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. 

 

##RVT1056

Why do ST type tires have more pressure gain than LT type tires?

 The main reason for ST type trailer tires to gain more pressure than the LT tires, is that they are forced to support more load relative to their size & inflation than LT tires are.

If you ever look at the Load & Inflation tables and find an LT and an ST type tire of identical physical dimensions you will see that the ST tire is “rated” to carry more load than the LT type tire is. The basic theory behind that increased capacity is that ST tires will be traveling slower because people should not be traveling as fast when towing as when just driving the car or truck. When ST tires were introduced they were limited to 65 MPH MAX in an effort to offset the damaging effects of higher loading than seen in LT tires.

Tire load capacity is basically a function of volume and pressure as seen here Load = K x (Air Volume x Air Pressure) with different type tires having different “K” factor. Tires in LT applications are required to support lower load as a percent of their volume and pressure so they do not have to “work”as hard so they do not generate as much heat.

More heat means a greater increase in pressure. (Approx 2% pressure increase for each increase in temperature of 10F)

Please note that the actual load calculation is much more complicated as the response to air pressure is not linear and different aspect tires i.e. 75 series vs 85 series etc have some different factors that are applied to the actual calculation.

 

 

Please note that the above is just one page of many used to calculate load capacity for tires not already published in the TRA Yearbook, so do not go and try to check every size. I show this to demonstrate that the calculations are complex.

##RVT1055