Go Back   iRV2 Forums > MOTORHOME FORUMS > Class A Motorhome Discussions
Click Here to Login
Join iRV2 Today

Mission Statement: Supporting thoughtful exchange of knowledge, values and experience among RV enthusiasts.
Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on iRV2
 
Thread Tools Search this Thread Display Modes
 
Old 07-27-2015, 03:12 PM   #15
Senior Member
 
jadatis's Avatar
 
Join Date: Oct 2012
Posts: 247
Used my calculator , but asumed some data .
Asumed you weighed your RV fully loaded so with all the persons and freight as you go on trip , in it.
Also assumed dual load behind so 1 axle 4 tires on the road.
Had another idea about your GAWR front and that is they forgot a 7 so 7700 lbs. Mostly GAWR's together is a bit more then GVWR and if 7000 its exactly the same.

Made picture of it , but if weight in use is more it needs to be higher pressure.
If tandem axle so 2 alxes with each 2 tires on the road, can have a little lower pressure.

The advice is for the maximum speed of tire wich is most likely N so 140km/86m/h . But then it can be higher pressure without bumping and so more reserve wich is never bad.

So check if my assumptions are richt , other give the corrections , and I will calculate again.

__________________

__________________
jadatis is offline   Reply With Quote
Join the #1 RV Forum Today - It's Totally Free!

iRV2.com RV Community - Are you about to start a new improvement on your RV or need some help with some maintenance? Do you need advice on what products to buy? Or maybe you can give others some advice? No matter where you fit in you'll find that iRV2 is a great community to join. Best of all it's totally FREE!

You are currently viewing our boards as a guest so you have limited access to our community. Please take the time to register and you will gain a lot of great new features including; the ability to participate in discussions, network with other RV owners, see fewer ads, upload photographs, create an RV blog, send private messages and so much, much more!

Old 07-27-2015, 03:53 PM   #16
Senior Member
 
Ray,IN's Avatar
 
Join Date: Aug 2001
Location: North America somewhere
Posts: 13,699
Quote:
Originally Posted by dan-nickie View Post
That document appears to be for passenger tires and light truck tires and sizes 15 or 16 inch.
What did you get from Ch 4? Did you find that same information on the Michelin website? It is stated there also.
__________________

__________________
2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member,FMCA."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert the Constitution. "Abraham Lincoln"
Ray,IN is offline   Reply With Quote
Old 07-28-2015, 02:58 PM   #17
Senior Member
 
Join Date: May 2011
Location: Mesa, AZ
Posts: 121
Very interesting chart, jadatis. Thanks for taking the time to put this together based on the figures that I had provided.

A lot of great posts on this thread, and as expected, a varying range of opinions. If I am reading the chart correctly that jadatis provided, it appears that I could get away with as low as 66 psi in the front tires, and 69 psi in the rears, based on my rig being well under its GVWR. Other posts have advised that I should never run any lower than what is stated on the RV placard, which is 80 psi.

Looking at the Hankook load limit chart that lllkrob posted, the range they provide goes from 80 psi up to 120 psi (which is also the max. load limit stamped on the tire). From that, I would surmise that Hankook does not recommend running this size/type of tire at under 80 psi.

Building in the safety factor that many have suggested to make up for other variables, it looks like 85 psi would be a good starting point until I get around to doing a 4 corner weight calculation.

Thanks again to everyone who contributed to this discussion.
__________________
terryl is offline   Reply With Quote
Old 07-28-2015, 07:42 PM   #18
Senior Member
 
Join Date: Sep 2012
Posts: 4,232
Think about this in regards to the mfg plate saying 80PSI: they don't tell you what brand tires to run. They say what size SHOULD be on it, or more like, what size was original and that it was designed for. But I've never seen where they state what brand. So they really can't say what the pressure should be, just what it should be for what they installed.
__________________
jesilvas is offline   Reply With Quote
Old 07-28-2015, 07:54 PM   #19
Member
 
Join Date: Mar 2015
Posts: 73
Formula: ( Axle Weight / # Tires ) / Tire Rating x Max. Pressure Rating = Recommended Tire Pressure

Example: ( 4380 /2 ) / 3415 * 80 = 51.3

This was for the steer axle of my unloaded Dodge Ram 2500 w/Load Range E tires (LT).

Formula came from Michelin.

Your PSI recommended by this formula for the tires and weight you gave would be 62 - front; 65 - rear. No way I would even come close to doing that. The stiff walls on large tires require a minimum pressure just to keep the beads sealed.
__________________
2003 Fleetwood Expedition 34W
2000 Thor Hurricane 29D Motor Home
2011 Lance 1050S Truck Camper
JumboJet2 is offline   Reply With Quote
Old 07-29-2015, 09:37 AM   #20
Senior Member
 
wa8yxm's Avatar
 
Damon Owners Club
Workhorse Chassis Owner
Join Date: Mar 2009
Posts: 22,799
Alas Cat scales only give you half the info you need. You really need the weight on each corner.. But since you have axle weights the best you can do is assume balanced and divide by two. to get wheel weights (Divide by 4 if you have duals to get tire load)

Go to the tire manufacturer's web site, Find the support line and obtain the weight/inflation chart for the make and model tire you have

Many like to add 5PSI. in your case I suggest you do.

Oh, if you can not find the chart on the website.. Try Contact US and ask for it from cuss-some-more service.
__________________
Home is where I park it!
wa8yxm is offline   Reply With Quote
Old 07-29-2015, 02:26 PM   #21
Senior Member
 
Ray,IN's Avatar
 
Join Date: Aug 2001
Location: North America somewhere
Posts: 13,699
Never forget, sidewall flex generates heat, and excess heat is the #1 cause of tire failures, whether it's caused by overloading or under-inflation.
__________________
2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member,FMCA."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert the Constitution. "Abraham Lincoln"
Ray,IN is offline   Reply With Quote
Old 07-29-2015, 03:09 PM   #22
Senior Member
 
Join Date: Sep 2012
Posts: 4,232
Quote:
Originally Posted by Ray,IN View Post
Never forget, sidewall flex generates heat, and excess heat is the #1 cause of tire failures, whether it's caused by overloading or under-inflation.
But the "over" or "under" is in reference to the number on the load chart, correct?
__________________
jesilvas is offline   Reply With Quote
Old 07-29-2015, 03:46 PM   #23
Senior Member


 
National RV Owners Club
Holiday Rambler Owners Club
Join Date: Sep 2013
Location: Southern Lancaster County Pa
Posts: 757
This is a very important safety topic and I want to do the right thing, but my head is spinning after reading all the calculations and gyrations that you guys are doing. I guess I've lost too many brain cells to comprehend all this stuff.

I have looked all over and in my MH to see if there is any recommended tire pressure for my MH. There is none. The tires (Kumho) say max load 4,500 at 110 psi. Originally, I had my tires set at 90 psi but a kid at the truck tire shop (where I get air in them) told me I should be at 110. I reset them to 100 and they will run at about 112 on the road. I have a tpms unit. This makes for a very hard ride.

My question is--does it sound safe to run 90 psi like I had been running previously without going through the scale thing. My MH runs true and does not wander and rode a lot better at 90psi.

We are not full time and only use the MH for weekend trips.
__________________
Gerry & Pat
2000 National Sea Breeze 1340
Gerryl is offline   Reply With Quote
Old 07-29-2015, 11:31 PM   #24
Senior Member
 
Ray,IN's Avatar
 
Join Date: Aug 2001
Location: North America somewhere
Posts: 13,699
Quote:
Originally Posted by jesilvas View Post
But the "over" or "under" is in reference to the number on the load chart, correct?
Actually no; the load/inflation charts only reflect the absolute minimum air pressure to support the corresponding load. Say you do use such a chart and inflate to that minimum pressure this afternoon. In the morning when you leave home and ambient temperature is 30* lower, your tires are likely under-inflated.
Michelin, Goodyear, Firestone, etc all say to never run less than the tire placard in your RV, yet they publish a load/inflation chart. This confuses many people into running their tires underinflated at times. Running a tire 20% underinflated is considered to having run the tire while flat, running a tire 2# under a load/inflation chart is running underinflated.
Open the link in my first post and really READ chapter 4 -MH tires, pay particular attention to page 51, RH column.
Many MH owners desire a very soft smooth comfy ride and choose to run minimum air pressure or add a few extra pounds and say that is their safety margin. I don't buy it. I run tire placard stated air pressure on all my vehicles, haven't had a flat in over 20 years (excepting the cheap ST tires on the trailer)
__________________
2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member,FMCA."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert the Constitution. "Abraham Lincoln"
Ray,IN is offline   Reply With Quote
Old 09-04-2015, 08:39 AM   #25
Member
 
Join Date: Mar 2015
Posts: 73
Weighed my 2003 Fleetwood Expedition this morning.

7040 lbs.
__________________
2003 Fleetwood Expedition 34W
2000 Thor Hurricane 29D Motor Home
2011 Lance 1050S Truck Camper
JumboJet2 is offline   Reply With Quote
Old 09-04-2015, 08:58 AM   #26
Member
 
Join Date: Mar 2015
Posts: 73
Weighed my 2003 Expedition this morning. Full fuel, water, and kitchen ware.

Front axle: 7,040 lbs.
Rear axle: 14,380 lbs.

The Yokohama calculator linked below suggested 80 psi for each tire.

265R75 22.5 Load Range G

Inflation Pressure Calculator¬*|¬*Yokohama Tire Corporation

Using my formula obtained from Michelin:

Formula: (( Axle Weight / # Tires ) / Tire Rating ) x Max. Pressure Rating = Recommended Tire Pressure

Rear Axle: (( 14,380 /4 ) / 5,510 ) * 120 = 78.29 psi
Front Axle: (( 7,040 / 2 ) / 5,510 ) * 120 = 76.66 psi

80 psi works! Add approx. 10% and 85 psi works too!
__________________
2003 Fleetwood Expedition 34W
2000 Thor Hurricane 29D Motor Home
2011 Lance 1050S Truck Camper
JumboJet2 is offline   Reply With Quote
Old 09-05-2015, 03:34 PM   #27
Senior Member
 
jadatis's Avatar
 
Join Date: Oct 2012
Posts: 247
Saw my picture of first post here gave broken link, and think its because of htpps instead of http.
So here again.


Have no time now to calculate again so will do that later after my hollydays.

But the 5500 you use for the rear is not right , for dual load maximum load is lower then single load wich you have at front .
__________________
jadatis is offline   Reply With Quote
Old 09-05-2015, 05:37 PM   #28
Senior Member
 
Smitty77's Avatar
 
Join Date: Aug 2010
Location: Over the next hill, around the next curve...
Posts: 3,309
Quote:
Originally Posted by JumboJet2 View Post
Weighed my 2003 Expedition this morning. Full fuel, water, and kitchen ware.

Front axle: 7,040 lbs.
Rear axle: 14,380 lbs.

The Yokohama calculator linked below suggested 80 psi for each tire.

265R75 22.5 Load Range G

Inflation Pressure Calculator¬*|¬*Yokohama Tire Corporation

Using my formula obtained from Michelin:

Formula: (( Axle Weight / # Tires ) / Tire Rating ) x Max. Pressure Rating = Recommended Tire Pressure

Rear Axle: (( 14,380 /4 ) / 5,510 ) * 120 = 78.29 psi
Front Axle: (( 7,040 / 2 ) / 5,510 ) * 120 = 76.66 psi

80 psi works! Add approx. 10% and 85 psi works too!
Just a caution, but that could be the wrong PSI to run with. Without four corner weight, where you can determine the highest weight of a specific side of the axle, you may have used the wrong Tire Manufactures PSI line to obtain PSI's.

I've read on this forum, and many others, of up to 800 lbs weight differences between sides of the same axle. (Now, who new discussion on why, and usually it is how the coach builder installed equipment or tanks...)

If you can not get an individual four corner scales weight. The try to go back to that same CAT scale, and run down the side with one side only of the coach being on the scale. Take that measurement for each axle position, and subtract it from your total axle weight, and you can determine which axle has the highest weight. It is that weight to use when going to the PSI Charts. I personally, do the math, and if that highest axle weight is with the top 25% of the range, I bump up to the next higher weight level on the PSI chart. I then add 5 PSI to whatever that PSI is. (The 10% rule is better, but with mine, I had bumped up the the next PSI Line Level for each axle, and the 5 PSI on top of that provided me with a good cushion.

I've mentioned on other threads, that about 5 years ago, I called to different tire manufactures tech support lines, and asked what kind of 'safety margin' did their PSI charts have? Both of the people I talked to said that they did have baked in safety margins. But neither would tell me what they were, or how the were calculated. (Possibly they did not know.). So the PSI charts though the minimum for that weight, does have some safety margin/cushion to it. I was concerned about 'coach weight creep' while traveling (Picture I Love Lucy picking up rocks to hide from Desi in The Long Trailer movie!), and I've traveled several times with hour upon hour of steady strong cross winds. That loads the leeward tire. Thus, my extra 5 PSI, or better yet, extra 10% of PSI, as a good safety margin on top of the tire manufacturers safety margin. And in my case, I'm still running between 10-15 PSI below the coaches tire placard.

Tireman9's blog, is one of the best easy reading sources in regards to tires, care and feeding, and all other sorts of tire related items... Strongly recommend it as a saved reference to access when needed!

Best to all,
Smitty
__________________

__________________
Roo II is our 04 Country Coach Allure 40'
OnDRoad for The JRNY! Enjoy life...
Smitty77 is online now   Reply With Quote
Reply

Tags
tire pressure



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Tire pressure question Lifemember Class A Motorhome Discussions 10 06-10-2015 09:40 AM
Weighing Coach for Tire Pressure? Sky Pilot Class A Motorhome Discussions 21 06-02-2015 09:03 AM
Recommended Minimum Tire Pressure carolinagirl Winnebago Industries Owner's Forum 43 01-02-2014 08:43 PM
Tire pressure question AWMIII Newmar Owner's Forum 1 06-12-2013 10:22 AM

» Virginia Campgrounds

Reviews provided by


Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 11:48 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.