Tire Pressure

[golden]

My manual for my MH says the max pressure on my 22.5" Michelins is 100 psi on the front and 90 on the back. I normally run 95 and 85 respectively. This time I will be carrying near max GVWR and GCVR, going from central Texas (el 790') to SW CO (at some places near 11,000'). I will be driving on good to excellent 2/4 lane roads and almost no interstates.

Is it better to have more pressure in tires with more weight or less pressure? In other words, should I increase the tire pressure to the max or lower it to 90/80?

[EMD_Driver]

If you are going by the MFR GVWR, they are rated to that capacity at the pressures they recommend. I don't own a MH, but my fifth wheel's tires are always inflated to the MFR's stated pressure.

[Sky_Boss]

Someone slap me if I am wrong.

Your GCVR doesn't really come into play for tire pressure. That assumes the coach is at or below GVWR. If that isn't the case (and I don't think so from your post) ya got bigger issues.

Now, I don't recall my manual telling me the highest tire pressure per se but instead puts those numbers in context of the tires that came with the rig. Since the previous owner change the front tires to something different, my owners manual for those tires does not apply. I have charts for my tires based on my tire manufacturer's tables it tells me that if I am at GVWR and at the max both front and back axles AND if I am balanced L&R also, I should use about 102 PSI in front and 93 PSI in the rear. Of course, those pressures or COLD tire. Since my tires max pressure is 110 PSI, I should be well within that range after they heat up to normal ops temps.

Now, as you probably know, we should get individual wheel weights and then use the highest weight on each axle to govern the tire pressure for both wheels or set of wheels on the same axle. Unless you have a major left side vs right side imbalance, I don't think you would see a huge difference in these kinds of numbers.

Now, that assumes that my interpretation of max pressure is based on cold tire and thus the manufacturer has been smart enough to calculate the fact that 110 cold will be a higher PSI when the tire heats up.

Regardless, I don't think there is a good reason to ever under-inflate a tire. Not that I can think of anyway.

Am I making sense?

[golden]

I just put the GCVR to add more detail to the post. It sounds pretty much like I'm on the right track as I have always inflated my tires about 5 psi less than the max tire pressure listed on the tire itself because of the natural heating of the tire when being driven. The funny thing is that the tag on the inside of my driver door states the max pressure should be 95/85 while on my Workhorse site it states 100/90 (so does the tire). I think I'll go with the Michelin recommendation, the same you listed. We have basically the same MH so I'll just piggy tail on yours.

[SteveLevin]

For ride comfort (and better handling) you really just want to inflate to the pressures required for the weight on that axle. On my motorhome max pressure is 20psi more than I need for my actual weight (even allowing for 1,000 pounds of random fudge factor).

Steve

[sdennislee]

Why not have the coach weighed and inflate based on actual corner weights, then you know for sure you are safe?

[Sky_Boss]

Originally Posted by sdennislee

Why not have the coach weighed and inflate based on actual corner weights, then you know for sure you are safe?

Long, maybe boring analytical thinking
I'm not sure if you were referring to me and my "assumptions" or not. I totally agree with your point about finding some place to get all 4 corners weighed. I was just working through a max weight scenario and thinking out loud how I would deal with it. I'm picky and need to understand the technical side of issues so that I can feel comfortable with the safe boundaries.

As a pilot, figuring weight and balance (W&B) is a bit easier in small planes. The plane has charts to determine center of gravity (CG) for front to back. Its manuals contain specific formulas to calculate the change of CG based on added fuel, location and weight of passengers, luggage and other kinds of stuff. Some tables take into consideration the change in CG based on fuel burn through the flight. In a plane the idea is to keep total weight and location of weight in balance. Too much weight...NOT GOOD. Weight too far forward...landings get a little harder to do because nose is harder to hold up. Not good but normally not too nasty if recognized and dealt with properly. Get the weight too far back...some really nasty things can happen such as tail spin which is not recoverable from. Too much weight on a wing tip and it can cause the wing to snap roll into a spin into that wing...another very nasty outcome.

W&B for a MH presents soooo many variables that it would take a lot of hours and a freely available scale system to develop similar data. Things like wheel weights with fuel tank vs 1/4 tank. Full fresh water vs minimal amount. Even the shifting of fresh water to waste water tanks can affect individual wheel weights asymmetrically. While violating proper W&B in a MH doesn't present the obvious dangers of falling out of the sky, a blow out caused by and/or exacerbated by poor weight management...nasty outcome.

So, once someone does get 4 corners weighed at "max" loading they should be able to reasonably figure out worst case scenario for left right balance also. With that data, they should then be able to figure the worst of the worst case.

Here is an interesting tidbit I ran across in my calculations. My front GAWR is 9000. The most I can shift my weight to one wheels is 4675. That means if I max out my front axle, one wheel can weight 4675 (52%) and the other 4325 (48%). I'm not advocating that, it is just an interesting exercise. The limiting factor being the 110 PSI tire limit.

Conversely, the rear axle could be allowed to be further out of L-R balance. One wheel set can hold 8820 (57%) and the other 6680 (43%). Again, I don't advocate such an imbalance, just pointing out the math.

Here is what I want to do:

LOAD ER UP (full fuel and fresh water and packed for a trip) and get it weighed. THEN...
UNLOAD fuel and water and weigh again with all else being the same. (I may be satisfied with a CAT scale full axle weight just to get a feel for the change front and back.)

By doing that, I should be able to create a W&B envelope to give me a good feel for tire pressures under this range of possibilities. It should also give me some help in figuring out if I need to reduce weight when dealing with mountainous terrain.

WHEW...just thinking of that makes me dizzy. And yes...I am a little crazy too.

Again, I know it sound complicated and it isn't for everyone. It works for me in the long run in developing theoretical understanding. Don't hold that against me. LOL

[Mr_D]

Originally Posted by golden

I just put the GCVR to add more detail to the post. It sounds pretty much like I'm on the right track as I have always inflated my tires about 5 psi less than the max tire pressure listed on the tire itself because of the natural heating of the tire when being driven.

ON TRUCK size tires the cold pressure on the sidewall is the MINIMUM required to support the maximum weight rating of the tires. Same with the tire charts, it's the MINIMUM cold pressure to support the weight.

From page 2 of the 06/07 Michelin RV Tire Guide:

"If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."

From the Firestone/Bridgestone RV tire guide:

Bear in mind that these are maximum ratings. The sidewall of the tire shows maximum load and minimum inflation pressure for that load

From the GoodYear RV Tire Guide:

How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load.

From TOYO

Q: What are the consequences of inflating the tires to accommodate the actual loads?
A: If the inflation pressure corresponds to the actual tire load according to the tire
manufacturer’s load and pressure table, the tire will be running at 100% of its rated load
at that pressure. This practice may not provide sufficient safety margin. Any air
pressure loss below the minimum required to carry the load can result in eventual tire
failure.
But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

[sdennislee]

I load mine full of normal items and fluids. I try to divide the basement weigh evenly left to right and a bit heavier to the rear.

I have the corners weighed then adjust tire pressure according to GY chart. I don't worry about weighing again unless I do something to drastically change my weights.

[Midniteoyl]

Originally Posted by golden

I just put the GCVR to add more detail to the post. It sounds pretty much like I'm on the right track as I have always inflated my tires about 5 psi less than the max tire pressure listed on the tire itself because of the natural heating of the tire when being driven. The funny thing is that the tag on the inside of my driver door states the max pressure should be 95/85 while on my Workhorse site it states 100/90 (so does the tire). I think I'll go with the Michelin recommendation, the same you listed. We have basically the same MH so I'll just piggy tail on yours.

NO NO NO.. You ALWAYS increase the PSI as your increase weight! Under inflating only allows the tire to flex more which cause more heat in the tire and a greater chance of a blow out. Please, if you have the actual weights, inflate the tires to the tire manufacturers recommendation, or, inflate to the MAX on the data plate for your coach. You ALWAYS want to do this with the tire COLD. Do NOT 'reduce by 5 psi to allow for heat'. You are in fact under inflating the tire when you do this.

As for the MAX PSI listed on the tire itself, that is a MAX COLD PSI. The amount of pressure increase due to tire heat has been calculated in. And, it is in fact the MINIMUM PSI needed to hold the MAX Wieght. Unfortunately, it is also the MAX PSI allowed, so the point is mute..

[Full.Monte]

Here is the Michelin tire inflation chart for RVs. You HAVE to weigh each corner of your MH to inflate your tires correctly.

Michelin North America RV Load & Inflation Tables

Over-inflation is not good for the tires either. They will wear poorly. If you are running at the max "just to be safe", you are likely to be over-inflated.

[golden]

My head is spinning! I don't have the opportunity to weigh all four corners as that capability is not available in my area. So, I just go by the weight I get on the CAT scales for front, back and overall and then go by the manufacturers recommendation.

[Wayne M]

Everything being equal, at the difference of altitude of 10,000 feet you will see an increased pressure of 4.8 psi. With a 30 degree drop in temperature a tire at 95 psi would loose 5.7 psi. So let's round those up to 5 and 6 psi respectively. At 95 psi you gain 5 psi for altitude change and loose 6 psi for temperature drop. Hmmm! My math is fuzzy, but it looks like your tires would be at 94 psi. Lets consider that there is only a 20 degree drop, you loss would be 4 psi so your pressure would now be 96 psi, and at a 10 degree drop it would be 99 psi. Nothing to be running out and changing pressure for, and then it must be done at the "cold inflation" point of the tire if you are going to change the pressure. That means it has to sit for a couple hours, or better yet do it early morning without the sun beating down on the tire(s) or late evening when the sun has set.

For every 1000 feet of altitude change there is a 0.48 PSI change. Up in altitude up in pressure. Down in altitude down in pressure.

For every 10 degrees of temperature change there is a 2% change in pressure. Up in temperature up in pressure, down in temperature down in pressure.

It would help to know the exact tire designation. Just 22.5" doesn't mean a thing when looking at load tables. A designation as stamped on the sidewall is needed. Example: 255/70R22.5 (and the load range)

Happy trails.

[DeWat]

Originally Posted by golden

My head is spinning! I don't have the opportunity to weigh all four corners as that capability is not available in my area. So, I just go by the weight I get on the CAT scales for front, back and overall and then go by the manufacturers recommendation.

I'm totally with you here everytime I read one of these tire pressure threads. It would really be nice if there was a centralized web page where one could go to to find the closest four corner scales. I would love to get a four corner weight, even if it cost me up to $50, but I just can't fnd a place in the Metropolitan DC area. I tried one suggestion I saw in another tire pressure thread, and called MD State Police, but the only thing they were able to give was the closest truck scale.