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Old 10-31-2015, 09:07 AM   #1
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PSI question

Example: The gross weight of dual rear axle is 8,000 lbs. with each end weighing 4000 lbs. I'm a bit confused on how to get the proper PSI from an inflation pressure table.


Do I enter the inflation table with my tire size, find the column with 4000 lbs. on the "D" line, then get the PSI at the top of that column?
or
Do I (as I've read somewhere) divide the 4000 lbs by 2 first, then enter the inflation table, find the column with 2000 lbs. on the "D" line, then get the PSI at the top of that column?
Using this method, the sample inflation tables I've looked at do not go down to a value as low as 2000.


I think my confusion comes from this: if you have to divide the weight on dual applications by 2 first,
what's the point for having both an "S" line and "D" line? It seems you would enter the table using tire size, go across line (no S or D) to weight, then up to top of column to get PSI.


Thanks for any help,
iagman
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Old 10-31-2015, 09:20 AM   #2
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What kind of RV has a dual rear axle with a GAWR of only 8,000 lbs? Or are you just making up numbers?

First, you need the actual weight on the corner, not the GAWR. Then you look up THAT weight in the table, in the D column, rounding up to the next column. That's your PSI.

The reason there's different columns for S and D is that dual application derate the tires over a single. Duals get hotter than singles (less air flow) so they can't carry as much weight as a single (per tire).
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Old 10-31-2015, 09:28 AM   #3
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There is no one answer, since inflation tables vary in how they display the information. Some show the combined weight for dual tires on the "D" line, while others show single tire weights. Michelin, for example, shows the weight capacity for the pair of tires, while Goodyear and many others always show the capacity for each tires, whether single or dual pairs.

The purpose of the "D" Dual line is to reflect the difference in load rating for a single tire configuration vs duals. When two tires are tightly paired like that, their weight carrying capacity is reduced because more heat builds up in the pair than when single. Notice that dual pair always has a lower rating than 2x the single tire rating.

You can usually tell the different in the two table formats just by eyeballing them. If the D line appears to be (roughly) almost double the S line, then it is a twin tire rating and you can divide the axle rating or weight by 2 instead of 4.

If you know the individual corner (axle end) weights, you can use that, divide by 2 if the table shows individual tire values. If you only know the total axle weight, divide that by either 2 or 4. Until you have some actual weight to use, use the axle GAWR as the estimate and divide by 2 or 4. When using an axle weight or rating, allow for the likelihood that one end is heavier than the other. Rarely is the split exactly 50/50, so assume it is more like 55/45 or even 60/40 and use the heavier weight to get the psi. All tires on the same axle must have the same psi, though.

In all cases add some extra psi to make sure you always have enough. For example, if it turns 20 degrees colder while you are using the RV, tire pressure will drop a little. If you have an extra 5-10 psi, there would be no need to stop and add air to allow for colder weather.
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Old 10-31-2015, 09:35 AM   #4
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I'm new to all this and probably expressing myself with incorrect terms. The values I used were just examples.
What I meant was the rear axle has dual tires. The weight carried
by a set of those tires is 4000 lbs. It seems that I read somewhere
that this value is divided by 2 to get the weight carried by each tire.
From this I understood you would then use a weight of 2000 lbs. to
find PSI.
If this is not correct, then it makes sense to look up PSI using the weight on the S or D line.
Thanks for reply,
iagman
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Old 10-31-2015, 04:57 PM   #5
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Assuming the rear axle is "Duals" you find the "Dual" chart, then divide the wheel weight by two (Since the chart is for Tires not wheels) and then follow that weight across to the pressure.

If you post the actual chart, I can highlight it for you. But It's been a long day and I'm not up to decoding your original post.
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Old 10-31-2015, 07:03 PM   #6
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Look at your tires sidewall. Inflate to max. You will be better off with max until you figure out what you weigh. There should be a recommendation on a sticker in your driver's door jamb.
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Old 11-01-2015, 06:00 AM   #7
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I've found the answer I was looking for. It was contained in this statement concerning dual tires on the rear axle,


"Placing two tires at both ends of an axle nearly doubles the weight that the axle can carry. Dual tires allows three properly inflated tires to temporarily carry the weight originally allocated to the four tires should a rear tire go flat."


From that statement I understand that each tire in a dual setup must be inflated to carry the entire weight of an axle end, NOT just half the weight. Otherwise, if a tire blew, the single good tire would not be able to support the load.


Thanks to all who responded,
iagman
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Old 11-01-2015, 11:31 AM   #8
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Quote:
From that statement I understand that each tire in a dual setup must be inflated to carry the entire weight of an axle end, NOT just half the weight. Otherwise, if a tire blew, the single good tire would not be able to support the load.
That is absolutely Not True. And it is easily verified by looking at the Single vs Dual load ratings for a given tire. Each dual is rated to handle about 10% (give or take) less than a single, for the reason I described in my earlier post.

And if you drive more than a mile or so on a single tire after one dual loses air, the other tire is probably ruined as well because it is severely overloaded, which is the same as underinflated. It may not blow out right away, but is has been damaged internally and will fail soon.
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Old 11-01-2015, 12:52 PM   #9
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That is funny, I have driven several miles with a flat on one set of duals to get to a tire shop without problem. That is on a fully loaded semi. The only time I have had trouble in that respect is if the other tire is bad or damaged. I would do the same with my Motorhome as I really don't think they are that different. The secret is to slow down and not cause the tire to over heat.
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Old 11-02-2015, 07:33 AM   #10
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Quote:
The secret is to slow down and not cause the tire to over heat.
Exactly. Go slow and as little distance as possible. That remaining tire is way overloaded, so be as kind as possible to it.

However, any professional tire engineer will tell you that the tire has been weakened by that occurrence and its lifespan has likely been shortened. Maybe only a little weakened, or maybe it will blow out next week. Hard to know, which is why most people insist that the tire was still ok.

Tire Engineer Roger Marble (tireman9) addresses the "limp home" scenario in his blog on tire safety. See RV Tire Safety: Can you "Limp" home on a failed dual?
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