THANKS TO OUR SPONSOR!

THANKS TO OUR SPONSOR!
Your Ad here
Be sure to sign up for the weekly RV Travel Newsletter, published continuously every Saturday since 2001. Click here.
Huge RV parts & accessories store!
You have never seen so many RV parts and accessories in one place! And, Wow! Check out those low prices! Click to shop or browse!

Sunday, February 2, 2014

What minimum inflation should I run in my RV tires.

This question gets asked for a variety of reasons. Here is one example...

"My Winnebago was recently weighed on the four corners.  The weights confirmed previous truck scale results.  The Goodyear tire chart for my tire begins at 80psi.  The weights are well below those shown in the chart for 80psi at all corners.  Is it OK to run 80 psi or would you recommend say 85 so any slow leakdown would keep the pressure above 80?  I don't recall ever seeing you address this issue.  In other words, what is the minimum safe pressure for a tire even if the load is less than what the minimum will support?"

Simple answer first. Yes you need to run at least 80 and I doubt there would be anything wrong with 85psi.


Now if you want the more involved answer here it is......You didn't give the numbers but I think you will understand the following..

Lets assume the corner weight is  4,000#   If you have a Goodyear 255/70R22.5 according to their chart you see that tire is rated 4,190 @80 and 4,370 @ 85 or an additional 180# more capacity for 5 more psi
Now the load capacity is a direct function of inflation pressure. While the formula involves exponential functions, I don't think that we have to get to higher math to answer your question. So lets consider it reasonably linear over small changes in inflation so if you look at the 90 psi rating you see 4,550 which is 220# more capacity for 5 additional psi if we go up to 95 psi we see an increase in load capacity of 125#.  So we see a 180#, 220# and 125#  increase in load capacity for each increase in pressure of 5psi. I would suggest we use 200# capacity for 5 psi as a reasonable compromise.

4,190 - 200 = 3,990#  which is almost equal to the estimated 4,000# so this implies that 75 psi might be theoretically OK but we need to stay with the numbers published for your tires from the manufacturer of your tires so 80 psi would be the minimum cold inflation.

So 80psi would give you about a 5 psi margin over what is needed for the load. If you have read other posts you will see that I normally suggest a 5 to 10% increase in inflation so you are not chasing the pressure when it goes up or down a couple of psi as you travel across the country and the changes in ambient temperature affect your tire pressure or with the 1 psi or so normal leak-down seen in almost all tires.
Now if the corner load is actually 3,000# then in theory the tire might only need  55 or 60 psi to carry the load. BUT running 80 will not hurt the tire and all that might happen is you would have a little harder ride.

Again, run no less than the 80psi shown in the charts unless you can get something in writing from GY corporate, NOT A LOCAL DEALER but from corporate engineers in Akron, OH.

It may interest you to know that on my Class-C RV, I am running between 10 and 15 psi above the minimum pressure needed based on my actual corner loads and have had no problems.

In closing you need to be thankful that Winnebago selected tires with plenty of extra load capacity.

 Hope this helps.

2 comments:

  1. Hmm, so, can you deduct the actual weight of the tire and rim from the weight the scale shows since the tire itself is sitting on the ground and is not "suspended" weight?

    ReplyDelete
  2. Sorry no. We are not talking about "unsprung weight". The issue is the heat generated in the structure of the tire due to bending. More load (or less inflation) results in more heat. Other than external causes such as punctures or cuts, Heat is the #1 killer of tires and the damage is cumulative and non-reversible.

    ReplyDelete

Thanks for your comment. We look at each one before posting to keep away the spammers.