Yet Another Tire/Wheel Question

[Norm Green]

I am getting ready to take our 08 Thunder out again, first time since my stroke on Sept. 23rd in Branson, and I notice that all my Michelin tires are showing significant sidewall cracking.  I have 315R80/22.5 on the steering axle and 295R80/22.5 on the other axles.  I have priced the exact replacement from FMCA Michelin Advantage Plan and local pricing for Toyos M144.  There is almost $2000 difference between the brands.  In looking at the specifics of the two brands, I found that the listed rim size for both sizes of the Michelin's is 8.25 to 9.0".  The rim size for the Toyos in the 295 size is also 8.25 to 9.0" but for the 315 size it is 9.0 to 9.75".  My question is what size rim do I have and will the 315R80/232.5 Toyos fit on them without issue?

[David T. Richelderfer]

The rim size is stamped on the rim.  My coach has 9x22.5 rims all around.  I'll bet yours does too.

[Norm Green]

Thanks, I checked the wheels and the rear wheels are marked 22.5 / 8.25 but nothing noted on the outside of the front wheels and the inside surface is so dirty I can't find any markings.

[David T. Richelderfer]

The various sized rims (9x22.5 versus 8.25x22.5) have different rim load ratings as well as tires mounted on the various rim sizes will have different tire load ratings.  We also have 315s (Dunlop, 22 ply) on the steering axle and with 9x22.5 rims those tires' load rating is 9,090 lbs at 130 psi.  If 315 tires are mounted on narrower (8.25x22.5) rims, then the tires' load rating is somewhat diminished as I recall.  I do not know why a wider rim would allow a tire to have a greater load rating, but that is what I recall the chart showing.

[Norm Green]

Thanks David.  I would be very interested if anyone has mounted the 315R80/22.5 Toyos on a 8.25 wheel successfully?  I am going to try to contact Toyo for their input.

[Frank Bergamo]

Norm, something else you need to be aware of is minimum dual spacing. It is my understanding that a 8.25" wheel needs a minimum wheel spacing of 13.2". The 315/80 tires I believe will rub on each other. The dual spacing is basically the measurement from center line of tire to center line on other tire. Check tire specs to confirm this measurement. Hope this helps!!

[Norm Green]

Thanks Frank, I'm only looking at 315s on the steer axle.  I'll put the standard 295s on the back axles.

[David T. Richelderfer]

I, and Norm I believe, did not consider mounting 315s on the drive or tag axles.  The 295s have plenty of load rating for the two rear axles.  But on the steering axle the 295s load rating didn't leave any tolerance for tire air pressure fluctuation due to temperature and/or altitude changes.

[Doug Allman]

Been doing a lot of investigating as I am going to move some of my coach tires around and really wanted to know about nitrogen. I have always had a loss of air from the front drivers steer.

From what I have read the air molecule is 1/4 the size of the nitrogen molecule. It also contains no moisture so is not a cause of deterioration, to rim or rubber tire. I am not sure deterioration of tire is an issue as we mostly change out tires in 6 year period or so. If you dont they will let you know you should have when they give that loud bang going down the highway.

To fill the 295 22.5, 8 of them, it would from the formula I have found and hopefully interpreted correctly take 582 cu feet of nitrogen. My local supplier of nitrogen indicates that a large tank holds 305 cu feet of nitrogen and the refill is approx $45.00. Large cylinder rental is $150.00 for 5 year interval. Hose and guages are another $155.00. So to fill 8 tires and own the needed items would cost  $395.00. Then if you want to keep a small tank on board your coach so you can add nitrogen if needed as you travel is another $165.00.

Now I want nitrogen just because of what it does for keeping the tire cooler 40%, supposedly getting better mileage due to correct pressure 10% and longer life 50% (this is non issue as we never wear out unless you are an officer of the club and go across this great land to rallys and meetings to meet your obligations). Although after 3 years and 50,000 miles on this set of Michelins I am wondering what they will look like in 3 more years as the tread has gone down more than I expected. BUT I think I will rely on the tire shops to install nitrogen as a more $ friendly way.

They do have a tank that you can use and only pay for the nitrogen you use and I may use this option to do the initial fill as I have my own tire changer and shop I can do it in. Airgas is a nation wide business and they say that there are 90 locations across the US where you can get nitrogen from them in the same way.

I am very interested in this thread as I will be putting on new tires in 2019 which is not far away. Have always been partial to Michelin due to its supposedly better ride characteristics.

[Edward Buker]

Doug,

Air is 78% Nitrogen, 18% oxygen, and trace gasses plus water vapor. If you use Nitrogen you get about 93% into a tire because they do not allow evacuation with vacuum to deform a tire before filling it because it damages it. So what you are getting in reality is roughly the same mix of gasses plus about 15% more nitrogen. When you look at gas expansion with temperature there really is no difference regarding the gasses, but there is a real difference in expansion of the water molecules. As the temperature rises while driving, the water molecules will vaporize into a gas, and that transition from a liquid to a gas is responsible for most all the rise in tire pressure.

I think the thing to focus on is the moisture and not so much the gas mix. Keep as much moisture out of your tires as possible.

I had several Michelin tire fails in my last coach and working with their reliability engineers we believed that moisture in the tire caused failure in the following way. I live along the gulf coast, very high humidity and moisture saturated air. My coach sits during the winter when the moisture condenses. I added air without a water separator using a small compressor that I carry with me. When my tire failed (sidewall zipper fail) I noticed that the steel cables had some rust on them. With steel cable roughness and the elongation and contraction motion of the steel cables with each revolution, Michelin believed they actually sawed themselves. So when I sat in the winter the moisture was being driven slowly through the sidewall and condensed on the steel cords and rusted them. Truckers do not have the issue because the tires do not sit, this fail is unique to RVs. Michelin increased the thickness of the barrier coat which is on the inner surface of the tire and is the layer responsible for maintaining pressure and is the barrier to gas and moisture migration through the sidewall.

I have added a water separator to my travel compressor and fill when the humidity is low.  I use my home compressor when ever possible given it has a better water separator system on it.

This is my two cents on focusing on what I think is important which is the moisture. In a round about way using nitrogen gets you to a low moisture state in a tire given it is a demoisturized gas, but it is an expensive way to get there. I can see no other benefit to having nitrogen in a tire for our application.

Later Ed