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Old 05-08-2009, 06:20 PM   #1
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Correct tire pressure?

Hi Folks!

I have a 2005 SeaBreeze on the 2006 F53 chassis. The coach was manufactured in April 2005 and the chassis January 2005. The sticker in the coach says the tires should have 80lbs of pressure in all 6.

I am kind of playing a little numbers game because I have yet to weigh the coach. I am, for the sake of argument, assuming we are at our weight limit which is 7000lbs on the front axle and 13500 on the rear.

I took the 7000 from the front axle, divided it by 2 and came up with 3500. Then, I looked at Goddyear's inflation chart and found that for my tire size (245/70R19.5- prior to 2/2006 manufacture date) I should have 70lbs and that is for 3650 which put me at a total of 7300lbs for the front axle.

I then took the 13500 from the rear axle capacity and divided by 4. That came to 3375. After looking at the Goodyear chart, the lowest recommended pressure is 70lbs. With 4 tires at 70lbs I'm good to 13660.

This gives me a total of 20960 based on tires pressure.

My question is: does this sounds right? Have I calculated correctly?

Here is a link to the chart:

http://www.goodyear.com/rv/pdf/rv_inflation.pdf

Thanks!
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Old 05-08-2009, 07:51 PM   #2
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It sounds right according to the charts but according to a lot of tire experts you need to always run 10% over the charts for safety. You will not damage your tires at 10% over the recommended inflation but you may damage them if 10% underinflated. You are correct in that you do need to weigh the MH fully loaded and fueled just like you are on a trip.
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Old 05-08-2009, 08:07 PM   #3
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I might suggest that the 10% figure is used purely to compensate for left/right weight imbalances **when you only weigh an axle**. There can be substantial left/right weight differences on an axle. So a 10% uptick would be fine until you get the 4-corner weigh.

The tire charts are produced by the folks that made the tire AND more importantly have the equipment and labs to validate what they publish. So called "tire experts" are quoted from opinion and then mis-quoted by continual repetition (you know the ol' pass what you heard around the room game).

Hence, once you do a 4-corner weigh stick to the charts. Period. Randomly inflating 10% for no reason is just that, randomly inflating for no reason. Letting hearsay from so called "experts" override the manufacturer's testing and specifications and lab analysis is plain hearsay and with no facts.

When in doubt go to the horse's mouth, the maker - or their designated lab. Anywhere else, unless also a fully accredited lab, and you only have opinion.
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Old 05-08-2009, 08:12 PM   #4
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After running for an hour, my front tires go from 105 PSI to 115/117 PSI depending on road surface temps. My rear tires go from 85 PSI to 97/99 PSI running for the same time. So why inflate 10% over recommended pressures when the pressure will increase that much just by running down the hiway? Pressures are checked with Pressure Pro.
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Old 05-08-2009, 08:17 PM   #5
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You're on the right track. However, I'd really get each corner of the coach weighed before going that close to the axle rating, as one corner of the coach may weigh a lot more than you think.

To be safe, I'd put 5 psi more in each tire than the minimum, that would get you a comfortable safety margin according to the chart. That is what Michelin recommends for constant high speed driving.

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Old 05-08-2009, 08:31 PM   #6
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Tire pressure sure is a subject that gets attention

There were 3 posts while I was composing my answer!

BigSkyBob, the tire manufacturers take heating and hot tire pressures into account when creating the charts, that's why they always tell you to measure pressures cold.

As long as you have your cold pressures at the proper level according to the charts you are OK. However, as RVDude hinted you really need to know your corner weights if you are going to reduce the pressures to the chart's level.

In my case, when I reduced my steer tire pressure to the chart's recommended level for my front corner weight my handling went south, so I'm back up to close to what the drive tires are at. Fortunately I have no ride problems at this pressure.

JMHO, YMMV,
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Old 05-08-2009, 08:35 PM   #7
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Tires heat up with use. The tire designers take that heat induced increase in pressure into account in their tables and loading designs. Never ever ever ever let the air out of a "hot" (as in **driven**) tire to meet what is a "cold" inflation spec. Tire pressures are always measured "cold" (cold means **undriven**) at the ambient temperature for the area you are in. Given you'll drive in the heat of the day and the cold of the night a good thing is to measure your pressures in the morning thus getting a good "average" ambient temp for your area. And yes, if you never lost a drop of air you would still have to fill your tires when the winter turns ambient cold and yes let air out again when the summer turns ambient hot. But pressure is ALWAYS measured "cold" that is **undriven** at the local temperature. And if you store your rig/car in a heated garage and drive it in freezing winter, you need to set the pressure when the tires are at the outside temp (not the heated garage temp) or consult the tire manufacturer's correction tables.

BTW - get a good manual gauge. You may be surprised at the error of even PressurePro. BTW - The SmarTire TPMS system accounts for tire heating and the resulting psi increase in it's temperature corrected algorithms thus allowing tighter high/low alarms than PressurePro and other external mount system can achieve.
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Old 05-08-2009, 08:39 PM   #8
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Big Sky Bob. The tires are always going to heat up and increase pressure so I am not talking about that 10% increase on a hot tire. All the Motor Home and Family Coach magazine in their tire tech articles always talk about increasing the tire pressure gained from weighing the MH and looking at the charts by 10% COLD pressures. Since the MHs are most likely always run a highway speeds this will make sure the tires run a little cooler and not result in compiling overheating damage over time that will cause belt separation.
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Old 05-09-2009, 07:26 AM   #9
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I have an 05 Dolphin 3555 on a w22 chassis. The tire pres.on the info sheet in the MH is for GVWR. I run 5 lbs over that and with over 20,000 miles and almost 5 years my tires show no signs of sidewall cracking or uneven wear. I have not weighed mine because every time I take it out I do not load it the same. I am sure I don't overload it. The worst thing you can do is under inflation.
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Old 05-09-2009, 08:48 AM   #10
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Thank you for all the replies. I guess what I'm saying is that, properly loaded, the tires at their minimum inflation requirement are sufficient for my coach's maximum weight rating by axle and in total.

See if I got this:
When I weigh it, if I am over weight, I will need to remove some items or re-distribute the weight to meet the ratings. Adding air won't help because I'll still be overweight.

If I am under maximum weight, there's nothing to do because the tires need to have at least 70lbs. of pressure in them.

I may or may not want to add 5lbs or 10% based on what makes me comfortable.

Is this right?
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Old 05-09-2009, 10:15 AM   #11
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Quote:
Originally Posted by tps0424 View Post
If I am under maximum weight, there's nothing to do because the tires need to have at least 70lbs. of pressure in them.
If I might amend this to say, if your actual weights are under GAWR for each axle thus also implying under GVWR fill the tire to what the tire table says to for the weight they are actually carrying.

And let me add one, never exceed the cold inflation max pressure printed on the sidewall.

Your other two comments are spot on.
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Old 05-09-2009, 10:36 AM   #12
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Quote:
Originally Posted by RVDude View Post
If I might amend this to say, if your actual weights are under GAWR for each axle thus also implying under GVWR fill the tire to what the tire table says to for the weight they are actually carrying.
This is where I think I need help. For my tire size, I can not let any air out because I need to have at least 70lbs. of pressure in the tires. According to the chart, 70bls is the minimum for the tire, if I am reading it right. 70lbs. is also appropriate because it is more than enough to support the GAWR and I should not exceed that rating.

In my case is 70 both my minimum and my maximum?

Sorry for the questions but, I'm concerned for my family's safety.
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Old 05-09-2009, 11:00 AM   #13
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tps - interesting, you got some big tires on that rig - yipee! Usually rig makers use the smallest tire size possible (max inflation required and harsh ride) as smaller tires = cheaper cost.

I'll first say I am surprised that the Goodyear tables are not set up for each of the specific Goodyear tire brands. I guess Goodyear has just the one RV tire??? Michelin for example has a table for each of their brands of RV tires (XZA, XRV, XZE, etc...)

You are correct, given the weights and tire size in your OP at 70psi your tires are able to handle BOTH the GAWR (7,000f/13,500r) and GVWR (20,500) AND everything less. Never seen a rig where the tires were sized at the factory like yours. That's a very good thing. I would have expected 225 tires on it and you can see where the inflation pressures make a big difference for your rig if you had 225's. 70psi all around in those 245's at the weights you report. Cheers!
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Old 09-25-2009, 01:45 PM   #14
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People are setting air pressures based upon axle weight, but what happens to your pressure if you lose a tire? All the weight is instantly re-distributed to the remaining tires. I think having some margin is wise by airing up to the max.

Also, duallies should be pressure balanced? You don't want 115 psi on the inside tire and 120 psi on the outside tire. Another issue, if tire pressures were so precise, wouldn't you need to reset the pressure constantly to accomodate load variances? that's why I just go with the max.
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