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Wednesday, September 19, 2012

Adjust tire pressure for better ride?

Got a question from Bob G. from somewhere in TX 

Let me start off by saying I have really enjoyed your informative articles on the Internet.  Your writing style is one that even the inexperienced can understand and learn from.  Keep up the good work, we need the help!

My question deals with inflation pressures.  I know in the past we have been instructed to inflate according to the manufactures weight limit charts.  With all the tire monitors giving us "instant" pressure readings could the cold inflation numbers be modified slightly where the manufacturer

Inflation Number is achieved after the tires warm up?  I know the NASCAR guys do this.

The real reason I am asking about this is a problem I currently have with "expansion joints" on the highway.  We are currently in the middle of a TX-AK-TX trip and my fillings are starting to fall out!  If my front pressure is set at a cold pressure of 75, which is chart recommended for my axle loading, the bump from expansion joints can be tolerated.  As the tires heat up and pressure goes to 85 and above the bump becomes more of a BANG which hits the front end so hard my dash has broken mounting screws in the past.  Now that I have secured it to the chassis the dash is developing a horizontal crack.  With the aid of my TPMS  I could adjust my cold pressure to reach 75 or 80 hot pressure after a few warmup miles on the tires.  I am currently seeing a temperature rise of 60 to 70 degrees above ambient between cold and hot tires.  This reading was taken with a laser temp gauge measuring in a valley between the tread.  I don't rely on myTPMS for accurate temp as it is a TireMinder with the screw on sensors.

I know I could change my shocks to something softer on the compression stroke to take care of the "bang" but my ride and handling characteristics are really good everywhere else.

What say Ye Great Tire Zen Master?

Bob G
Thanks for the compliment Bob, and for reading the blog.

You pose a reasonable question Bob, but you fail to understand that the load/inflation tables are based on the fact that the pressure does increase as a tire is run. 
If you were to lower your “hot pressure” to match the inflation in the table you would probably soon experience tire durability problems and certainly see a hit in fuel costs and your rolling resistance, which is the force it takes to roll a loaded tire down the road, would immediately increase.

If you are seeing 60 to 70 degree increase over ambient you may be running right at the low limit for tire inflation. You didn’t say how old or what size your RV was or the number of miles on it but shock absorbers do wear out and many times even when the RV is new there are better riding shocks available than the low cost ones that might have been selected by the RV manufacturer.

You are correct about the temperature information from your external TPMS. The temperature you are seeing will most certainly not reflect the tire internal temperature. Watch my blog as I am conducting a test right now to collect the facts. I hope to post the report soon.

Bottom Line
Never bleed down hot pressure. You should consider the inflation on the load tables to be your minimum cold inflation. I usually recommend people run plus 10 % over the table long as they are not exceeding the inflation molded on the tire or the max inflation rating for the wheel. If you can’t run the +10% then you probably have tires and wheels that are lower capacity than what you need based on your actual loading.

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