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Old 04-30-2012, 06:25 PM   #1
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Tire Pressure and weights

I just had my coach weighed (not each tire but each axle) and this is what I came up with:

Front 8860 (10410 limit)
Rear 15860 (17500 limit)
Gross 24660 (27910 GVWR limit)

I went to the Michelin site and found my tires (22.5 XRV). Do I divide in half my front and rear weights? That doesn't look right to me.

Sorry if this is a redundant thread, but I couldn't really find a definitive answer. I'm currently running 100 pounds in each tire.
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Old 04-30-2012, 06:47 PM   #2
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Usually, you divide your fronts by 2, the rears by 4 (or 6 if a tag)... HOWEVER, Michelin looks to be doing a 'per side' thing on their Dual listing, so, just divide the rear by 2 also

You dont say which size you have, but I'm gonna assume 255/80R22.5, in which case 85PSI front will be just even with your weight.. most boost the PSI by 10 to for safety, so in your case, 95PSi should do.

I dont think I see a tag and the 17500lbs says a single axle rear, so 7980lbs per side also equates to 85PSI per tire.. adding 10 nets the same 95PSI as the fronts..

Your 100PSI was really close and you prolly dont need to change it you dont have the need too..
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Old 05-01-2012, 12:13 AM   #3
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Mark , were these weights taken with the coach fully loaded for travel ?
If you were not loaded leave the tire presures at 100PSI until you get the loaded weights. Then re-calculate and adjust accordingly.
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Old 05-01-2012, 08:49 AM   #4
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Quote:
Originally Posted by Skip426
Mark , were these weights taken with the coach fully loaded for travel ?
If you were not loaded leave the tire presures at 100PSI until you get the loaded weights. Then re-calculate and adjust accordingly.
Yes. It was fully loaded with a half tank of water and a full fuel tank.
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Old 05-01-2012, 07:47 PM   #5
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Another $.02 for you.

If you can go back, weight the coach from one side only for each axle. Say the left. Then subtract that from the full axle weight.

Then look at the side that has the highest weight per axle, and use this to determine the tire PSI.

If the tire MFG does not have a Dually weight noted, then divide that axle weight, whichever is the highest, by 2.

A few other factors. Going around corners. Driving in extended cross winds. What I call 'I may need that, and it will fit here...' coach weight creep. Etc.

As extra safety margin on PSI, I do two things:
1) Go the next highest weight range on the chart. If close to the top of a range, I jump two spots. (Say 20 lbs below the top, of a range. Jump to ranges.
2) I then take that PSI, and add an extra 5 psi contingency on top of that.

I usually find on my vast experience of two rigs (!) - that I'm still 15 to 30 PSI below the Coach Manufacturers PSI label values.

If you can not go back to weight the side. Then I suggest you make another fudge factor, and assume one side of each axle is heavier then the other. Say on the fronts, divide by two, and then add an extra 250lbs. On the rears, for duallies, add an extra 500lbs. (Better to do the weight of the side if you can. But it is very common for one side of both front and rears to have differences from left to right side... Compensate for that, if you can't get the actual weight.

Sorry for being so long winded, but PSI is important for safety!!

Best,
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Old 05-01-2012, 07:54 PM   #6
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... then divide ... the rears by 4 (or 6 if a tag)...
ABSOLUTELY NOT! REPEAT. ABSOLUTELY NOT!

You will NEVER divide by 6 with a tag. The tag is an independent axle and is weighed separately and divided by 2 and use the single inflation spec.

The drive is divided by 2 (not 4) and you then use the DUAL inflation spec.
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Old 05-02-2012, 01:33 AM   #7
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Originally Posted by ottffss View Post
ABSOLUTELY NOT! REPEAT. ABSOLUTELY NOT!

You will NEVER divide by 6 with a tag. The tag is an independent axle and is weighed separately and divided by 2 and use the single inflation spec.

The drive is divided by 2 (not 4) and you then use the DUAL inflation spec.
Not if you only have a rear weight, not a separate tag axle weight, which he didnt mention..
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Old 05-02-2012, 06:54 AM   #8
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When recently in Arizona, I went to a CAT scale with my 2011 Phaeton. This was the first time I ever had the coach weighed and the guy performing the weighing job showed a STEER AXEL weight of 11,660 lbs, DRIVE AXEL weight of 0 lbs and a TRAILER AXEL weight of 20,100 lbs for a total GROSS WEIGHT OF 31,700 lbs.

I have Michelin 275/80/R 22.5 XZA 3 tires. If i have used the Michelin tire weight info on the internet correctly, I assume to get proper front tire pressure, I divide the FRONT by 2 and BACK by 4?

I do not have a TAG AXEL.

If i do this, I come up with about 80 psi per tire which seems really low to me. I have been running about 105 psi, is this too high? I note max pressure cold for the tires is not to exceed 120 psi.

I hope I don't sound too ignorant on this subject, I am doing the best I can to get some proper info so that I do this right as I am aware of the safety issues.

Thanks for any feedback anyone might offer.

I emailed Michelin asking for some help on this subject but got no response from them WHATSOEVER!
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Old 05-02-2012, 11:29 AM   #9
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Yes. It was fully loaded with a half tank of water and a full fuel tank.
Mark, I just had to ask , as your loaded weights look the same as my empty weights. I weighed the coach empty to check the factory listed 3345lbs CCC. Loaded I'm running within 1000 lbs of max GVW. 28000lbs chassis.Skip
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Old 05-02-2012, 12:37 PM   #10
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...I assume to get proper front tire pressure, I divide the FRONT by 2 and BACK by 4?...
You are not reading the prior posts. To repeat, on the drive (a dually setup and no tag) you divide that axle weight by TWO, yes TWO and use the DUAL line in the charts. On the steer you divide by 2 and use the SINGLE line.

Now, you need to provide more tire info. There are 2 models of Michelin 275/80/R 22.5 XZA3 tires a NON XZE (LRG) and an XZE (LRH) model. I'll use the LRG table for the examples below.

So...
For your steer axle, a single, weight = 11,660 / 2 = 5830 go to tables on SINGLE line (for LRG model) and find the first column equal or greater than 5830. That is 105psi (to carry a max of 5980lbs per side).

For your drive, a dual, weight = 20,100 / 2 = 10,050 go to tables on DUAL line (for LRG model) and find the first column equal or greater than 10,050. That is 95PSI (to carry a max of 10,140lbs per dual side).

Always go to the higher PSI column if it is close.

If you ever get each "corner" individually weighed (which you should at some point) you will use the greater of the 2 weights measured for THAT axle and just go immediately to the tables with that greater of the two weights number (no division) as you already have the axle "end" weight with a four corner weigh.

And again, COLD means UNDRIVEN (and not heated by the sun). Checking in the morning or evening is the time to do it. If you drive from Miami to Fairbanks you WILL need to check and reset your pressure due to the change in AMBIENT temperature of those locations.

You will NEVER, EVER release air on a hot (driven or sun heated) tire. No matter what psi you measure when hot.
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Old 05-02-2012, 12:50 PM   #11
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As a repeat caution, if only obtaining full axle weights (not 'four corner'), compensate a safety margin on the divided weight, to allow for the very probable reality that one side of the 'full axle' will carry a higher load.

Say Front axle = 11,000 X 2 = 5,500. In this example add a safety value of say 500lbs to the 5,500 and use 6,000 for PSI checking. Again, if close for sure bump up a range. And as I noted above, sometimes bump two if real close to a range value.

Most scales that provide weights, have enough room to drive back over and take one more 'one side only' measurement. Do this, and then do the math to get a 'four corner' weight.

Best,
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Old 05-02-2012, 01:07 PM   #12
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Mark, I just had to ask , as your loaded weights look the same as my empty weights. I weighed the coach empty to check the factory listed 3345lbs CCC. Loaded I'm running within 1000 lbs of max GVW. 28000lbs chassis.Skip
Yes, these are my fully loaded weights.
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Old 05-02-2012, 03:23 PM   #13
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You are not reading the prior posts. To repeat, on the drive (a dually setup and no tag) you divide that axle weight by TWO, yes TWO and use the DUAL line in the charts. On the steer you divide by 2 and use the SINGLE line.

Now, you need to provide more tire info. There are 2 models of Michelin 275/80/R 22.5 XZA3 tires a NON XZE (LRG) and an XZE (LRH) model. I'll use the LRG table for the examples below.

So...
For your steer axle, a single, weight = 11,660 / 2 = 5830 go to tables on SINGLE line (for LRG model) and find the first column equal or greater than 5830. That is 105psi (to carry a max of 5980lbs per side).

For your drive, a dual, weight = 20,100 / 2 = 10,050 go to tables on DUAL line (for LRG model) and find the first column equal or greater than 10,050. That is 95PSI (to carry a max of 10,140lbs per dual side).

Always go to the higher PSI column if it is close.

If you ever get each "corner" individually weighed (which you should at some point) you will use the greater of the 2 weights measured for THAT axle and just go immediately to the tables with that greater of the two weights number (no division) as you already have the axle "end" weight with a four corner weigh.

And again, COLD means UNDRIVEN (and not heated by the sun). Checking in the morning or evening is the time to do it. If you drive from Miami to Fairbanks you WILL need to check and reset your pressure due to the change in AMBIENT temperature of those locations.

You will NEVER, EVER release air on a hot (driven or sun heated) tire. No matter what psi you measure when hot.
All that ottffss has posted in this quote is good, but have some exception:

ottffss has given you the table weights, but looking at those weights for 105 psi will give you a fudge factor of 150 pounds on the front axle, so if you pick up an extra girlfriend you "may" be over weight.

For the rear the fudge factor is even lower at 90 pounds for 95 psi (Difference between your weight of 10050 lbs and the 95psi table weight of 10140 lbs. If you pick up a 100 pound bag of dog food and put it in the rear, rover will need to eat fast, and find grass quickly to get you down to n weight supported by 95 psi. The front may do well at the fudge factor of 150 pounds, but you could go up to 110 psi and carry two extra girlfriends.

The back however is border line, in my opinion, and I would use at least 100 psi for a bigger fudge factor. Look at the charts for that fudge factor.

Good luck.
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Old 05-02-2012, 05:13 PM   #14
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...The back however is border line, in my opinion, and I would use at least 100 psi for a bigger fudge factor. Look at the charts for that fudge factor.
..
Spot on... And hence why many folks will jump to the next column on the chart or more simply, just add 5psi or 10 psi to the table inflation!

Good points Wayne!

And it not just be any additional cargo, but the fudge may be needed simply due to side-to-side weight differences which we won't know until a four corner weigh is done.
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