Tag axle tire pressure ?

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93southwind

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Jan 22, 2008
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Incase you havent seen my last couple posts, Im working a deal on a 93 southwind 37'er. I'll be going over today to take some good pics, look over it some more and i want to drive it as well, and thats the reason for my question.  It has the dual rear axles, and i want to check the tire pressure and make sure i have them set properly so that i get an honest feel of how it drives.  Thanks for any help you folks can be.
 
The tag axle tires should have the same pressure as the duals in front of them. Unless there is some serious weight difference (doubtful, and which you'd only know about if you were able to weigh each axle independently) all three on a side will carry approximately the same weight and require the same pressure.
 
Be sure to use the appropriate dual or single axle pressures for the two sets of tires.  They won't be the same for the same load.
 
Look at the charts.  For the same load on the tire, they run at different pressures in dual vs. single use.
 
Example:  The GY 275/70R22.5 with 5180# per tire in dual use requires a minimum of 95psi.  With 5170# in single use it requires only 85psi.
 
Here's an example of tire inflation tables. Scroll down to the second table and note the S and D annotations, which is what Ned was referring to.

Be sure to use the inflation tables for your brand of tire. The example was from for GoodYear.
 
Understood, but the de-rating of a tire in a dual configuration is done to take into consideration the possibility of one of the duals going flat, and the remaining tire having to carry twice the weight. By reducing the pressure of the tag tire, you also reduce the maximum it can carry and the safety margin it provides at full inflation pressure for the weight. I think I'll stay with inflating all three to the same pressure.

Anyone else have an opinion?
 
I would use the tire manufacturers recommendations.
 
Monaco's comments about the slideout side being heavier is too simplistic.  If you load the motorhome properly, you'll put more weight on the opposite side to balance the side to side weights.  Just assuming one side is heavier is misleading and could cause problems.  There is no substitute for actual wheel position weights.

I'm glad we don't have a tag too.  I've never wanted one and probably will never have one.  Some motorhome manufacturers added tag axles to try to carry more weight than the chassis was designed for.  That's not the same as the chassis manufacturer designing the chassis for a tag axle.
 
Karl, I have always used your logic on the air pressures for the tag and dual axles and plan to continue.  lou
 
In Monaco's case, of course, they design and build their own chassis and each one is deigned for a defined set of coaches.
 
but the de-rating of a tire in a dual configuration is done to take into consideration the possibility of one of the duals going flat, and the remaining tire having to carry twice the weight.
Click to expand...

That's not what was explained to me, Karl, but I don't know if the source was "authoritative" or not.  In my version, the pressure in a dual configuration is higher to keep the sidewalls straighter, i.e. less bulge under pressure, so as to avoid contact between the two tires and reduce heat build-up.  An extra 10 psi or so does not seem to me to be significant in the case of one tire failing.
 
Good article (one of many excellent ones at the Tire Rack site) but my reading of it does not find anywhere that it says the dual pressure difference is intended to provide additional carrying capacity in a failure situation.  In fact I see no explanation at all of why the load rating is different for single vs dual.  We may have to settle for our conjectures and leave it at that. The important thing is that there is a difference - all sources agree on that.

If I weighed the rear axle pair as a unit, I would probably set all the rear pressures the same, tag and dual. But if I were able to get separate weights for the tag axle and the drive axle, I would set the pressures of each per the inflation table and use the dual rating for the drive axle and single rating for the tag.  On the only tag rig I owned (a 96 Southwind), the tag carried only about 4000-5000 lbs, considerable less than the dual drive axle. The tag GAWR was only 5k, while the drive axle GAWR was 11k, as I recall.  I inflated them all the same, as you suggest and had no problems in two years of use.  Perhaps this is a non-issue, something for Rvers to debate around the campfire but signifying little in the great scheme of things.  ???
 
Gary,
As you said, there's probably no one definitive answer. I've read about things like road crowning/sloping, wheel offset, rapid tire deflation, sidewall flexing, and probably others long forgotten.
something for Rvers to debate around the campfire
Click to expand...
ANd here I thought the only acceptable topics were black water tank sensors, toilet paper, the weather and, of course, people who are not there. ::)
 
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