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Old 05-21-2018, 10:43 PM   #1
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Am I reading the Michelin tire pressure correctly?

2014 Dutchstar 2014 (4018) with a tag axle. Only have axle weight from CAT scale - steer axle 12,700 lbs, drive axle 14,440 lbs, trailer axle (assuming that means tag axle) 8020 lbs. Tires are Michelin 305/70 22.5. When looking at the Michelin chart at https://www.michelinrvtires.com/refe...tion-tables/#/ .....

Steer axle...half of 12,700 lbs is 6,350 lbs....pressure probably closet to 95 psi
Drive axle...half of 14,440 lbs is 7,220 lbs...the lowest weight for dual is 9,530 lbs...which is considerably higher than my 7,220 lbs. Do I use the psi associated with the minimum weight listed...which is 75 psi?
Trailer/tag axle...half of 8020 lbs is 4010 lbs...which again is below the minimum weight (5,375 lbs)...so do I use the minimum psi of 75 psi?

I currently have the steer at 110 psi, drive at 90 psi, and the tag at 85 psi.

According to the Michelin chart, Iím over inflated on all tires. Thoughts?
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Old 05-21-2018, 11:17 PM   #2
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Most tire folks will recommend you do not go below 85 psi for fear of rolling the head off in a turn. Also one side is almost certainly heavier than the other so you need individual weights on each of the 6 positions to get the proper pressure. I would not change from your present settings until you get individual position weights. All JMHO
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Old 05-21-2018, 11:28 PM   #3
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You look pretty good the steer maybe a little high, look on the placard on wall next to drivers seat and see what the factory had written on them for pressure when built.
This important Newmar link has a link for your tire pressures near bottom of link.
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Old 05-21-2018, 11:29 PM   #4
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Before we get into the inflation numbers there are two other important things to consider.

First, its very possible that your drive / tag tires are smaller than 305s. Please take a close look at the sidewalls of each and every tire and ensure they're not 295s or 275s.

Second, dividing the total weight of the axle in half may leave you with tires under inflated for the load. Our coaches are known to carry more weight on one side than the other given their construction, how we load them, etc. If a 4-corner weight is possible that's a much better option for determining safe tire pressures. If not - some allowance should be made to accommodate a load that may be as much as 500 lbs heavier on one side of the axle vice the other.

In general - on a tire that has a max inflation of 120 lbs air - most of us keep no less than 90 lbs even when the table suggests lower.

Let us know what you find out about tire sizes and a possible 4-corner weight. Meantime I'm sure others will join the conversation.
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Old 05-22-2018, 03:30 AM   #5
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If you first add 10% to the weighed axle-loads, and then search back, you have some reserve with still acceptable comfort and gripp.

Have you weighed fully loaded? Otherwise your weights in use are higher.

If the pressure/ loadcapacity - list was made with the ideal formula to mother nature ( michelins calc comes close to that) , you could even go back to zero pressure if the load was low enaugh ( about 2% of maxload) .
If the rimms have no humps to keep the tire on them, then even 30 psi would be enaugh to keep them on rimm. Ofcource if the load then is low enaugh.


That michelin does not go lower is that the list gets to long and lower is not used for these tires to thwir opinion.
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Old 05-22-2018, 09:06 AM   #6
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Quote:
Originally Posted by Dhilbe View Post
2014 Dutchstar 2014 (4018) with a tag axle. Only have axle weight from CAT scale - steer axle 12,700 lbs, drive axle 14,440 lbs, trailer axle (assuming that means tag axle) 8020 lbs. Tires are Michelin 305/70 22.5. When looking at the Michelin chart at https://www.michelinrvtires.com/refe...tion-tables/#/ .....

Steer axle...half of 12,700 lbs is 6,350 lbs....pressure probably closet to 95 psi
Drive axle...half of 14,440 lbs is 7,220 lbs...the lowest weight for dual is 9,530 lbs...which is considerably higher than my 7,220 lbs. Do I use the psi associated with the minimum weight listed...which is 75 psi?
Trailer/tag axle...half of 8020 lbs is 4010 lbs...which again is below the minimum weight (5,375 lbs)...so do I use the minimum psi of 75 psi?

I currently have the steer at 110 psi, drive at 90 psi, and the tag at 85 psi.

According to the Michelin chart, Iím over inflated on all tires. Thoughts?
You are in the same boat as me. I have went across the CAT scales twice but have not been able to get a 4 corner weight completed yet. So I'm just winging it by the charts. You are reading them correctly.

I had my steer set at 110 cold which was 10 psi over what the charts said according to my weight. The last two legs of traveling the temps were in the 90's and my tire psi stayed up around 136 psi. I'm adjusting down to 105 psi in my steers until I can get 4 corner weight. IMHO you are probably good on your drives and tag until you get different numbers.
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Old 05-22-2018, 10:38 AM   #7
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Ali way's remember to read PSI when tires are cold in morning, when your driving coach over miles the tires PSI will increase 15 to 20 PSI. do not make any adjustments when driving to check PSI.'s.
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Old 05-22-2018, 02:31 PM   #8
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When I weigh my coach at a cat scale, after I weigh the 3 axles I pull forward and move over a little to get one side off the scales then back up until only the one side is on the scales. The additional charge is only $4. A little math and I have individual wheel weights.
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Old 05-22-2018, 03:55 PM   #9
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Ok. Didn't know that.
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Old 05-22-2018, 03:58 PM   #10
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Rather than dividing the axle weight in half, use 55% of the axle weight to establish an estimated wheel weight. That will give you a safety margin until you can get actual wheel/corner weights. And, round up to the next weight on the pressure table when you fall between pressures.

Obviously, actual corner weights are the best. I accomplish this by using the state weigh station scales when the station is closed. They usually leave the scale and display active for truckers to check their loads. A couple of passes through the station, getting axle weights on the first pass and single-side weights (one set of wheels off the scale) on the next. Some quick calculations will yield individual wheel weights.

TJ
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Old 05-26-2018, 02:02 PM   #11
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Finally got over to the RV, 305 tires all around the coach. Keeping the pressure at 110/90/85 for now
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Old 05-26-2018, 04:11 PM   #12
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Quote:
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Finally got over to the RV, 305 tires all around the coach. Keeping the pressure at 110/90/85 for now
That is what I run on my 2014 4018 as well.
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