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 01-01-2016, 07:39 AM   #1
Member
 
gjurrens's Avatar
 
Join Date: Aug 2014
Location: Punta Gorda, FL but not there for a few years!
Posts: 47
Exclamation Calculating Proper Tire Inflation Pressures

Gang, I tried replying to some older posts on this topic, but none were recent enough. So maybe it's time to revisit this. It sure is for me, anyway. As always, I really appreciate fresh wisdom on this forum. This is a really important topic to me.

I think I'm doing the right thing with my new-to-me 43' DP with a tag axle (2005 Newmar Mountain Aire 4304). I'd like some help here, though, in checking my tire inflation pressure calculations based on actual weights I got from a CAT scale yesterday...

Here are the coach's ratings:
Front GAWR: 14,200
Rear GAWR: 30,000 (including tag)
GVWR: 44,200
GCWR: 54,200

Here's what I weighed her at this morning on a CAT scale:
Steer axle: 13,120
Drive axle (including tag): 22,760

Check me out here:

I run Bridgestone R250F 290/80R22.5 all around. I've included the relevant tire inflation table numbers in this post.

The front axle seems a pretty straightforward calculation. At 13,120 lbs. of weight on the steer axle, that weight is spread over two tires at 6,560 each (assuming weight distribution is pretty even across the corners, which I couldn't weigh separately yesterday). The inflation table reads 100 PSI at 6510 lbs (single configuration, which is the front axle, i.e., one tire at each end of the axle) and at 6810 lbs, they recommend 105 PSI. Since my wife and I weren't in the cockpit when the coach was weighed, I now know that I should inflate each of the front (steering) tires at105 PSI for the current load.

Now here's where I get kinda confused...

The drive and tag axles weighed in at 22,760 lbs total. Those two axles comprise six tires, four of which are on the drive axle (a dual configuration) and two are on the tag axle (a single configuration). So I figure each axle must bear half the weight (11,380 lbs). For the tag, I know I need to read off the "single" row on the inflation table since there are only two tires on that axle and only a single tire at each end of the axle. If this axle needs to carry 11,380 lbs, to me that means that each tire (assuming similar corner weights) must carry half that, or 5,690 lbs. That puts me between the reading on the table for 85 PSI (5,640 lbs) and 90 PSI (5,930 lbs). If my assumptions are correct about the calcs, this seems to make sense. Correct?

On the drive axle tires in a dual configuration, however, I really need some validation here. Since there are four tires on this axle, I know I need to use the "dual" row in the inflation table. If this axle is also carrying half of the 22,760 lbs (true?), which is 11,380, but I'm spreading that weight over four tires instead of two, and I'm using the dual line in the chart, that says each tire is carrying 1/4 the load, or 2,845 lbs each. If true, on the chart, I'm between 105 PSI (2,780 lbs) and 110 PSI (2900 lbs). So I think I need 110 PSI on the dually configured drive axle (rounding up), again, assuming my math isn't all screwed up. The max inflation rating for these tires at max load is 125 PSI, so I'm good, right?

Another question: is it "kosher" to have the tag tires at one PSI and the drive tires at a different pressure?

Now another factor is that these weights include the tongue weight of my empty 29' enclosed car hauler trailer (3,600 lbs empty), so as I load that trailer up (to approx 9,000 lbs total including the weight of the trailer - tongue weight 10-15% of that, I'm assuming), I know I'll have to probably increase inflation to accommodate that increased tongue weight, obviously. This doesn't affect my calculation questions above, but just FYI.

OK, guys, you're my go-to crew. Am I close or way off? Thanks!!!! Gene
__________________

__________________
gjurrens 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 01-01-2016, 08:06 AM   #2
Moderator Emeritus
 
Gary RVRoamer's Avatar


 
Join Date: Jan 2000
Location: Silver Springs, FL. USA
Posts: 18,062
Generally the tag is not adjusted to carry half the rear end weight, but you would need to consult your coach chassis builder to see what is recommended. Also, isn't there a placard by the driver seat with a recommended pressure for tag and drive axle? That would give you an idea of the proportion.

You really should have weighed tag & drive axles separately - not hard to do on a segmented scale, which most truck stop scales will be.
__________________

__________________
Gary Brinck
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC
Gary RVRoamer is online now   Reply With Quote
Old 01-01-2016, 08:10 AM   #3
Senior Member
 
Tiffin Owners Club
Winnebago Owners Club
Join Date: Feb 2012
Location: Las Cruces, NM
Posts: 2,271
First off, find a scale where you can weigh all three axles separately. You cannot treat the tag and rear axle as one. The tag typically carries approx. a third of the drive axle depending on how you are loaded.

Inflate your drive axle tires per the tire charts and then inflate the tag axle tires to the same pressure as your drive axle tires.

I think that your trailer is going to impact you tag axle more that the rear. However, it will be properly inflated to handle the load if you inflate it to the rear axle weight. The only way to tell is reweigh the three axles again with the trailer hooked up.
__________________
Roger & Mary
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (Sold)
luvlabs is offline   Reply With Quote
Old 01-01-2016, 08:13 AM   #4
Senior Member
 
Tiffin Owners Club
Join Date: Oct 2010
Posts: 267
For a surprise go back and re weigh your rig, and weigh your tag and drive axles separately. They are nowhere near close in weight, by design, so your assumption on the rear end is not valid. If at all possible weigh each axle end individually. Then you use a tire pressure appropriate for the heavier side of the axle on BOTH ends of the axle. You want both ends of the axle the same.

You can always inflate above the chart pressures, up to the maximum pressure stated on the tire. But NEVER under inflate vs the chart pressures. The higher the pressure, the stiffer the ride. With a tag axle you will likely have a lot of room to play around with higher (above chart level) pressures back there to get your best ride. And there is no problem carrying different pressures on the tag and drive axles as long as you are within tire chart parameters.
__________________
Larrikin is offline   Reply With Quote
Old 01-01-2016, 12:26 PM   #5
Senior Member
 
jimbo2013's Avatar
 
Newmar Owners Club
Join Date: Aug 2013
Posts: 1,699
You're really doing this without 4 corner weights?

But you want to do right?

You can not just take 50% of the axle.

Find a cat scale and drive one side only on 3 scale tables.
__________________
Newmar Ventana 3933 | Miata close behind
1,060 Watts of Solar | 8 T-105 RE Batteries | Outback controls
HR 38 Endeavor
jimbo2013 is offline   Reply With Quote
Old 01-01-2016, 03:03 PM   #6
Senior Member
 
Fleetwood Owners Club
Join Date: Sep 2015
Location: Near San Antonio, TX
Posts: 430
Quote:
Originally Posted by jimbo2013 View Post
You're really doing this without 4 corner weights?

But you want to do right?

You can not just take 50% of the axle.

Find a cat scale and drive one side only on 3 scale tables.
As far as i know they won't let you do that
__________________
Montyhp
2005 Discovery 39s Cat C7 330hp, AF1 brakes,
2013 Silverado Crew Cab Toad, Blue Ox Towbar
montyhp is offline   Reply With Quote
Old 01-01-2016, 03:44 PM   #7
Senior Member
 
2 Stroker's Avatar
 
Join Date: Nov 2009
Location: Warren, Oregon
Posts: 1,714
In 2013 I got a new set of Michelin 295/80R22.5 XZA2 with a max PSI of 120lbs. I had my coach weighted at the FMCA Rally in Albany,Or (4 corner wt) and I'm over wt in the rear by 1150lbs and under in the front by 730lbs. With a combined wt of 33,620lbs FR 6,405lbs-FL 6,065lbs RR 10,195lbs-LR 10,955lbs I have to run 105 PSI in the front and 100 PSI in the rear. They give you a print out of your coach wt's and the air pressure to run. This is the only way to get a true wt reading and know what PSI to run, money well spent.
2 stroker
__________________
2006 Country Coach Inspire 360 40ft Genoa Designer Series, Samsung 197 RR
Cat C9--STEEL & COPPER Bolt Together Radiator w/ updated rubber mounting
SilverLeaf 330 Magnum 2812 PSW 2011 Ford Edge Sport-Air Force One
2 Stroker is online now   Reply With Quote
Old 01-02-2016, 05:39 AM   #8
Member
 
gjurrens's Avatar
 
Join Date: Aug 2014
Location: Punta Gorda, FL but not there for a few years!
Posts: 47
Thumbs up

OK guys. This is why I come here. Great stuff. Nothing like the voices of experience!

My first assumption that the tag carries half of the weight on rear of the coach - bad assumption. Gone.

Second assumption that its valid to weigh both tag and drive axles together - also bad. Also gone. I guess my first weighing was better than nothing, but maybe not much. Next time I weigh, I guess I'll ask the weigh master for two successive weights. The scale I was on had three segments, so I weighed (1) steer axle, (2) drive/tag axles, and (3) both trailer axles. I guess I'll weigh without the trailer and do (1) steer axle, (2) drive axle, and (3) tag axle. Now I gotta do this as soon as I get the chance. I wondered about how different the tag would be from the drive axle weights. And inflating the tags the same as the drive, even though they'll theoretically be carrying less weight (per tire and total)... another pearl of wisdom.

Third assumption that I need corner weights. Good assumption. I knew this was important as indicated in my original post. I'll clearly do so when I get the chance. Meantime, I don't want to run under-inflated and I don't want to just push maximum into every tire. Some info is better than none. Be aware that aside from shop time for preventative and fixative maintenance, I've only had my coach actually available to me for a total time span measured in days, but message received... corner weights it is. The scale I used didn't really have enough clearance to drive off one side and weigh the other. Maybe some others will. And I won't be getting to a rally any time soon. When I do, you betcha I'll pay for corner weights! Meantime, I feel some approximations are better than nothing.

But when I do get corner weights, another piece of info you've given me is to use the pressure for the heavier end of the axle on all tires for that axle (if redistributing weight is not an option).

Next new piece of info you guys have provided is that the trailer weight will impact the tag more than the drive axle (not sure how true this will be with a weight distribution hitch - Blue Ox SwayPro is on my wish list), but if I'm inflating the tag tires same as the drive axle, should still be acceptable, and only get better with a WD hitch. I gotta think about this more. Maybe once loaded, I'll weight (1) drive axle, (2) tag axle, and (3) trailer axles, until I can get corner weights, of course.

Another question: any issue using a WD hitch on a Class III 10K lb hitch on the coach that likely isn't really designed for a WD a hitch? This is a general question only. I'll research the Hidden Hitch brand of hitch the manufacturer put on my MADP. I should be OK on weight, just not sure what a WD hitch does to a receiver not specifically designed for it. Big deal or nothing to worry about (before I spend 600 bucks on a WD hitch)?

Opinions on a gadget? I saw a cool device, not sure it's worth the price tag of $140, but I'm considering getting a Sterling Trailer Tongue Weight Scale (2,000 lb. capacity). Since I'm towing as much as 10K lbs. I thought this might safely bridge me between CAT scales and rallies as my payload changes (mostly in the trailer). The reviews on this device are numerous with top marks. Any words of wisdom here?

Thanks again, y'all. You never disappoint! Gene
__________________
gjurrens is offline   Reply With Quote
Old 01-02-2016, 07:15 AM   #9
Senior Member
 
Join Date: Dec 2013
Location: Lake Havasu City, AZ & Plover, WI
Posts: 1,709
You need to start over. Get a six position weigh. Then if the side to side numbers are too far off, have the ride height checked to see if the difference can be corrected. There are proportioning valves for the drive and tag axles. Those valves determine the air psi in the air bags and thus the weight distribution between each. My guess is that they are a 66/33 ratio or something close to that. 66% of the weight is on the drive and 33% on the tag. That ratio won't change when you hook the trailer up. The tongue weight will increase the total weight on the rear, but the ratio will remain the same. Some weight will also come off of the steer axle. I have manual valves on our tag coach, so I can adjust what each axle carries. I tow a 12,000# trailer with a 1500# tongue weight on a WD hitch. My son tows a similar trailer without a WD hitch. Both work fine, as we have 15,000# hitches. I think the WD hitch smooths out the bumps and dips in the road. I have done a ton of weight checks and I have recorded all of the position weights with several ratios and prefer a 75/25 ratio. It has worked well for two years with and without the trailer and takes some of the weight off of the steer axle.
__________________
2013 43 QGP Allegro Bus
Crasher is online now   Reply With Quote
Old 01-02-2016, 07:31 AM   #10
Senior Member
 
Robmat's Avatar


 
Winnebago Owners Club
Ford Super Duty Owner
Join Date: Aug 2010
Location: North Wildwood, NJ
Posts: 443
Quote:
Originally Posted by jimbo2013 View Post
You're really doing this without 4 corner weights?

But you want to do right?

You can not just take 50% of the axle.

Find a cat scale and drive one side only on 3 scale tables.

January's Motorhome Magazine has an article on this very topic along with pictures, etc. The author went to a CAT scale and weighted the entire MH, then left side and right side and got three receipts and went over them. Pretty good article contain much that was said here.
__________________
Bob and Cathy
2015 Itasca Sunova 33c towing a 2008 Jeep Wrangler Sahara
Member FMCA F421963, GS Life, SKP#127220, WIT, PA,
Robmat is offline   Reply With Quote
Old 01-02-2016, 09:04 AM   #11
Senior Member
 
Tiffin Owners Club
Winnebago Owners Club
Join Date: Feb 2012
Location: Las Cruces, NM
Posts: 2,271
Most CAT scales are constructed in suc a manner that driving less than all the wheels across them is not possible. The numbers can be quite inaccurate even if you could.

Individual wheel weighing is best done on flat level ground with people who know what they are doing. You can get individual wheel weights at rallies - Escapees and FMCA for example, Freightliner in Gaffney, SC, and maybe a few other places. Having the ride height set to specs is an absolute must before doing individual wheel weights.

What do individual wheel weights get you -other than pointing out some obvious mis-loading that the owner can correct? You need to inflate all the tires on the axles to the pressure required for the heaviest wheel on that axle. You can also estimate that weight by adding 5% of the axle weight to the number that you use to look up your inflation pressure from the tables. Do your axle weights first, then when it is convenient, get your individual wheel weights.
__________________
Roger & Mary
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (Sold)
luvlabs is offline   Reply With Quote
Old 01-02-2016, 09:54 PM   #12
Senior Member
 
Ray,IN's Avatar
 
Join Date: Aug 2001
Location: North America somewhere
Posts: 13,686
Make this simple.
Load/inflation charts reflect the absolute miminum air pressure to safely support the corresponding weight, at least that is what every one of them state.
All major tire mfgrs. are members of the RMA - Rubber Manufacturers Association. They published a pdf titled, "Care and service of passenger and light truck(LT) tires". Chapter 4 of that pdf is titled, " Recreational Vehicles Applications"(pg 47).
Within that chapter (pg 51) is a paragraph titled " Determining Proper Inflation Pressure", that states:
"
However, never use inflation pressure lower than
specified by the vehicle tire placard, certification
label or owner’s manual."

Some folks choose to ignore that because they desire a soft comfy ride from lower air pressure, at the risk of tire damage or failure.
The RMA states that over 90% of all tire failures are due to underinflation or overloading.
__________________
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 01-03-2016, 07:53 AM   #13
Senior Member
 
jimbo2013's Avatar
 
Newmar Owners Club
Join Date: Aug 2013
Posts: 1,699
Quote:
Originally Posted by montyhp View Post
As far as i know they won't let you do that
They certainly do

Quote:
Originally Posted by luvlabs View Post
Most CAT scales are constructed in suc a manner that driving less than all the wheels across them is not possible.

The numbers can be quite inaccurate even if you could.
I disagree, I weighed the total axles, then did each side and added together were spot on.

The Cat scales work great for true 4 corner (or 6) weights.
__________________
Newmar Ventana 3933 | Miata close behind
1,060 Watts of Solar | 8 T-105 RE Batteries | Outback controls
HR 38 Endeavor
jimbo2013 is offline   Reply With Quote
Old 01-03-2016, 01:59 PM   #14
Member
 
gjurrens's Avatar
 
Join Date: Aug 2014
Location: Punta Gorda, FL but not there for a few years!
Posts: 47
Hey Jimbo,

I can only speak to the only time I've weighed my coach/trailer so far. I've only had her on the road a few days total. I was at CAT Scale #458 at the Pilot Travel Center, I-75 Exit 161 in Punta Gorda, Florida. There was hardly enough room to clear my mirrors, so really no room at all to drive one end of all axles off the scale platforms on either side. Others must be different. Where I can, I'll do that to get corner weights. And I'm going to look at other options, like moving and storage companies, farm suppliers with grain elevators, recycling companies, etc. I want this to be a thoughtful process where I can take my time.

Yellow pages here I come!

Thanks again. Gene
__________________

__________________
gjurrens is offline   Reply With Quote
Reply



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
Valve stems and tire pressures TexasSprinter Sprinter Chassis Forum 7 10-13-2015 07:37 PM
02 DSDP 3872 Tire Pressures Eric Lis Newmar Owner's Forum 16 07-03-2015 08:52 AM
Tire Inflation Procedures broadrun Monaco Owner's Forum 9 07-02-2015 05:40 PM
Class A tire pressures JOC1212 Class A Motorhome Discussions 18 06-28-2015 02:04 PM
Tire pressures oildriller83 Class A Motorhome Discussions 99 06-27-2014 05:30 PM

» Virginia Campgrounds

Reviews provided by


Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 12:08 PM.


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