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Friday, June 10, 2022

What margin should be used for Low Pressure warning on TPMS?

 In response to a reader from Australia with a question on TPMS Low Pressure Warning level, I responded...

We need to separate the settings specified by US DOT for regular passenger car applications and the more specialized world of Caravans (RVs for us Americans) (both motorized and pull behind).

DOT is trying to protect the worst case which is operating a tire in overload. Most cars have their tire inflation set based on the goals of the manufacturer to meet fuel economy goals while delivering acceptable ride & handling the expectations of the consumer. With our current Federal fuel economy standards, this results in fuel economy being #1 or very near the top of the priority list. This means that almost all tires have sufficient pressure to significantly exceed the load capacity requirements. I have seen some data that shows +30% is not an extreme level. So that means a loss of 25% of air pressure still keeps the tire out of the "overload" situation.

Now when we move to the Caravan market, there are no regulations for fuel economy. So marketing pressure of low cost takes priority. This means the RV manufacturer selects the lowest cost tires possible. While in many cases this means tires that just barely meet the DOT safety requirement tests, it also means the smallest tire size sufficient to support the load as this translated to smaller (lower cost) wheel well space and smaller (lower cost) wheels. This results in most tires needing to specify higher inflation (more load capacity) than if the tire were applied to a car. The bottom line is lower levels of actual reserve load capacity with data showing that the average user ends up overloading the tires or axle or both.

I am a strong supporter of tires & inflation that provides at least a 10% reserve load for motorhomes with towables goal of 15% reserve load being needed to address the Interply Shear forces seen in multi-axle towables. These are what I would consider MINIMUM reserves.

To achieve these smaller margins we need to recommend the Low Pressure setting of TPMS is closer to 0% air loss from the cold "set pressure". If we were to allow a 25% air loss before the TPMS Low Pressure sounded, the owner could have been driving hundreds of miles in an overloaded tire condition.
Here is a post on how I set the TPM system on my Class-C motorhome. 


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