Tire Pressure by Axle weight

[jenglish92]

I just purchased a new Tiffin Allegro 32SA on a 24,000 lbs chassis with Michelin 255/80R22.5 GXRV tires.

Michelin publishes a load and inflation table for the tires here.


If I am reading this chart correctly, the loads listed are for Axle Ends, therefore, the Single load row needs to be doubled to determine total Front axle load and the Double (dual tire) load row needs to be doubled to determine the total Rear axle load. This is needed as some scale locations will only do front and rear measurements and not each tire (axle end) separately. Obviously, this isn't perfect as not each side of the coach will be loaded exactly the same, but you work with what you can.

You can see the table in the attached file as well.

Again, if I am reading this correctly, the maximum tire pressure I should ever run in my coach in 80 PSI, since the total Front and Rear Axle weight would total 24,040 lbs, which is my Gross Vehicle Weight Rating.

In reading other forum posts, I have seen people recommend 90 or 100 PSI for 24K chassis rigs, but that would seems to be for loads well above their GVWR. What are the downsides of over-inflating the tires in relation to the load chart? I would think it would improve gas mileage but reduce ride quality.

I guess it is possible to have a heavily loaded rear axle at 16k lbs and need 85 PSI per rear tire, and a lightly loaded front axle at 8k lbs and need 75 PSI per front tire.
Further reason why getting the coach properly weighed both loaded and unloaded is important.

Anyway, wanted to confirm the tire chart reading and usage, and see what other other people on doing on similar tires and chassis size.

[D Lindy]

The Michelin tire pressure charts I've read list the tire pressure per tire in either a single or dual tire configuration, not by the axle weight but the individual 4 corner weights.

[Betr2Trvl]

I will assume for arguments sake, that your door sticker shows 100#s front and rear (which is what my door sticker reads on my 31 SA on a 24k chassis).

Looking at the Michelin chart I have for my 255/80-22.5 XRVs it would suggest:
9000#s front axle max weight equates to ~87#
15500# rear axle equates to 80#

However, when you get axle weights, what you don't know is how that is distributed left / right. Given that deviation, most recommendations have you add roughly 10% to the Michelin chart to the pressures for a given axle weight, which would get you to 96# front and 88# rear.

I now have my fronts at 96 and rears at 92, which I just reset on a very chilly morning based n my actual weights below. I'm also looking at how the pressures compare going down from the 100#s door sticker based on my weights, instead of going up from the chart numbers, and I feel pretty comfortable with these numbers.

This also gives me more overhead for greater cold temp. deviations this time of the year. There are also a couple other nuances that I considered, but won't get into that.

These weight are from this summer, when I had the coach as loaded as I ever expect it will be, including a full fresh tank which I seldom have. Waste tanks wee empty.

8420# front on the 9000# axle
14160# rear on the 15500# axle
22580 total vs 24k GVW

Keep in mind also, that a 32SA is about 500# more than a similarly equipped 31SA and that is all on the rear axle given the added length / weight is all from the added foot to the bedroom.

The added pressure does increase the spring rate of your tires, which of course provides a bit harsher ride. But under inflated is dangerous.........

Better to be at the door sticker until you get a good weight with a loaded coach...... And better to err to more air pressure than less.

[Betr2Trvl]

Just want to quickly clarify a point above regarding 31SA / 32SA weights.

I noted that a 32SA is ~500#s more than a similarly equipped 31SA, that would be a 31SA on the optional 24k chassis. That is based on my CCC being a bit over 3500#s on the 24k chassis and most 32SAs have about a 3000# CCC for what I have seen posted in several 32SAs.

[wildtoad]

One can over think, over manage, and over worry tire pressures and by golly many people do. If you know the axle weights then you have enough information to use the chart to set the minimum pressure needed to support the weight. It is of course better to know the four corners weights but if you don't have them you don't have them. i suspect that if the difference between left front versus right front causes more that a lb or two of pressure difference you have bigger problems.

[CLIFFTALL]

I agree . I'd add 3-5 lbs for good measure until you can get the four corners weighed

Originally Posted by wildtoad

One can over think, over manage, and over worry tire pressures and by golly many people do. If you know the axle weights then you have enough information to use the chart to set the minimum pressure needed to support the weight. It is of course better to know the four corners weights but if you don't have them you don't have them. i suspect that if the difference between left front versus right front causes more that a lb or two of pressure difference you have bigger problems.

[Betr2Trvl]

As noted, the pressures in the charts are the minimum to support the weight, so in that perspective, more pressure is ok (to the point of over inflating the tires, and ignoring other considerations)

In addition, by adding some pressure you keep yourself from chasing too low pressure on colder days vs. when you set pressures. 10 degrees equates to ~1.5#-2#s of air pressure in "100 psi" RV tires.

Adding a minimum of 10% to the chart pressures is the rule of thumb that I have consistantly seen as a general safety factor. There is also something about not being any lower than X% of another factor, but don't recall off hand what that % is, nor the baseline since it wasn't relevant for me.

And, as I noted, I am also looking at given my actual axle weights, how do my pressures relate to the chassis sticker which is based on a fully loaded coach. I think that is a very relevant second data point to consider........

[Betr2Trvl]

I just dug up a thread from another forum on my original weighing last year.

Edited for brevity.......

Gas full (80 gal)
Water tank 1/3rd
Black and Gray both empty
Residential fridge empty
1/3 of our normal clothing, linens,and dry goods on board
No one in the passenger seat during the weighing

We have the 24k chassis (9k front / 15.5k rear axle ratings)
255/80 - 22.5 XRVs

Total Weights:
Front 8,100#
Rear 13,440#
Total 21,540 (For those with the standard 22k chassis don't panic, the 24k chassis adds about 500#s)

Based on max weights, my reading of the Michelin table looks to be about 77# front and 75# rear, plus the buffer factor and consideration for food, dry goods,and more clothes equates to about 85#s or so front and rear.....

Here is the response from the tire expert on that site, and it references his recommendation for a minimum pressure based on max. tire pressure that I did not remember above.

So what I have done is use the most accurate weight available which is your axle weights. Divide by 2, multiply by 5% to compensate for left to right imbalances, look that up on the Michelin chart and add 5 psi for safety. Additionally, I don't like to see radial tires run at less than 75% of their max. pressure (in your case not less than 82.5 psi) so I recommend exactly the same pressure as you mention 85 psi all the way around.

So you can better see how I ended up at my new pressures of 96#F / 92#R based on a fully loaded coach with my new weights (1,000#s more, with the full fresh water tank being a signifcant chunk of that) as shown in the previous post in addition to my thoughts about also looking at going down from the door sticker, and generally being safer (and easier to manage) with more pressure than pushing the low limits.......

Hope all this helps.

Regards