Michelin Load & Inflation Tables

[davidbrady]

This info came from the Michelin Truck Tire Data Book April 2013. They spec a minimum dual spacing of 13.8" for the Michelin XZA2-Energy, 315/80R22.5, on 9" wheels (page 37). On 8.25" wheels they spec a minimum dual spacing is 13.5" (page 97).

They also go on to say:

DUAL SPACING
It is also important that sufficient space is provided between dual tires to allow air to flow and cool the tires and to prevent the tires from rubbing against one another.

To make sure dual spacing is correct, simply measure from the outside edge of the outer tire to the outside edge of the inner tire of the dual assembly. This will give you the center to center distance of the duals across that axle. Refer to the minimum dual spacing column in the application data books.

If you're contemplating a switch to 315's on 8.25" wheels it's probably a good idea to measure your current "dual spacing". (You can use any reference point on the tread that you like, but you have to measure at the same place on both tires. If you're running 8.25" wheels and the spacing is greater than or equal to 13.5" then you can run 315's on your 8.25" wheels.

For a point of reference, I measured the dual wheel spacing on my LXi. I'm running XZA2 Energy's 315/80/22.5's LRL on 9" wheels, Alcoa part number 893603 (hub pilot).

Dual Wheel Spacing: 13.88"
Space Between Tires (Loaded): 0.65"

I put a copy of the Michelin Truck Tire Data Book April 2013 in our Document Library at Document Library > Chassis > Tires and Wheels.

[mrkane]

Measuring with a steel tape, my drives show 13.2" spacing. (Since purchasing this coach, 295/80R22.5 LRH tires have been fitted on the 8.25" width wheels. These appear to be the closest in size to the original 12R22.5s supplied with the coach.)

This coach weighs approximately 44,000 lbs, so the LRH tires are sufficient.

Would it be feasible to use a spacer with the 8.25" wheels to increase the spacing? Would changing to the 9" wheels give enough additional spacing? Would longer lugs be required in both cases?

NB: My wheels are Firestone 8.25 x 22.5 inch, marked "7000 LBS max load at 120 PSI cold."


M.R.Kane
1987 PT40 'Sleipnir'
currently near Elmo, TX

[DonB]

The total weight of the coach is not meaningful for this discussion, only the axle weights. It is *extremely* unlikely that any 80s or 90s WB needs heavier tires on the drive axle, so why fight the spacing issue? The 295 is smaller in diameter than the 315, which in turn is smaller in diameter than the 12R; the 295 varies from tire to tire, but generally about an inch less in diameter than the 12R.

The big issue is purely the steer axle. What do you weigh there? Many Birds, including my previous and current ones, are at or over 14K on the front axle, which is where the 315 really, really, helps.

I changed Blue Thunder to 315s on the steer, and Chuck changed all of the ones on this one to 315s. I don't plan to run spacers, just greater pressure to maintain separation until it is time to replace the tires again.

[mrkane]

Michelin tables show 12R as 485 rev per mile, 315 as 492 rpm and 295 as 503 rpm.

Last time Sleipnir was weighed, both waste tanks were nearly full, as was the fuel tank. Steer was 12220/13200 GAWR, drive was 23,960/23,000 and the tag was 8380/12,100, total 44,560/44,000 GVWR. Of course, I rarely operate on the highway with full waste tanks, and the diesel tank doesn't remain full for long, either.

The load tables show 105 PSI is required for the 295/80s and 95 PSI for the 315/80s to carry 14,000 lbs. Using 295s on all axles would seem to give the advantage of being able to swap between any position, while also avoiding spacing issues on the drive axle.

It is unclear to me whether up-sizing the tires brings real benefit with low-capacity wheels, such as mine. Granted the 295/80s are an uncommon size, but I was able to get one in the middle of SD last fall to replace a damaged tire, with no waiting (except for the tech to drive ~150 miles from Rapid City.)


M.R.Kane
1987 PT40 'Sleipnir'
currently near Elmo, TX

[DonB]

Your numbers are odd - it is not normal for the sum of the AWR to exceed the VWR, and yours do, dramatically (48300/44000). Your AWR for tag and drivers are both higher than expected.

In any case, I would do something about the actuals. Your tag suspension pressure should be increased, forcing weight to both tag and steer and off the drivers.

Later Wide-Bodies are much heavier, and the AWR on the steer is higher. Blue Thunder had a steer GAWR of 14300 as I recall, and the M380 has a GAWR of 14600 (some are upgraded to 16000). My actual weight on the steer this past Friday morning was 14,000 at an Oregon weigh station, with fuel down 50 gallons and the fuel tank is right behind the steer axle. Once you get into weights that high the 12R and 295 both have to be run with high pressure, leaving little room for error and a much harder ride. That is the incentive to move to the 315.

[davidbrady]

You'd definitely see an improvement in ride quality by going to 315's everywhere. If I got the load tables right your steer axle pressures would drop from 105 to less than 95 psi and your drives would drop from 100 to less than 90 psi. (These are rough estimate numbers).

A wheels dual spacing is completely dependent upon the wheels outset dimension. Double the outset to get the wheel spacing. For the 8.25" Alcoa wheel the outset is 6.6 inches. Doubling this gives us the wheel spacing measurement that you found earlier of 13.2", which we know isn't quite enough if you're going by the book.

Alcoa does allow a high temperature nylon spacer to be used between aluminum and steel wheels and between aluminum wheels and steel drums. I don't have one in front of me but I'd guess the thickness to be as much as 0.10" which would bring your spacing to 13.3. I'd get on the phone with Alcoa to see what the actual thickness of the spacer is and what their recommendation might be, or maybe someone here has a spacer in front of them.

Amendment: Well, looks like I didn't get the load tables right. The pressure numbers that I cite above are for the LRL 315/80R22.5" Michelin tire on a 9.0" wheel. On an 8.25" wheel mrkane's steer pressures drop to 101.5 psi and his drive tires drop to about 90 psi. Still a good ride improvement, but not quite as dramatic on the steers. (Thanks mbulriss)!

[mbulriss]

Guys, keep in mind that you are looking at the load chart for 9" wheels at the start of this thread. Per Michael's comments, he has 8.25" wheels on his 87. You need to use the alternate chart from Michelin for the load factors on the 315/80 with the 8.25" wheels. On that chart, 100psi gets you 13,820 for singles, 105psi gets you 14,380. So Michael would need approximately 103psi on the steers to have 14,000 on the steers, per that chart. The 8.25 load chart is on page 87 of the Michelin book, but I have it in pdf format and can't seem to copy the chart to include it here.

BTW, I am running mine at 110 on the front with the 8.25 and the 315/80. (My normal rides are a Hummer or a Corvette, so I don't really understand a soft ride anyway! LOL)

[davidbrady]

(05-21-2013 19:07)mbulriss Wrote:  Guys, keep in mind that you are looking at the load chart for 9" wheels at the start of this thread.

That's a really good point Mike. Thanks, I'll go back and amend my mistake. Here's the Michelin 315/80R22.5 (on an 8.25" wheel) table from the April 2013 Michelin Truck Tire Data Book:

[DonB]

I put the 2007, 2010, and 2013 8.25-inch tables in an earlier post in this thread. I noted that the early table called for 13.2 inches, and then they changed it to 13.5 inches while adding a 75PSI column. Pure conjecture that the two moves are related, and higher pressures decrease the separation requirements.

Over the years Alcoa has made a great many variants of the 8.25-inch wheel, so I'm not going to assume that they all have the same outset. Certainly they have changed other aspects of the wheel - for example the wheels used to have valve in the center of the flat area, and then they moved it to the inner curve. They have also steadily increased the maximum load rating on that wheel size.

[davidbrady]

For completeness what are your Alcoa part numbers? Don's and mrkane's and anyone else running 8.25" x 22.5" wheels.