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Sunday, March 22, 2015

Repeat failures on Airstream trailer. Why?

Saw this question of an RV forum on trailers

"We just acquired 16" Sendel wheels and a set of XPS Ribs for our 2009 28' International to help deal with our tendency to have a front left wheel blowout. We have been through a set of Goodyear Marathons and a set of Towmarks (e-rated, 80 psi cold max). I have not had access to individual scales, but I have reason to believe that the left front is carrying more weight because we have 130 lbs of extra battery forward in the coach on the left side. We will be addressing that by replacing all 260 lbs of battery with 84 lbs of lithiums. This will result in an overall tongue weight reduction of close to 100+, but most importantly, it will take at least 50 pounds off the left front, too.

The question is proper inflation. Our Airstream weighs 5960 on the axles when the WD hitch is engaged. Our tire dealer divided by four, which of course, is not accurate, as we can guess that there is much more weight on that particular wheel. But I'm getting input to inflate to less than 80psi when checking on a cool or cold early morning based on the reasoning that the 80psi recommendation is for cold pressure at around 68F. The theory is that as the ambient pressure rises, that same air will increase in pressure, with the coach just sitting there. Therefore, if one inflated to the max 80psi at 68F, that would be way over a "cold" tire pressure taken later in the day when the temp hits 100F. This is what they say all there 18 wheeler customers do when running on a day that they know will be considerably hotter later--and I've had the same advice from three different tire dealers.

Would our resident tire expert care to "weigh in" on this subject? It would be most appreciated.

You are correct that simply dividing by 4 does not give you the actual load on each tire.

The worksheet on THIS page will show you how to do the math.

I have a blog post  with an example how to get your weights for free. Some folks have had success in contacting their State Police and asking if they could go to a location where they are checking trucks.

But setting aside the need to confirm your load balance.
I suggest that to lower the internal structural forces that are trying to tear the belts apart, you need to set your "cold" inflation to the max on the tire sidewall. The "why" of this was covered in this post. My position was developed based on the work done by Turner & Ford as seen in the book "Mechanics of Pneumatic tires" pub by U.S. Department of Transportation, National Highway Traffic Safety Administration, 1981 .

Now to your specific question on how to set the "cold" pressure.

I covered pressure vs temperature in THIS post  and when you review that post you will see that the pressure in a tire will change by about 2% for each 10°F change so in tour example of 68 vs 100F I would expect about 6% change. The intention of the "Set the pressure in the morning" is to not use a warm (higher than ambient) tire pressure when it has been in the Sun.

I think the reality is that if you check your tires in early AM before you start your travels and before the tire is in full Sunlight, you will be fine for the rest of that day, no matter what the temperature is over the rest of the day.
Tires generate heat internally (not on their surface as many think). This heat is transferred to the moving air around the tire and wheel until a steady state is reached where the heat being generated equals the heat flowing away from the tire.



Hope this helps



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4 comments:

  1. Worksheet link is bad. You have two web page links combined. Just remove the first one, and it works fine, taking you to the intended Bridgestone web page.

    ReplyDelete
    Replies
    1. Got it fixed, bbadwolf. Thank you very much for letting us know. We're sorry for the inconvenience. Have a great day!
      Diane at RVtravel.com

      Delete
  2. We had new tires put on our 5th wheel, went about 60 miles, one blew. Seems the tire shop put the wrong valve stem on and there was a very s l o w leak, only while moving..They checked all tires and replace the wrong one, now all is well.

    ReplyDelete
  3. Your example of valve failure shows how easy it would be for some to incorrectly declare the tires were "defective"

    ReplyDelete

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