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Old 05-05-2010, 12:30 PM   #15
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Quote:
Originally Posted by Gary - K7GLD View Post
If a person is willing to *maybe* sacrifice a small bit of ride comfort, a REASONABLE degree of added PSI also reduces rolling resistance, and delivers better MPG - a trade-off *I* am sure willing to accept!
Like Gary said. I run 90PSI in the fronts even though Michelin says I only need 75PSI (note my tires are 22.5", 110PSI max cold inflation). The added pressure actually made it track better and the ride still passes the "spouse comfort" test.

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Old 05-05-2010, 01:16 PM   #16
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Originally Posted by msmorto View Post
To All
I need to make a clarification on my original post as follow

This is for (245/70R19.5 LRF XRV) tires
The weight of my motor home fully loaded (Steering axle = 7220 lbactual weight
Drive axle = 12520 lb actual weight) not GAWR
The GAWR from Michelin is Single - 4080 lbs at 95 PSI
dual - 3860 lbs at 95 PSI
Steering axle = 7220 lb
Acording to my calculation ea. Front wheel can carry 3610 lb plus/minus
Drive axle = 12520 lb
Ea.Rear wheel can carry 3130 lb plus/minus
Acording Michelin chart I shoul run 80 PSI
Would 85 PSI be OK for all tires ?
Thank you.
Mario
Mario, Lets see if we can clear up some of the confusion. I believe this is the situation.
1 You have a real weight for the Front axle of 7220# but you do not have the individual RF or LF loading.
2. For some reason Michelin chooses to publish Axle loads instead of tire loads in their tables but the Micheline Axle Rating for your 245/70R19.5 is 4080# per tire Single and 3860# Dual at 95psi cold inflation for a Load Range F tire. This matches published industry guidelines for this size and Load Range.
3. The load capacity for this size tire at 90 is 3890# & 3655# and at 85 3740# & 3515# and at 80psi would be 3640# & 3415# These are the Single & Dual load ratings.
4. Your Rear Axle load is 12,520" total but you do not have the side to side loading.
Now with this information this is how I would proceed:
First until you get data that indicates otherwise I would assume that one side is loaded to 55% of the total or 3971#, So we now look at the load inflation tables and find the load in the table that is above your actual. This means that you need a MINIMUM 95psi in each Front. Normally I also suggest that once you find the inflation in the table you increase the inflation by 5 psi for a "Safety Factor" but in your case that would exceed the Maximum inflation for your tire and possibly the max inflation for your wheel so you can not do this.

Next looking at the rear. 55% on one side is 6886# for both tires or 3443 each. The tables indicate you need a minimum of 85psi in the rears and in this case I would increase this to 90psi cold.

Suggestions. Since you are right at your Max load and Max inflation for the front, I would try and move some stuff to the rear. Also I would try and get a side to side loading to see if you need to use the 55% rule of thumb or not. I would also make sure that your pressure gause is accurate to 1% or in your case +/- 1psi and digital again because you are so close to the Max.

I will answer Flossy's question about temperature in a separate post.

Hope this helps.
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Old 05-05-2010, 01:25 PM   #17
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Is there a standard temperature that the tire inflation tables are based on? I've looked at a couple of different manufacturers but cannot find it. When I air up my tires in Yuma it could be 105 degrees outside, when I air them up in Michigan it's more like 40 degrees. It seems to me that has to affect the weight calculations.

Dave
Flossy, your logic is sound but based on the concept that people should check the air in their tires at least once a month and before each long trip most do not see a 60 degree swing, this is not a stated issue. Plus passenger cars usually have a 15% to 20% margine for additional load.

Assuming you know the REAL minimum inflation you need to carry your load (see my process in the post to mario) and Assuming your pressure gauge is accurate (At a recent Rally I tested over 32 gauges after my Tire Seminar and found 20% off by more than 5 psi) and Assuming tou are not going to drive from 40 degree Michigan to 100 degree Yuma in one day then you have no problems.
Lots of assumptions there aren't there?

Tire pressure only changes about 1 or 2 psi for every 10 degrees change in temperature so in any single day travel you should seldome see more than a few psi change due to temperature. This si not an issue if you use my +5 psi margin as I mentioned in my reply to Mario.

Hope this answers your question.
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Old 05-05-2010, 06:48 PM   #18
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To Bob King
I donot have a GAWR on the printed sheet in the closet. What I do have is this

GVWR--- 20,500 LBS
UVW-----16,800 lbs
SCWR----770 LBS
CCC------2,158 LBS
GCWR---26,000 LBS

Mario
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Old 05-05-2010, 11:09 PM   #19
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Treman9, possibly I wasn't clear in my question. What I was trying to figure out is what is the ambient temperature the tire makers use when they create their load tables. If it is 70 degrees then I can compensate for that when I air up my tires. I haven't in the past tried to compensate for the various temperatures encountered on our trips as we re going down the road. But I wold like to be able to set my initial air pressure with some degree of certainty because the conditions do vary depending on where I'm at. i.e, I would like to air up my tires before I leave and free reasonably certain they were not under inflated or overinflated to begin with.

It is interesting to watch the change in pressure as we are rolling down the road, Fortunately I've finally got sensors that are somewhat accurate compared to my tire gauges.

Thanks for the input

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Old 05-06-2010, 07:40 AM   #20
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Quote:
Originally Posted by msmorto View Post
To Bob King
I donot have a GAWR on the printed sheet in the closet. What I do have is this

GVWR--- 20,500 LBS
UVW-----16,800 lbs
SCWR----770 LBS
CCC------2,158 LBS
GCWR---26,000 LBS

Mario
Mario:

If your MH is on a recent Ford F53 chassis, the front GAWR is 7500lb, rear is 13,500lb. Workhorse W20 should be about the same.

This information should be on a plate provided by the chassis mfr, on our coach it is located on the wall next to the drivers seat.

Stewart
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Old 05-07-2010, 10:59 AM   #21
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Flossy
You are reasonable in thinking that there must be some "Standard" temperature for the load calculations but in reality the answer is to set the pressure based on your real load at the ambient temperature you are at.
When tires are tested the pressure is set based on ambient and not adjusted by some table.

Yes ambient can change and yes this will affect the pressure in the tires when you measure them "cold".

BUT, Cold inflation really means when the tires are at ambient temperature and not artificially heated by external sources such as engine exhaust or sunlight. That is the way tires are tested.

The reality is that unless you wake up some morning and its very cold like less than 40 and you expect to be driving where the ambient is HOT like over 90 - that same day - You probably are just going to make yourself crazy trying to adjust up or down and compensate for the the 3 or 4 psi you might observe each day.

One thing I find is that most people do not have an accurate digital pressure gauge. i.e. read to 1 psi and accurate to +/- 1 psi so trying to set inflation when your stick gauge reads at 5 psi increments and is off by 5 psi is a bit of wasted effort.

Now if you want to entertain yourself you could keep a chart. Each morning record the ambient temperature to within 1 degree and also check your inflation with your digital gauge and record the inflations. Tires should not be in the sun for this test and you need to give tires at least three to four hours to reach ambient so waiting all night is easiest.
Remember that each time you check your air you are letting a little escape so you will have to top off the tires every week or so even if you are not traveling. If you do this for a week of traveling maybe you can let us all know what you learn and if your data suggests a need to adjust I will change my opinion.


Finally before someone asks about adjusting inflation every time you buy something and add it to your load.
Assuming you had your RV loaded to its normal full load i.e. full of fuel, water, propane, food, clothes, tools, people etc, etc and that you got a corner by corner load and you inflated all tires on an axle to the same inflation based on the heaviest loaded side after doing the proper calculations. You also increased the inflation by 5psi for a safety margin, while not exceeding the max rating for the tire.

You do not need to get your rig weighed again unless you are either at the MAX for a tire or you are a rock hound and have taken on hundreds of additional pounds load.
If you are a rock hound then you need to get your rig weighed when fully loaded.

I have weighed my Class-C twice. When I first bought it on my first trip I confirmed loading with front and rear axle loads. Later at my first opportunity I got a corner by corner weight by RVSEF. I have a TPMS system and know the individual sensors are within +/- 3 psi of my calibrated 0.5 psi reading gauge.
Because my unit is light I only need 55psi in my tires. I inflate to 63 using my hand gauge which gives a range of 60 to 65 cold and 70 to 75 hot based on my TPMS readings.
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Old 05-07-2010, 11:42 AM   #22
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How do you know you have an accurate gage unless it is calibrated. Try to find one of those at your local PepBoys. None of my four or five gages test to each other within 5 pounds. I always wonder how accurate the gages are at a tire store. Just because it LOOKS accurate doesn't mean it is. Easy to test thermometers before you buy one, just look around until you find 3 that read alike and buy one of those. Can't do that with tire gages. (Ok maybe it is tire gauges, whatever).

I wouldn't trust a gage to be accurate just because the mfr says it is unless they can provide some type of certificate for that particular (exact) gage. Now if Tireman can point us to a gage that he is comfortable as being accurate, that would help.
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Old 05-07-2010, 11:46 AM   #23
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Tireman9, thanks for the clear explanation, that's the first time I've seen a really concise explanation of the situation. I suspected that was true but it nice to have it confirmed. I like to keep track of my inflation mainly to ensure that my tire pressure monitors are not leaking. I have a TST system installed and have a new set of valve senders.

Like everyone else I'm also concerned about my GVWR. As you probably know the Newmar Mountainaire (gas) is already near it's GVWR coming from the factory. At any rate I think I've got my tire pressures set based on the load I carry. Hopefully at the next rally I can get all four corners weighed to see how balanced I am.

Thanks again

Dave
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Old 05-07-2010, 11:49 AM   #24
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Tire pressure gauges

Concerning tire gauges, does any one have a recommendation of a good one to get. I have a dial one from Camping World and it appears to agree with my older stick type and my tire pressure monitor but they all could be wrong.
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Old 05-08-2010, 01:13 AM   #25
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Quote:
Originally Posted by Flossy View Post
Concerning tire gauges, does any one have a recommendation of a good one to get. I have a dial one from Camping World and it appears to agree with my older stick type and my tire pressure monitor but they all could be wrong.
i recently purchased the psiclops gauge and inflator. it is a very well built unit. it comes in a plastic case. there is enough hose so that you can inflate both front tires on opposite sides of the moho at the same time.
i have used it once and it worked very well.

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Old 05-09-2010, 02:43 PM   #26
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Yes having an accurate gauge is important. I have a couple of Accutire 5-150 psi gauges I got from Sears but they are also available from Tire Rack & Amazon.
When I checked them against an ISO calibrated master gauge one read +0.5 psi and the other -0.5 psi. My TPMS readings are +/- 2 or 3 psi of the Accutire so I know the range of readings I should always see on my TPMS.

I found an interesting series in a forum at Edmunds Car Space by searching the Forums on Tire Gauges. A guy did a series back in 2001 on different gauges ranging from cheap $1 pocket car tire gauges (some off by 25%) and dual head truck tire analog gauges at +/- 2 to 5% and even some digitals including Accutire were found to be accurate to within +/- 1 psi or better.

So bottom line it appears you get what you pay for so expect to spend at least $14 or more.
In general Digital appear to be more accurate than Analog and analog dial are more accurate than stick type.
This doesn't mean you can't get an accurate $3 stick gauge but unless you check it out and keep it clean and exercised as the article mentions you may not be setting the aire pressure correctly.
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Old 05-27-2010, 04:51 PM   #27
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Smile tire psi

I use the pressure recommended by the coach manufacturer, I have 22 1/2 in tires on a 38ft Winne, recommended is 105 front 95 rear
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Old 05-28-2010, 08:03 AM   #28
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I use the pressure recommended by the coach manufacturer, I have 22 1/2 in tires on a 38ft Winne, recommended is 105 front 95 rear
Sounds reasonable but since we all carry around different amounts of "Stuff" I and the other Tire engineere lurking here and the tire manufacturers and the RV Safety Education Foundation and the guys that write the tech articles ALL strongly recomend that you get the real loads on each corner of your coach.

This can best be done at a large RV Rally where this service is often available. Here is some more info.

Some rigs are out of "balance by 5% to 10% side to side. I have seen some reports indicating that over 50% of RVs have one or more of the following components overloaded. Tires, Axle, Springs, Wheels.
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