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Old 11-05-2014, 03:20 PM   #1
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Axle weights and Inflation PSI

With the coach loaded, full water and appr. 75 gals of Diesel fuel, I had the coach weighed at a CAT Scale near South Bend.

My question: Do I divide the Steer Axle weight by 2 and the Drive axle weight by 4 to determine what my cold Inflation PSI should be?

It seems logical since my Toyo Tires "RV Load Inflation Tables" shows:
Max Load Single/Dual Max Load (Cold) PSI
5205/4805 110/110

Steer Axle (Single) = 8940 lbs. X .5 = 4470 lbs. .
Chart says: 4440 lbs. 85 PSI 4620 lbs. 90 PSI
Should I Inflate to 85 lbs. - 87.5 lbs. PSI ?

Drive Axle (Dual) = 18,520 lbs. X .25 = 4630 lbs.
Chart says: 4525 lbs. 100 PSI 4685 lbs. 105 PSI
Should I inflate to 100-102.5 PSI ?
Hope this makes sense, I'd love to run with less PSI in front. Road strips pound quite a bit at 110 PSI.
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Old 11-05-2014, 03:49 PM   #2
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That looks O.K., but feel sidewalls after a short drive for excessive heat. It's usually better to be 5 psi over the recommended pressure than running lower.
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Old 11-05-2014, 03:52 PM   #3
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I would go to the next pressure up. In this example 90 and 105.

As Bob says 5 over is not an issue and is certainly better than 5 under.
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Old 11-05-2014, 03:53 PM   #4
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Quote:
Originally Posted by michguy View Post
With the coach loaded, full water and appr. 75 gals of Diesel fuel, I had the coach weighed at a CAT Scale near South Bend.

My question: Do I divide the Steer Axle weight by 2 and the Drive axle weight by 4 to determine what my cold Inflation PSI should be?

It seems logical since my Toyo Tires "RV Load Inflation Tables" shows:
Max Load Single/Dual Max Load (Cold) PSI
5205/4805 110/110

Steer Axle (Single) = 8940 lbs. X .5 = 4470 lbs. .
Chart says: 4440 lbs. 85 PSI 4620 lbs. 90 PSI
Should I Inflate to 85 lbs. - 87.5 lbs. PSI ?

Drive Axle (Dual) = 18,520 lbs. X .25 = 4630 lbs.
Chart says: 4525 lbs. 100 PSI 4685 lbs. 105 PSI
Should I inflate to 100-102.5 PSI ?
Hope this makes sense, I'd love to run with less PSI in front. Road strips pound quite a bit at 110 PSI.
When you can, get each wheel position weighed. Then inflate per table to the heavier side of each axle. Sometimes one side is quite a bit heavier than the other: our sure is!
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Old 11-05-2014, 03:55 PM   #5
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Good info! Thanks. Any idea where I can get each wheel weighed?
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Old 11-05-2014, 04:18 PM   #6
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Originally Posted by michguy View Post
Good info! Thanks. Any idea where I can get each wheel weighed?
Escapees do at some of their locations. Also, if you can find scale that you can get just one side on, that will work, too. (Weigh entire coach; then just one side; then just subtract the one side from total).
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Old 11-05-2014, 04:23 PM   #7
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Thanks again! I'll start checking scales.
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Old 11-05-2014, 07:01 PM   #8
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I called the WI State Patrol and they came to my home with six individual scales We did two weighs, one with the tag dumped and one without. Only took 20 minutes and no charge. He said it was my tax dollars at work for me.
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Old 11-05-2014, 07:11 PM   #9
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Thanks again! I'll start checking scales.
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Old 11-06-2014, 08:58 AM   #10
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Until you get the individual position weights you can't get the correct psi. Once I had the corner weights, I found that I had to adjust the ride height to get them closer to the same side to side. Then I went to the chart and inflated the tires to 5 psi above the recommended pressure. I also did it in the morning with the OAT at 60*. I have not had to add or release pressure in over a year. The starting pressures are above the minimum down to 35* and below the max at 85*. They are also correct at 1000' or 11,000' altitude.
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Old 11-06-2014, 09:06 AM   #11
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There are two things that would improve your results:
1. Add 5% after dividing the axle weight by 2 or 4. That's to allow for the likelihood that the axle weight is not evenly distributed, i.e. one end is heavier than the other. You need enough psi to handle the heaviest end, but both ends need to have the same psi.

2. After finding the inflation table entry that matches your estimated load, go up one or two entries in the table, i.e. increase pressure by 5-10 psi. That's so that you have some extra margin and don't have to worry about day-to-day variations in ambient temperatures. A little bit higher than optimum doesn't hurt, but a little low can quickly cause problems. Make sure you stay on the high side.

Here is an example of how to do it from Roger Marble, a recognized tire safety engineer. He suggest 10% where I used 5-10 psi in step #2, but 10% is about 10 psi on a typical large motorhome tire.
http://www.rvtiresafety.com/2014/11/...inflation.html
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Old 11-07-2014, 05:00 PM   #12
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Quote:
Originally Posted by Gary RVRoamer View Post
There are two things that would improve your results:
1. Add 5% after dividing the axle weight by 2 or 4. That's to allow for the likelihood that the axle weight is not evenly distributed, i.e. one end is heavier than the other. You need enough psi to handle the heaviest end, but both ends need to have the same psi.

2. After finding the inflation table entry that matches your estimated load, go up one or two entries in the table, i.e. increase pressure by 5-10 psi. That's so that you have some extra margin and don't have to worry about day-to-day variations in ambient temperatures. A little bit higher than optimum doesn't hurt, but a little low can quickly cause problems. Make sure you stay on the high side.

Here is an example of how to do it from Roger Marble, a recognized tire safety engineer. He suggest 10% where I used 5-10 psi in step #2, but 10% is about 10 psi on a typical large motorhome tire.
RV Tire Safety: Goodyear RV tire Load / Inflation calculation
In one of the threads Roger said he grossed his axle weight up 20% and then went to the inflation tables.
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Old 11-07-2014, 05:15 PM   #13
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For now: YES. but you need to get corner weights, not axle weights.

And remember pressures are different for single and dual tires in many cases. So be sure you use the right hart.

The problem with simply divide by 2.. IF the RV is perfectly balaned side to side,,, No problem.. But how many RV's do you think are out there that are that well balanced? (2, perhaps 3).
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