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Old 02-26-2016, 04:59 PM   #1
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Need a math teacher for tire pressure confirmation

I have weighed the coach, done the math and checked the Michelin pressure chart, but it doesn't seem right. Please confirm that I am correct or point out my error. Thanks.

Michelin 275/80R22.5 H XZA3 tires.

150 Gallons of Fuel
Fresh Water Full
Fridge Full
Liquor cabinet Full

Steer axle: 12800#
Drive Axle: 15020#
Tag Axle: 7380#
Total 35200#

When I check the Michelin chart, I get:
110 psi on Steer
75 psi on Drive
75 psi on Tag.

I know it is a big difference on load per tire from the front to the back, but it just seems like a big difference in pressure on front and rear.

PLEASE NO GUESSING. PLEASE NO OPINIONS. JUST PURE KNOWLEDGE.
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Old 02-26-2016, 05:09 PM   #2
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I'm assuming the rears are dually tires on each axle? If so the load is spread over many more tires than the steering ones. That would both make sense and give more sidewall flex and smoother ride in the rear.
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Old 02-26-2016, 06:40 PM   #3
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The sidewall of the tire tells the true story. For optimum tread wear and tire life run your tires at 110 front 90 on the rear. These are TRUCK tires and are to be treated as such. If you run a tire that size at 75psi you are basically 60% deflated. Running to low of pressure will cause overheating leading to early tire failure.


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Old 02-26-2016, 07:06 PM   #4
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Dual's always run less than a single, I would run what is rated by the tire manufacturer for the weight you have on them. I always add a little more for a margin factor. I don't just put in the max pressure just to run max pressure.
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Old 02-26-2016, 08:57 PM   #5
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Quote:
Originally Posted by Blackrock View Post
The sidewall of the tire tells the true story. For optimum tread wear and tire life run your tires at 110 front 90 on the rear. These are TRUCK tires and are to be treated as such. If you run a tire that size at 75psi you are basically 60% deflated. Running to low of pressure will cause overheating leading to early tire failure.


This ain't a Cadillac so don't expect a Cadillac ride.

This absolutely the wrong advice.
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Old 02-26-2016, 09:01 PM   #6
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Twice, I don't have a tag axle but I run less pressure in my dual rears than I do in my steer tires. If you are getting these pressures from the Michelin tire chart then I would say you are correct.
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Old 02-26-2016, 09:31 PM   #7
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I share Campin Cajun's opinion, and would add that you may want to consider left and right weights and base pressure on the heavier side of the axle. Tire pressures should be the same on both sides of the axle, but that pressure should relate to the heaviest side.


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Old 02-27-2016, 08:04 AM   #8
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I share Campin Cajun's opinion, and would add that you may want to consider left and right weights and base pressure on the heavier side of the axle. Tire pressures should be the same on both sides of the axle, but that pressure should relate to the heaviest side.


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Yes, exactly!


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Old 02-27-2016, 08:14 AM   #9
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Welp, looks like I got Flamed pretty good on this one. I only been in the heavy truck fleet repair business for thirty plus years so I guess I need to bow out of any tire related issues on this forum.
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Old 02-27-2016, 09:15 AM   #10
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Originally Posted by Blackrock View Post
Welp, looks like I got Flamed pretty good on this one. I only been in the heavy truck fleet repair business for thirty plus years so I guess I need to bow out of any tire related issues on this forum.
Seems many from the heavy truck world recommend max pressure in all tires. Motor homes are different thing and the manufacturers sometimes recommend adjusting the tire pressure based on the way to give the best ride.
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Old 02-27-2016, 09:15 AM   #11
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So much for not guessing ...

Too much riding on this for guessing and opinions. I don't want to sound insulting or unappreciative, but "Do you have duals?". This is not helpful.

I know it is not a Cadillac. It actually rides better than my Cadillac XTS and the weight on the Cadillac tires actually varies very little. Michelin says they are RV tires not truck tires.

CC, Paulie, NV dn: Thanks for the input. Maybe we need a separate forum for Berkshire owners and a separate forum for pickup owners.

I trust that Michelin knows that of which they speak. On my 2014, I carried 95 front and 90 rear, which corresponded with the Michelin chart. However, the tag seems to make the chart appear low for the drive and tag.

Marty, Tom, Phil and the rest of you Berkshire guys, please weigh in here. (Yes that was a pun.). As I originally requested, please verify that I am reading the chart correctly.
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Old 02-27-2016, 09:21 AM   #12
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Welp, looks like I got Flamed pretty good on this one. I only been in the heavy truck fleet repair business for thirty plus years so I guess I need to bow out of any tire related issues on this forum.
I also somewhat disagreed with your advice, but didn't want to sound snarky so I kept it to myself.

I've always understood tire sidewall #'s to be the tire mfg max cold psi for safe operation.

This *usually* to best tire life and coolest operating temps and generally what I do in my car, especially if I know I'll be loaded up and where ride quality is a secondary concern.

However, I've also seen strong recommendations in the case of RVs or other 'specialty vehicles' to refer to the data plate inside the door of the vehicle for proper tire pressures.

The vehicle manufacturer (not the tire manufacturer) has developed these numbers based on the axle distribution, gross weight and other oddball factors the tire maker could not foresee in a specialty application.

Ride Quality also being one of those factors.

I would expect by following the data plate in the door you sacrifice some tire life for improved (less jittery or 'skatey') ride quality.

Thanks.
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Old 02-27-2016, 10:38 AM   #13
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Originally Posted by Blackrock View Post
Welp, looks like I got Flamed pretty good on this one. I only been in the heavy truck fleet repair business for thirty plus years so I guess I need to bow out of any tire related issues on this forum.

All that experience in heavy trucks does not qualify you to comment on motorhome tire pressures. It's a whole different animal.
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Old 02-27-2016, 10:42 AM   #14
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Byron, the tire stickers placed in motorhomes is only a guide. A person that owns a motorhome needs to get it weighed (best is a four corner weight) then adjust tire pressures stated on the TIRE manufacturers pressure charts. FR does dictate tire pressure.
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Old 02-27-2016, 11:05 AM   #15
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Quote:
Originally Posted by Byron in TX View Post
I also somewhat disagreed with your advice, but didn't want to sound snarky so I kept it to myself.

I've always understood tire sidewall #'s to be the tire mfg max cold psi for safe operation.

This *usually* to best tire life and coolest operating temps and generally what I do in my car, especially if I know I'll be loaded up and where ride quality is a secondary concern.

However, I've also seen strong recommendations in the case of RVs or other 'specialty vehicles' to refer to the data plate inside the door of the vehicle for proper tire pressures.

The vehicle manufacturer (not the tire manufacturer) has developed these numbers based on the axle distribution, gross weight and other oddball factors the tire maker could not foresee in a specialty application.

Ride Quality also being one of those factors.

I would expect by following the data plate in the door you sacrifice some tire life for improved (less jittery or 'skatey') ride quality.

Thanks.
We don't have a "data plate inside the door". It is a Motorhome NOT a car or truck! Tires on a Motorhome are different. The whole situation is different. In the case of a Motorhome tire, it IS the tire manufacturer that sets the numbers.

To Twice: 4 corner weight.

Karen
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Old 02-27-2016, 11:12 AM   #16
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I agree the psi on the tag and drive axle sounds light. One issue is you are one of the first to have a tag so info is light. I'm sure you added extra to the weight to compensate for unbalanced loads and extra passengers. Aside from that I would be very comfortable with the guidelines given by Michelin. The front axle surprised me. What is the capacity of that axle? Also is there a compensating valve on the tag to adjust the pressure of the airbags? I know Newmar is having an issue with the tag pushing to much weight to the steer axle. Not saying you have an issue but wonder if that is adjustable.
I would run the psi given by Michelin. I trust that guide and it always has given me a safe comfortable ride.
One other thing Cadillac wishes their cars ride as good as our Berkshires.



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Old 02-27-2016, 11:41 AM   #17
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Thanks, Phil

Phil, thank you for a cogent reply. Axle ratings are: Steer, 14320; Drive, 20000#; Tag, 10000#.

I belive the seemingly heavy weight on the front comes from 150 gallons of fuel and the 10k Generator.

Yes, I have made allowance for side to side differences and included a safety margin. I have the four corner weights on this particular unit when it left the factory and they are in line with my recents loaded weights.

The bag pressure on the tag is not user adjustable. The tag weights of dumped vs. undumped are not very significant.

If you haven't already done so, please put your eyes on the Michelin chart and verify my reading of it. Thanks.
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Old 02-27-2016, 11:55 AM   #18
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What size tires do you have Twice?
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Old 02-27-2016, 12:07 PM   #19
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Michelin 275/80R22.5 H XZA3 tires.
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Old 02-27-2016, 12:09 PM   #20
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Sorry Twice I saw you posted tire size in the OP, looking at my tire pressure chart it calls for 105 psi steer, and if I am correct about tag axle coaches it calls for 100psi drive.

My reasoning - the drive axle should be inflated to hold at least the max weight of the rear of the coach. So I added your drive axle weight & the tag axle weight then divided by 2 to get a psi of 100.

Does that make sense? I may be wrong, I'm doing some research now.
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