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RVNotebook · 07-21-2005, 12:09 PM

I'm looking for a consensus of opinions about the placard posted in my Mountian Air with Workhorse 22 chassis. It has been my belief through what I have read that the PSI posted on the placard is the minimum that the coach manufacturer recommends. This sometimes conflicts with the tire manufacturer load charts.
Whats the correct answer?

By the way, can I get my karma changed?

RVNotebook · 07-21-2005, 12:09 PM

I'm looking for a consensus of opinions about the placard posted in my Mountian Air with Workhorse 22 chassis. It has been my belief through what I have read that the PSI posted on the placard is the minimum that the coach manufacturer recommends. This sometimes conflicts with the tire manufacturer load charts.
Whats the correct answer?

By the way, can I get my karma changed?

edgray · 07-21-2005, 12:27 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I have read that the PSI posted on the placard is the minimum that the coach manufacturer recommends. This sometimes conflicts with the tire manufacturer load charts.
Whats the correct answer? </div></BLOCKQUOTE>
RVNOTEBOOK: I believe you should get accurate actual axle weights, and set the pressure according to the tire mfg. weight charts. It won't hurt to add an additional 5 psi to allow for unequal loading, up to the tire's max. inflation limit, of course.
I tried to help with your Karma request, let me know if you want something different. ED

tmitchell · 07-21-2005, 05:05 PM

I had the same question on my 2002 DS W22. The sticker showed 100 lbs. front and rear. The tire chart showed 80 lbs for my weight. I called Newmar customer service and asked them where they came up with the 100 lb. figure. They said the tire company gave them the number. I called the tire company (Goodyear) they said for my weight 80 lbs. would be right.

RVNotebook · 07-22-2005, 03:07 AM

Thanks Ed and Tom.
The reason for my confusion is the following excerpt from the Goodyear's web site.
"Tire pressure should never be reduced below the vehicle manufacturer's recommended levels to support load conditions in order to improve the ride quality of a vehicle. The difference in ride quality is not significant. When minimum inflation pressure requirements are not met, tire durability and optimum operation can be affected."
So I'm still a little confused but I assume you should follow the tire manufacturers load chart else they wouldn't provide them for RV tires.

Ed, thanks for changing my karma.

Mickey

edgray · 07-22-2005, 05:26 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">So I'm still a little confused but I assume you should follow the tire manufacturers load chart else they wouldn't provide them for RV tires.

Ed, thanks for changing my karma. </div></BLOCKQUOTE>
Yes, I think you've got it. and you are welcome. always glad to help out a fellow Newmar owner

ED

Cruzer · 07-22-2005, 05:45 AM

There's generally 3 different PSI ratings for tires. There's the tire manufacturer's "minimum pressure" for a given load, there's the coach builders placard rating, and the maximum pressure/load rating stamped into the sidewall of the tire.

You never want to exceed the maximum pressure stamped on the sidewall. That (and the rating of your rims) is the high end that you should not exceed.

The tire manufacturer's charts are accurate. They will tell you, on a per tire basis, exactly how much air pressure is required to sustain any given load. If you drop under this number the tire will overheat and can fail because it's working too hard. If you go over this number (but still under the sidewall "max" rating) the tire won't fail but it will tend to ride higher in the middle. This can reduce the tire contact patch and also tends to wear out the center of the tire faster than the outside tread but generally this isn't a big issue with RVs because the tires tend to wear out from age rather than mileage.

Note that the tire charts are based on a "per tire" minimum PSI for a given load. The biggest problem is that RVs aren't equally or consistently loaded. There can be differences in side to side weights and not everyone goes through the trouble of doing a 4 point weight check so you don't really know what each tire is supporting. Plus, coaches vary quite a bit between unloaded and fully loaded weights. Water, fuel, LP, and holding tank levels vary and also the cargo, and weight of the occupants can change. For this reason the coach builder has built in a certain "fudge factor" to allow for safe operation under these conditions. This will be higher than the tire pressure charts. However, if you have an accurate 4 point scale reading under the heaviest of all possible conditions you can use these numbers when airing your tires. That's why the common consensus is to weight it, check the chart, and add another 5-10 PSI for safety. Or, you can just go with the coach builder's placard.

RVNotebook · 07-22-2005, 08:32 AM

Thanks all,
The reason for my request for input is for use with my Mountain Aire and a feature of a program I wrote which calculates the PSI for a given axle load. The program produces an alert if the recommend PSI from the tire manufacturer is less than the RV manufacturers placard information.
It looks like I will go with the tire manufacturer's load charts for the Mountain Aire and revise the alert to encompass Cruzer's remark about weight distribution.

The input has been very helpful (as always from irv2)