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Friday, December 27, 2019

I run TPMS but the tire still came apart. Why?

I found a post from a fellow RV owner who has a TPMS but still had the tire come apart as seen below.

The tire in the picture shows clear signs of a "Run Low Sidewall Flex Failure" i.e. that sharp line around the sidewall with the melted body cord showing.
Yes, the driver got a warning of air loss but I wonder what the Low-Pressure warning level is set at, or if the system has a "rapid air loss" setting.

The "melt line" takes a few miles to develop with significant loss of air (maybe 25 to 40%) while still running at highway speeds. Just driving a mile or so with a 10% loss as you slow down to pull over is usually not enough time to get the body cord to the 350 to 400F required to melt the Polyester cords.

As I have covered how I program my TPMS in this blog back in August 2017. I suggested that the Low-Pressure Warning level be set such that the driver gets a warning as soon as the pressure has dropped to the pressure needed to support the tire load based on scale load and tire Load / Inflation table numbers.  Hopefully, those who subscribe to the blog or those who have gone back and read the posts followed my procedure and have adjusted their TPMS settings according to their RV vehicle needs.
This may take a little thought and effort depending on the brand TPMS. Some may allow you to directly set the low-pressure level while others may require you to calculate the required "set" level as they have a fixed percentage of air loss before they start the warning "beep".

Some systems provide a warning as soon as just a few psi is lost (from the higher hot pressure level) from a puncture.
Example  Your cold set pressure is 80 based on a minimum required inflation to support the load of 70 psi.  When running down the road at 60 mph your tire is probably at 88 to 90 psi. If your system only warns with a 25% pressure drop from the "set" pressure that means you can travel many miles before you get a warning at 60 psi. At that point, you have overloaded your time for many miles.
"Early Warning" or "Rapid Loss" feature might warn you as soon as your pressure drops from 90 to 87. This allows you to monitor the pressure as you look for a location to pull off the road.
TPMS is a powerful safety device but to get the maximum benefits it provides you need to do a little more than just screw the sensors onto your metal valve stems.


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