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Old 05-12-2012, 11:18 AM   #1
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Weights and pressures.

Couple of questions. First here are my MH Weights and Michelin recommended pressures from their product spec sheet for the XA2 Energy tires (315/80R22.5 on the front 295/80R22.5 on the drive and tag)
Left
Front 3470kg - 7650lbs - 115psi
Drive 3810kg - 8400lbs - 85psi
Tag 2160kg - 4764lbs - 80psi

Right
Front 3120kg - 6870lbs - 115psi
Drive 4510kg - 9943lbs - 85psi
Tag 1990kg - 4387lbs - 80psi

First, does it seem odd to anyone that I have more weight on the right had drive axle and more weight on the left had tag? I would have expected the coach to be heavier on the same side.

Second, I posted the recommended pressures above, but I run it at 125 psi on the front and 110 on the rear. Maximum inflation is 130psi for the 315s and 120 for the 295s. Anyone aware of a downside from running a little hard - I don't mind the ride, it is better than it was with the Goodyear G670s at lower pressure.
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Old 05-13-2012, 04:05 AM   #2
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Hi,

It does not seem strange to me that you have an uneven weight distribution.
A lot depends on where the coach designers put major weight items like water tanks, holding tanks, fuel tanks and spare tires. It does seem strange to me that despite your doing good work on figuring out the recommended tire pressures, you choose not to believe the numbers and set tire pressures at some intuitive number. One simple way to make yourself believe the recommended tire pressures is to set them at the listed numbers, then take a chalk and draw a line across each tire tread. Drive a block or so and check to see if you have any chalk left on the treads. If you have chalk left on the sides of the tread, you are over-inflated.
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Old 05-13-2012, 07:46 AM   #3
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Running over-inflated adversely effects handling and braking. You end up with uneven contact pressure across the tire, which reduces the effective contact patch. And it put more stress on the rest of the suspension components, because the tires are doing their designed share of absorption.

It's pretty common for there to be weight distribution differences between the front and the back. Not so much between drive and tag axles. It's quite possible the load sharing valve between the drives and tags are mis-adjusted. That's something your chassis shop can check.

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Old 05-13-2012, 11:07 AM   #4
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Distaff,
I looked at several Michelin RV tire inflation charts and none had those numbers you posted.

However, here is my take. The number I have for that tire, single is 7920lb@115psi for the 9" rim.(Same for 8.5" but does not go higher than a 120psi rating)
Your post indicates you are at 7650lb@115 psi. That, to me, gives you a "fudge factor" of 270 pounds. The fudge factor is that you could add an additional 270 pounds to that tire and still be within a safe load range. Evenly distributed you could add 540 pounds to the axle and still be within the very minimum safe load. (Air in tires on the same axle should be set at the heavier load rating for the heaviest tire for psi)

You are running 125psi. That pressure will support 8810 lbs of weight (X2=17620lbs), and that gives you a fudge factor of 1160 pounds per tire or 2320lbs distributed across the axle. Do you need that high a fudge factor?

At 120 psi the fudge factor would be 620lbs per tire, 1240 lbs evenly distributed across the axle. To me the 120 would be a smoother ride, as the closer you get to the tire max pressure the rougher the ride. However, the handling at the higher pressures is a little better than at a low pressure.

Are you good? Yes, at pressure of 115, 125 you are most definitely within the load specifications of the tire manufacturer and you have some leeway, up or down, in pressure. However the leeway "down" at the 115 psi rating and dropping 5psi would put you 40 pounds under the load weight rating for that tire.

For every degree of temperature increase change there is a 2% increase change in air pressure. For every 1000 feet of increase in altitude there is a 0.48 degree increase in pressure.

Just my 2 cents.
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Old 05-14-2012, 06:10 PM   #5
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A couple of things, since I didn't keep a note of the pressures with the coach, they tend to creep UP over time. This is why I thought I'd revisit it.

Next up, Wayne, I'm attaching the sheet from Michelin that I used. I note something odd about it - they give the load per tire in LBS and kPa. kPa is NOT a unit of weight, it is a unit of pressure, but they clearly mean KG. I pulled the same sheet down today and it correctly shows LBS and KG.

I think I have more of a cushion than I need, but was curious if anyone had a specific reason that over inflation causes damage - I am aware that it can cause the center of a tire to wear prematurely, but mine have a pretty healthy foot print even running them hard.

I note that I misread the table the first time, it only calls for 75 PSI on the tag axles and 80 psi on the drive - this table gives the weights per pair of tires in the dual application. I have seen another where it divides it in half and gives the weight per single tire.

I think I'll still run them a little harder than called for, but not quite so hard.
Attached Files
File Type: pdf XZA2 ENERGY.pdf (41.9 KB, 23 views)
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Old 05-15-2012, 11:50 AM   #6
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Quote:
For every degree of temperature increase change there is a 2% increase change in air pressure.
I believe it is 2% per 10 degrees of temp change (not every single degree).
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Old 05-15-2012, 12:31 PM   #7
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I'm having all 6 tires replaced next month and adding a TPMS. I've done all my calculation based on charts and tend to fudge a little high. However, I think once I get my TPMS running, I will validate through tire temp readings.

That being said, a small upwards fudge factor seems "OK".
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Old 05-15-2012, 12:46 PM   #8
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Quote:
Originally Posted by distaff View Post
First, does it seem odd to anyone that I have more weight on the right had drive axle and more weight on the left had tag? I would have expected the coach to be heavier on the same side.

Second, I posted the recommended pressures above, but I run it at 125 psi on the front and 110 on the rear. Maximum inflation is 130psi for the 315s and 120 for the 295s. Anyone aware of a downside from running a little hard - I don't mind the ride, it is better than it was with the Goodyear G670s at lower pressure.
No, ours is heavy on the passenger side front and drivers side read.

You wheels probably have a maximum pressure of 120 psi if they're aluminum. That's for both our steel and aluminum wheels.

Running a tire at higher than chart pressure is OK, except that makes it more susceptable to impact damage from a chuckhole or curb.
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