determine the correct MINIMUM pressure. All tires on an axle get the same pressure based on the heavier wheel position. Many add 5-10 PSI to that as a safety cushion (so you do not have to go and recheck when your wife stops at the outlet mall).

Next best-- look at your GVWR plaque which by law will be in the driver's area. On it will be recommended PSI BASED ON MAXIMUM GAWR. Here you are assuming your weights are not higher or lower than maximum. You had better hope that this number is wrong, or you have no safety margin in tire carrying capacity.

Further down still-- put in what is on the sidewall of the tire. ASSUMES you have NO LOAD INFORMATION ON WHAT THE TIRE WILL BE CARRYING. Could be right on (hopefully not) or could be off by 20 or more PSI.

All tire manufacturers recommended tire pressure are based on COLD (have not been driven) AT THE AMBIENT TEMPERATURES YOU WILL BE EXPERIENCING

ANYONE WHO TELLS YOU WHAT PSI TO CARRY WITHOUT WEIGHTING YOUR COACH DOES NOT KNOW WHAT THEY ARE DOING. Example, the "correct" minimum tire pressure from the Michelin book for a 275/80R22.5 is 75 pounds if the load is 4,500 pounds and 115 if the load is 6,175 pounds. BIG DIFFERENCE.

Brett Wolfe

[tbutler]

Brett has hit the nail on the head. Or should I not mention nails when discussing tires? Here is another factor to consider when checking your tire pressure. You should check tire pressure each day you travel in the RV. Why? The more frequently you check your tire pressure the better you will know your tire pressures. If one tire is suddenly low, it indicates a problem that should be investigated. You will know that the problem needs to be investigated because you have checked the tire for many days and it is always at the same pressure as the other tires on that axle and now suddenly one tire is different. Alarms go off in your head and you do more than just add a little air to bring it up to the correct pressure.

Another reason you check air pressure each day you drive is because the temperature each day will affect the pressure of the tire. When Brett mentions ambient temperatures, that means the temperature each day you drive. Because each day will vary slightly in temperature, the pressure will vary also and you want to be sure that the pressure stays at your desired value.

Now here is a gotcha for checking pressure. If you check tire pressure after the sun rises and the sun is shining on any of your pretty black tires, those tires will have an unreliable pressure reading because they are warmer than the other tires, just as if they had been driven. How much is the difference? Check your tire pressure before sunrise and then check again an hour or two after sunrise and see how the pressure changes. Compare that pressure with the pressure of tires on the same axle with the same starting pressure but in the shade. You will find significant differences in pressure readings. In reality, the tires should all be at equal pressure but you won't have any way to verify that with your pressure gauge. If you now try to equalize the pressure on an axle, you will end up with the tires in the sunlight being at lower pressure (the gauge will read the same but the warmer tires will have less air in them) than the cool tires in the shade. So the only time you can check tire pressures and be sure you are getting accurate pressure readings is when the tires are all at an equal temperature, before sunrise! This same principle applies to tires that are near a running generator or other heat producing device. If something in your coach warms tires unevenly, you can not adjust tire pressures until that heat source is removed from the equation and the tires have equalized in temperature.

If you weigh your coach with RVSEF you will receive an analysis that mentions another complication for motor homes. Your coach never weighs the same one day to the next. No I'm not suggesting your diet isn't working... I am pointing out that each day you drive, you use significant amounts of fuel from your fuel tank. A 100 gallon fuel tank holds fuel weighing about 600 pounds. Your propane tank may hold another 200 pounds of propane if it is full. You carry fresh water, sometimes more, sometimes less. You have waste tanks that are sometimes empty and at other times may be nearly full. So when your coach is weighed you need to know what the status of all your tanks are. Then you need to compensate the actual weight of your coach to the tanks full weight. In my coach, the fuel, propane and fresh water are all near the front axle. The waste tanks are near the rear axle. So when it is weighed I have to add weight for each axle to equal the missing liquids in each of the tanks. That way the tire pressure will be sufficient for an all tanks full situation.

So, weigh your coach, add weight for empty or partially full tanks, look up the weight for the tires on the manufacturers tire chart and add 5% or 10% for a safety measure. This becomes your target pressure for the tires on one axle. Each axle will be different, based on the weight on the tires on that axle. Also, be aware that tire charts will show a different weight allowance for single tires vs. dual tires. Dual tires run hotter due to the adjacent hot tire and thus need a different pressure for the same weight a single tire carries. The charts make all this easy, just look for the dual weight scale for those tires and use the single weight scale for single tires.

Finally, when you drive, your tire pressure will likely go above the rated pressure stamped on your tires and rims. The pressure stamped on the tires and rims is for the cold pressure before you start driving. This rated pressure assumes normal heating during driving and thus the associated pressure increase is also factored into the rating.

[tireman9]

As a Tire Design and Quality Engineer with 40 years experience, and as a retired professional race car driver and driving instructor, I can say the I agree with almost everything TBUTLER and wolfe10 have said.

A couple of additional details.

For about 99% of all tires, you will find that the Load & Inflation table for manufacturer A is identical to that of Manufacturer B and these both match industry guidelines as published in Tire & Rim Association (US) or ETRTO (European) or JATMA (Japan). This means that you don't have to worry if you cannot find the information from your tire manufacturer.

If any of you are going to Bowling Green I am giving a seminar "Tire Basics for the RV owner" and will be offering to compare your tire gauge to one of my calibrated digital gauges. Stop in and say Hi.

Regarding checking your tire pressure every day you intend to drive is a good suggestion. Personally I do that most of the time BUT I have a Tire Pressure monitor system on my RV so I know my tire pressure about every hour (in addition to the automatic checks done by the system.

As far as getting up before sunrise to check your tires. Personally I would not bother. BUT I would check my tires once, sometime late in the evening at least 4 hours after driving and being out of the sun. Be sure you record the inflation at that time. Then when you check the tires after they have warmed up in the sun you will see the increase (about 3 to 10psi I would expect). Make a note of the "Inflation in the sun" number.

If I had checked all my tires (out of the sun) at the start of a trip and then checked each day I would not worry about a few PSI increase due to warming in the sun. I also would not worry about the 1 or 2 PSI increase if the ambient temperature went up from the day before. NOTE tire inflation will increase or decrease by about 1 PSI for every 10 degrees change in air temperature.

Running your tires 3 to 10 PSI higher that required to carry the MAXIMUM load they see in actual use is something I suggest. It gives you a bit of a margin for all the variables.

You should NEVER run your tires at an inflation that is less than specified by the tire Load & Inflation tables.

Also, if I had set all my tires to the correct pressure at the start of a trip I would probably never let any air out to lower the pressure as long as I was not exceeding the Max Cold Pressure molded in on the sidewall of the tire.

[robertcalvert]

Hello Tireman9:

Could you please advise of a manufacturer name or model number and cost for your "' calibrated digital tire gauge". I must own 6 gauges of which I now only trust one.

Cheers,

Robert

[revleroy]

Will they have weighing devices at the Six State Rally in Shawnee ?

[tireman9]

Robert and others

I have only tested a few gauges so my suggestion should NOT be taken as meaning these are the only gauges that are accurate.

First, If you have six gauges but only trust a couple I am courious why. Did you go to one tire and get a reading from all six and compare the readings?

The brand of gauge I use is Accutire. These are digital gauges and have tested as accurate to +/- 0.5 psi. If your gauge is +/- 1% then you are probably OK.

Mine is a single head 150 psi unit. I have had my coach weighed at each corner. I looked up the minimum inflation needed to carry that load by rounding up on the chart. I then increase my inflation by 5psi. This is my "IDEAL COLD" inflation I set the tires to every day I drive. The extra 5psi gives me just the extra cushion I need to account for normal day to day temperature changes. I also do not get my shorts in a bind if I am one or two "low" from my "IDEAL". If I drop to three low then I add some air.

Since I am not a full timer I have access to plenty of air at home. If I were on the road and started seeing three psi drop and added a bit of air then saw another three psi drop I would be suspicious and watch my air in that tire very closely.

Of course since i also run a tire pressure monitor I can check my pressure and even temperature a few times a day but I am a tire engineer so I am a bit strange when it comes to taking care of my tires.

You may need a dual head gauge so will need to shop a bit. I would simplu Google "Tire gauge digital" and "Tire gauge dual" and "Tire gauge dual digital"

I would look at the specifications and claimed accuracy. If no data is given then i would not consider that gauge.

You need to have a gauge that reads at least 25 psi higher than your tires stated maximum.

[wolfe10]

Tireman9

Good post. I use the same "Chart plus 5 PSI formula" with 4 wheel position weights-- using the heavier wheel position on each axle to access the inflation chart.

If all one has is axle weights, I add 10 PSI to compensate for left to right imbalance as well as safety reserve.

Once a year I take my tire gauges- (boy, I feel shorted, I only have TWO double footed gauges) to a friend who has a large tire store with a shop air system with high quality calibrated built in gauge. I write deviation on a piece of tape and affix it to the gauge (i.e. gauge reading of 90= 94 actual).

Brett Wolfe

[CAPKELLY]

When checking the Cold tire pressure before a trip. Does it make a difference IF the coach is still on the Jacks or to get a true pressure must the Rv be off the Level jacks.. Thanks your input. Kevin

[wolfe10]

When checking the Cold tire pressure before a trip. Does it make a difference IF the coach is still on the Jacks or to get a true pressure must the Rv be off the Level jacks.. Thanks your input. Kevin

No, it doesn't make a measurable difference.

But you can prove this yourself. Check with the coach on jacks, stow jacks, recheck.

Brett Wolfe

[yogi115@aol.com]

Have any of you ever seen OTR truckers at a rest stop carrying a big stick and walking around hitting the tires on their rigs? Its called "thumping" and it is a quick way to see if you have a change in pressure. An experienced driver will tell you that a tire that has lost air will make a different sound than the "normal" pressure tires. Also doesn't hurt to carry a big stick when you are refueling!!!!

[wolfe10]

Have any of you ever seen OTR truckers at a rest stop carrying a big stick and walking around hitting the tires on their rigs? Its called "thumping" and it is a quick way to see if you have a change in pressure. An experienced driver will tell you that a tire that has lost air will make a different sound than the "normal" pressure tires. Also doesn't hurt to carry a big stick when you are refueling!!!!

If you have a reasonably "tuned" ear, "Thumping" with a tire bat will tell you if tire pressure is WAY out of spec (at or below "Run Flat" PSI), so it is better than nothing, but does not substitute for checking PSI.

I do it (along with the temp check below) every time we pull into a rest area, fuel stop, etc.

Another accepted method of ROUGHLY checking PSI while driving is to check tire temperature-- either by lightly placing your hand on the sidewall or for those with all the high-tech toys, shooting with an IR gun.

What you are looking for here is NOT absolute temperature, but a tire that is significantly hotter than other tires that SHOULD be the same. What this means is that you would expect some tires to be hotter than others-- for example a tire in the sun and you need to take that into consideration.

Brett Wolfe

[CAPKELLY]

I am thinking of adding a tire pressure monitoring system for my Monaco Knight and toad. There are many units out there.as you know. Do you have a specific brand and type that you think works best.. I really do not want to have the tires removed and inside monitors added. Are these units accurate enough to even tell your tire pressure when cold so you do not need to check the pressure? Thanks you for your help. Kevin

[tireman9]

No, it doesn't make a measurable difference.

But you can prove this yourself. Check with the coach on jacks, stow jacks, recheck.

Brett Wolfe

I have seen a controlled contest where a number of experiensed OTR drivers were asked to evaluate three tires. OK, Low or High.

So with w 33% change of being correct with random guess why did only 27% get the OK tire correct???

[tireman9]

If you have a reasonably "tuned" ear, "Thumping" with a tire bat will tell you if tire pressure is WAY out of spec (at or below "Run Flat" PSI), so it is better than nothing, but does not substitute for checking PSI.

I do it (along with the temp check below) every time we pull into a rest area, fuel stop, etc.

Another accepted method of ROUGHLY checking PSI while driving is to check tire temperature-- either by lightly placing your hand on the sidewall or for those with all the high-tech toys, shooting with an IR gun.

What you are looking for here is NOT absolute temperature, but a tire that is significantly hotter than other tires that SHOULD be the same. What this means is that you would expect some tires to be hotter than others-- for example a tire in the sun and you need to take that into consideration.

Brett Wolfe

Well since I am a bit anal about tire pressure I installed internal sensors on my unit as I wanted to know both Temp and press. BUT I drive a Class-C and my required inflation is only 55PSI I could go with a unit that is designed for passenger tires (75 psi max) and I have the tools at home to de-seat one bead to replace the valve.

I cannot address which brand is best as I haven't tested them.

I can tell you that you should expect the TPM gauges to be off by a few PSI.

If you get the screw-on-cap style I would suggest that you inflate your tires and set them with your best digital gauge. Then screw on the TPM cap and when all done run a system check. This will give you a reference number for each position. ( I would not worry about +/- 3 to 5 psi because.....

1. You have weighed each corner

2. Checked the table for the lowest inflation needed to carry the heavyest side

3. Added 5 psi to get the proper COLD inflation pressure.

With a TPM you probably can use those numbers for your daily check but I believe that you will still need to "Top-Off" your tires once a month. You can use that opportunity to keep an eye on the accuracy of your TPM valves.

[CAPKELLY]

well thanks there Tireman. Your comments were very helpful...Kevin