RV Tire Safety
with RV tire expert Roger Marble
Here is a question posted on a travel trailer forum:
“Why so many trailer tire failures and so few, if any, tow vehicle tire failures?”
This post could be considered Part 2 of my post of last week, How do you figure an ST-type tire’s maximum safe speed?
The primary reason, IMO, is staring everyone in the face and the information is molded onto the tire sidewall so you do not need to be an Einstein, but almost everyone chooses to ignore the science so they can save a few bucks.
Fact: Tire load capacity is a function of air volume (tire size) and inflation pressure. This is well established and is confirmed by every Load and Inflation chart printed. Increase size or increase pressure and you increase load.
If we all drove around on tires with LT-level sizing and inflation there would be almost no failures other than those caused by road hazard or punctures.
But people want a softer ride, better fuel economy and lower cost, so alternatives were introduced into the market.
LT tires can be considered the standard for tires designed to carry load at highway speeds.
P-type tires have a higher load capacity than identical size LT tires, but if you check the actual load on most car tires, you will see they have 15% to 25% or more “Reserve Load” capacity (more capacity than needed). Inflation is set for cars based not on load but ride, handling and fuel economy. You also know that P tires must have their load capacity de-rated when P tires are used in truck, SUV or trailer application. When P-type tires are run with zero reserve load, you will probably get more failures. (There was a giant recall in 2000 of every Ford Explorer made from 1990 – 2000 to replace the tires that didn’t have the capacity to support the weight of an Explorer when there were more than two people in it who weighed more than 150 lbs. each.)
ST tires have a higher claimed load capacity than LT tires, and travel trailer companies use the smallest (lowest cost) tire that can, most of the time, meet the specification for load.
How then is it that ST tires are rated to carry more load?
The load formula for ST tires is based on a 65 mph Max operation speed and an expected shorter life. The formula was developed back when bias tires that wore out at less than 15,000 miles were the norm.
Since 1990, when ST tires were still “Alpha” sized (H78-15ST), the load formula has not been changed as far as I can determine.
The science indicates that if you want the tire durability on your trailer as you get on your truck maybe you need to consider running the same type tires, i.e., LT type. Of course, that means you also need to use LT Load and Inflation limits.
Here is some info I posted September 14, 2011, about tire sizes:
“Here are some facts from the Tire & Rim Association industry standards book:
P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112 mph on a Passenger car
1844# 35 psi 112 mph on a SUV or P/U or trailer (no Dual)
1512# single 1377# Dual 50 psi 85mph
2340# single 2040# Dual 50 psi 65mph“
NOTE I did not mention the actual load on a tire but only the specified max. Weight data indicates many TT have one or more tires in overload.
Read more from Roger Marble on his blog at RVtiresafety.net.