Yet Another Tire Pressure Question

[Dan McMartin]

Got the Bay Star weighed today. I'm on a 24k chassis and came in at 21640, 7100 front axle and 14540 in the rear. So, I go look at the tire inflation charts for my 255/80 22.5 XRV tires and come to find I don't even reach the low end of the chart.

So, I'm at 94 psi and the chart's lowest is 80 psi. Before I go let 12-14 lbs of air out of each tire, I just wanted to verify I'm not reading something wrong. Am I correct in my assumption that I'm below the chart minimum and I should be running 80 lbs?

[RV Pirate]

Check the tire inflation sticker in your RV. 80 sounds way too low. I'm guessing you're gonna need to be around 100 psi. Our Challenger likes 100 psi and I'm running the same size tires you are.

[cruzbill]

Just a reminder that any pressure listed in an inflation table is the MINIMUM pressure required to support the listed weight in the table.

[Unplanned Tourist]

It's not unusual to have tires that are rated for a much higher load than you normally have. That's a good thing in my mind. Less stress and longer life if properly cared for than tires that are on the edge of or over loaded.
Just play with different pressures and find what works best for ride and handling, then keep them close to that.
Happy Glamping.

[F76Marion]

Dan,

This is a bright thing for you to check out.
But you are right, none of the tire charts that I have cover this.
That means that your best bet is to call the manufacturer Monday with all your data ready. Those people are good at this and you're having all the real data will delight them.

Frank

[l1v3fr33ord1]

Dan-

I'm reading off the 18th edition of the Michelin charts. Assuming no side-to-side variation, but instead allowing an extra 500 pounds on each corner, I read 75 psi for the fronts and 81 psi for the rears.

What are the maximum axle ratings for that chassis?

What does the placard in the coach say?

[wildtoad]

So now you know that you have some leeway in the tire pressures for your unit. I would not run anything below the chart minimum. What I would do is set the pressures to what the placard in the unit says and IF you think the ride is too harsh adjust the pressures down in stages but not below the minimum on the chart.

For mine I see no rhyme or reason for what Newmar says to run and the chart for the GAWR’s. Perhaps there is a reason ( other than CYA ) why they have pressures higher than what the tire chart says is the minimum needed to support the weight.

[Dan McMartin]

The placard says 100 psi. Seems a bit high. The dealer sent me home at 92-94. And nothing in the charts would indicate I need to be at 100.

My axle ratings are 9000 and 15500. Even fully loaded, I'd be at 85 psi on the front axle and 80 on the rear. As Mark said, add 500 lbs for uneven loading and I'm at 75 front and 80 in the rear at my current weight.

I might try 85 psi front and back and see how it works.

[ADS43264ME]

Dan,

What is the load rating of your tires? I quickly looked at the Michelin RV tire charts and it goes down to 70 psi for the G load version. There was a retired engineer/tire company CEO on the Tiffin forum and his advice was to always take your axle weights and divide by 2. Then multiply that weight by 1.05 and look that weight up on the chart. Lastly, add 5 psi to the chart recommendation.

As someone said, the chart is the minimum recommended pressure. Did you weigh your coach with full fuel, water, loaded and with passengers. Under inflated tires are dangerous and the most likely source of a tire blow out. Better to have them over inflated than under inflated.....

["007"]

You must have missed this link.
Tire pressures to high in 90's will be a hard ride.

[Vanslam]

I ride with 80 in the front and 90 in the rear of my Canyon Star witha 24,000 lb chassis. I did have it weighed and used the tire chart and added about 5lbs extra. It has worked for me.

[l1v3fr33ord1]

Originally Posted by Dan McMartin

The placard says 100 psi. Seems a bit high. The dealer sent me home at 92-94. And nothing in the charts would indicate I need to be at 100.

My axle ratings are 9000 and 15500. Even fully loaded, I'd be at 85 psi on the front axle and 80 on the rear. As Mark said, add 500 lbs for uneven loading and I'm at 75 front and 80 in the rear at my current weight.

I might try 85 psi front and back and see how it works.

Interesting.

Michelin values taken from 18th edition of the Michelin Truck Tire Data Book (2016) (link here). For the specific tire, values are on page 26 of the book (page 28 of the PDF file).

Please confirm, as errors are possible (even likely).

As weighed:
Front axle: 7100
Calculated weight to carry: (7100/2 + 500) = 4050
Minimum tire pressure to carry: 75 psi
Rear axle: 14540
Calculated weight to carry: (14540/2 + 500) = 7700
Minimum tire pressure to carry: 81 psi

At axle ratings:
Front axle: 9000
Calculated weight to carry: (9000/2 + 500) = 5000
Minimum tire pressure to carry: 101 psi
Rear axle: 15500
Calculated weight to carry: (15500/2 + 500) = 8250
Minimum tire pressure to carry: 83 psi

[Dan McMartin]

Originally Posted by 007";4009675]Tire pressures to high in 90's will be a hard ride.[/QUOTE]

I hadn't seen that. My big surprise after casually reading tire pressure threads was that I fell below the lower end of the charts. I just wanted to make sure I wasn't missing something and it appears I wasn't.

[QUOTE=l1v3fr33ord1;4009852]Interesting.

Michelin values taken from 18th edition of the Michelin Truck Tire Data Book (2016) (link [URL="http://util.michelintruck.com/databook/DataBook.pdf

here[/URL]). For the specific tire, values are on page 26 of the book (page 28 of the PDF file).

Please confirm, as errors are possible (even likely).

As weighed:
Front axle: 7100
Calculated weight to carry: (7100/2 + 500) = 4050
Minimum tire pressure to carry: 75 psi
Rear axle: 14540
Calculated weight to carry: (14540/2 + 500) = 7700
Minimum tire pressure to carry: 81 psi

At axle ratings:
Front axle: 9000
Calculated weight to carry: (9000/2 + 500) = 5000
Minimum tire pressure to carry: 101 psi
Rear axle: 15500
Calculated weight to carry: (15500/2 + 500) = 8250
Minimum tire pressure to carry: 83 psi

I'm reading page 20 of that link for RV tires. I'm getting roughly the same pressures. I wasn't adding the 500 to the max weights in my post above.

And BTW, the forums are freaking out. I can't get rid of all that bold and my reply box is about half the width it should be. Weird

["007"]

I see your fronts at 75 psi and rears at 80 psi some people will add 5 psi to all tires for safety factor.
All not over the placard settings from factory of 110 psi.