RVtravel.com and RV forums have many posts on the question of “How much pressure?” for tires. Also, there is much made of the adjustment of the cold pressures after fitting new tires. We tire design engineers know that new tires will “grow.” The tire can lose more than 5% of that initial inflation when first installed. Here is an example of my recommendation.
Assuming we want 70 psi inflation in a “cold” tire, what should we do?
All “new” tires do “grow or stretch” after the initial inflation. The amount of stretch depends on construction (radial or bias) and the reinforcement material (nylon, polyester or steel). When developing new tire designs, the measurement of tire dimensions (OD [outer diameter] and max width) is one “test” that is run with new molds or new constructions.
Testing new tires
Our standard procedure is to inflate tires to the number on the sidewall and place the tire in a 100°F chamber and wait 24 hours. The 100°F softens the rubber and allows the tire to take a “set” as it grows.
While we are talking about rubber, the “recovery” to the original size is not 100%. So the tire will now be a bit larger than it was when first mounted, even if all the sir is let out of the tire.
After the 24-hour “growth” period, we would then adjust inflation to the desired level for the rest of the initial testing to take place, before we continue with other testing.
Don’t forget, we are trying to measure a new tire to an accuracy of +/- a hundredth of an inch. If you check the tire industry standards books or the data books published by tire companies, you will see the dimensions are shown to the hundredth.
This “new tire growth” is normal and is observed in all brands and types of tires.
Whenever I get a new set of tires, I always request the tire dealer set the initial inflation on them to at least 10% to 15% greater than my goal. That way I can easily bleed the pressure down slightly to where it’s just right.
RELATED
- Learn the basics of setting tire pressure and testing TPMS
- The importance of knowing your RV’s ‘4-corner weights’
- If I upsize my RV’s tires, can I lower their inflation?
- A relevant question (plus much info) on 4-corner weights
- What does RV “weight creep” have to do with tire safety?
*****
If you have tire questions, check out Roger’s posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.
If you still have a question for Roger after searching the above posts, send your inquiries to him using the form below.
RVT1239
While you’re at it, ask the tire store if you can return soon and have them retorque the lug nuts. They did use a torque wrench on your wheels didn’t they?
Good idea.
I had a weird thing happen last week putting air in my RV tire. When I got to the 3rd tire. The Dewalt portable air compressor stopped. Changed batteries, same thing happened. When I unscrewed the compressor hose off the valve stem. Seemed like the valve stem innards stuck open. Screwed the hose back on. Got my valve stem tool, checked the tightness of the innards. Pushed it in a couple times. Air stopped. Couple days later I was able in inflate to 80# when temperature was warmer. Is it possible moisture Froze in the valve stem keeping it open?
I guess that’s possible. If you unscrew the valve core and get drops of water “spitting out” that would indicate that you need to use an air dryer when inflating your tires. I have a couple posts on my blog on the topic of Dry Air and how to get dryer air for your tires.
Thanks
Roger, as always, an interesting and informative article. I was today years old when I learned that new tires “grow” after initial inflation. I am about to have two new front tires, with Rettrobands, installed in TN. I now know to ask the installer to inflate to a little more pressure than I usually run, and I will check them carefully to see where they stabilize. Thank you!
You are welcome. I have read good things about Retrobands.