with RV tire expert Roger Marble
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 F to 400 F required to melt the polyester cords.
I 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.
An “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.
A 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.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
A LT tire dually axle set at 70-psi has an axle capacity of 1000# less than same axle at 80-psi; considering 5-10% variance/ tolerance issues on gauges, including TPMS sensors, TIGHT alarms can be critical, and [lowering jamb label to weight chart to improve ride] puts you much closer to CRITICAL pressures …I.e. LESS room for error/ less time for action?
One rule to ALWAYS follow, never rely on technology. In this instance inspect tires at every fill up. Before you start the season, go over the tires carefully, especially if they are getting to the outside date range. Not a criticism, just a practise.
After reading Roger’s article a long time ago I quickly changed my TPMS Low Pressure Aim to be the minimum psi required for the weight of my coach. When first starting out it may alarm a little bit until the tires warm up and the psi increases above the LPA but at least I know exactly what I am dealing with. That early morning alarm is a small inconvenience in order to drive on safe tires.
Dr4Film Not sure if I completely understand. If the scale weights indicated you need 70 psi in a tire I would set the low pressure level to 70. From reading my http://www.RVTireSafety.Net blog you also know I recommend that people add 10% pressure above the minimum required to avoid having false alarms with the normal daily +/- a couple psi